Patent Publication Number: US-2021175683-A1

Title: Optically pumped tunable VCSEL employing geometric isolation

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 16/409,272, filed on May 10, 2019, which claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 62/670,423, filed on May 11, 2018, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     MEMS (Micro electro-mechanical systems) tunable VCSELs (Vertical cavity surface emitting lasers) are useful in optical coherence tomography (OCT) because of their tuning speed, large coherence length [ 1 , 2 , 3 ], and lack of coherence revival artifacts [ 4 ]. While VCSELs at many wavelength bands are possible, most work to date has occurred in the 1550 nanometer (nm) [ 5 , 6 , 7 ], 850 nm [ 8 ], 1310 nm [ 9 , 10 , 11 , 12 ]; and 1060 nm [ 9 , 10 , 13 , 14 ] wavelength bands. The 1550 band is useful in optical telecommunications, as well as the 850 and 1310 nm lasers. The 1310 and 1060 bands are popular for use in OCT. The 1060 band, in particular, is of interest because of applications in ophthalmology, including imaging of the retina [ 14 ] and in biometry (distance measurement of structures in the whole eye) [ 14 ]. The 850 nm band is also interesting for ophthalmology because of the transparency of water in that range and compatibility with silicon photodetectors. 
     Optically pumped MEMS tunable VCSELs generally have a wider tuning range than electrically pumped ones [ 2 , 9 , 13 ]. In optical pumping, pump laser light is used to power the VCSEL. Pump light absorbed in the VCSEL is then reemitted at a longer wavelength as tunable VCSEL light. 
     Optical pumping, however, presents the challenge of exciting the VCSEL with a low noise pump laser light, Generally, the RIN (relative intensity noise) of the pump is transferred to the VCSEL light. Pump lasers can be noisy because of (1) fundamental RIN [ 15 ], (2) mode hopping in Fabry-Perot lasers, or (3) because of feedback of pump light reflected back from the VCSEL destabilizing the pump. 
     SUMMARY OF THE INVENTION 
     There are several ways of attacking these pump noise problems. Pump feedback can be reduced through (1) Faraday isolation and (2) geometric isolation. Single frequency pump lasers (Distributed feedback lasers (DFB), distributed Bragg reflector lasers (DBR), discrete mode lasers [ 16 , 17 ], volume Bragg grating (VBG) stabilized lasers [ 18 , 19 , 20 ] can eliminate wavelength jitter and amplitude noise that accompanies mode hopping. 
     Eliminating wavelength changes or uncertainty in the pump light is also important for other reasons. Optics with wavelength dependent transmission in the path to the VCSEL can convert optical frequency shifts into pump power changes (FM-to-AM conversion). This can happen with noise, also. Wavelength jitter can be converted to effective pump power noise. 
     Shaping and controlling noise, such as through laser pumps brought into coherence collapse [ 21 , 22 ] are potentially useful. Instead of narrowing the emission pump bandwidth to a single cavity mode, another method of obtaining low noise is to use a super-luminescent light emitting diode (SLED) which is a broad band emitter. Since RIN≈1/Δν [ 23 ], the RIN goes down in proportion to the emission bandwidth [ 23 ]. 
     In terms of pump noise,  1060  nanometer VCSELs present a special problem. This is because they are typically pumped in the 750-850 nm wavelength range where Faraday isolators are large, heavy, and expensive. 
     As an alternative to isolation based on Faraday rotators, geometric isolation ideas presented here can at least reduce and possibly prevent optical feedback from the VCSEL to the pump laser. These solutions are particularly useful in miniature bulk optical packages where the VCSEL, and possibly a SOA (semiconductor optical amplifier) and/or pump are integrated into one hermetic package (co-packaged). This is also applicable where just the VCSEL and a WDM (wavelength division multiplexor) filter formed by a dichroic mirror are co-packaged. 
     Moreover, while the following description concerns noise control in  1060  nanometer VCSELs, this approach can be applied to other VCSEL wavelengths as well. 
