US20050215985A1 - Method of generating an ultra-short pulse using a high-frequency ring oscillator - Google Patents

Abstract

The present invention provides a method of generating an ultra-short pulse in a ring oscillator by amplifying a series of wavelength-swept-with time pulses using one or more amplifiers, compressing the amplified wavelength-swept-with time pulses, reducing the compressed pulses to sub-picosecond pulses, stretching the sub-picosecond pulses into wavelength-swept-with time pulses and returning the stretched pulses to the one or more amplifiers.

Description

REFERENCE TO RELATED APPLICATIONS
• This application is a continuation patent application of U.S. patent application Ser. No. 10/916,367 filed on Aug. 11, 2004, which claims the benefit of U.S. Provisional Patent Application Nos. 60/494,275 filed on Aug. 11, 2003 (now abandoned) and 60/503,578 filed on Sep. 17, 2003 (now abandoned). U.S. patent application Ser. No. 10/916,367 incorporated the contents of U.S. Provisional Patent Application No. 60/539,024 filed on Jan. 13, 2004 (now abandoned) by reference. The entire content of U.S. patent application Ser. No. 10/916,367 filed on Aug. 11, 2004 is hereby incorporated by reference.
TECHNICAL FIELD OF THE INVENTION
• The present invention relates in general to the field of light amplification and, more particularly to generating an ultra-short pulse in an oscillator.
BACKGROUND OF THE INVENTION
• Without limiting the scope of the invention, its background is described in connection with ultra-short pulse in an oscillator, as an example. Ablative material removal is especially useful for medical purposes, either in-vivo or on the outside surface (e.g., skin or tooth), as it is essentially non-thermal and generally painless. Ablative removal of material is generally done with a short optical pulse that is stretched amplified and then compressed. A number of types of laser amplifiers have been used for the amplification.
• Machining using laser ablation removes material by disassociating the surface atoms and melting the material. Laser ablation is efficiently done with a beam of short pulses (generally a pulse-duration of three picoseconds or less). Techniques for generating these ultra-short pulses (USP) are described, e.g., in a book entitled “Femtosecond Laser Pulses” (C. Rulliere, editor), published 1998, Springer-Verlag Berlin Heidelberg New York. Generally large systems, such as Ti:Sapphire, are used for generating ultra-short pulses.
• The USP phenomenon was first observed in the 1970's, when it was discovered that mode-locking a broad-spectrum laser could produce ultra-short pulses. The minimum pulse duration attainable is limited by the bandwidth of the gain medium, which is inversely proportional to this minimal or Fourier-transform-limited pulse duration. Mode-locked pulses are typically very short and will spread (i.e., undergo temporal dispersion) as they traverse any medium. Subsequent pulse-compression techniques are often used to obtain USP's. A diffraction grating compressor is shown, e.g., in Patent 5,822,097 by Tournois. Pulse dispersion can occur within the laser cavity so that compression (dispersion-compensating) techniques are sometimes added intra-cavity. When high-power pulses are desired, the pulses are intentionally lengthened (e.g., to a nanosecond) before amplification to avoid internal component optical damage. This is referred to as “Chirped Pulse Amplification” (CPA). The pulse is subsequently compressed to obtain a high peak power (pulse-energy amplification and pulse-duration compression).
• As a result, there is a need for generating ultra-short pulses at very-high frequency repetition rates, which allows pulse selection to be used to accurately vary such very high ablation pulse repetition rates.
SUMMARY OF THE INVENTION
• Ultra-short optical ablation systems can be operated more efficiently when pulse-energy is controlled by varying the ablation pulse repetition rate. The present invention provides a method of generating an ultra-short pulse in a ring oscillator, (sub-picosecond) pulses at very-high frequency repetition rates, which allows pulse selection to be used to accurately vary such very high ablation pulse repetition rates. The oscillator may include an amplifier (e.g., a Semiconductor Optical Amplifier (SOA)), an output coupler, a compressor (e.g., a chirped fiber Bragg grating with an associated circulator), a nonlinear optical element (e.g., a saturable absorber, such as a carbon nanotube saturable absorber), a stretcher (e.g., a chirped fiber Bragg grating with an associated circulator) arranged in a ring configuration to provide ultra-short (sub-picosecond) pulses of repetition rates of 25 MHz to 1 GHz or more. Note that the components can be connected together using optical fiber. Other embodiments of the present invention may have an ablation pulse repetition rate of between about 1 MHz and 25 MHz. The oscillator of the present invention has relatively few components, is relatively inexpensive, can be easily miniaturized, and is also useful for other systems.
