US20050069007A1 - Q-switched CO2 laser for material processing - Google Patents

Abstract

A simultaneously super pulsed Q-switched CO₂laser system for material processing comprises sealed-off folded waveguides with folded mirrors that are thin film coated to select the output wavelength of the laser. The system also comprises a plurality of reflective devices defining a cavity; a gain medium positioned within the cavity for generating a laser beam; a cavity loss modulator for modulating the laser beam, generating thereby one or more laser pulses; a pulsed signal generation system connected to the cavity loss modulator for delivering pulsed signals to the cavity loss modulator thereby controlling the state of optical loss within the cavity; a control unit connected to the pulsed signal generation system for controlling the pulsed signal generation system; and a pulse clipping circuit receptive of a portion of the laser beam and connected to the pulsed signal generation system for truncating a part of the laser pulses.

Description

CLAIM OF PRIORITY
• This application is a continuation of U.S. patent application Ser. No. 10/116,392, filed Apr. 4, 2002, entitled “Q-Switched CO₂Laser For Material Processing,” which claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/281,516, which was filed on Apr. 4, 2001 and is incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
• This invention relates to short pulse Q-switched and simultaneously super pulsed and Q-switched CO₂lasers and more particularly to such lasers in material processing.
BACKGROUND
• It has become well appreciated in the laser machining industry that machined feature quality is improved as one utilizes shorter laser pulse widths and higher laser peak intensity in drilling holes. More specifically, the geometry of holes drilled with lasers become more consistent, and exhibits minimal recast layers and heat-affected zone around the holes as the laser pulses become shorter and their peak intensity becomes higher (XiangLi Chen and Xinbing Liu;Short Pulsed Laser Machining: How Short is Short Enough, J. Laser Applications, Vol. 11, No. 6, December 1999, which is incorporated herein by reference).
• It is desirable to have the highest quality at the lowest cost but often one must choose a compromise. High-machined feature quality means low recast layer and heat-affected zone thickness, small surface roughness, accurate and stable machined dimensions. Low cost of ownership means a quick return on the investment made in the purchase of the laser machining equipment. Low cost of ownership also involves low maintenance, low operational costs, and high process speeds and yields in addition to low equipment cost. The choice of the laser parameters such as wavelength (IR, near IR, visible or UV lasers) and operational pulse format (milliseconds, microseconds, tenths of microseconds, nanoseconds, picosecond or femtosecond duration pulses) depends on the particular process, material design tolerance, as well as cost of ownership of the laser system.
• Moving from lasers that function in the IR region (i.e. CO₂) to the near IR (i.e. YAG or YLF), to the visible (i.e. doubled YAG or YLF), to the near UV (i.e. tripled YAG, YLF or excimer lasers), the trend is toward higher equipment cost in terms of dollar per laser average output power and lower average power output (which are disadvantages) while also having a trend toward higher power density (w/cm²) because of the ability to focus shorter wavelengths to smaller spot sizes (which is an advantage).
• Moving toward shorter pulsed widths, the laser costs and the peak power per pulse and therefore power density (W/cm²) both tend to increase, while the average power output tends to decrease which results in the cost in terms of dollars per laser output power to increase.
• The recast layer and heat-affected zone thickness are greatly reduced when using nanosecond pulses over millisecond and microsecond wide laser pulses. (XiangLi Chen and Xinbing Liu;Short Pulsed Laser Machining: How Short is Short Enough, J. Laser Applications, Vol. 11, No. 6, December 1999) These improvements result from the higher laser beam intensity associated with the higher peak powers that are obtained with shorter laser pulses that utilize Q-switching, mode locking and other associated techniques and the fact that the pulse duration is shorter than the thermal diffusion time. For example, the typical thermal diffusion time for a 250 micron diameter hole is approximately 0.1 millisecond. In spite of the lower energy per pulse, high drilling speeds can still be cost effectively obtained because of the high pulse repetition rate obtained with these technologies. The high laser beam intensity provided by short laser pulses technology results in vaporization-dominated material removal rather than the melt-expulsion-dominated mechanisms using millisecond wide laser pulses. It is also known that shorter pulse width yield more limited heat diffusion into the surrounding material during the laser pulse. Hole-to-hole dimensional stability is also improved because the hole is drilled by the material being nibbled away by tens to hundreds of laser pulses of smaller pulse energy but occurring at a high pulse repetition frequency rather than by a few high-energy pulses. For the same reason, thermal and mechanical shocks from nanosecond pulses are also reduced compared with millisecond pulses. These advantageous effects obtained with nanosecond laser pulses have been detected by observing fewer micocracks occurring when holes were drilled in brittle materials such as ceramic and glass when utilizing nanosecond laser pulses.
• When the intensity is further increased through laser mode locking techniques to get down to the subnanosecond pulse width (i.e. picoseconds and femtosecond region), additional reductions in the recast and heat-affected zones are observed. Since a typical electron energy transfer time is in the order of several picoseconds, femtosecond laser pulse energy is deposited before any significant electron energy transfer occurs within the skin depth of the material. This forms a plasma that eventually explodes and evaporates the material leaving almost no melt or heat-affected zone. Due to the small energy per pulse (˜1 ml), any shock that is generated is weak resulting in no microcracks even in brittle ceramic alumna material. Femtosecond pulses are not presently obtainable with CO₂lasers due to the narrow gain of the laser line. Femtosecond pulses are presently obtainable with solid-state lasers.
• For the same total irradiated laser energy, femtosecond pulses remove two to three times more material than the nanosecond pulses. However, even “hero” type, one of a kind experimental, state of the art laser research and development systems that operate in the femtosecond range deliver only several watts of average power, while nanosecond lasers yield one or two order of magnitude higher power output. Consequently, femtosecond lasers are still too low in average power to deliver the required processing speeds for most commercial applications. It has been reported (XiangLi Chen and Xinbing Liu;Short Pulsed Laser Machining: How Short is Short Enough, J. Laser Applications, Vol. 11, No. 6, December 1999) that a 1W femtosecond laser requires more than a minute to drill a 1.0 mm deep hole of 0.1 mm diameter. Present femtosecond lasers have such high cost that their use is cost effective for only special high value applications that unfortunately have relative low unit volume market potential. For example, Lawerance Livermore National Lab has made use of the fact that femtosecond laser pulse energy is deposited essentially with no thermal transfer to cut and shape highly sensitive explosive materials without denotation.
• It is well known that the trend for optical absorption in metals as a function of wavelength is toward lower absorption with increasing wavelengths as shown inFIG. 1. Consequently, the near IR, visible and ultra violet wavelength regions are most effective in machining most metals. This advantage does not exist in plastic material. The data contained inFIG. 1 is not relevant once a plasma is initiated on the metal surface because all of the laser energy is absorbed in the plasma, which in turn imparts the energy to the material. Once the plasma is initiated, the absorption as a function of wavelength variation for metals becomes essentially flat. Consequently, one can paint the surface of the metal for greater absorption at longer wavelengths and the higher absorption advantage of shorter laser wavelengths is effectively eliminated.
• The electronics industry has needs to shrink the size of semiconductor and hybrid packages, and greatly increase the density of printed circuit boards because of the market desire for smaller cellular phones, paging systems, digital cameras, lap top and hand held computers, etc. These needs have resulted in interest in the use of lasers to form small vertical layer-to-layer electrical paths (via) in printed circuit boards. The short pulse CO₂laser is particularly attractive for drilling via holes in printed circuit boards because of: 1. the high absorption of the printed circuit board or hybrid circuits resin or ceramic material at the CO₂wavelength when compared to YAG or YLF lasers which operate in the near IR and in the visible and UV wavelength regions with harmonic generating technique; 2. the lower cost per watts associated with CO₂lasers when compared to YAG lasers, and 3. because of the high reflectivity of copper at CO₂wavelengths, which enables CO₂laser via hole drilling equipment to drill through the resin layer down to the copper layer where the drilling is stopped because of the high reflectivity of the copper interconnect material at the CO₂laser wavelengths. These are called “blind via,” which connect the outer layer of a circuit to the underlying inner layer within the multi layer board. The major disadvantages of CO₂lasers in via hole drilling is the larger spot size obtainable with its 10.6 micron wavelength when compared to shorter wavelength laser. Another disadvantage is that pulse widths below several nanoseconds are difficult to obtain with CO₂lasers. The major advantages of CO₂Q-switched lasers are: they offer lower cost per watt of laser output when compared with solid state lasers, higher absorption of their radiation by resin and ceramic board materials, their ability to operate at high PRF, their ability to generate substantial output power under Q-switched operation, and their ability to stop drilling when the radiation gets to the copper layer.
• The advantages of drilling via holes in printed circuit boards with laser systems have enabled laser systems to capture 70% of the via hole machine drilling market in 1999 (David Moser;Laser Tools For Via Formation, Industrial Laser Solutions, p. 35, May, 2000, which is incorporated herein by reference), with the remaining 25% of the market held by photo-via and the remaining 5% by other techniques, such as mechanical drilling, punch and plasma etching.
• The upper CO₂laser transition level has a relatively long decay rate for storing larger than normal population inversion (385 torr⁻¹sec⁻¹at 300 K and approximately 1300 torr⁻¹sec⁻¹at 500 K). The lower CO₂laser levels for both the 9.4 and 10.4 transitions are approximately an order of magnitude faster so a large population inversion between the lower levels and the upper level can be easily maintained. Laser mediums that have transitions with long lifetime upper energy levels are good candidates for application of Q-switched techniques (A. E. Siegman;Lasers, Chapt. 26, University Science Books, 1986, which is incorporated herein by reference). The long lifetime of the upper levels store energy by building up a higher than normal population with respect to the lower laser level. Consequently, CO₂lasers are good candidates for performing Q-switching (G. W. Flynn et al;Progress and Applications of Q-switching Techniques Using Molecular Gas Lasers, IEEE J. Quant. Electronics, Vol. QE-2, p. 378-381, September 1966, which is incorporated herein by reference).
• Q-switching is a widely used technique in which a larger than normal population inversion is created within a laser medium by initially providing for a large loss within the feedback cavity. After a large inversion is obtained, one quickly removes the large optical loss within the feedback cavity, thereby quickly switching the cavity Q back to its usual large value (i.e. low loss value). This results in a very short intense burst of laser output, which dumps all the excess population inversion into the short laser pulse (A. E. Siegman;Lasers, Chapt. 26, University Science Books, 1986).
• FIG. 2A typically illustrates the time dependent variation of the losses within the feedback cavity that can be obtained with either a rotating feedback mirror, an electro-optics modulator (i.e. switch) or with an acousto-optics switch inserted in the lasers feedback cavity under continuous pumping condition, P_RFCW, ofFIG. 2B.FIG. 2A also illustrates the time dependent gain variation experience by the continuously excited laser under the internal cavity loss variations illustrated. The gain is allowed to rise for an optimum time of about one or two population decay time of the upper CO₂laser level. At such an optimum time, the cavity loss is switched from a high loss to the normal loss condition (i.e. the Q of the cavity is switched from a low to a high value condition) by applying a high voltage pulse, say to the electro-optic modulator (EOM) as shown inFIG. 2B. Since the gain greatly exceeds the losses at this point, laser oscillations by stimulated emission begins with the output building up exponentially, resulting in the emission of a giant laser output pulse whose peak power is hundreds of times larger than the continuous power of the laser. The pulse has a long tail, which will eventually decay down to the lasers' CW power level as long as the gain exceeds the feedback cavity loss. In most cases, this tail is detrimental to a hole drilling process. This invention will provide a solution to this long pulse tail problem. When the high loss cavity condition is again switched on, the laser action stops and the described dynamic process of gain build up is repeated.
• To the present time, Q-switched CO₂lasers have not found extensive commercial application, as have solid-state lasers (whose upper state life times are measured in seconds instead of tenths of seconds as for the CO₂laser). Nearly all of the Q-switched CO₂laser applications to date have addressed predominately military applications. Some of the reasons for the lack of interest in commercial CO₂Q-switched lasers are high cost of the electro-optic crystal (namely CdTe), limited suppliers for the electro-optic (EO) crystals, large performance variation between different optical paths within an EO crystal and large performance variation between different crystals. There is also difficulty in obtaining good anti reflection thin-film coatings on CdTe crystals. In addition, electro-optic modulators cannot be easily replaced by acousto optic modulators in the IR because they have higher attenuation and poorer extinction performance than in the visible region, as well as larger thermal distortion and poorer reliability. Q-switched CO₂lasers were also considered to have poorer reliability than the Q-switched solid state laser which was mostly caused by the CdTe crystals. Consequently, superpulsed or externally gated CW laser operation of diffusion cooled CO₂lasers or TEA laser techniques have been utilized with CO₂lasers to satisfy most short pulse CO₂laser needs to date (A. J. DeMaria;Review of CW High Power CO₂Lasers, Proceedings of the IEEE, pg. 731-748, June 1973, which is incorporated herein by reference). Mechanically Q-switched CO₂laser have also been utilized but they do not have the pulsing flexibility of electronically Q-switched lasers.
• For these reasons, techniques such as gated CW and super pulse, along with acousto-optic deflection external to the optical cavity of either CW or super pulsed lasers into a aperture have been predominately utilized to date with CO₂lasers to obtain IR laser pulses for industrial applications, even though each of these techniques are deficient when compared with CO₂Q-switching techniques in one or more of the following: longer pulse widths with slower rise time, lower pulse repetition frequencies (PRF), lower over all laser efficiencies, long duration tails associated with the pulses and lower peak powers. TEA lasers have also been used to date, but they suffer from higher time jitter from pulse to pulse, higher pulsed voltage requirements along with associate acoustic shock noise and non-sealed off laser operation which requires gas flow.
• Thus it is desirable to make the Q-switched CO₂laser lower in cost, more reliable, enable the cost effective utilization of the present state of the art of CdTe electro-optics crystal technology without sacrificing Q-switching performance, and obtaining higher peak power and shorter pulses by simultaneously utilizing super pulse and Q-switching techniques as well as cavity dumping techniques, and utilizing the same EO modulator to clip off the long tail of the laser pulses usually obtained with Q-Switching techniques. It is desirable to make Q-switched CO₂lasers commercially practical for numerous hole drilling applications, especially for via hole drilling of printed circuit board and for laser marking of stressed glass containers holding a vacuum or partial vacuum or a pressure higher than ambient pressure such as automobile headlights, flat panel displays, cathode ray tubes for TVs and computers, street lights, light bulbs stressed plate glass in automobiles or pressured glass or plastic containers containing soft drinks, beer, etc.
BRIEF SUMMARY
• FIG. 3 illustrates a block diagram of a laser material processing system. The system includes the laser head and its power unit, which may or may not have an internal controller. An RF power unit is preferred. The RF unit can be operated CW or in a super pulsed mode. The super pulsed mode of operation is used to obtain increased peak power laser pulses. The laser head and its power unit and controller are usually provided by a laser supplier, while the controller for the XY scanning system, the scanners, the keyboard, the optical shutter and a display unit are usually the responsibility of the original equipment manufacturer. The original equipment manufacturer (OEM) controller commands the scanning system and the display unit and sends signals to the laser controller, which in turn commands the laser head. If the laser is liquid cooled, a chiller is required which either the laser manufacturer or the systems OEM can supply. Usually, the OEM chooses to supply the chiller. Laser beam shaping optics are usually required between the laser head and the scanners.
• Either the laser manufacturer or the OEM system manufacturer can supply the laser beam shaping optics. This overview block diagram is essentially identical to a block diagram used to describe laser engraving, marking, cutting and drilling systems for desk top manufacturing type applications with the software being basically the differentiating portion of the system. The system OEM normally is responsible for the optical scanner, the system controller and its software and the displays.
• The OEM system controller tells the XY optical scanning system where to point and informs the laser head through the controller within the laser's head power unit when to turn on or off and how much power is to be emitted. The OEM system controller also monitors and supervisors the chiller, and displays the desired information on the display unit to the system operator who usually enters commands through the keyboard that address the system controller. The system controllers and the laser power unit controller also perform appropriate diagnostics to protect the system from inadequate cooling, RF impedance mismatch between the laser discharge and the RF power supply, and safety features such opening and closing the systems optical shutter, etc.
• FIG. 4 illustrates the modifications toFIG. 3 for the case when a Q-switched laser is utilized in the material processing system. In addition to commanding the laser power supply, the system controller performs calculations utilizing the input from the operator provided through the keyboard and issues commands regarding the laser modulation format (i.e. gated output or super pulse output for example, the timing of the Q-switched laser pulse along with pulse duration and repetition frequency, etc.) and monitoring the status of the laser head and its power supply as well as the chiller. The system controller also issues commands (and may receive signals) from the Q-switched power module. The system controller receives signal from an operator through a keyboard and commands as well as monitors the status of the optical shutter, which can be inserted either before or after the optical scanners. In some cases, the optical shutter is specified for inclusion at the direct exit of the laser beam out of the laser housing. If the shutter is included as part of the laser housing, the laser manufacturer supplies the optical shutter and its circuitry. The status of the system is displayed to the operator of the keyboard by an appropriate display unit. The Q-switching module ofFIG. 4 is in principle the same for either a solid state or gas laser system with the major difference being the use of a different electro-optical crystal.
