TITLE OF THE INVENTION
OPTICAL SUBASSEMBLY FOR TRANSMITTING AN OPTICAL SIGNAL WITH A CONTROLLED DIVERGENCE ANGLE AND OUTPUT POWER
FIELD OF THE INVENTION
This invention generally relates to optical signal transmission assemblies. More particularly, the present invention relates to an optical subassembly for transmitting an optical signal with a controlled divergence angle and output power to maximize the power of an optical signal which is launched into an optical fiber, while complying with established international safety standards and regulations for optical systems.
BACKGROUND OF THE INVENTION
Referring to Figure 1 , a conventional optical fiber communication system is generally indicated at 20, and consists of a transmitter 22, an optical fiber or waveguide 24, and a receiver 26. The transmitter 22 comprises an electronic drive circuit to modulate a transmitted optical signal and an optical subassembly which includes an optical signal source, such as a laser diode or a vertical cavity surface emitting laser ('VCSEL"), for example, and any lenses, mirrors, waveguides, or optical fibers that are used to couple the light from the optical signal source to an optical fiber. The optical subassembly provides a stable mechanism by which the light emitted from the optical signal source is collected and launched into an optical fiber.
Conventionally, an optical signal source is coupled to an optical fiber, by an optical subassembly, in a manner such that an axis of the optical signal source along which an optical signal propagates is aligned with a central axis of an optical fiber. This type of an alignment scheme is employed to obtain maximum coupling efficiency and high output powers. However, in some applications such an arrangement may fail to comply with established safety standards or regulations for a particular laser system classification.
Established safety standards and regulations exist that govern the manufacture of optical systems. Such safety standards and regulations are promulgated by the U.S. Center for Disease and Radiological Health under the Food and Drug Administration (21 C.F.R. §1040); the International Electrotechnical Commission (I.E.C. 825) whose document is closely aligned with the European Norm (EN60825), and the American National Standards Institute (ANSI Z136)
The rationale for the ANSI Z136 standard is discussed in an article written by R C Petersen and D H Sliney, "Toward the Development of Laser Safety Standards for Fiber-Optic Communication Systems," Applied Optics, 1 April 1986, volume 25, number 7, pages 1038-1047 As stated therein, the optical power level in a typical optical fiber communication system is a few milliwatts per fiber At this power level, even if the end of the fiber is placed in contact with the skin, there is little potential hazard There are, however, potential risks of injury to a human eye if the end of an energized fiber is viewed For wavelengths between 400nm and 1400nm, the hazard is to the retina of a human eye
Laser systems are labeled Class 1 through 4 according to criteria stated in applicable standards A Class 1 laser system is considered eye safe (i e , non-hazardous) whereas a higher class laser system is not considered eye safe and requires additional safety controls A laser system manufacturer must measure the output power of the transmitter according to criteria stated in an applicable standard and compare the measured power to the accessible emission limit set by the applicable standard Laser systems which are not classified Class 1 require the implementation of costly safety measures, such as interlocks, key control or beam stops, warning devices and training
As stated hereinabove, in a conventional optical subassembly, the optical axis of an active optical device, such as a laser, is aligned with the optical axis of an optical fiber Such an alignment scheme is employed to obtain maximum coupling efficiency and high output powers However, a shortcoming of such an alignment scheme is that a high output power may require a manufacturer to classify the laser system at a higher classification than desired, which as previously discussed is costly
One method to overcome such a shortcoming is to reduce the output power of the laser For example, the output power of a laser diode can be reduced by reducing the drive current Thus, it is possible to attain the desirable Class 1 designation by reducing the output power of the laser However, an attendant shortcoming of such a system is reduced signal-to- noise ratio ("SNR"), and 2) an increased bit error ratio ("BER") in certain signal transmission applications The foregoing illustrates limitations known to exist in present optical subassembly designs Thus, it is apparent that it would be advantageous to provide an improved optical subassembly directed to overcoming one or more of the limitations set forth above Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter
SUMMARY OF THE INVENTION