• More specifically, the present invention provides a method of generating an ultra-short pulse in a ring oscillator by amplifying a series of wavelength-swept-with time pulses using one or more amplifiers, compressing the amplified wavelength-swept-with time pulses, reducing the compressed pulses to sub-picosecond pulses, stretching the sub-picosecond pulses into wavelength-swept-with time pulses and returning the stretched pulses to the one or more amplifiers. The one or more amplifiers may include one or more semiconductor optical amplifiers (SOAs).
• The oscillator may also include an electrical impulse generator (EIG) to drive the amplifier (or drive an electro-optic modulator) and/or a polarization controller to provide cleaner pulses. The electrical impulse generator can be used to activate a device in the ring to synchronize the oscillator and the pulse selector (e.g., the electrical impulse generator may be used to activate the SOA), or the electrical impulse generator may be used to activate an electro-optic modulator to produce a temporal window of net positive pulse amplification within the ring.
• The compressing may be preformed by one or more gratings, the stretching may be preformed by one or more gratings, or both the compressing and the stretching may be preformed by one grating or more than one grating. For example, the compressor and stretcher can be a single grating having a circulator connected to each end of the grating. The grating can be a chirped fiber Bragg grating.
• Generally, the output pulses from the ring oscillator are coupled out through an output coupler (e.g., a portion of each of the amplified pulses is coupled out through the output coupler; either stretched or compressed pulses may be coupled out through the output coupler). A pulse selector may be used to give a pulse-selector output with a repetition rate of less than one-tenth the oscillator repetition rate.
BRIEF DESCRIPTION OF THE DRAWINGS
• For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures and in which:
• FIG. 1 illustrates a schematic of the ring oscillator in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
• While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.
• To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a”, “an” and “the” are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.
• In many applications, an ablation pulse repetition rate of about 1 MHz or more is desirable. The ring oscillator of the present invention provides ultra-short (sub-picosecond) pulses at very-high frequency repetition rates. Ultra-short optical ablation systems can be operated more efficiently when pulse-energy is controlled by varying the ablation pulse repetition rate, which allows pulse selection to be used to accurately vary such high ablation pulse repetition rates.
• The present invention provides a method of generating an ultra-short pulse in a ring oscillator, (sub-picosecond) pulses at very-high frequency repetition rates, which allows pulse selection to be used to accurately vary such very high ablation pulse repetition rates. The oscillator may include an amplifier (e.g., a Semiconductor Optical Amplifier (SOA)), an output coupler, a compressor (e.g., a chirped fiber Bragg grating with an associated circulator), a nonlinear optical element (e.g., a saturable absorber, such as a carbon nanotube saturable absorber), a stretcher (e.g., a chirped fiber Bragg grating with an associated circulator) arranged in a ring configuration to provide ultra-short (sub-picosecond) pulses of repetition rates of 25 MHz to 1 GHz or more. Note that the components can be connected together using optical fiber. Other embodiments of the present invention may have an ablation pulse repetition rate of between about 1 MHz and 25 MHz. The oscillator of the present invention has relatively few components, is relatively inexpensive, can be easily miniaturized, and is also useful for other systems.
• More specifically, the present invention provides a method of generating an ultra-short pulse in a ring oscillator by amplifying a series of wavelength-swept-with time pulses using one or more amplifiers, compressing the amplified wavelength-swept-with time pulses, reducing the compressed pulses to sub-picosecond pulses, stretching the sub-picosecond pulses into wavelength-swept-with time pulses and returning the stretched pulses to the one or more amplifiers. The method may also include synchronizing the oscillator and the pulse selector.
• Now referring toFIG. 1, a schematic of aring oscillator100 in accordance with one embodiment of the present invention is shown. Theoscillator100 includes one or more amplifiers (Semiconductor Optical Amplifier (SOA)102) driven by an electrical impulse generator (EIG104), a Faraday Isolator (FI106), an output coupler (Output108), a temporal compressor (first chirped fiber Bragg grating (CFBG-110) connected to the ring with circulator112), a nonlinear optical element (saturable absorber (NL114)), a polarization controller (PC116) and a temporal stretcher (second chirped fiber Bragg grating (CFBG+118) connected to the ring with circulator120). All of the connections between these components can made through an optical fiber as opposed to previous oscillators that used free-space optical components. In other embodiments, the electrical impulse generator (EIG104) may drive an electro-optic modulator. The output coupler (Output108) provides ultra-short (sub-picosecond) pulses having repetition rates of 25 MHz to 1 GHz or more. The present invention can produce pulses having a duration of 100 to 200 fs, which may set a new benchmark for the shortest pulse generated by a diode laser. Other pulse durations may be produced.