• In addition to utilizing Q-switched lasers and even shorter pulsed laser systems, such as mode locked or cavity dumped short pulse laser systems for hole drilling applications, the Q-switched laser system ofFIG. 4 can also be utilized to mark, encode or drill stressed glass vessels or structures as well as to perforate or punch holes in paper without charring. The advantage of utilizing Q-switched or cavity dumped lasers to mark or encode stressed glass containers, which have a pressure difference between the inside and outside surfaces of the containers, has not been appreciated nor recognized because laser systems for such applications have not been presently commercially available. Such containers include, for example, sealed glass automotive headlights, streetlights, cathode ray tubes, flat panel displays and beer, soda, and champagne bottles. Tempered glass surfaces of safety glass doors, windows, and automotive side windows are also good candidates for laser marking or encoding with short laser pulses because microcracks in brittle materials such as glass and ceramic materials are not generated by Q-switched or shorter laser pulses. If longer pulsed laser radiation is used to mark or encode such stress containers and glass surfaces, micro cracks are created at the location where the laser marks or encodes the glass. These microcracks become enlarged and propagate with time under the stress load to which the brittle material is subjected. CO₂laser radiation is strongly absorbed by glass and ceramics so they are the laser of choice for such applications. Because of their size, power, cost and processing speed CO₂lasers are preferred for non-metal processing of materials. UV radiations are also absorbed by glass material and are considered alternate lasers for such applications, but at higher cost and slower processing speeds.
• The high laser beam intensity provided by short pulse laser technology results in the vaporization-dominated material removal rather than the melt-expulsion-dominated mechanisms using longer duration pulses. Thermal and mechanical shocks are reduced with the short laser pulse system ofFIG. 4 when compared with longer pulse systems ofFIG. 3. Consequently, micro cracks do not occur under laser marking or encoding with short pulse lasers. Cutting off the long Q-switched pulses long tail will prevent the development of micro cracks at the glass location, which is marked or encoded. The application of the laser system ofFIG. 4 thereby opens up the market of direct marking or encoding on stressed glass containers and structures. Currently ink jets or other similar devices are used to mark or encode such glass containers and structures. Inkjets have well known disadvantages over laser marking/encoding system. Some of these disadvantages are their mark is not permanent and can rub off through handling and exposure to the environment, the inks and solvents are consumables and recurring costs can be high, the inks and solvents are toxics and dirty up the factory environment and the down time of inkjet marking systems is high which adds to their operating costs. The major advantage of inkjet marking systems for this application is low initial capital cost.
• The drilling of numerous small holes in paper or plastic parts without charring the edges of the paper or plastic material is desired in many industries. Some examples are in the tobacco filtration, and in the banking and billing industries for perforating checks and other financial documents. In the past TEA lasers have been used for these applications. It has not been appreciated that Q-switched lasers can be utilized to perforate such materials. If higher energies are required than available with sealed-off Q-switched lasers, then a laser amplifier can be used to increase the pulse energy of the Q-switched laser. Q-switched lasers have output pulse repetition rates exceeding 100 kHz, while TEA lasers have practical PRR having an upper limit of about 500 Hz.
• A Q-switched CO₂laser system for material processing is disclosed. The system comprises a plurality of reflective devices defining a cavity. A gas discharge gain medium is positioned within the cavity for generating a laser beam and an electrical power supply is used to excite the discharge. A cavity loss modulator modulates the laser beam, generating thereby one or more laser pulses. A pulsed signal generation system is connected to the cavity loss modulator for delivering pulsed signals to the cavity loss modulator, thereby controlling the state of optical loss within the cavity. A control unit is connected to the pulsed signal generation system for controlling the pulsed signals delivered to the cavity loss modulator. A pulse tail clipping circuit is receptive of a portion of the laser beam and is connected to the pulsed signal generation system for truncating a part of the laser pulses.
• The electro-optical crystal is birefringent. Consequently, stress will cause polarization changes in a laser beam, independent of any voltage applied to the crystal. The holder of the electro-optic crystal is designed to minimize stress on the crystal while holding the crystal firmly in place. The electro-optic crystal is piezoelectric, so the packaging of the crystal is such as to absorb the ultrasonic energy generated by the cavity loss modulator when a voltage is repetitively applied and removed. A thin Indium plate is utilized for this purpose.
• The laser system includes a system for automatically terminating the generation of the laser. This feature is used to protect the laser from back reflection from the workpiece into the laser cavity, thereby preventing optical damage. This feature is also used to stop the laser from operating once the high reflecting surface, such as copper, is encountered. InFIG. 29, a first polarizing device receives the laser beam. A second polarizing device, receptive of the laser beam from the first polarizing device, is operative thereby to change the polarization of the laser beam from a first state to a second state. The second polarizing device is receptive of the laser beam in the second state of polarization, reflected from an object or work piece and is operative thereby to change the polarization of the laser beam from the second state to a third state. The first polarization device is also receptive of the laser beam, in the third state of polarization from the second polarizing device. A detector is receptive of the laser beam from the first polarization and provides an output signal indicative of the reflectance of the object. A comparator is provided for comparing the output signal of the detector with a reference signal. This generates an output signal indicative of the greater or lesser of the detector output signal or the reference signal. The laser system also includes a shutter system connected to the control unit for alternately blocking and passing the laser beam.
• The pulsed signal generation system comprises a pulse receiver, connected to the control unit, providing electrical isolation. A pulsed signal generation switching circuit is receptive of pulsed signals from the pulse receiver and is operative thereby to charge or discharge the cavity loss modulator. A power supply powers the pulse receiver and the pulsed signal generation switching circuit. The pulsed signal generation switching circuit comprises a first switch connected to the power supply and to the cavity loss modulator and is receptive of a pulsed signal from the pulse receiver and is operative thereby to charge the cavity loss modulator when the first switch is in the closed position. A second switch is connected across the cavity loss modulator. The second switch is receptive of a pulsed signal from the pulse clipping circuit and operative thereby to discharge the cavity loss modulator when in the second switch is in the closed position and the first switch is in the open position.
• The cavity loss modulator comprises an active optical crystal having an entrance surface receptive of the laser beam and an opposing laser beam exit surface. A first optical window has an optical entrance surface receptive of the laser beam and an opposing laser beam exit surface. The exit surface of the optical window is in physical contact with the entrance surface of the active optical crystal and thereby defines a first optical interface. An optical reflector is in physical contact with the laser beam exit surface of the active optical crystal thereby defining a second optical interface. The optical reflector is operative to receive the laser beam from the active optical crystal and to redirect the laser beam into the active optical crystal.
• The laser system includes a multiple pass optical assembly comprising a first reflective device positioned within the cavity. The first reflective device is receptive of the laser beam from the cavity loss modulator and operative to redirect the laser beam into the cavity loss modulator. A second reflective device is positioned within the cavity receptive of the laser beam from the cavity loss modulator. The second reflective device is operative to redirect the laser beam into the cavity loss modulator. Thus multiple passes of the laser beam through the cavity loss modulator are realized. A plurality of mirrors which are wavelength selective mirrors comprise an output coupling mirror having high transmission at non-lasing wavelengths for coupling the laser beam out of the optical cavity; a plurality of laser beam turning mirrors having high reflectivity at lasing wavelengths for directing the laser beam between the waveguide channels; and a feedback mirror providing optical feedback to the laser cavity.
• The laser system includes a phase grating for receiving the laser beam. The phase grating thus diffracts a portion of the laser beam away from the laser beam at a prescribed order and controls the amplitude of the Q-switched pulses.
• A method of maintaining constant phase retardation induced in a laser beam by an electro-optic crystal in a repetitively Q-switched CO₂laser is also disclosed. The method comprises maintaining zero voltage across the electro-optic crystal during the high optical loss interval of the Q-switching cycle; and maintaining a prescribed non-zero voltage across the electro-optic crystal during the low optical loss interval of the Q-switching cycle.
• A method of operating a Q-switched CO₂laser having a gain medium and a cavity loss modulator for material processing is disclosed that comprises energizing the gain medium for a first prescribed time duration; and energizing the cavity loss modulator for a second prescribed time duration causing the laser cavity to switch from a high loss state to a low loss state generating thereby one or more laser pulses.
• A method of controlling the amplitude of the output pulses of a repetitively Q-switched CO₂laser which includes a diffraction grating is disclosed that comprises diffracting a portion of the laser output pulses into a diffraction side order; and varying frequency of the diffraction grating.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a graphical depiction of a comparison between the absorption rate of several different metals as a function of wavelength for several laser sources.
• FIG. 2A is a graphical depiction of the cavity loss, gain and laser output pulse in a repetitively pulsed Q-switched laser.
• FIG. 2B is a graphical depiction of the continuous wave radio frequency power input to a Q-switched CO₂laser and the high voltage signal delivered to an electro-optic modulator to effect Q-switching.
• FIG. 3 is a block diagram of a generalized laser material working system.
• FIG. 4 is a block diagram of a generalized Q-switched laser material working system.
• FIG. 5 is a first schematic diagram of a Q-switched CO₂laser system for material processing.
• FIG. 6 is an isometric view of a laser head, with a heat exchanger, mirror holders and a housing for a RF phase matching network used inFIG. 5;
• FIG. 7 is a graphical depiction of the output power of CO₂lasers at various laser wavelengths.
• FIG. 8 is a graphical depiction of the transmission of an output coupling mirror for a CO₂laser as a function of wavelength used inFIGS. 5, 6 and10.
• FIG. 9 is a graphical depiction of the transmission of a turning mirror for a CO₂laser as a function of wavelength used inFIGS. 5, 6 and10.
• FIG. 10 is a second schematic diagram of a Q-switched CO₂laser system for material processing showing a stop drilling option.
• FIG. 11 is a first cross-sectional side view of a high optical damage threshold electro-optical modulator ofFIGS. 5 and 10.
• FIG. 12 is a second cross-sectional side view of an electro-optic modulator illustrating greater detail than inFIG. 11 on how not to stress the electro-optic modulator crystal.
• FIG. 13 is a cross-sectional end view of an electro-optic modulator ofFIG. 12.
• FIG. 14 is a diagram of the arrangement of an electro-optic crystal receptive of a laser beam polarized in the X axis as shown and subject to a constant applied voltage.
• FIG. 15 is a graphical depiction of the variation in the optical phase shift in the laser beam passing through the electro-optical crystal ofFIG. 14 as a function of position across the face of the electro-optical crystal.
• FIG. 16A is a first schematic diagram of an electro-optical modulator within a laser cavity with no voltage applied across the electro-optical crystal resulting in a state of high optical loss within the laser cavity.
• FIG. 16B is a first schematic diagram of an electro-optical modulator in a laser cavity with a nonzero voltage applied across the electro-optical crystal resulting in a state of low optical loss within the laser cavity.
• FIG. 17A is a second schematic diagram of an electro-optical modulator within a laser cavity with no voltage applied across the electro-optical crystal resulting in a state of high optical loss within the laser cavity.
• FIG. 17B is a second schematic diagram of an electro-optical modulator within a laser cavity with a nonzero voltage applied across the electro-optical crystal resulting in a state of low optical loss within the laser cavity.
• FIGS. 18A, 18B and18C depict multiple pass configurations of the electro-optical modulator ofFIGS. 16A-17B.
• FIG. 19 is a first graphical depiction of oscilloscope traces of a high voltage pulse applied to an electro-optical crystal in a Q-switched laser and the resultant output pulse of the laser.
• FIG. 20 is a graphical representation of the time interrelationship of a high voltage pulse applied to an electro-optical crystal in a Q-switched laser and the resultant output pulse of the laser.
• FIG. 21A is a first schematic diagram of an electronic circuit for truncating a portion of the output pulse in a CO₂Q-switched laser.
• FIG. 21B is a graphical depiction of the variations in the triggering of the output of the circuit ofFIG. 21A as a function of the variations in the rise time and amplitude of the output pulse in a CO₂Q-switched laser.
• FIG. 22A is a second schematic diagram of an electronic circuit for truncating a portion of output pulse in a CO₂Q-switched laser.
• FIG. 22B is a graphical depiction of the variations in the triggering of the output of the circuit ofFIG. 22A as a function of the time delay and attenuation of the output pulse in a CO₂Q-switched laser.
• FIG. 23A is a graphical depiction of an oscilloscope trace of the output pulse of a CO₂Q-switched laser with a long tail.
• FIG. 23B is a graphical depiction of an oscilloscope trace of the output pulse of a CO₂Q-switched laser with a truncated tail.
• FIGS. 24A and 24B are schematic depictions of the housing and the arrangement of the electronic circuits ofFIGS. 21A and 22A.
• FIGS. 25A, 25B,25C and25D are graphical depictions of the relative timing of the charging and discharging signals, the high voltage signal applied to an electro-optical modulator and the resultant output pulse of a Q-switched laser.
• FIG. 26 is an electrically equivalent schematic diagram corresponding toFIG. 25 utilizing the positions of mechanical switches for charging and discharging the EOM to explain the switching circuit processes for an electro-optic modulator in a Q-switched laser.
• FIG. 27 is a more detailed electrical schematic diagram of the switching circuit inFIG. 26.
• FIG. 28 is a schematic diagram of the housing and the placement of the assembly of the elements of the switching circuit ofFIG. 27.
• FIG. 29 is a schematic diagram of an automatic laser stop circuit for a Q-switched laser used in drilling blind via holes.
• FIG. 30A is a graphical depiction of the relative laser output powers for various modulation techniques wherein format I is for CW pulsed, gated modulation, format II is for super pulsed, format III is for Q-switched with tail clipping, format IV is for super pulsed Q-switched with tail clipping, format V is for Q-switched cavity dumped and format VI is for super pulsed Q-switched cavity dumped.
• FIG. 30B is a graphical depiction of the relative timing of the application of a super pulsed RF pump power to a laser gain medium and a single pulse applied to an electro-optic crystal in a simultaneously super pulsed pumped and Q-switched laser.
• FIG. 31 is a graphical depiction of the relative timing of the application of super pulsed RF pump power to a laser gain medium and the repetitive pulses applied to an electro-optic crystal in a simultaneously super pulsed pumped and repetitively Q-switched laser.
• FIG. 32 is a graphical depiction of oscilloscope traces of the experimental results of the application of super pulsed RF pumped power to a laser gain medium and the repetitive Q-switched output laser pulses resulting from repetitive electrical pulses applied to an electro-optic crystal in a simultaneously super pulsed pumped and Q-switched CO₂laser whose output was depicted in ofFIG. 31.
• FIG. 33 is a graphical depiction of the relative timing of the application of super pulsed RF pump power to a laser gain medium and the increasing amplitude of repetitive pulses applied to an electro-optic crystal in a simultaneously super pulsed pumped and repetitively Q-switched laser with the Q-switched pulses increasing in amplitude.
• FIG. 34 is a schematic block diagram of the electronic circuit controlling the high voltage applied to the electro-optic modulator ofFIGS. 5 and 10 for controlling the peak output pulse power of a Q-switched laser.
• FIG. 35 is a schematic block diagram of an acousto-optic system for controlling the output peak power of Q-switched CO₂lasers pulses.
• FIG. 36 is a first graphical depiction of oscilloscope traces of the experimental results of a simultaneously super pulsed RF pulsed pumped and Q-switched CO₂laser with increasing amplitude Q-switched output pulses.
• FIG. 37 is a second graphical depiction of oscilloscope traces of the experimental results of a simultaneously super RF pulsed pumped and Q-switched CO₂laser showing varying repetitive Q-switched output pulses.
DETAILED DESCRIPTION
• Q-Switched CO₂Laser Housing
• FIG. 5 illustrates a schematic overview of alaser assembly100, including alaser housing102 containing a sealed-off, folded waveguide, electro-optically Q-switched CO₂laser head400 and various electronics, optical, an electro-optical modulator and electro-mechanical switches. A multiple pass zig-zag folded waveguide is shown at806 within thelaser head400 for illustration purposes. A three pass or more than five pass folded waveguide configuration could also be used in the hermetically sealedlaser head400. An output coupling mirror (OC)406 and turning mirror (TM)414 utilize a metal O-ring to maintain the hermetical seal as disclosed in U.S. patent application Ser. No. 09/612,733 entitled High Power Waveguide Laser, filed on Jul. 10, 2000 (which is incorporated herein by reference in its entirety) and in U.S. provisional Patent Application Ser. No. 60/041,092 entitled RF Excited Waveguide Laser filed on Mar. 14, 1997 (which is incorporated herein by reference in its entirety). TheOC406 and theTM414 transmit radiation out of the hermetically sealedlaser head400 with theOC406 transmitting approximately 51% andTM414 approximately 1% or less. Themirror holder flange804 for theOCM406 and theTM414 are as disclosed in U.S. Provisional Patent Application No. 60/041,092 entitled RF Excited Waveguide Laser filed on Mar. 14, 1997. Turning mirrors416 (TM1, TM2, TM3), do not transmit radiation out of thelaser head400. Themirror holder flange802 for turningmirrors416 are the same as the disclosed in U.S. Provisional Patent Application Ser. No. 60/041,092 entitled RF Excited Waveguide Laser filed on Mar. 14, 1997.
• Themirror holder flange802 for the thin film polarizer (TFP)404 mounted on thelaser head400 is a modified version forOCM406 andTM4414. The modification is needed because of the larger diameter and the angle required forTFP404. It is also a modified version of the mirror holder forthin film polarizer114. The modification over theTFP114 holder is required because of the need forthin film polarizer404 to be mounted on thelaser head400, which requires a hermetical seal.TFP114 is mounted on thelaser housing102 where a hermetical seal is not required. There is an option to place awindow404bin place ofTFP404 and then placeTFP404 outside of thelaser head400.FIG. 6 illustrates an isometric view of thelaser head400 withmirror holder flanges802 and804 containing theTFP404 and its retainer ring404a, as well asmirrors416 and414.