     In general, according to one aspect, the invention features an optically pumped tunable VCSEL swept source module, comprising a VCSEL, and a pump for producing light to pump the VSCEL, wherein the pump is geometrically isolated from the VCSEL. 
     In different embodiments, the pump is geometrically isolated by defocusing light from the pump in front of the VCSEL, behind the VCSEL, and/or by coupling the light from the pump at an angle with respect to the VCSEL. In the last case, angle is usually less than 80 degrees. 
     The pump can be a VBG or FBG stabilized laser, a discrete mode laser, a DFB laser, and/or DBR laser. 
     The pump could also be a super luminescent diode (SLED). 
     The module can further comprise an integrated dichroic 
     It can also include an SOA and/or possibly an integrate pump chip. An isolator is also useful to isolate the VCSEL from back reflections from the SOA. 
     In general, according to another aspect, the invention features a method for optically pumping a VCSEL. This method comprises producing pump light with a pump source, coupling the pump light into the VCSEL from the pump source, and preventing the pump light from being coupled back into the pump source by geometric isolation. 
     In some embodiments, the pump source is geometrically isolated by defocusing, by focusing pump light in front of the VCSEL or by focusing pump light behind the VCSEL. 
     The pump source can also be geometrically isolated by coupling the pump light at an angle with respect to the VCSEL. Typically this angle is less than 88 degrees. 
     The pump source can be a VBG or FBG stabilized laser, a discrete mode laser, a DFB laser, FP laser and/or DBR laser. 
     The pump source can also be a super luminescent diode (SLED). 
     The pump source can also be operated in coherence collapse. 
     In some modules, the pump light from the pump source is coupled to the VCSEL and a swept optical signal generated by the VCSEL separated using a dichroic filter. 
     Amplifying the swept optical signal with an SOA is also a possibility. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIGS. 1A and 1B  are schematic views showing two approaches, by defocusing, for creating geometric isolation between the pump and the VCSEL in a tunable VCSEL swept source. 
         FIG. 2  is a top plan view of an optically pumped tunable VCSEL swept source module and also showing a third approach to geometric isolation whereby the returning pump beam is offset and then possibly blocked. 
         FIG. 3  is a plot of various performance metrics against a common time scale in microseconds over the course of wavelength sweeps of the VCSEL, in which the clock plot  310  shows the k-clock sampling frequency over the course of both VCSEL sweeps, the trigger plot  312  shows the trigger voltage for each of the two sweeps, the power plot  314  shows the power output from the VCSEL, the first spectrogram plot  316  is a spectrogram of the optical power output from the  808  nanometer pump laser operating in the coherence collapse regime with feedback from a fiber Bragg grating placed one meter away from the chip in the fiber, and the second spectrogram plot  318  is a spectrogram of the optical power output of the  808  nanometer pump laser operating in the coherence collapse regime with a fiber Bragg grating placed 0.14 meters away in the fiber. 
         FIGS. 4A and 4B  are plan views of three different optically-pumped tunable VCSEL swept source modules. 
         FIG. 5  is an exploded perspective view of a MEMS tunable VCSEL showing one example of the VCSEL  115  and its gain substrate  116 . 
         FIG. 6  is a cross-section of the VCSEL of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and/or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present. 
     Unless otherwise defined, all terms (including technical and scientific terms) used. herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     In general, geometric isolation takes advantage of the alignment and/or defocusing of the coupling optics between the pump and the VCSEL to suppress the level of reflections that can couple back into the pump chip. In the case of defocusing, it is helpful to note that the pump spot size on the VCSEL and the mode size of the VCSEL cavity itself do not necessarily need to be the same. This allows the pump light to be slightly defocused on the VCSEL and consequently the fed back light is not perfectly back-focused on the pump, This reduces the effective amount of fed back light. 
       FIG. 1A and 1B  show two examples of geometric isolation by pump defocusing. The pump laser  110  is effectively a point source  114 , The pump light aperture  114  might be the pump chip exit facet or the optical fiber that transmits the light from the pump chip to the lens train  122  of the coupling optics. In either case, this pump light is defocused at the gain substrate  116  of the VCSEL  115  by the coupling lens train  122  of two lenses LensA and LensB. 