The present invention advances the art of optical subassemblies beyond which is known to date In one aspect of the present invention, an optical subassembly is provided having a housing, an active optical device for generating an optical signal along an optical axis and a multimode optical transmission medium for receiving the optical signal The multimode optical transmission medium has an optical transmission axis The optical signal is coupled into the multimode optical transmission medium at an angle greater than 0° and less than a critical acceptance angle of the multimode optical transmission medium The coupling angle is determined between the optical axis of the active optical device and the optical transmission axis of the multimode optical transmission medium
It is, therefore, a purpose of the present invention to maximize the power of an optical signal which is launched into an optical fiber while meeting established international safety standards and regulations for optical systems Another purpose of the present invention is to effectively control optical signal transmission such that a consistent and repeatable numerical aperture of an optical transmission assembly is obtained Another purpose of the present invention is to provide an optical subassembly which is capable of selectively launching higher order modes in an optical fiber
BRIEF DESCRIPTION OF THE DRAWINGS The foregoing summary, as well as the following detailed description of a preferred embodiment of the invention, will be better understood when read in conjunction with the appended drawings For purposes of illustrating the invention, there is shown in the drawings an embodiment which is presently preferred It should be understood, however, that the invention is not limited to the precise arrangement and instrumentality shown In the drawings Figure 1 is a schematic view of a prior art optical fiber communication system,
Figure 2 is a theoretical graph illustrating output per channel versus numeπcal aperture Figure 3 is an end view of an MT fiber optic connector,
Figure 4 is a table illustrating accessible emission limits ("AEL") and measurement distances according to Amendment A11 of European Standard EN 60825-1 for 0 0625mm core fibers on a 0 25mm pitch for up to twelve fibers with a numerical aperture ("NA") of 0 275 and a wavelength of 850nm, Figure 5 is a graph illustrating percentage of power collected through a
7mm aperture versus number of channels,
Figure 6 is a graph illustrating output power per channel versus launch methods (e g , a vertical cavity surface emitting laser ("VCSEL") 13 degree launch, a light emitting diode ("LED") full mode launch, and a VCSEL zero degree launch),
Figure 7 is a graph illustrating far field distribution of a zero degree launch,
Figure 8 is a graph illustrating far field distribution of an LED launch,
Figure 9 is a graph illustrating far field distribution of a 13 degree launch, Figure 10 is a graph illustrating numerical aperture versus lateral displacement for a VCSEL optical signal source which is launched at various angles with respect to the axis of the optical fiber,
Figure 11 is a cross-sectional view of an optical subassembly in accordance with the teachings of one embodiment of the present invention, Figure 12 is an enlarged, partial sectional view of an optical subassembly in accordance with an alternate embodiment of the present invention, and
Figures 13-18 are schematic illustrations representing several alternate embodiments of an optical subassembly made in accordance with the teachings of the present invention
DETAILED DESCRIPTION OF THE INVENTION Conventional laser transmitters, and in particular, conventional optical subassemblies, are not designed to overcome the trade-off that exists between high output power and low laser classification Understanding how lasers are classified provides the key to understanding this limitation International safety standards and regulations for optical systems have been premised upon an aperture-based benchmark with respect to the human pupil As is well known, a pupil is the varying aperture which limits the amount of light that enters the eye After passing through the pupil, the light is focused onto the retina The international safety standards and regulations for optical systems are based on measuring the optical power that passes through an aperture at a fixed distance from a source The power that is collected through the aperture is measured and compared to the applicable standard's accessible emission limit ("AEL") The accessible emission limit is the maximum level of power allowed through the aperture as defined by the applicable laser safety standard For example, if the measured power is below the Class 1 AEL, then the laser system can be classified as a Class 1 laser system