• For many plastic materials, CO₂laser operation at a wavelength of approximately 9.2 microns is preferred over a wavelength of 10.6 microns or other wavelengths because of the increased absorption of the material at 9.2 microns. When operation at a lower CO₂gain line is desired as in this case, it becomes necessary to suppress lasing at higher gain lines. This is especially true under Q-switching laser operations because of the very high gain that is built-up under the high optical loss (i.e. laser hold-off) condition.FIG. 7 illustrates the relative power output of the various gain lines that can be emitted by a CO₂laser. Note that high gains occur at wavelength of approximately 9.3, 9.6, 10.25 and 10.6 microns. One can utilize a grating to select one of these gain lines and discriminate against the rest. Unfortunately gratings are expensive, easily damaged by high intensity laser radiation and optically lossey. Fortunately, one can utilize state-of-the-art mirrors that have thin-films deposited on them to reflect the desired gain line back into the cavity and/or transmit undesired gain lines out of the laser cavity. For example, the optimum transmission of theoutput coupling mirror406 for a CO₂laser having a 226 cm long unfolded waveguide gain length operating at a wavelength of 9.249 microns under Q-switched conditions is approximately 50%. Consequently, for the partly reflectingOCM406 ofFIG. 5, one would desire all other wavelengths to have a higher transmission so as to provide higher attenuation for these other wavelengths.
• FIG. 8 shows the functional relationship of transmission vs. wavelength of a coated ZnSe mirror that has a 50% transmission at a wavelength of about 9.25 microns, approximately 90% transmission at wavelengths of about 10.25 and 10.6 microns and approximately 70% transmission at a wavelength of 9.6 microns. These are the highest gain lines for CO₂lasers. This mirror performance is well suited for theoutput coupling mirror406 of thelaser head400 inFIG. 5 because it transmits more of the undesired wavelengths out of the laser cavity, defined betweenOC406 and feedback mirror (FBM)408, if oscillation at a wavelength of around 9.2 microns is desired.
• For additional discrimination against undesired gain lines, turning mirrors416 can also be thin-film coated so that they have higher reflectivity at the desired wavelength and less at the undesired wavelengths. This characteristic would also beneficially contribute to the oscillation at the desired wavelength while assisting in the prevention of oscillation at undesired wavelengths. It may not be necessary to coat all three of these mirrors, one can coat only as many as required to prevent undesired oscillation on other gain lines.FIG. 9 illustrates the functional relationship of the transmission vs. wavelength of a ZnSe thin-film coated mirror that has only 1% transmission (i.e. 99% reflectivity) at a wavelength of 9.25 microns and higher transmission (i.e. lower reflectivity) at the higher CO₂gain lines with wavelengths of 9.6, 10.25 and 10.6 microns. These characteristics favor the oscillation at a wavelength of 9.25 micron and discriminate against the oscillation on the other gain lines. This coating can also be used on turning mirrors TM₁, TM₂andTM₃416 as required for additional wavelength discrimination.
• Mirror414 ofFIG. 5 has high reflectivity at the desired wavelength (e.g. 9.25 microns). It none-the-less transmits a small amount of radiation out of the cavity (about 1% or less at a wavelength of 9.25 microns). The small radiation output ofmirror414 is detected by adetector302 such as a pyro-detector whoseelectrical signal304 is fed to an Automatic Down Delay Circuit (ADDC)306. The purpose of theADDC306 is to clip the long tail of the Q-switched laser pulse after a selected time delay, τ_pc, from the beginning of the laser pulse. The amount of time delay, τ_pcis selected by the laser operator.Mirror414 is positioned and hermetically sealed in an identical mirror holder asOCM406.
• From a cost and reliability standpoint it is wise to minimize openings in a laser head that has to be hermetically sealed. Consequently, as seen inFIG. 10, one may use the option of having feedback mirror (FBM)408 ofFIG. 5 to have a small amount of transmission (about ½ to 1%) (i.e. by not using a metal coated mirror for example) and detecting this output radiation with thedetector302 for supplying theelectrical signal304 to theADDC306. This alternative allowsmirror414 to utilize the same mirror holder as formirror416, and allowsmirror414 to be placed inside thelaser head400 so that a hermetically sealing metal O-ring will not be required formirror414 since it no longer needs to emit radiation outside thelaser head400. It also allowsmirror414 to serve as an optical filter to discriminate against other wavelengths if needed.
• The thin-film polarizer (TFP₁)404 ofFIGS. 5 and 10 serves the same function as theTFP404 ofFIGS. 16A and 16B as will be explained in the narrative associated with those figures. InFIGS. 16A and 16B, the polarized laserradiation exiting TFP404 ofFIGS. 5 and 10 passes through an electro-optic modulator (EOM)140 through a ¼wave polarization rotator412 to theFBM408. InFIGS. 17A and 17B thepolarization exiting TFP404 passes through the electro-optic modulator140 off the reflective phase retarder (RPR)410 to theFBM408.FIGS. 5 and 10 illustrate how thepulsed signal106 from the laser head's powersupply controller unit104 is feed to theEOM140 by apulse receiver202 such as a RS422 differential transistor/transistor logic (TTL) circuit. Thiscircuit202 provides good noise immunity for the rest of the electronics interfacing with thelaser assembly100 from the high voltage (i.e. several kV's)pulse switching circuit206 that drives theEOM140. Thepulse receiver202 provides electrical isolation by not having a common ground with thelaser head400. The highvoltage power supply208 provides DC power to both thepulse receiver202 and to theswitching circuit206.
• The powersupply controller unit104 of thelaser system100 provides signals to adriver110 of anoptical shutter112 to block or unblock theoutput402 from thelaser head400. This optical shutter is added so that the operator can manually open the shutter to operate the laser, as is well known in the art.
• High Optical Damage Threshold Electro-Optic Modulator
• Active optical CdTe crystals are utilized extensively as electro-optics modulators for CO₂lasers. It is generally difficult to get anti-reflection coatings to adhere well to the entrance and exciting surfaces of the CdTe modulator crystals. These films can easily be damaged when inserted into CO₂laser feedback cavities. Anti-reflection coatings are used to reduce optical losses when these crystals are inserted within a laser feedback cavity to switch the cavity losses from a higher loss condition (i.e. low cavity Q) to a low loss condition (i.e. high cavity Q). Peeling and optical damaging of these coatings by the intense laser radiation is a common damage failure for these modulators when used to Q-switch CO₂lasers. Solving the thin film optical damage problem of the EO modulation crystals would result in a significant increase in the failure damage safety margin of Q-switched CO₂lasers and in the material drilling systems in which they are utilized.
• The anti-reflecting thin film damage problem is much less severe with passive optical IR window materials such as ZnSe or GaAs (U.S. Pat. No. 5,680,412, which is incorporated herein by reference). Since CdTe and ZnSe have refractive indices, n, which are close to one another (i.e. n=2.6 and 2.4 respectively) at CO₂laser wavelengths, optically polished ZnSe windows can be placed in optical contact with the entrance and exit surfaces of a CdTe EO crystal and not experience high transmission losses through either direction of the CdTe/ZnSe interface. The losses for a ZnSe/CdTe/ZnSe electro-optical modulator structure are a little higher than for thin film coated CdTe EO modulators, but the trade-off between the improvement in the reliability of the laser and the slightly higher losses is worth it.FIG. 11 illustrates such a ZnSe/CdTe/ZnSe EO modulator structure at140. High optical damage threshold anti-reflection thin film coatings on ZnSe are easily deposited and are commercially available.Item142 is the CdTe EO crystal anditems148 and150 are the ZnSe transparent windows in optical and thermal contact with theCdTe EO crystal142.
• The problem experienced in the deposition of anti-reflection coatings on CdTe is believed to arise from the fact that in order to get good adherence films, the CdTe needs to be heated at such a temperature that the material decomposes; i.e. Te is driven off of the surfaces of theCdTe crystal142 to be coated. This leaves a Cd enriched surface that presents an electrical conducting path betweenelectrodes146 and144 of the EO modulator ofFIG. 11. Under the high voltage applied to the electrodes of theCdTe crystal142 this poor electrical conducting path along the surfaces of the crystal causes electric current to flow between theelectrodes144,146 which in turn causes non-uniform heating at the interface of the thin film anti-reflection coatings and the CdTe crystal surface. This leads to a weakened bond between the two materials. In addition, the periodic stress imposed by the pulsating laser radiation also contributes to the optical damage in the poor bond between the anti-reflecting films and the CdTe substrate, which causes the Q-switched laser performance to deteriorate with time. ZnSe can be heated to the necessary temperature for the deposition of good adherence anti-reflecting films without decomposition of the material. It is fortuitous that the refractive index between ZnSe and CdTe crystals is sufficiently close so as to yield low optical losses at their contacting surfaces if these two materials are placed in optical contact.
• FIG. 11 illustrates in a side view of the basic components of an electro-optical modulator140 containing, for example, aCdTe crystal142 havingconductive electrodes144,146 applied on opposite sides thereof. TheCdTe crystal142 is disposed between twotransparent windows148,150 whose refractive index matches or comes close to matching thecrystal142. For the case of the CdTe EO crystal, ZnSe is suitable. The outer faces148aand150aof thetransparent windows148,150 are anti-reflection coated. It is assumed that the deposition of a high optical damage threshold thin film coating oncrystal142 is difficult if not impossible while such coatings are easily depositable onwindows148,150. TheEO modulator housing152 includesdielectric support member154, which has anopening156 so that electrical contact can be made toelectrodes144,146. TheEO modulator housing152 hasend support members158,162, each having anopening160,164 through whichlaser radiation402 can pass through the window/EO crystal assembly.End members158,162 are secured tomember154 byfasteners166,167,168,169. Thelaser beam402 is positioned with respect to the optical axis418 ofcrystal142 for amplitude or phase modulation.
• FIGS. 12 and 13 illustrate side and end views, respectively, of theEO modulator assembly140 in greater detail. Since CdTe is birefringent, mechanical stress can cause changes in the polarization rotation. Consequently, to obtain optimum optical performance, it is important not to stress theEO crystal142 by holding it so tight that normal thermal expansion and contraction will stress thecrystal142 thereby causing changes in the polarization of the laser radiation propagating through thecrystal142, independent of any voltage applied across thecrystal142. Thecrystal142 is contained in ametal housing2, which is fabricated, for example, of Aluminum (Al).Item18bis a thin (e.g. 0.10 inches in thickness) cushion, such as an Indium strip, which is placed between the metal housing2 (which serves as a ground electrode) and theCdTe crystal142 and the ceramic spacers (items4 and5).Item18ais also a thin cushion, such as an Indium strip, that is placed between theCdTe crystal142 and the entire length of thehot copper electrode144. A spring associated withscrews17 provide cushions for thecrystal142 from being over stressed by the tightening ofbolts14 and17. Since CdTe is also a piezoelectric material, the Indium strip also acts as an acoustic absorber for the ultrasonic energy generated by the CdTe crystal as the voltage is repetitively applied and removed across thecrystal142. This acoustic absorption is important for proper operation of the CdTe electro-optic modulator140. The electrically hotpositive electrode144 is pressed against theEO crystal142 by a dielectric6, which has a hole in it to enable making an electrical contact to thehot electrode144. The dielectric6 is spring loaded15 so as to gently press thehot electrode144 against theEO crystal142 by the threadedbolts14. Thebolts14 are threaded through ametal cover8 fabricated from the same material as themetal housing2. Thiscover8 is bolted into themetal housing2 bybolts20. There is a thin plate ofIndium metal18bbetween themetal housing2 and theEO crystal142 and betweendielectrics4 and5 and theEO crystal142 to cushion thecrystal142 against thehousing2, to absorb the acoustic energy generated by the piezoelectric action of theCdTe crystal142 when the voltage is repetitively applied and removed from thecrystal142 and also to ensure good electrical contact between the electrodes and theEO crystal142.Dielectrics4,5 hold theEO crystal142 sideways, again byspring action16 upon which pressure is exerted by the threadedbolt16. Optically polishedtransparent windows148,150 are pressed up against the optically polished end faces172,176 of theEO crystal142 by the use of wave springs13 which are compressed by theretainer spring holder11 bybolts21. Theretainer spring holder11 has ahole11ain it to provide passage of alaser beam402 through the ZnSe window/CdTe EO crystal/ZnSe window arrangement172,142,176. Theouter surfaces22 of thetransparent windows148,150, which are not in contact with the EO crystal end faces172,176, are coated with an anti-reflection coating to minimize optical transmission loss through thestructure140.Items9 and10 are an insert and a window holder, respectively, to ensure that excessive compression cannot be directed toward thetransparent window148,150 and crystal interface.Springs15 and16 are used to prevent stressing theCdTe crystal142 so that its birefringence does not cause undesired rotation of the polarization of the laser radiation passing through thecrystal142.
• Maintaining Zero DC Bias on CdTe EO Crystals for O-Switching IR Lasers
• CdTe electro-optic modulator crystals contain traces of impurities at very low concentration levels, which adversely affect the performance of these crystals in electro-optical modulator applications. The concentrations are so low that they are difficult to control in the crystal growing process. Consequently, the yield in growing these crystals with the same phase retardation performance for a given applied voltage from crystal to crystal is not high, especially if the crystal is operated by requiring that an external DC bias be maintained across the crystal for a long time. The reasons why these impurities adversely effect EO modulator performance in Q-switched lasers are as follows. It is well known that the voltage, V_o, required to be placed across an EO crystal in order to change the phase of the optical radiation propagating therethrough by ½λ (or 180 degrees) is given by:$\begin{array}{cc}{V}_O=\frac{{\mathrm{<figure-callout\; label="\lambda \; O"\; filenames="US20050069007A1-20050331-D00024.png,US20050069007A1-20050331-D00026.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}_O}{2{\mathrm{<figure-callout\; label="n\; O"\; filenames="US20050069007A1-20050331-D00007.png"\; state="\{\{state\}\}">n</figure-callout>}}_O^{}{r}_{41}}\times \frac{d}{l}& \left(1\right)\end{array}$
  where λ_ois the wavelength of the radiation×10⁻⁴cm,$\frac{d}{l}$
  is the aperture/length ratio of the crystal, no is the refractive index of the crystal (=2.6 in CdTe) and r₄₁is the electro-optic coefficient (=6.8×10⁻¹⁰cm/volt in CdTe) (A. J. Beauliea;Transversely Excited Atmospheric Pressure CO₂Lasers, Applied Phys. Letters, Vol. 16, pg. 504-506, June 1970, which is incorporated herein by reference). Typically a CdTe crystal of d=0.5 cm and l=5 cm requires a voltage of V_o=4.35 kV to obtain ½λ phase retardation.
• When a DC voltage calculated from Eq. 1 is applied across acrystal142, as illustrated byFIG. 14, the residing charge carriers within thecrystal142 move slowly through thecrystal142 and become captured within the unevenly distributed traps caused by the aforesaid impurities. Besides being unevenly distributed, the sizes of the traps also vary. These localized, captured charges set-up a DC bias within thecrystal142. This in turn causes variations in the phase retardation suffered by the radiation propagating through thecrystal142 as a function of the location of the propagation path through thecrystal142. In addition, these internally generated phase retardations vary with ambient temperature and with time. The yield in producing crystals that do not demonstrate these effects is low and consequently the cost is high for obtaining crystals having acceptable performance. These problems have been a big influence toward limiting the application of Q-switched CO₂lasers to primarily military applications and not toward industrial applications.
• An approach that addresses the difficulties discussed above in order to obtain a Q-switched CO₂laser suitable for industrial material processing applications is disclosed that ensures that an external DC bias is not required on theEO crystal142 in order to obtain a high loss state within thelaser cavity406,408.FIG. 14 illustrates an experimental arrangement to determine the effects, at various locations across theface172 of acrystal142, of the trapped charges on the polarization of the optical radiation propagating through aCdTe crystal142, at a given time and temperature, when subjected to a ½λ retardation DC voltage. Such a variation in polarization across the face of thecrystal142 is indicated by thesurface182 ofFIG. 15. If the crystal were perfect, one would obtain a 180° rotation of the polarization vector such that the polarization exiting of the crystal would equal the polarization entering thecrystal142. Instead a rotation of the polarization by a phase angle, θ, varying from spot to spot172aacross theface172 of thecrystal142 is found.
• FIG. 15 illustrates phase retardation data taken on a CdTe crystal subjected to a 4,400V DC bias voltage. Note the large variation away from the uniform 180° phase retardation that would be expected from a perfect crystal. Under this situation, one has no choice but to select a given location over the face of the crystal and adjust the voltage to obtain the desired phase retardation. This results in the use of a narrow laser beam whose diameter is much smaller than the cross sectional area of the crystal. If a large beam is used, then different portions of the cross section of the beam would experience different phase retardations as the beam progresses through the crystal. This effect would also require that each Q-switched laser must have the voltage and location across the EO crystal be individually adjusted to obtain the required phase retardation. Because of the nature of the trapped charges not being tightly captured and the fact that they have a slow mobility, the phase retardation shown inFIG. 15 varies with time and the temperature of the crystal. This compounds the difficulty of using EO Q-switched lasers in industrial applications. In addition, if the polarity of the applied DC voltage is reversed, an entirely different phase retardation pattern across the face of the crystal is obtained.
• One approach to getting around these CdTe material problems in order to obtain a CO₂Q-switched laser suitable for industrial material processing applications comprises operating the EO modulator under a zero DC voltage condition during the high cavity optical loss portion of the pumping interval of the Q-switching cycle indicated inFIGS. 2A and 2B. The DC voltage is only applied to theCdTe crystal142 during the short time of the pulse output interval shown inFIGS. 2A and 2B when the cavity loss is low (i.e. the cavity has a high Q). The short time that the voltage is applied to the crystal minimizes the charge carriers congregating in traps and generating an internal DC bias unevenly across thecrystal142.
• The arrangement for operatingCdTe crystals142 with no external DC voltage applied to theEO modulator crystal142 and inserted within a Q-switched laser is shown schematically inFIG. 16A. The output beam of a CO₂laser is polarized parallel to the plane of themetal electrode730 that is exposed to the gas discharge within thelaser gain medium726. Consequently, in the side view shown inFIG. 16A, the optical electric field is polarized perpendicular to the plane of the paper as shown with the dots “•” inFIG. 16A. A thin film polarizer (TFP)404, is inserted between thelaser gain medium726 and the high optical damage thresholdEO modulator assembly140 as shown inFIGS. 16A and 16B. TheTFP404 is positioned so that theoptical radiation402 polarized perpendicular to the plane of the paper propagates through theTFP404 with minimum optical losses (FIG. 16B). However,optical radiation402 polarized in the plane of the paper does not propagate through TFP404 (FIG. 16A). Polarization in the plane of the paper is shown by the arrows “⇄” and “