• Some of the mode-lockeddiode laser cavity100 components are generally commercially available, including the semiconductor optical amplifier102 (e.g., InPhenix, Covega, Kamelian, or Exalos). Theelectrical impulse generator104 can be originated from an integrated electronics module (IEM) based on a digital signal processor (DSP) platform. DSP circuits are currently available as evaluation boards from several manufacturers (e.g., Texas Instruments, Motorola, Analog Devices). The IEM can generate short current pulses for themodelocked laser cavity100 coordinated with similar current pulses for biasingsubsequent amplifiers102. Chirpedfiber Bragg gratings110 and118 have been developed by 3M Company. The large group velocity delays (GVD) imposed by the one or more chirpedfiber Bragg gratings110 and118 (e.g., >1000 ps/nm) are much greater than the current intracavity stretcher configuration, which will further linearize the amplification inside the laser. Nonlinear loss modulation in themodelocked cavity100 is established using compound cavity geometries (e.g., additive pulse modelocking, colliding pulse modelocking, nonlinear optical loop mirror) using standard fiber optic components and a novel design.
• Features of femtosecond pulse creation of the present invention includes the electrical impulse bias of the semiconductoroptical amplifier device102, the ultrafast nonlinear loss mechanism of thecarbon nanotubes114, and the dispersion management using one or more first chirpedfiber Bragg gratings110 and one or more second chirped fiber Bragg gratings118. Actively modelocked diode lasers have been injected with very high frequency sinewaves (>1 GHz) to synchronize longitudinal mode phase and to provide a brief window of net positive gain in the cavity. Electrical impulse (delta function) bias enables lower repetition rates (≦10 MHz), while maintaining a limited positive gain window in the cavity. Nonlinear loss mechanisms, such as saturable absorption or Kerr lensing, provide additional pulse shortening in the cavity by shearing off the low intensity edges of the modelocked pulses. Chirpedfiber Bragg gratings110 and118 have low insertion loss at 1550 nm and can provide sufficient group delay to establish extreme chirped pulse amplification inside the fiber ring cavity. Laboratory demonstrations thus far have been limited by use of traditional Treacy grating stretchers and compressors (not shown), which are limited in their magnitude of chirp.
• Theamplifier102 may be a semiconductor optical amplifier that provides optical gains. The Faraday isolator106 insures unidirectional beam propagation and prevents feedback from theoutput coupler108. The first chirped fiber Bragg gratings (CFBG-110) unchirp and compress the optical pulse subsequent to amplification in theamplifier102.
• Alternately, an electro-optic modulator (not shown) can provide cavity loss modulation. The short electrical pulse from theelectrical impulse generator104 makes the EOM temporarily transparent which creates a narrow (200 ps) temporal window of net positive gain inside the laser cavity. The short electrical pulse prevents build up of amplified spontaneous emission inside the cavity and synchronizes the modelocked pulse train to an external timing signal.
• The nonlinearoptical element114 is a passive optical loss mechanism, such as a carbon nanotube based saturable absorber, that shears off the long leading edge of the optical pulse thereby dramatically shortening the pulse temporal width. The second chirpedfiber Bragg gratings118 chirp and stretch the optical pulse, which facilitates nearly linear amplification inside the semiconductoroptical amplifier102. Thepolarization controller116 maintains linear polarization along the preferred cavity axis for stable laser operation. Theoutput coupler108 can couple a portion of the pulse energy out (typically for further amplification) and retains the remainder (e.g., 5%) in the ring as feedback. Being an oscillator, the ring is self-starting and the second chirpedfiber Bragg gratings118 stretcher provide wavelength-swept-with time pulses for amplification by theamplifier102.
• Note that thecirculators112 and120 may be connected to opposite ends of a single chirped fiber Bragg grating (not shown), such that a single circulator acts as both a stretcher and a compressor. Moreover, a Treacy grating (not shown) can be used for the compressor or stretcher as opposed to one or more chirpedfiber Bragg gratings110 and118. The carbonnanotube saturable absorber114 implementation is novel, as is the use of an impulse-driven semiconductoroptical amplifier102.
• The present invention may be used in systems along with the co-owned and previously filed provisional applications noted below by docket number, title and (generally) provisional number, and are hereby incorporated by reference herein:

  Docket		US Serial
  Number	Title	Number	Filing Date
  ABI-1	Laser Machining	60/471,922	May 20, 2003
  ABI-2	Laser Contact With W/Dopant/Copper Alloy	60/472,070	May 20, 2003
  ABI-3	SOAs Electrically And Optically In Series	60/471,913	May 20, 2003
  ABI-4	Camera Containing Medical Tool	60/472,071	May 20, 2003
  ABI-5	In-vivo Tool with Sonic Locator	60/471,921	May 20, 2003
  ABI-6	Scanned Small Spot Ablation With A High-Rep-	60/471,972	May 20, 2003
  	Rate
  ABI-7	Stretched Optical Pulse Amplification and	60/471,971	May 20, 2003
  	Compression
  ABI-8	Controlling Repetition Rate Of Fiber Amplifier	60/494,102	Aug. 11, 2003
  ABI-9	Controlling Pulse Energy Of A Fiber Amplifier By	60/494,275	Aug. 11, 2003
  	Controlling Pump Diode Current
  ABI-10	Pulse Energy Adjustment For Changes In Ablation	60/494,274	Aug. 11, 2003
  	Spot Size
  ABI-11	Ablative Material Removal With A Preset	60/494,273	Aug. 11, 2003
  	Removal Rate or Volume or Depth
  ABI-12	Fiber Amplifier With A Time Between Pulses Of	60/494,272	Aug. 11, 2003
  	A Fraction Of The Storage Lifetime
  ABI-13	Man-Portable Optical Ablation System	60/494,321	Aug. 11, 2003
  ABI-14	Controlling Temperature Of A Fiber Amplifier By	60/494,322	Aug. 11, 2003
  	Controlling Pump Diode Current
  ABI-15	Altering The Emission Of An Ablation Beam for	60/494,267	Aug. 11, 2003
  	Safety or Control
  ABI-16	Enabling Or Blocking The Emission Of An	60/494,172	Aug. 11, 2003
  	Ablation Beam Based On Color Of Target Area
  ABI-17	Remotely-Controlled Ablation of Surfaces	60/494,276	Aug. 11, 2003
  ABI-18	Ablation Of A Custom Shaped Area	60/494,180	Aug. 11, 2003
  ABI-19	High-Power-Optical-Amplifier Using A Number	60/497,404	Aug. 22, 2003
  	Of Spaced, Thin Slabs
  ABI-20	Spiral-Laser On-A-Disc	60/502,879	Sep. 12, 2003
  ABI-21	Laser Beam Propagation in Air	60/502.886	Sep. 12, 2003
  ABI-22	Active Optical Compressor	60/503,659	Sep. 17, 2003
  ABI-23	Controlling Optically-Pumped Optical Pulse	60/503,578	Sep. 17, 2003
  	Amplifiers
  ABI-24	High Power SuperMode Laser Amplifier	60/505,968	Sep. 25, 2003
  ABI-25	Semiconductor Manufacturing Using Optical	60/508,136	Oct. 02, 2003
  	Ablation
  ABI-26	Composite Cutting With Optical Ablation	60/510,855	Oct. 14, 2003
  	Technique
  ABI-27	Material Composition Analysis Using Optical	60/512,807	Oct. 20, 2003
  	Ablation
  ABI-28	Quasi-Continuous Current in Optical Pulse	60/529,425	Dec. 12, 2003
  	Amplifier Systems
  ABI-29	Optical Pulse Stretching and Compression	60/529,443	Dec. 11, 2003
  ABI-30	Start-Up Timing for Optical Ablation System	60/539,926	Jan. 23, 2004
  ABI-31	High-Frequency Ring Oscillator	60/539,924	Jan. 23, 2004
  ABI-32	Amplifying of High Energy Laser Pulses	60/539,925	Jan. 23, 2004
  ABI-33	Semiconductor-Type Processing for Solid State	60/543,086	Feb. 09, 2004
  	Lasers
  ABI-34	Pulse Streaming of Optically-Pumped Amplifiers	60/546,065	Feb. 18, 2004
  ABI-35	Pumping of Optically-Pumped Amplifiers	60/548,216	Feb. 27, 2004