If the output beam of the optical signal source has a large divergence angle, then only a portion of the total output power passes through the aperture The divergence angle describes the amount of power that is measured a distance away from the source at angles off the optical axis of an active optical device Those skilled in the art will recognize this as a far field distribution of power The larger the divergence angle of the emitted light, the higher the total output power of the optical signal source which can be achieved while still keeping the power that is collected through the aperture constant This concept is best illustrated in Figure 2 which is a plot of the accessible emission limit versus numerical aperture The numerical aperture is equal to sιn(θ/2) where θ is the divergence angle of the five percent intensity points of the far field distribution plot The plot shown in Figure 2 is based on example theoretical calculations described in Appendix A6 of European Standard EN 60825-1 , with Amendment A11 , published in October 1996 (This document is similar to IEC 825-1 "Safety of Laser Products Part 1 Equipment Classification, Requirements and User's Guide ) As can be seen in Figure 2, a higher numerical aperture source allows higher output powers Similarly, if the optical fiber communication system uses a high numerical aperture optical fiber, higher output powers can be transmitted in the fiber A high output power is desired at the end of the fiber where the receiver detects the transmitted light signal The high numerical aperture fiber will only be a benefit if the higher order modes of a multimode optical fiber are launched such that the light exiting the fiber diverges at the full numerical aperture angle
As is well understood by those skilled in the art, fibers with large core diameters generally 50 μm or greater, are called multimode fibers Literally hundreds or thousands of modes propagate in these fibers These modes can be envisioned as being rays that reflect at different angles along the core/cladding wall The higher-order modes or rays propagate at large angles relative to the fiber axis Lower-order modes propagate at small angles The lowest-order mode is the fundamental mode and it propagates along the fiber axis, as if it were a ray at a 0° angle A single-mode fiber has a small core diameter, typically 10 μm or less, and only this fundamental mode is capable of being guided by the fiber
The critical acceptance angle of a multimode optical fiber or waveguide is the maximum angle at which light will be accepted and propagated into the fiber or waveguide It also defines the angular spread of radiation at which light exits the fiber The critical acceptance angle is determined through Snell's Law of Refraction and the critical angle of the waveguide which restricts the angle of incidence at which total internal reflection takes place Total internal reflection is the principle behind all dielectric waveguides, including optical fibers The numerical aperture is the sine of the acceptance angle of the fiber A graded-index fiber, in which the refractive index in the core is graded from a maximum value at the center to a minimum value at the core/cladding boundary, has a varying acceptance angle across the core In the center of the core, on the fiber axis, the acceptance angle is at a maximum, and as the position is moved off axis, the acceptance angle decreases
The center of the core of the optical fiber defines the fiber axis and is the axis along which light is guided The fiber or waveguide is typically straight, thus, the optical transmission axis is a straight line defined by the center of the core and extends into free space outside the confines of the fiber or waveguide A far field scan of the light exiting the multimode optical transmission medium would also identify the transmission axis as the axis upon which the energy is centered In a typical far field plot, this would be identified as θ=0°
The optical axis of a laser is also defined in this manner That is, the light being emitted from the laser is centered about an axis, and thus, defines the optical axis of the optical source. A far field scan of the emitted light would identify the optical axis as θ=0° in the far field distribution plot.
It is an object of the present invention to launch substantially more power in the higher order modes of the optical fiber relative to the lower order modes. This is accomplished by coupling or launching an optical signal into a multimode optical transmission medium at an angle greater than zero degrees and less than a critical acceptance angle of the multimode optical transmission medium. This angle is determined between the optical axis of an active optical device and an optical transmission axis of the multimode optical transmission medium.
The type of optical source plays a large role in determining which modes of the multimode optical fiber are launched. A light emitting diode, which has a broad divergence pattern, will typically launch all of the modes with equal power distribution. Furthermore, an LED has a short coherence length which causes the energy carried in the modes of the fiber to couple readily to each other. Therefore, even if the launch of an LED was restricted to only launch a subset of the modes, after a short distance there would be sufficient mode coupling such that the all of the modes would carry substantially equal powers. Lasers have longer coherence lengths than LEDs, and therefore, exhibit minimal coupling between the modes in an optical fiber. Tests have found that when a low divergent VCSEL source is aligned with conventional zero tilt angle to a multimode optical fiber, there is insufficient mode-mixing in the fiber to transfer energy from the low order, on-axis (low angle) modes, to the higher- order, higher angle modes. The numerical aperture of the light exiting the fiber is the same as the numerical aperture of the VCSEL source. In this case, the higher angle modes of the fiber are not launched.