  

  .” Radiation exiting to the left ofTFP404 passes through thelaser gain medium726, partially reflected back into thelaser gain medium726 by the partially reflectingmirror406, and back throughTFP404 into theCdTe EOM assembly140.
• InFIG. 16A, theradiation402 polarized perpendicular to the plane of the paper emitted by thelaser gain medium726 propagates through theTFP404 and continues through the high damage threshold ZnSe/CdTe/ZnSeEO modulator assembly140 ofFIGS. 11, 12,13 and14 and apolarization rotator412 such as a quarter wave plate. The ¼plate412 is utilized to convert linear polarization to circular polarization as shown by the circular arrows “

  

  ” “

  

  .” Other polarization rotating devices can also be used, such as quarter wave rhombs, prisms or reflective phase retarders.
• The linearlypolarized beam402 propagating through the¼λ plate412 inFIG. 16A becomes circularly polarized and is in turn reflected off the reflectingmirror408 back through the¼λ plate412 thereby experiencing another 90-degree rotation in polarization. The optical radiation propagating back toward the ZnSe/CdTe/ZnSeEO modulator assembly140 is now polarized parallel to plane of the paper. This radiation propagates through theEO modulator assembly140 back toward theTFP404. TheTFP404 reflects this polarization component out of the laser cavity comprised ofmirrors406,408. This in effect maintains a high loss condition for thelaser cavity406,408 with no voltage applied to theEO crystal142. This high loss condition prevents the laser from oscillating which in turn enables the population (i.e. the gain) in the upper laser level to build-up to a much larger than normal value. This population build-up acts as an optical energy storage process for the laser. This optical stored energy is released by applying a voltage to theEO modulator crystal142 in order to induce a ¼λ (i.e. 90 degree) polarization rotation. The application of a pulsed voltage to the EOM crystal avoids the phase retardation variation problems caused by the impurities within the CdTe material as mentioned previously.
• The switch to a low cavity loss condition of thelaser cavity406,408 can be made to occur as follows. The spontaneous emission radiation emitted by thelaser gain medium726 that is polarized perpendicular to the plane of the paper as shown inFIG. 16B, propagates through theTFP404 and through the high optical threshold ZnSe/CdTe/ZnSeEO modulator assembly140. The difference this time is that a voltage applied to theEO modulator140 provides a 90-degree (i.e. ¼λ) polarization rotation to the radiation exiting to the right of theEO crystal142. The voltage applied in this arrangement is ½ of V_oof Eq. 1 because the radiation makes two passes through theEOM crystal142. Consequently, the radiation leaving theEO modulator140 is circularly polarized. When this circularly polarized radiation propagates through the¼λ plate412, it becomes linearly polarized in the plane of the paper as shown inFIG. 16B. This linearly polarized radiation is reflected from the reflectingmirror408 as linearly polarized light, back through the¼λ plate412, which again rotates the radiation by 90 degrees and converts it to circular polarization. This circularly polarized light is directed back through theEO crystal142. Since a ¼λ voltage is still experienced by theCdTe crystal142, the circular polarization is again rotated by 90 degrees, which converts the radiation back to the original polarization perpendicular to the plane of the paper as seen inFIG. 16B. This polarized radiation is propagated through theTFP404 into thelaser gain medium726 and amplified therein. The optical intensity within thelaser cavity406,408 can now build up rapidly thereby depleting the larger than normal optical energy stored in the upper laser level which results in a short, high peak powerlaser output pulse402a. By the above described process, the radiation is rotated 360 degree by making two passes through theEO modulator140 and thequarter wave plate412.
• Reflective phase retarders have found extensive applications in the laser material processing industry to avoid variation in the Kerf width (or cross-section of the laser cut) caused by how the linearly polarized laser beam making the cut in the material is oriented with respect to the direction in which the beam is traveling. It is well known that the orientation of the polarization in relation to the direction of the cut significantly affects the cut cross-section. The conversion of the linear polarization into circular polarization eliminates the cross-section variation of the laser cut with direction of travel of the laser beam. RPRs are capable of handling the intensity within a laser cavity and are preferred for use in Q-switched CO₂lasers suitable for industrial material processing applications.FIGS. 17A and 17B illustrate the use of aRPR410 in place of the¼λ plate412 ofFIGS. 16A and 16B. The explanation for theRPR410 is the same as for thequarter wave plate412.
• FIGS. 17A and 17B illustrate the operation of a Q-switched laser with a 90 degree reflectivephase retarder device410 in place of the ¼λ plate ofFIGS. 16A and 16B. The alignment of theRPR410 is not very sensitive where as the alignment of the laser's reflectingmirror408 with the lasers partially reflectingmirror406 is sensitive. Consequently, the reflectingmirror408 and theRPR410 can be pre-aligned and placed in one housing (not shown). This housing can then be aligned with respect to the partially reflectingmirror406.
• The above paragraphs describe how the cavity loss and gain variations illustrated inFIGS. 2A and 2B are experimentally obtained for Q-switched laser operation. The ¼λ DC voltage can be applied in a periodically or randomly “on-command” pulsed format. The low loss enables the large energy stored in the upper state of the laser medium to be emitted in a single pulse of radiation with several orders of magnitude greater peak power over the continuous wave average power and more than one order of magnitude greater peak power over the RF super pulsed pumped operation. The pulse rise time is determined by the gain achieved during the high loss phase of the cyclic Q-switching process, the optical losses within the cavity, and the cavity length. Typically, we find that pulse rise times of 50 to 75 nsec and pulse widths at the half power points of 100 to 130 nanosecond are obtained from Q-switched diffusion cooled, sealed-off CO₂utilizing an NV shaped saw tooth foldedwaveguide configuration806 having a total gain length of 226 cm and excited by 1100 watts of RF power. It has been found that a time delay of approximately 300 nsec occurs before significant laser radiation is emitted from the time the cavity is switched from a high loss state to a low loss state. Peak powers of approximately 8 kW are typically obtained at a 20 kHz pulse repetition rate. Typically such a laser would normally emit over 100 W but when operated with the insertion of theTRP404,EO crystal142 and aRPR410 within thefeedback cavity406,408, such a laser typically emits 50 to 60 W of average power when operated in a Q-switched mode at high repetition rates.
• Multiple Passes to Reduce EO Crystal Size and Voltage Requirements
• The larger the EO crystal that is required, the more difficult it is to obtain good quality crystals. Furthermore, the cost of the crystal increases with size. On the other hand, the smaller the ratio of d/l, in Eq. 1, the lower the voltage, V_o, required to be applied across the crystal to obtain the desired polarization rotation. Consequently, making two or more passes through the crystal is, in many cases, advantageous from the standpoint of cost or from the standpoint of the reduction in the DC voltage applied across the crystal. This is so in spite of the additional optical losses suffered by multiple passes through the window/crystal/window assembly; assuming that one does not increase d appreciably in order to utilize the multiple pass approach.
• FIGS. 18A, 18B and18C illustrate some multiple pass configurations in a side view format.FIG. 18A illustrates a double pass configuration with a high reflection coating150bdeposited on the outer surface of theZnSe window150 furthest from thegain medium726. TheZnSe window148 closest to thelaser gain medium726 is coated with ananti-reflection coating148a. If d remains the same as in the single pass configuration ofFIGS. 16A, 16B,17A and17B, then one has the choice of reducing l by ½ so that a shorter crystal is utilized, or, if l remains the same, as in the above single pass configurations, then the double pass configuration ofFIGS. 18A and 18B reduces the voltage, V_o, by ½ for the same phase retardation. By this process one can reduce the voltage by ⅓ for the triple pass configuration ofFIG. 18C.
• FIG. 18B illustrates another version of the doublepass EO modulator140. This version utilizes aZnSe prism182 replacing theZnSe window150 and the reflectingcoatings150bthereon shown inFIG. 18A. The double pass versions require ½ V_oof Eq.1 to be applied to theEO crystal142 for an 180 degree phase retardation, whereas the triple pass version ofFIG. 18C requires ⅓ V_oto be applied; assuming the d/l ratio of theEO crystal142 is maintained constant. For a 90 degree phase retardation, the voltages required are ¼V_ofor a double pass and ⅙V_ofor a triple pass arrangement of thecrystal142.
• The inclusions of theTFP404 and theRPR410 optical components are also indicated inFIGS. 18A, 18B and18C. The¼λ plate412 ofFIGS. 16A and 16B or other polarization retardation devices can also be used in place of theRPR devices410.
• Automatic Down Delay Circuit (Pulse Tail Clipping)
• FIG. 19 illustrates the operation of the Q-switched laser system ofFIG. 5 orFIG. 10. The square shapedwaveform342 is the high voltage pulse applied to theCdTe EO crystal142 by the switchingcircuit206 ofFIGS. 5 and 10. The pulse-like waveform344 is the Q-switched output pulse of the laser displaying along tail344a. The horizontal scale is 250 nsec per major division. Both waveforms are bandwidth limited in this figure. The peak voltage of thehigh voltage pulse342 applied to theCdTe EO crystal142 is 2.7 kV and its width is approximately 1.5 microsec. Approximately 500 nsec after this voltage is applied to theEO crystal142, laser action is initiated. The average power of the Q-switchedoutput pulse344 of the laser, at a 20 kHz pulse repetition frequency (PRF) for the voltage pulse applied to thecrystal142, and with a long tail, is approximately 15 W for 110 nsec wide pulses. This yields about 6.8 kW of peak power per pulse (15W÷(10×10⁻⁹sec×2×10⁴Hz)). The energy per pulse is approximately {fraction (3/4)} mJ. Notice that thepulse342 extends out to beyond 1 microsecond because of thelong tail344a. Thistail344acontains appreciable energy which can circumvent the advantages of using short laser pulses to drill holes or mark or encode stressed glass containers or surfaces, or to perforate or drill small holes in paper or plastic. Notice also that when thehigh voltage344 applied to theEO modulator crystal142 goes to zero thelaser pulse344 is clipped or truncated at344b. At 20 kHz, and with tail clipping such that little or no pulse tail occurs, the average laser power experimentally obtained was 10 W with pulse energies of approximately 0.5 mJ per pulse.
• The major function of the Automatic Down Delay Circuit (ADDC)306 ofFIGS. 5 and 10 is to realize the full benefit of short Q-switched laser pulses in material processing applications, such as drilling, by truncating thelong tails344aof the Q-switched laser pulses. This prevents unnecessary heating of the material during processing, such as hole drilling. The use of Q-switched pulses with tail clipping has not been previously utilized in industrial material processing applications such as via hole drilling of printed circuit boards.
• One method by which to clip or truncate the tail of thelaser pulses344 in the laser system ofFIGS. 5 and 10 is depicted byFIG. 20.FIG. 20 follows the data shown inFIG. 19. The width of thehigh voltage pulse342 applied to theEO crystal142 is preselected or adjusted at a set value so as to obtain approximately the desired amount of tail clipping of the long tail Q-switchedlaser pulse344. This is accomplished when theswitching circuit206 turns off thepulse342. The turning off of thehigh voltage pulse342, causes the laser to transition from a low loss state to a high loss state, thereby causing laser oscillation to cease. By preselecting or adjusting the width of thehigh voltage pulse344, the amount of tail clipping can be preselected or varied as desired, yielding a laser shape and pulse width (LPW) at the half power points as shown inFIG. 20. The width of the high voltage pulse can be decreased to the point where the back end of the simultaneously super pulsed Q-switched laser pulse can be truncated to yield pulses shorter than 100 nanoseconds.
• For some applications this pulse width preselection or adjustment approach may not be suitable because of the variation that can occur in the laser oscillation build time (BT) and the pulse rise time (PRT). BT is the time required for the laser to begin to lase after building up from spontaneous emission after the laser has transitioned from a high loss state to a low loss state by the application of thehigh voltage pulse344 to theEO crystal142. The laser pulse rise time (PRT) and the laser buildup time (BT) can vary primarily due to gain changes that can occur within the laser medium. This can be caused by the aging of the laser, variations in the temperature of thelaser head400, varying the pulse repetition rate, loss changes within thelaser feedback cavity406,408, the amount of appliedRF power716 driving thelaser head400 because of power line variations, and in the variation in the polarization of thelaser radiation402 caused by changes in theEO modulator140. All of these effects will cause a variation in the amount of laser pulse tail clipping that will occur when utilizing the preselected high voltage pulse width approach. The largest variation will occur with changes in the pulse repetition rate of the laser.
• A second method of tail clipping is illustrated byFIGS. 21A and 21B. This laser pulse tail clipping approach offers less variation in pulse length with changes in laser gains and/or pulse repetition rate. This approach can be utilized in the laser systems depicted inFIGS. 5 and 10 for industrial material processing applications.
• In the ADDC ofFIG. 21A,radiation402cemitted by eithermirror414 inFIG. 5 orFBM408 ofFIG. 10 is detected by anoptical detector302 such as a pyro-detector. An outputelectrical signal304 from thedetector302 is applied to the input of one or morecascaded preamplifiers310. Theoutput signal312 from thepreamplifier310 is applied to oneinput terminal314aof avoltage comparator314. Anadjustable DC bias318,320 is applied to theother input terminal314bof thevoltage comparator314. When thepulsed signal312 from thepreamplifier310 exceeds the preselectedvoltage bias level320, thevoltage comparator314 provides anoutput signal316 to aprogrammable timer324. Programmable gate arrays can be configured to perform the programmable timing or counting function of thetimer324. Provision is made at324ato allow the laser operator to manually provide the appropriate time delay, τ_pc, of theprogrammable timer324, thereby enabling the laser operator to select how much of the Q-switchedtail344ais clipped. After the selected time delay, τ_pc, theprogrammable timer324 emits asignal308 to thepulse receiver202 or theswitching circuit206 ofFIGS. 5 and 10. This turns off thehigh voltage342 applied to theEO crystal142 thereby transitioning the laser from a low loss state to a high loss state and causing the laser action to cease.