• Although the present invention and its advantages have been described above, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification, but only by the claims.

Claims (19)

1. A method of generating an ultra-short pulse in a ring oscillator, comprising the steps of:

amplifying a series of wavelength-swept-with time pulses using one or more amplifiers;

compressing the amplified wavelength-swept-with time pulses;

reducing the compressed pulses to sub-picosecond pulses;

stretching the sub-picosecond pulses into wavelength-swept-with time pulses; and

returning the stretched pulses to the one or more amplifiers.

2. The method ofclaim 1, wherein the one or more of the one or more amplifiers comprise one or more semiconductor optical amplifiers.

3. The method ofclaim 1, wherein the compressing is preformed by one or more gratings.

4. The method ofclaim 3, wherein the one or more gratings comprise a chirped fiber Bragg grating.

5. The method ofclaim 1, wherein the reducing is preformed by one or more nonlinear optical elements.

6. The method ofclaim 5, wherein the one or more nonlinear optical elements comprise a carbon nanotube saturable absorber.

7. The method ofclaim 1, wherein the stretching is preformed by one or more gratings.

8. The method ofclaim 7, wherein the one or more gratings comprise a chirped fiber Bragg grating.

9. The method ofclaim 1, wherein the ring oscillator is coupled through an output coupler.

10. The method ofclaim 9, wherein a portion of each of the amplified pulses are coupled out through the output coupler.

11. The method ofclaim 9, wherein a portion of each of the compressed pulses are coupled out through the output coupler.

12. The method ofclaim 1, wherein the oscillator runs at a repetition rate of at least 25 MHz.

13. The method ofclaim 12, wherein the oscillator further comprises a pulse selector to give a pulse-selector output with a repetition rate of less than one-tenth the oscillator repetition rate.

14. The method ofclaim 13, further comprising the step of synchronizing the oscillator and the pulse selector, wherein an electrical impulse generator is used to activate a device in the oscillator.

15. The method ofclaim 14, wherein the electrical impulse generator is used to activate one or more semiconductor optical amplifiers.

16. The method ofclaim 14, wherein the electrical impulse generator is used to activate an electro-optic modulator to produce a temporal window of net positive pulse amplification within the oscillator.

17. The method ofclaim 1, wherein the oscillator contains one or more polarization controllers.

18. The method ofclaim 1, wherein the step of stretching and the step of compressing are preformed by one or more chirped fiber Bragg gratings.

19. The method ofclaim 1, wherein one or more optical connections between components are made through optical fiber.

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