A series of experiments were performed to determine how mode launching in a multimode fiber impacts the eye safety measurements. The first experiment was to launch only the lower order modes of the fiber; the second experiment was to launch all the modes of the fiber; and the third experiment was directed at launching only the higher-order modes of the fiber. The first and third experiments used 4-wide 25mm aperture VCSEL arrays which were pigtailed to 4-wide 62.5/125 fiber MT to MT assemblies. The difference between the first and third experiments was the launch technique. More particularly, the first expeπment employed a zero degree launch and the third a 13 degree launch These angles were determined with respect to the optical axis of the laser and the transmission axis of the fiber The second expeπment used an array of 850nm FC connectoπzed LEDs, from PD-LD Inc of Princeton, New Jersey, that was connected to a 12-wιde MT to 12 FC connector fanout assembly, using 62 5/125 fiber The second experiment was termed "full mode launch" because of the use of LEDs In each experiment, the open MT output end of the assembly was measured
In order to proceed with the measurements according to the European Standard EN 60825-1 with Amendment A11 , published in October 1996, the spatial extent to the source must be known A drawing of an endface 50 of an MT connector 48 is shown in Figure 3 Up to twelve 0 0625 mm core fibers 52 are spaced on a 0 250 mm pitch The spatial extent of the source is thus 0 0625 for one channel, 0 3125 for two channels, and up to 2 8125 mm for 12 channels A spreadsheet (Figure 4) was created to facilitate these calculations and the outcome for this particular source end face 50 at a wavelength of 850nm
The spreadsheet depicts the angular subtense of the source depending on how many of the twelve fibers or channels are in use (i e , actively emitting light) The correction factors C4 and C6 are used to determine the accessible emission limit The AEL is 0 44mW (441 67μW in column 7 of Figure 4) for up to 9 channels after which the angular subtense increases thereby increasing the AEL Column 8 of Figure 4 describes the output power per channel that would be required if all of the light emitted from the fiber end faces were collected through the aperture Columns 9 and 10 of Figure 4 show the calculations of the two measurement procedures outlined in § 8 2h of Amendment A11 to EN 60825-1 The remainder of the table shows theoretical calculations based on the formulae outlined in Annex A 6 "Accessible emission limits for diverging beam, point-type sources" of IEC 825-1 Columns 11 and 12 of Figure 4 show the beam diameter at the measurement distance based on a diverging Guassian beam of numerical aperture (NA) 0 275 Column 13 shows the percentage of power from the beam that is collected through the aperture and is used to calculate the attainable output power while still remaining under the AEL For example, if fifty percent of the light is collected through the aperture then the total output power could be up to twice the AEL Colu n 14 is thus column 8 divided by column 13, and column 14 is a similar calculation using the other method (fixed measurement distance rather than fixed aperture diameter) which indeed shows the two measurement methods are equivalent. The output powers were measured according to European Standard EN
60825-1 , with Amendment A11 , published in October 1996. Specifically, the fixed diameter aperture method was chosen, using a 7mm aperture detector Model 268LP and a Model S370 Optometer power meter, from Graseby Optronics of Orlando, Florida. A four-wide array was used. When all four optical sources were powered on, the 7mm aperture detector was placed
22.96mm away from the MT ferrule end face. The MT ferrule was mounted on an optical rail which had "mm" designations on it to measure the distance from the 7 mm detector aperture within about 0.25mm. The 7 mm aperture detector was mounted on a micropositioner and at each measurement distance the positioner was adjusted to maximize the power reading on the power meter.
With four lasers on, a power reading was taken at 0mm and 23mm. With three lasers on a power reading was taken at 0mm and 20mm. With two lasers on a power reading was taken at 0mm and 16.7mm. Finally, with one laser on a power reading was taken at 0mm and 16mm. At each distance the ratio of the "z"-distance power reading to the 0mm power reading was calculated in order to determine the percentage of power collected by the aperture. This would be similar to column 13 in Figure 4 except that it is using measured results rather than theoretical calculations.
Figure 5 shows a graph of these results for the three different launch techniques. By using the percentage of power collected by the aperture number and dividing it into the AEL (0.44mW), the attainable power per channel can be calculated. These results are graphed in Figure 6.
Specifically, Figure 6 shows that higher output powers can be attained by using the 13 degree launch as opposed to either the full mode launch or the zero degree launch. This can be also seen in far field distribution plots as measured by a Rifocs Far Field Scanner Model 611 R, manufactured by Rifocs, Inc. of Camariilo, California. Figure 7 shows the far field plot for the zero degree launch. Figure 8 shows the similar plot for the full mode launch. Finally, Figure 9 shows the plot for the 13 degree launch. As can be seen in Figures 7-9, a higher NA distribution translates into a higher attainable output power as less power is collected through the aperture
A benefit of this higher output power being emitted from the module while still remaining Class 1 laser system status is that this power can then be launched into the cable plant fiber Thus, the potential of having the highest transmitted power fiber optic links while still attaining Class 1 status can be attained More power in the fiber causes more power to reach the detector Of course having more power at the detector means higher signal to noise ratios (SNR) and lower bit error ratios (BER)