• Since the beginning of the sequence for issuing thesignal308 to clip thetail344aof the Q-switchedpulse344 is started by detecting the Q-switched pulse itself and not by the beginning of thehigh voltage pulse342 as inFIG. 20, the approach ofFIG. 21A is not sensitive to the laser oscillation build up time (BT) ofFIG. 20 and therefore provides a better control of the laser pulse width (LPW).
• As illustrated inFIG. 21B, as the Q-switched pulse rise time and/or Q-switched pulse amplitude changes, shown for illustration purposes as326a,326band326c, there is some variation in the time at which thetimer324 starts running because of changes in the laser pulse rise time (PRT). Changes in the laser pulse rise time and amplitude will occur because of the same reasons given above. This approach will cause a much smaller variation in the laser pulse width (LPW) than the approach ofFIG. 20.
• If the LPW variation provided by the approach ofFIGS. 21A and 21B still cannot be tolerated in the application of interest, a third method of tail clipping is illustrated byFIGS. 22A and 22B. InFIG. 22A, thecomparator314 issues asignal316 at a pre-selected position on the Q-switched pulse. Examples of possible pre-selected pulse positions are at 50% of the pulse rise time or at the peak of the pulse. InFIG. 22A, adetector302 is used to detect the Q-switchedpulse402c. Theelectrical output signal304 of thedetector302 is provided to one or morecascaded preamplifiers310. Theoutput signal312 of thepreamplifier310 is split in twosignals312a,312b. Onesignal312ais propagated through a time delay, τ_Sdevice328, which yields asignal S₁330 as shown. Signal S₁has the pulse shape shown at340ainFIG. 22B. The time delay, τ_S, of up to about 60 nsec was found to be adequate. Theother signal312bis propagated through anattenuator336, which provides asignal S₂338 as shown. Signal S₂has the pulse shape shown at340binFIG. 22B. Signal S₁along with a negativeDC bias voltage334 is applied to oneinput terminal314aof thevoltage comparator314. Signal S₂is applied to theother input terminal314bof thecomparator314. When the value of signal S₁equals the value of the signal S₂(340cofFIG. 22B), thevoltage comparator314 issues asignal316 to thetimer324. After the time delay, τ_pc, at324a, theprogrammable timer324 issues thesignal308 to thepulse receiver202 or switchingcircuit206 ofFIGS. 5 and 10 to turn off thehigh voltage342 being applied to the EO modulator140 ofFIGS. 5 and 10. The turning off of thehigh voltage342 applied to theEO modulator140 causes the laser cavity to transition from a low loss state to a high loss state, thereby stopping the laser action and clipping the Q-switchedlaser pulse tail344a. The amount of the tail clipped is determined by the time, τ_pc, applied to theprogrammable timer324 at324aby the operator of the laser system.FIG. 23B illustrates the relationship of the signals S1 and S2 provided to thevoltage comparator314.
• FIGS. 23A and 23B illustrate typical Q-switchedpulses344 obtained from the laser ofFIG. 5 under long pulse and short pulse tail conditions, respectively utilizing theADDC306 ofFIG. 22A. The data is not bandwidth limited. The data was taken with a HgCdTe detector (not shown) at the output of thelaser housing102 ofFIG. 5 or10.FIG. 23A shows an approximately 1.1 microsecond pulse length at the baseline including the long tail. By decreasing the value of the delay, τ_pc, in theprogrammable timer324 ofFIG. 22A, ashort pulse tail344bis obtained as illustrated inFIG. 23B. In this case, the total Q-switched pulse width at the baseline is now about 450 nsec. The Q-switched pulse width at the half power points of the pulse is 100 nsec for both cases. The data inFIGS. 23A and 23B was taken at a PRF of 20 kHz for thevoltage342 applied to the electro-optic crystal142.
• In the housing arrangement for theADDC306, high electromagnetic interference (EMI) immunity is desired because of the closeness of the nearby high voltage pulse circuitry required to drive theEO crystal142. The EMI immunity is obtained by inserting thedetector302 and the rest of theADDC circuitry306 within a tightly sealedmetal housing346 and itscover348 and making exceptionally good electrical contact to the covers and electrical connectors that enclose thecircuitry306 within themetal housing346 and itscover348.
• FIGS. 24A and 24B present a side and end views that illustrate where theADDC306 components are placed within themetal housing346. InFIG. 24A,item348 is the top metal cover anditem356 is the rear metal cover. These are tightly bolted onto themetal housing346 with good electrical contact gaskets (not shown) between thecovers348,356 and thehousing346 to eliminate spurious electrical signals from getting into or out of thehousing346. Item350 is a DC to DC converter to convert 28 volts DC from thecontroller104 to the appropriate DC voltage value to power all the circuits of theADDC306. The DC to DC converter350 is placed on the signal processing printedcircuit board352. This printedcircuit board352 contains the programmable gate arrays and associated components comprising theprogrammable timer324.Item354 is an electrical connector that provides electrical signal access into and out of thelaser housing102. The Q-switched laser pulse tail-clipping signal308 from theADDC306 is delivered to thepulse receiver202 through this EMI protectedconnector354.Item358 is the printed circuit board that contains thepreamplifiers310, the time delay, τ_S,328 theattenuator336, theDC bias332 and thevoltage comparator314 circuits illustrated inFIGS. 21A and 22A.Item360 is a bottom height adjustment plate.Item302 is the optical detector which can be a pyro-electric detector as illustrated inFIGS. 21A and 22A.Item364 is an optical diffuser to ensure uniform illumination of the pyro-detector302.Item364 is inserted into a separate opticalcomponent barrel housing372 which fits into themain ADDC housing346. Inserted in the opticalcomponent barrel housing372 is anaperture370, anoptical attenuator368 and a beam-concentratinglens366.Items368,346 and364 are inserted only if needed.Item374 is a BNC coaxial connector, which provides an output signal from thedetector302 so that one can monitor the Q-switchedlaser pulse344 outside theADDC assembly housing346.
• High Voltage EO Crystal Electronics Design
• FIGS. 25A-25D illustrate the operation of the highvoltage switching circuit206 for theEO crystal142 ofFIGS. 5, 10,11,12 and13.FIGS. 25A-25D show the relative timing relationships between the waveform signals to command the charging214 of theEOM140, to discharge216 theEOM140, thehigh voltage218 applied to theEOM140 and the resultant Q-switchedpulse344 emitted by the laser. For illustrative purposes, the fundamental operation of the highvoltage switching circuit206 is depicted inFIG. 26 in an electromechanical switching format with cross-reference toFIGS. 25A-25D. During the period of time denoted by “t_a” inFIGS. 25A-25D, there is no signal applied by the highvoltage switching circuit206 ofFIGS. 5 and 10. Consequently, inFIG. 26 the chargingswitch220 and the dischargingswitch222 are both open and theEOM140 is not charged-up (i.e. no voltage is applied to the EOM140). When asignal214 to charge-up the capacitance of theEOM140 is provided by thesystem controller104 ofFIGS. 5 and 10, chargingswitch Sc220 inFIG. 26(b) is closed at time “t_b” thereby permitting the capacitance of theEOM140 to be charged up to the full high voltage value available from theHV switching circuit206 ofFIGS. 5 and 10. After approximately 100 nsec, signal214 to charge theEOM140 is turned off at time “t_c.” TheEOM140 is fully charged and switch220 is opened as illustrated at “t_c” inFIG. 26(c). The laser is now in a low cavity loss condition and laser oscillation is initiated. After the cavity build-up time (i.e. CBT ofFIG. 25D) the Q-switchedlaser pulse344 is emitted as shown. At time “t_d” thesignal308 from theADDC306 of FIGS.5 or10 orFIG. 21A orFIG. 22A, discharges theEOM140 by closingswitch222 as shown at “t_d” inFIG. 26(d). After approximately 100 nsec, signal216 to discharge theEOM140 is turned off and switch222 is opened at time “t_e,” thereby leaving thehigh voltage switch206 as inFIG. 26(e), which is the same as in the original state during the time “t_a.” The discharge of theEOM140 clips thetail344aof the Q-switchedpulse344 as previously discussed (seeFIGS. 25A-25D).
• FIG. 27 illustrates the electronic implementation ofFIG. 26. The high voltagepulse switching circuit206 operates in conjunction with the Automatic Down Delay Circuit (ADDC)306 ofFIGS. 21A and 22A to generate the high voltage waveform required for driving theEO modulator crystal142 to produce the Q-switchedlaser pulse344. The high voltage pulsedsignal generation system200 ofFIGS. 5 and 10 includes a high voltageDC power supply208 and the highvoltage switching circuit206 as shown in detail inFIG. 27. TheHV switching circuit206 accepts either apulsed signal214 to charge theEO modulator140 or apulsed signal216 from theADDC306 to discharge theEO modulator140.
• Thepulsed signal214 to charge theEO modulator140 initiates a charge cycle, which applies high voltage to theEO modulator crystal142 as depicted in time “t_b” ofFIGS. 25A-25D. Thepulsed signal216 to discharge theEO modulator140 initiates a discharge cycle where the EO modulator voltage is returned to zero as depicted in time “t_d” ofFIG. 25A-25D.
• In order to produce fast high voltage pulse rise times of ˜10 ns or less, the design of the highvoltage pulse switch206 requires careful attention in minimizing parasitic capacitance and inductance while still providing the necessary high voltage insulation to prevent electrical arcing. In addition, reducing parasitic capacitance results in lower power dissipation, which significantly effects the thermal management and ultimately overall size of the highvoltage switching circuit206.
• The desirable features for the highvoltage switching circuit206 driving theEO crystals142 are: 1) reliable high voltage operation in a small size and at high PRFs, 2) low parasitic capacitance for fast pulses rise times and reduced power dissipation, 3) low propagation delay to allow Q-switched or cavity dumped operation and 4) the ability to adjust the optical pulse amplitude by varying the high voltage pulse amplitude.
• To achieve the above performance, the highvoltage switching circuit206 is constructed using a plurality of highvoltage power MOSFETs224,226 ofFIG. 27 for charging and discharging theEO crystal142. The highvoltage power MOSFETS224,226 fulfill basic the functions of themechanical switches220,222 ofFIG. 26. The switchingMOSFETs224,226 are selected for their high operating speeds and avalanche high energy capabilities. The high speed characteristic is used to generate fast high voltage pulses while the latter characteristic is used to obtain reliable, fault tolerant operation. For example, MOSFET's such as Philips Electronics BUK456 are suitable for this application. Drive for the charging MOSFET switches224 is provided by a series arrangement of ncwideband pulse transformers228. Thesetransformers228 are constructed ontoroidal ferrite cores228ausing high voltage wire and potting compounds to obtain the required high voltage insulation.FIG. 27 illustrates a plurality of n_cstep downpulsed transformers228 with a n:1 ratio such as 2 to 1 or 3 to 1 in order to obtain current gain. When a “charge” pulsedsignal214 is applied to the pulsegeneration switching circuit206 through the amplifier/control circuit206a, thesetransformers228 provide a positive signal to the gate (G) and a negative signal to the source (S) connectors of each of the n_ccharging MOSFETs224. This causes current to flow from the high voltageDC power supply208 through the drain (D) to the source (S) of each of theMOSFETs224 then through theresistor232 and on to theEO crystal142. The number of chargingMOSFET switches224 used in thecircuit206 is determined by the voltage rating of theMOSFETs224 divided into the maximum voltage applied to theEO crystal142 plus a factor to ensure high reliability. For a voltage of 4 kV, five to six MOSFETs of the Philips BUK456 variety appears to suffice. When thecharge pulse signal214 goes to zero, theMOSFETs224 are turned off as illustrated at time “t_c” ofFIGS. 25A-25D and26 and theEO modulator140 maintains its charge until it is commanded to discharge bysignal216.
• When thedischarge pulse signal216 is applied to the high voltagegeneration switching circuit206 through the amplifier/control circuit206a, apulsed signal216 is applied to each of the nd step downtransformers230 which in turn applies a turn on signal across the Gate (G) and Source (S) connections of each of the “n_d” dischargeMOSFETs226. This enables current to discharge from theEO modulator crystal142 through each of the “n_d” dischargeMOSFETs226 to ground as schematically illustrated at time “t_d” ofFIGS. 25A-25D and26. When thedischarge signal216 goes to zero, theMOSFETs226 are turned off and the status of theswitching circuit206 is as depicted at time “t_e” ofFIGS. 25A-25D and26.
• An impedance, Z_n,234 can be connected across each of the n_ccharging MOSFETs224 to provide a voltage balance across theMOSFETs224. The resistive part of theimpedance234 across each of the chargingMOSFETs224 can typically each have a 10 megaohm value to balance the DC between each of the chargingMOSFETs224. If the avalanche properties of the chargingMOSFET224 present a problem then a capacitor across each of the resistors can be used. The value of the required capacitance across each of the chargingMOSFETs224, needs to be different. The capacitors are also a balance for the AC portion of the charging signals214. From a cost standpoint the capacitor can be done away with if the chargingMOSFETs224 are selected not to be sensitive to avalanching problems.
• Resistors236 (typically 150 kilo ohms) are used to ensure DC balancing during theEO modulator140 discharge portion of the cycle. The chargingresistor232 inFIG. 27 serves the purpose of minimizing cross conduction problems between the charging and discharging portion of the circuits.
• Discussed above are the basic elements of this highvoltage switching circuit206. Variations upon these basic elements are possible to those versed in the fast electronic circuits state of the art. For example, a single transformer with multiple secondary windings can be used to drive the MOSFETs instead of individual cores as shown inFIG. 27. Positive feedback windings can also be added to produce faster pulse transitions. Various opto-isolated gate drive techniques could also be applied. Active off drive of the non-conduction MOSFETs can also be employed which allows this isolation resistor to be minimized or eliminated. Resonant charging techniques can also be used to reduce power consumption.
• The high voltage pulse output from the highvoltage switching circuit206 may be varied in order to obtain a desired output optical power from the Q-switchedlaser pulse344. This can be accomplished by varying the output voltage of the highvoltage power supply208 in either an open or closed electrical loop fashion. In addition, theADDC circuit306 allows extended variability in the output optical pulse width thereby permitting pulse energy variation on a pulse-to-pulse basis.