An additional shortcoming of conventional zero tilt launch coupling schemes is that the numerical aperture is not controlled Figure 10 shows a plot of the numerical aperture versus lateral displacement of the center of the fiber core with respect to the center of the active emission area of the VCSEL When the fiber core center is aligned with the laser's active area center, the numerical aperture is at a minimum equal to the VCSEL numerical aperture When the fiber core is translated such that the centers are offset, the numerical aperture increases and the output power decreases This result is due to some of the higher order modes in the fiber being launched Offsetting the centers of the fiber cores could be a method to launch the higher order modes and thereby attain a larger numerical aperture However, this method is not preferred because there are very tight tolerances on the offset that would be difficult to maintain over temperature and time in a packaged optical subassembly If the alignment were to drift such that the offset were decreased, the numerical aperture would also decrease thereby creating the possibility of an unsafe laser transmitter Figure 11 shows one embodiment of an optical subassembly made in accordance with the teachings of the present invention An optical subassembly 71 includes a housing 72, a lens 73, and an active optical device 74, such as a laser diode, for example, supported in a cylindrical mount 75 The housing 72 has several bores having defined shoulders for positional stops for the lens 73 and the mount 75 Bore 76 is disposed at an off-axis angle relative to the optical axis of the laser diode 74 Bore 76 is a receptacle for ferrule 77 of an optical fiber 78 Bore 76 may have a tapered entrance 79 and a positional stop 70 The positional stop is locate such that the end face of the optical fiber 78 and ferrule 77 intersects the optical axis of the laser diode 74 Figure 12 an alternate embodiment of an optical subassembly mde in accordance with the teachings of the present invention An active optical device, such as a VCSEL 81 , emits light along an optical axis which is normal to a planar top surface 80 of the VCSEL 81 Ferrule 82 contains an optical fiber 83 and is aligned to the laser at an angle such that the intersection of the optical axis and the fiber axis occur at the fiber core center at a front surface of the fiber ferrule 82 Ferrule stop 84 is a plastic overmold on each side of the laser array 81 that has a front surface 85 at an angle to the planar surface 80 of the VCSEL array The ferrule stop 84 maintains the angle between the optical axis and the fiber axis as well as provides a fixed z-height from the laser where the fiber ferrule 82 is pressed against the ferrule stop 84 The fixed z- height allows some free space between VCSEL array 81 and ferrule 82 for robustness and for wirebonds 86 which is the method by which current enters the VCSEL 81 The wirebonds are attached to transmission line 87 Metal backing plate 88 provides some heat sinking capability to the VCSEL array Various alternate embodiments of the optical subassembly of the present invention are schematically illustrated in Figure 13-18
Figure 13 schematically illustrates an embodiment of the optical subassembly for the present invention wherein in active optical device 56 generates an optical signal along an optical axis A-B A multimode optical transmission medium 58, having a core/cladding region 60, defines an optical transmission axis C-D A means is provided for coupling the optical signal into the multimode optical transmission medium at an angle of 13 degrees, wherein the angle is determined between the optical axis A-B of the active optical device and the optical transmission axis C-D of the multimode optical transmission medium The multimode optical transmission medium is polished or cleaved or cut at a zero degree angle (i e , 90 degrees relative to the optical transmission axis C-D) This embodiment is shown in more detail in Figure 12
Figure 14 illustrates an optical subassembly in accordance with an alternate embodiment of the present invention The optical subassembly schematically illustrated in Figure 14 is similar to the subassembly illustrated in
Figure 13 except that the multimode optical transmission medium 58 is polished or cleaved or cut at a 13 degree angle (i e , a 77 degree angle relative at optical transmission axis C-D)
Figure 15 schematically illustrates an alternate embodiment of the optical subassembly of the present invention having a lens 64 which focuses the optical signal generated from the active optical device 56 at the intersection of axes A-B - C-D In the embodiment schematically illustrated in Figure 15, multimode optical transmission medium 58 is polished or cleaved or cut at a zero degree angle as descπbed in the embodiment schematically illustrated in Figure 13 This embodiment is shown in more detail in Figure 11
Figure 16 illustrates an alternate embodiment of the optical subassembly of the present invention wherein an active optical device generates an optical signal along an optical axis A-B A mirror 66 redirects the optical signal along axis A'-B' A multimode optical transmission medium 58 has an optical transmission axis C-D The generated optical signal is coupled into the multimode optical transmission medium at an angle of about 13 degrees, wherein this angle is determined between optical axis A'-B' and the optical transmission axis C-D
Figure 17 is an alternate embodiment of the present invention similar to that schematically illustrated in Figure 16 As may be appreciated by comparatively referencing Figure 16 and 17, the angular attitude of mirror 66 is varied
Figure 18 illustrates an embodiment of the present invention combining lenses and mirrors to couple an optical signal into the optical fiber at an angle greater than zero and less than a critical acceptance angle of the multimode optical transmission medium
Although a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages which are described herein Accordingly, all such modifications are intended to be included within the scope of the present invention, as defined by the following claims