• Thepulse receiver202, the highvoltage DC supply208 and the highvoltage switching circuit206 ofFIGS. 5 and 10 and27 are packaged in ametal housing236 as illustrated inFIG. 28. Because of EMI considerations, careful attention is given to preventing electromagnetic radiation from either leaking out of or into the metal housing by techniques well known in the art.Item238 is a cover for thehousing236.Items240 and242 are heat sinks for the high voltage switching circuit printed circuit board.Item202 is the printed circuit board containing the pulse receiver.Item244 is a filter capacitor anditem246 is the box containing the high voltageDC power supply208 such as an Ultra volt Model 4C24-P60.Item250 is the input/output electrical connector andItem252 is the end plate.
• Automatic Stop Drilling (ASD) Module
• An attractive addition to a via hole laser drilling system that uses repetitive laser pulses, such as a Q-switched laser, an acousto-optic gated cell, a gated or super pulsed laser, or a TEA laser, is an automatic stop drilling (or processing) system operative to direct the laser to stop drilling operations, thus, stopping the process when a reflective surface, such as copper or other metal, is reached in the drilling processing. For example, an automatic stop drilling (ASD) (e.g., an automatic stop laser operation) system may utilize the large back reflectance from copper or other metals when the laser has drilled through a dielectric material in a printed circuit board. The detection of the signal from the back reflected radiation provides a signal to the laser to stop pulsing at that location on a work piece and to start pulsing again after the laser beam has been moved to another spot on the work piece. Such an ASD system prevents wasted time thereby increasing the throughput of the via hole drilling system.FIG. 29 illustrates a schematic of anASD system500 connected to the Q-switchedlaser housing102 ofFIG. 10. Such anASD system500 can also serve to prevent back reflection radiation from theworkpiece548 from entering thelaser cavity406,408 and causing optical damage to the optical components within thecavity406,408.
• FIG. 29 illustrates one method of implementing an ASD system. It also illustrates, as an example, how a pulsed via hole drilling laser has drilled through awork piece548. As an example, thework piece548, such as a printed circuit board, has fourdielectric layers536,540,542,546, one openedcopper interconnect538 and asecond copper interconnect544 from which a strong back reflected signal is detected. The polarized pulsed laser beam is directed onto the work piece by theRPR506 and a focusinglens508. The pulsed laser beam drills through thefirst dielectric layer536, past the opened copper interconnect,538 and throughdielectric layers540 and542 until thelaser beam402 is stopped bycopper interconnect line544. Once thecopper interconnect line544 is reached a large amount of radiation is reflected back out of the viahole550 toward the focusinglens508 and back toward the laser cavity. TheASD system500 utilizes the back-reflected radiation to automatically stop the laser from continuing to pulse at this location. The laser re-initiates pulsing and therefore drilling action again after it is moved to a new location and commanded to restart pulsing.
• TheASD system500 functions as follows. Thelaser pulse402 from the Q-switched laser is polarized in the plane of the paper as shown at560 inFIG. 29. Thelaser pulse402 passes throughTFP114 which is part of thelaser housing102 ofFIGS. 5 and 10.TFP114 then passes thatradiation560apolarized perpendicular to the plane of the paper with little loss, and reflects outother polarization components560bof the radiation. The same occurs forTFP504. Consequently, by rotating theTFP504, one can vary the attenuation of the laser beam propagating onto thework piece548. This is a manual option that can be incorporated into thelaser housing102 if a manual attenuation adjustment module is desired instead of adjusting the voltage applied to theEO crystal142 to be described later, either in a closed loop format or by manually adjusting avariable resistor212 ofFIGS. 5 and 10. The twoThPs114,504 are positioned as indicated (i.e. tipped toward each other) in order to maintain a straight-line alignment from thelaser head400 to theRPR506. If the option for such a manual attenuation beam adjustment is not selected, then onlyTFP114 is required inFIG. 29. In either case,TFP114 and504 are used to pass the radiation ontoRPR506, onto the focusinglens508 and then onto thework piece548. TheRPR506 converts thepolarization560athat is perpendicular to the plane of the paper into a circularlypolarized beam560cso that when the circularly polarizedradiation562ais reflected562 from thecopper544 in thework piece548 is again reflected by theRPR506, the radiation is translated intoradiation562bwhose polarization is parallel to the plane of the paper. This polarization is reflected by theTFP504 as shown and detected by asuitable detector510, such as a pyro-electric detector. The use of the RPR also provides protection to the optics within the laser cavity from damage by radiation reflected back into the cavity by rotating the polarization of the back reflected radiation so that it is reflected away from the laser cavity byTFP3504.
• Theelectrical signal512 from thedetector510 is applied to apreamplifier514 and then to oneinput516aof avoltage comparator516. A variable DC bias520,522 is applied to thesecond input516b. In order to avoid false alarms, thebias voltage522 is adjusted to a high enough voltage so as to have thevoltage comparator516 emit anoutput signal524 only on strong back-reflected signals arising from the metal reflection and not from the much weaker reflection from the dielectric. When theoutput signal524 emitted by thevoltage comparator516 is larger than the bias voltage522 asignal524 is emitted by the voltage comparator. The electrical signal from thevoltage comparator524 is used to inform thesystem controller102 that a “stop laser pulsing” command has been sent to the EOmodulator switching circuit206. This allows thesystem controller102 to move the laser beam to another spot and reinitiate drilling action. Thesignal524 from thevoltage comparator516 is also applied to aninverter526 and then to a logic ANDgate532. The ANDgate532 supplies a “stop drilling”signal534 to thepulse receiver202, which insures that no voltage is applied to theEO crystal142, if thesignal528 from theinverter526 and acommand pulse530 from thepulse receiver202 both applied to the ANDgate532.
• The AutomaticStop Drilling System500 ofFIG. 29 operates as follows. If the drilling operation has not reached the secondcopper interconnect line544, then there is little or no back reflected signal detected at thedetector510. As a consequence there is no input signal512 provided to thecomparator516, norsignal output524 from thecomparator516, i.e., theoutput524 of thecomparator516 is at a “logic low.” However, if the drilling operation has not reached the secondcopper interconnect line544, thelaser system100 must continue drilling (i.e., continue Q-switching). In order for thelaser system100 to continue drilling, there must be aninput528 to the logical ANDgate532 so that the ANDgate532 can provide anoutput signal534. This can occur because theinverter526 converts the “logic low” ofsignal524 to a “logic high”528. The ANDgate532 logically ANDs the “logic high” ofsignal528 with the “logic high” ofsignal530 and theoutput534 of the ANDgate532 directs thelaser system100 to continue drilling. If the drilling operation has reached the secondcopper interconnect line544, then there is a large back reflected signal detected by thedetector510. As a consequence there is aninput signal512 provided to thecomparator516 through thepreamplifier514 and anoutput signal524 from thecomparator516, i.e., theoutput signal524 of thecomparator516 is now at a “logic high.” When the drilling operation has reached the secondcopper interconnect line544, the electronics instruct thelaser system100 to discontinue drilling (i.e., discontinue Q-switching). In order for thelaser system100 to discontinue drilling, theoutput534 of the logical ANDgate532 must be a “logic low.” Thus, theinverter526 converts a “logic high”524 to a “logic low”528. The ANDgate532 logically ANDs the “logic low” ofsignal528 with the “logic high” ofsignal530 and the lack of anoutput signal534 from the ANDgate532 results in no signal to the pulse receiver so that no voltage is provided to theEOM crystal142, thereby directing thelaser system100 to discontinue drilling.
• Simultaneously Super-Pulsed and Q-Switched Laser Operation
• FIG. 30A pulse formats I and II summarize the various CO₂laser pulse techniques that are presently used to drill or perforate materials namely, amplitude modulated CW operation (I) or super pulsed (II) mode of operation. Pulse formats III through IV ofFIG. 30A illustrate the shorter pulse, higher peak power approaches disclosed by this invention, namely Q-switched and simultaneously super pulsed and Q-switched operation. The advantages arising from the use of short, high peak power, high repetition rate, low energy per CO₂laser pulses are well recognized as arising from the fact that as laser pulses become shorter one obtains cleaner holes and the drilling process is conducted more efficiently with minimum adverse thermal effects on the material. Even though the energy per pulse is lower, the high pulse repetition obtained with Q-switched and cavity dumping techniques increase the speed of the drilling process. Consequently, it is believed that the pulsing format of IV inFIG. 30A is better than III, and III is better than II and II is better than I in drilling via holes.
• Pulse formats IV and III, where the Q-switched approach provides pulse widths of about 0.1 μsec to several μsecs, are better than pulse format II. The high repetition rate gain switch approach of pulse format II yields pulse widths of one to tens of microseconds. Pulse format II is better than pulse format I. The pulse gated format I is the normal laser pulsing condition. It yields pulses of a few microseconds duration to CW. The disadvantage of the smaller energy per pulse associated with shorter pulses when compared with the higher energy per pulse for the wider pulses is made up by the higher repetition rate and higher peak power of the shorter pulses which expel the material out of the via hole being drilled by the laser pulses. These parameters result in better-defined, smaller diameter and smoother via or perforation holes. Which of the formats shown inFIG. 30A is chosen can depend on the trade off between the cost of the laser, the speed of the drilling process, and the quality of the holes drilled.
• FIG. 30A illustrates the higher peak power advantages realized in operating a laser in a simultaneous super pulsed and Q-switched mode and also in a simultaneous super pulsed, Q-switched cavity dumped mode. Advantages include higher peak power, higher energy per pulse, and faster laser pulse rise time outputs for the sameaverage RF power606 supplied to thelaser head400 at716. The higher RF drive power in the super pulsed mode also enables CO₂lasers to operate at higher gas pressures which can yield higher energy per output pulse over non-super pulsed operation.FIG. 30B illustrates repetitive pulse operation of simultaneously super pulsed and Q-switched waveforms. In particular,FIG. 30B shows the RF peak power pulse (P_RFP)602 applied to the CO₂gain medium726 (at716 inFIGS. 5 and 10), thehigh voltage pulse604 applied to theEO crystal142, the maximum average RF power (i.e., continuous wave radio frequency power606 (P_RFCW)) that can be applied to thelaser gain medium726 and the resultant Q-switchedlaser output pulse344, and their respective timing relationship with each other at a duty cycle of approximately 50%. In this example, one Q-switchedpulse344 is generated persuper RF pulse602 exciting the CO₂laser discharge (gain medium726).
• The laser drilling system operator determines the laser's super pulsed repetition frequency, P_RF, and the duty cycle η.FIG. 30B illustrates the condition where the peak power of the RF pulse (P_RFP)602 is about twice the average RF power capability of the power supply (P_RFCW)606. This is typical for a duty cycle of 50%. Thehigh voltage604 applied to theEO crystal142 is selected to provide a 90-degree polarization rotation as described forFIGS. 16A-17B,5 and10. Thehigh voltage604 is applied to theEO crystal142 after allowing time, PBT, to elapse after the super pulsed RF is turned on in order for the population inversion in the laser medium to rise for an optimum time, which is about one to two population decay times, τ_d, of the upper laser level. This time is denoted as the population build-up time (PBT) inFIG. 30B. Thehigh voltage604 causes thelaser cavity406,408 to switch from a high loss state to a low loss state thereby allowing laser action to take place. This results in a Q-switched laser “spike” of radiation to be emitted by the laser after a cavity build-up time demoted as CBT inFIG. 30B. The system operator determines the delay time PBT between the initiation of the RF super pumpingpulse602 and the initiation of theHV pulse604 applied to thecrystal142. This is done by inputting this information through the keyboard ofFIG. 4 into the system controller, which provides the desired electrical signal to theRS422pulse receiver202 and to thehigh voltage switch206.
• InFIG. 30B, T_RFis the pulse width of the super pulse RF power (P_RFP)602, T_HVis the pulse width of thehigh voltage604 applied across theEOM crystal142, and T_PRPis the super pulse repetition period. The duty cycle is defined as T_RF/T_PRPand the super pulsed repetition frequency P_RF=¹/T_PRP.
• If thelaser beam402 is moved from one processing location (i.e. a via) to another processing location on the order of a millisecond or longer, then the super pulse operation can be performed at a much lower duty cycle. The lower duty cycle enables the application of higher RF power (i.e.,604 ofFIG. 30B) to the laser discharge within the foldedwaveguide806 during the super pulse operation as well as enabling the use of higher gas pressure in the gain medium. Both of these changes result in obtaining higher pulse energies. As a general rule, the amount of power P_RFP, applied to the gas laser discharge can be approximately equal to P_RFCW/Duty cycle. As an example, for a duty cycle of ⅙, the power P_RFPcan be up to 6×P_RFCW. This enables obtaining higher peak powers and energies per Q-switched pulse over the operation illustrated inFIG. 30B. A low duty cycle super pulsed and Q-switched operation can operate in a single Q-switched pulse operation or in a multiple Q-switched pulse operation during a single superRF excitation pulse602 duration T_RF.
• Operation of a low super pulsed duty cycle, simultaneously super pulsed and repetitive Q-switched laser during one super RF pulse excitation of thegain medium726 is schematically illustrated inFIG. 31. Typical performance parameters obtained for a pulse excitation duty cycle of 0.14 to 0.17, P_RFP=4 kW to 5 kW and T_RF=50 μsec are 20 kW to 25 kW.peak power for a single Q-switched laser pulse with an energy of 7 to 8 mJ per pulse. For P_RFP=4 kW to 5 kW, T_RF=100 μsec, and generating 10 Q-switched pulses during the excitation period T_RF=100 μsec, the average energy for the 10 pulses was 30 mJ. This output performance repeats at a repetition frequency equal to¹/T_PRP. These results were obtained with the laser system depicted inFIG. 5 or10. The NV foldedwaveguide configuration806 for the CO₂laser head400 is normally specified as a 100 W output laser when operated continuously.
• When thetails344aof the Q-switchedpulse344 are clipped as inFIG. 23B, the energies normally present in the pulsedlong tails344aofFIG. 23A are not extracted from the laserdischarge gain medium726. Consequently, this energy is saved and thus available for extraction in future Q-switchedpulses344, which follow in the repetitive Q-switching operation. With the tail clipped pulse width ofFIG. 23B (i.e. 310 nsec wide at the base of the pulse) and with T_RF=100 μsec, and assuming a cavity build-up time of 500 nsec, as many as 123 Q-switched pulses can be repetitively obtained if one allows a 810 nsec time interval between the train of Q-switched pulses (i.e. 100×10⁻⁶/810×10⁻⁹=123).
• The output energies available from such a repetitive Q-switched pulse train during a super pulse excitation is sufficient to drill the most difficult plastic materials such as those impregnated with glass fibers (e.g., FR4 printed circuit board materials).
• FIG. 31 schematically illustrates the operational events that occur for example where P_RFP=7×P_RFCWand aRF pulse excitation602, of duration T_RF, driving thelaser discharge726 with four repetitive Q-switchedpulses344 occurring during T_RF. After the population build-up time (PBT) required to populate the upper laser level of thedischarge726 under the P_RFPRF pulse power, high voltage is applied to theEOM crystal142 thereby turning thelaser cavity406,408 from a high loss state to a low loss state. This in turn causes laser action to begin and, after the cavity build-up time (CBT), a Q-switchedlaser pulse344 rises rapidly to a peak value and then drops to a low level with along pulse tail344a. Thelong pulse tail344awould decay down to a value equal to the continuous wave output power level of the laser if allowed to do so. After a time duration T_HV, thehigh voltage pulse604 applied to theEOM crystal142 is turned off, thereby converting thelaser cavity406,408 from a low loss state to a high loss state. This in turns stops laser action, thereby clipping thetail344aof the Q-switchedlaser pulse344. Clipping thetail344aoff thelaser pulse344 stops the depopulation of the upper laser level, thereby enabling the population of the upper laser level to again begin to build-up under the RFsuper pulse excitation602 of thedischarge726.
• FIG. 32 illustrates four Q-switched pulses at a 100 kHz PRF during one super pulse excitation of the laser discharge having a duration of approximately 60 μsec.
• Laser pulse trains comprising of repetitive short pulses contained within a pulse train envelope are superior in drilling materials compared to a single long pulse. (Steve Maynard,Structured Pulses: Advantages in Percussion Drilling; Convergent Prima Newsletter, The Laser's Edge, Vol. 11, Winter 2000, pg. 1-4, which is incorporated herein by reference). Such structured pulse trains are known to achieve higher drilling speed, better hole taper control, better debris control, and enable finer tuning or adjustment of the drilling process when the pulse amplitude is sequentially increased from the beginning to the end of the laser pulse train. Repetitive Q-switching a laser naturally leads to the desired pulse train for hole drilling applications (seeFIGS. 30A, 30B and31 for two repetitive pulse train examples). Additional examples are schematically illustrated inFIG. 30A. They are CWRF pumped or gated (I), super RF pulsed pumped (II), CW pumped Q-switched (III) and super RF pulsed pumped Q-switched (IV) pulse trains.
• By varying the amplitude of thehigh voltage604 applied to the EOM crystal142 a convenient non-mechanical, and therefore fast, variation of the amplitudes of the individual Q-switched pulses can be obtained (FIG. 33). This requires an electronic control of the voltage output of the highvoltage power supply208 ofFIG. 5 or10. This will be described later in this disclosure.
• The first Q-switched laser pulse starting the drilling of a hole does not require as much energy as the last Q-switched laser pulse. This is so because the last Q-switched laser pulse is drilling deeper within the material. A larger pulse energy is required for drilling deeper within a material because it takes more laser energy to bring the material out of the hole. By sequentially increasing the laser energy as the pulse train progresses, the surface debris can be better controlled and excessive tapering of the hole can be prevented. Drilling with a structured pulse train results in the ability to begin drilling with low energy per pulse, which causes little or no “bell mouthing” of the hole, and minimal debris splattering. Once below the surface of the material, the energy of the pulses can be increased incrementally as shown inFIG. 33 and the drilling process continues with minimum tapering.
• FIG. 33 schematically illustrates the events during the simultaneously RF gated super pulsed and repetitive Q-switched pulse train with progressively increasing laser pulse peak power.FIG. 36 illustrates experimental data of the operation of a Q-switched laser in this mode of operation.
• Another method of adjusting the output power of the laser is to provide means for rotatingTFP504 ofFIG. 29 about its center axis thus varying the amount of polarized laser radiation that can leave the laser housing. This mechanical adjustment is slow and does not allow the adjustment of peak power from pulse to pulse, except for very slow pulse repetition rates.
• Control of Output of Individual Q-Switching Pulse Peak Power
• One can vary the peak power or amplitude of thelaser output pulse344 by varying thevoltage604 applied to theEO crystal142 thereby not permitting a full 90-degree polarization rotation of the radiation within thelaser cavity406,408. Thevoltage604 applied to theEO crystal142 can be changed by applying a signal to the high voltage power supply (HVPS)208 to lower the voltage supplied to the high voltage switch which applies voltage to the EOM crystal142 (seeFIGS. 5 and 10). This can provide a fast, prescribed variation in the output Q-switched peak power from pulse to pulse.
• The output optical power from a laser is a relatively sensitive function of the laser intra-cavity losses. As such, using a method that allows the intra-cavity losses to be adjusted would permit the user to vary the laser output power in response to process demands. Since a Q-switched laser utilizes an internal variable optical loss modulator, such as theEOM140, one method for accomplishing this task is to control the amplitude of the high voltage pulse applied to the electro-optic modulator140. One method of varying the pulse amplitude of the laser output is to adjust the output of a low voltage DC-to-AC-to-high voltage DC power converter power supply266 shown inFIG. 34. This drives theEOM crystal142 through theHV switching circuit206 ofFIGS. 5 and 10. In this approach, the operator of the laser material processing system ofFIG. 4 supplies a low voltageDC setpoint command260 to apower supply controller262 which then adjusts thehigh voltage output264 of the power converter266 to minimize the difference between theoutput voltage264 and the user suppliedsetpoint260 by way of thevoltage divider270. This is illustrated in the block diagram208 ofFIG. 34.
• It should be noted that the power converter266 can be a linear regulator, switching regulator, or a hybrid. In addition, in cases where a free running power conversion stage is employed, theuser setpoint260 can be used to adjust theinput supply voltage268, which feeds the power converter266. Since this approach adjusts theoutput264 of the power converter266 in response to user suppliedsetpoint command260 the response time will be relatively slow, e.g., 10 kHz and lower. The power converter266 plus thepower supply controller262 and thevoltage divider network270 comprise the high voltage power supply (HVPS)208 ofFIGS. 5 and 10.
• Thehigh voltage output264 from the power converter266 is applied to the highvoltage switching circuit206 ofFIGS. 5 and 10 which turns thehigh voltage signal178 across theEOM modulator140 on and off as explained forFIGS. 5 and 10. The operation of the electronics schematically illustrated byFIG. 34 enables one to vary the Q-switchedpulse604aamplitudes of each pulse within the pulse train as exemplified byFIG. 33.
• Control of the Output of Individual Q-Switched Pulses within a Train with an Acousto-Optic Cell
• An alternative method of varying the amplitude of individual Q-switchedlaser pulses344, within the train ofpulses604604ainFIGS. 31 and 33, is to use an optical loss modulator located external to thelaser cavity406,408 ofFIGS. 5 and 10 in order to obtain low taper in holes drilled by the laser.FIG. 35 schematically illustrates one approach by the use of an acousto-optic cell272.
• The Q-switchedpulse train344 emitted by the laser is transmitted through an acousto-optic (AO)cell272. Germanium (Ge) is a good AO cell material for CO₂laser radiation; although their higher optical loss and slower switching times when compared with EO modulators does not recommend them for intra-cavity loss modulation applications in CO₂lasers utilized in industrial applications. Varying the RF signal274aapplied to theAO cell272, varies the strength of an optical phase grating276 generated within the Ge material. Such an optical phase grating276 is generated by the ultrasonic wave propagating through the Ge material as is well known in the art. The optical phase grating276 diffractslaser radiation278 at the Bragg angle, ψ, out of thelaser radiation402 propagating through theAO cell272. The Bragg angle, ψ, is determined by the ratio of the laser wavelength to the ultrasonic wavelength as is well known in the art. The higher theRF power274aapplied to theAO cell272, the greater the amount ofradiation278 diffracted out of thelaser beam402 which is performing the drilling into a side order of the acoustically variable optical phase grating276. This results in progressively larger amplitudes with time of the Q-switchedpulses344c, as seen at times t₁, t₂, t₃, and t₄. The diffracted laser beam is diverted onto an optical absorber or stop280.
• By synchronizing theRF power274aapplied to theAO cell272, at a prescribed power level, to coincide with the arrival at276aof a prescribed Q-switchedlaser pulse344 propagating through theAO cell272, one can vary the amplitude of eachlaser pulse344 to a desired level. Since there is a time delay, t_o, necessary for the acoustic radiation to travel to thepoint276awhere thelaser beam402 passes through theAO cell272, a comparable delay is induced in thesignal106 applied to the pulse receive202, so as to obtain synchronization between thelaser pulse train344 and theRF power274aapplied to theAO cell272 as shown inFIG. 35. This results in the increasing amplitudes of the Q-switchedpulse train344cas time increases from t₁to t₂to t₃to t₄. The envelope of the RF power applied to theAO cell272 is shown at282 inFIG. 35. As can be seen, at time t₁theenvelope282 of RF power is relatively high so that it causes the correspondinglaser output pulse344cat time t₁to be relatively small because of the large diffraction out of the laser pulse by the acoustic wave generated by the RF power when the acoustic wave intersects the laser pulse in the AO cell. However, as the magnitude of theRF envelope282 diminishes at times t₂, t₃and t₄, the corresponding amplitudes of the Q-switchedpulse train344cincrease respectively because of the smaller amount of diffraction caused by the lower amplitude acoustic waves generated by the decreasingRF power envelope282.
• Control of Individual Q-Switched Pulse Peak Power By Control of Timing Between the RF Super Pulse Exciting the Discharge and the High Voltage Applied to the EOM
• Another method for obtaining variable peak power in thelaser output pulse344 as inFIG. 33 and the upper portion ofFIG. 35 is to utilize the fact that the gain of thelaser medium726 begins to increase up to a maximum value in a time T_PBTafter the input power602 (FIGS. 31 and 33) energizing the laser is turned on. Consequently, the timing between the initiation of theinput power602 energizing the laser and the switching of the lasercavity comprising mirrors406,408 from a high loss state to the low loss state (i.e. in the Q-switched case) will vary the peak power of an emitted laser pulse. If a shorter time is provided between the initiation of the laser pumping energy and the switching of thelaser cavity406,408 from a high loss condition to a low loss condition, then the gain of the laser medium will not have peaked. Consequently the first pulse to be emitted will not have as large a peak power as it could have. After the Q-switched pulse is emitted by the laser, the gain of the medium has dropped down to the value required to maintain CW oscillation for the cavity loss condition. The gain of thelaser medium726 then begins to build up again when thecavity406,408 is switched to a high loss condition. The time required to exceed the laser gain at which the first laser pulse was emitted is now shorter because the gain build up begins at a larger population level than was the case for the first Q-switched pulse. If one then again switches from the high loss condition to a low loss condition at a time so that the gain is higher then when the first Q-switched pulse was emitted, then the second emitted laser pulse will have a higher peak power than the first pulse. These events can continue for a number of Q-switched laser pulses, thereby obtaining the increasing peak power of succeeding subsequent pulses until a maximum gain condition is reached. The number of pulses capable of being emitted by the laser is determined by the pumping intensity, the gain threshold of the laser, the time interval desired between the Q-switched pulses, the gas pressure of the laser, the cooling capability of the laser design and the amount of time delay between the initiation of the RFsuper pulse602 pumping of the laser and the switching of thelaser cavity406,408 from a high loss state to a low loss state.
• FIG. 36 illustrates the results when the RFsuper pulse602 ofFIG. 32 is shortened from 60 microsecond to approximately 48 microseconds, the time interval between pulses is maintained at approximately 10 microseconds for both cases, and the time between switching from a high cavity loss to a low cavity loss condition is shortened so that the first Q-switched pulse is emitted approximately 14 microseconds as indicated inFIG. 36. Note that under these conditions, four Q-switched pulse are obtained with each succeeding pulse increasing in amplitude. The advantage of this approach over that ofFIGS. 33 and 35 is simplicity and cost.
• For example, if the RF pumpingsuper pulse602 is increased to 72 microseconds and the time interval between pulses and the initiation of the high loss to low loss optical cavity switch after the initiation of the RF super pulse pumping process remain essentially the same, then the number ofoutput pulses344 increases to 6 pulses but the peak power of the last two pulses are decreasing in peak power because of the heating of the laser gas (seeFIG. 37). Such performance is generally not desirable in hole drilling operations. The time interval between pulses is selected based on the optimum time for the material removal from the hole before the arrival of the next pulse on the material. The approaches ofFIGS. 33 and 35 provide for greater freedom and therefore for simpler optimization of the time interval between emitted laser pulses and more independent control of the peak power of the laser pulses. None-the-less, if the approach ofFIG. 36 satisfies the application, it is lower in laser implementation cost.
• The concept presented is described in terms of a CO₂Q-switched laser but the principle is applicable to a Q-switched/cavity dumped CO₂lasers as well as to other lasers such as semiconductor diode pumped or flash lamp pumped solid state laser such as YAG lasers.
• While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

Claims (24)

1. A super pulsed, Q-switched CO₂laser system, comprising:

a plurality of mirrors defining an optical cavity;

a gaseous gain medium positioned within the optical cavity for generating laser light;

an RF source operable to energize the gaseous gain medium with a radio frequency pulse;

a Q-switch operative to switch the Q of the optical cavity; and

a system controller operable to control a relative timing of the RF source and the Q-switch in order to create at least one output pulse of laser light during an energized period of the gain medium.

2. A laser system as recited inclaim 1, wherein:

the system controller operates the Q-switch such that each successive output pulse of laser light during the energized period of the gain medium has increased energy relative to a previous output pulse.

3. A laser system as recited inclaim 1, wherein:

the RF source energizes the gaseous gain medium with a power that is greater than a maximum average radio frequency power corresponding to a stable continuous wave radio frequency power.

4. A laser system as recited inclaim 3, wherein

the system controller is operable to increase the power of the RF source relative to the maximum average radio frequency power by an amount that is inversely proportional to a change in the duty cycle.

5. A laser system as recited inclaim 3, wherein

the RF source energizes the gain medium with a radio frequency pulse having a power equal to twice the maximum average radio frequency power.

6. A laser system as recited inclaim 3, wherein

the RF source energizes the gain medium with a radio frequency pulse having a power equal to a integer multiple of the maximum average radio frequency power.

7. A laser system as recited inclaim 1, wherein:

the Q-switch is operable to switch the optical cavity between a high loss state and a low loss state, wherein during the high loss state a gain in the gain medium increases and thereafter when the Q-switch is changed to a low loss state the gain in the gain medium is depleted creating a high peak power pulse coupled out of the cavity as the pulse of laser light.

8. A laser system as recited inclaim 1, wherein:

the system controller includes a time delay circuit for detecting a portion of the laser pulse and providing a control signal for operating the Q-switch.

9. A laser system as recited inclaim 1, wherein:

the system controller triggers the Q-switch by applying a high voltage to an optical crystal of the Q-switch after a population build-up time, the population build-up time allowing a population inversion in the laser medium to rise to a predetermined level after application of the radio frequency pulse.

10. A method of Q-switching a super pulsed CO₂laser for material processing, the laser having a gaseous gain medium and a Q-switch, the method comprising the steps of:

energizing the gaseous gain medium with a radio frequency pulse; and

triggering the Q-switch to switch the Q of the optical cavity to create at least laser pulse during the radio frequency pulse.

11. A method as recited inclaim 10, wherein:

triggering the Q-switch includes operating the Q-switch to switch the optical cavity to a high loss state once the gain medium is energized in order to increase a gain in the gain medium, and switching the optical cavity to a lower loss state in order to deplete the gain in the gain medium and create a high peak power pulse that is coupled out of the cavity as the laser pulse.

12. A method as recited inclaim 10, wherein:

triggering the Q-switch includes applying a high voltage pulse to an optical crystal of the Q-switch.

13. A method as recited inclaim 10, wherein:

energizing the gaseous gain medium includes applying the radio frequency pulse with a power equal to an integer multiple of a maximum average radio frequency power.

14. A method as recited inclaim 13, further comprising:

increasing the power of the radio frequency pulse by an amount that is inversely proportional to the duty cycle.

15. A Q-switched CO₂laser system, comprising:

a plurality of mirrors defining an optical cavity;

a gain medium positioned within the optical cavity for generating laser beam radiation;

a Q-switch within the optical cavity and including an optical crystal, the Q-switch being operative to create a high loss state in the cavity when zero voltage is applied to the crystal and a low loss state when a prescribed non-zero voltage is applied to the crystal, the gain medium being depleted when the Q-switch switches to a low loss state whereby a high peak power pulse is coupled out of the cavity as a laser pulse; and

a system controller for triggering the Q-switch.

16. A system as recited inclaim 15, wherein:

the system controller includes a time delay circuit for detecting a portion of the laser pulse and providing a control signal for triggering the Q-switch.

17. A system as recited inclaim 15, wherein:

the system controller includes a pulsed signal generation system configured to receive the control signal and connected to the Q-switch to apply a high voltage pulse to the Q-switch, thereby controlling the state of optical loss within the optical cavity.

18. The laser system as set forth inclaim 15, wherein the Q-switch includes:

an optical crystal having an entrance surface receptive of the laser beam and an opposing laser beam exit surface;

a first optical window having an optical entrance surface receptive of the laser beam and an opposing laser beam exit surface, the exit surface of the first optical window being in physical contact with the entrance surface of the optical crystal thereby defining a first optical interface; and

an optical reflector in physical contact with the laser beam exit surface of the optical crystal thereby defining a second optical interface, the optical reflector receptive of the laser beam from the optical crystal and operative to redirect the laser beam into the optical crystal.

19. The laser system as set forth inclaim 15, wherein:

the optical cavity includes a multiple pass optical assembly for directing the laser beam through the Q-switch multiple times.

20. A method of operating a Q-switched laser system including an optical cavity, a gain medium located in the cavity, and a Q-switch located in the cavity operative to switch the cavity between a high loss state and a low loss state, the method comprising the steps of:

applying zero voltage to an optical crystal of the Q-switch to create a high loss state in the optical cavity;

applying a prescribed non-zero voltage to the optical crystal of the Q-switch to create a low loss state in the optical cavity, the gain medium being depleted when the Q-switch switches to a low loss state whereby a high peak power pulse is coupled out of the cavity as a laser pulse; and

detecting the laser pulse using a time delay circuit and providing a control signal for triggering the Q-switch; and

receiving the control signal to a pulsed signal generation system connected to the Q-switch for delivering one of the zero voltage and non-zero voltage to the Q-switch to control the state of optical loss in the optical cavity.

21. A method as recited inclaim 20, further comprising:

comparing a level of the output of the Q-switch pulse to a predetermined level and thereafter initiating a predetermined delay time between the detection of the portion of the laser pulse and the providing of the control signal.

22. A super pulsed, Q-switched CO₂laser system comprising:

a resonant cavity;

a gaseous gain medium located within the resonant cavity;

an RF power supply electrically coupled to the gain medium for exciting the gain medium;

a Q-switch located within the resonant cavity; and

a controller coupled to the RF power supply and the Q-switch, said controller for triggering the RF power supply to generate a sequence of pulses of RF energy, said controller further operating to trigger the Q-switch at least once during each pulse of RF energy, said Q-switch being triggered after the RF energy has built up gain in the gain medium, with the triggering of the Q-switch resulting in the generating of a high peak power output pulse.

23. A laser system as recited inclaim 22, wherein:

the controller functions to trigger the Q-switch at least two times during a selected pulse of RF energy to generate a sequence of output pulses.

24. A laser system as recited inclaim 23, wherein:

the controller operates the Q-switch so that the energy of each of the sequence of output pulses generated within one pulse of RF energy successively increases.

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