     In more detail, as shown in Fig, IA, light  112  from the pump  110  diverges as it propagates away from the pump light aperture  114 . 
     The pump light aperture  114  in one context is the exit facet of a pump chip. One example chip is a single spatial mode, edge-emitting, ridge waveguide GaAlAs or InGaAs chip. 
     In another context, the pump light aperture  114  is the exit facet of an optical fiber, such as a single mode optical fiber, that carries light from the pump chip to the lens train  122 . 
     In both of these contexts, pump light aperture  114  approaches a point source, only having an extent of less than a few micrometers in diameter in many cases. 
     The diverging pump light  112  from the pump source  110  is relayed to the gain substrate  116  of the VCSEL  115  by the coupling lens train  122 . Specifically, the pump light is collimated by first lens (LensA) and then focused toward the surface  116 S of the gain substrate  116  of the VCSEL  115  by a second lens (LensB). 
     The characteristics of the coupling lens train  122  such as the power of the first lens and the second lens at the wavelength of light of the pump source  110  along with the distances between the pump light aperture  114 , the first lens, the second lens, and the exit facet  118  are selected so that the focal point  120  of the pump light  112  is in front of the proximal surface  116 -S of the gain substrate  116  of the VCSEL  115 . 
     Arranging the coupling lens train  122  to focus the pump light  112  in front of the proximal surface of the gain substrate  116  of the VCSEL  115  causes the reflected pump light  124  or pump light exiting the VCSEL  115  to be defocused at the pump source  110 . 
       FIG. 1B  shows another arrangement of the coupling lens train  122 . In this embodiment, the focal point  120  of the pump light  112  is behind the surface  116 S of the gain substrate  116  of the VCSEL  115 . 
     In more detail, light  112  from the pump light source  110  diverges as it propagates away from the pump light aperture  114 . 
     The diverging pump light  112  from the pump source  110  relayed to the gain substrate  116  of the VCSEL  115  by the coupling lens train  122 . 
     The characteristics of the coupling lens train  122  in this example such as the power of the first lens and the second lens at the wavelength of light of the pump source  110  along with the distances between the pump light aperture  114 , the first lens, the second lens, and the exit facet  118  are selected so that the focal point  120  of the pump light  112  is behind the proximal surface  116 S of the gain substrate  116  of the VCSEL  115 . 
     Arranging the coupling lens train  122  to focus the pump light  112  behind the proximal surface  116 S of the gain substrate  116  of the VCSEL  115  causes the returning pump light  124  from the VCSEL  115  to also be defocused at the pump source  110 . 
     In either example, because of pump light defocusing at the VCSCL gain substrate  116 , the fed back beam does not focus to a point back at the pump source  110 . This reduction in power density reduces the total power of light destabilizing the pump. 
     This type of geometric isolation can be applied to more complicated beam paths that include mirrors, WDM couplers, and other optical elements. The essential part is the defocusing. 
       FIG. 2  shows how the coupling lens train  122 , including Lens A and Lens B are integrated into an optically pumped tunable VCSEL swept source module  100 . 
     In one example, the VCSEL  115  is fabricated by bonding a microelectro-mechanical system (MEMS) tunable mirror die  130  to the optical gain/bottom mirror gain substrate  116 . In the preferred embodiment, the VCSEL is as described in United States Patent Application US2014/0176958A1, by Flanders, Kuznetsov, Atia, and Johnson, “OCT System with Bonded MEMS Tunable Mirror VCSEL Swept Source”, which is incorporated herein in its entirety by this reference. 
     That said VCSELs with integrated MEMS tunable mirrors are another option. An early example of such an integrated VCSEL is described in U.S. Pat. No. 6,645,784 by Tayebati, et al. 
     Nevertheless, almost any configuration of optically pumped VCSEL could be used. 
     A dichroic mirror (filter) allows separation of the VCSEL beam  134  emitted by the VCSEL  115  from the pump light  112 ,  124 . 
     In the illustrated embodiment, light from a pump chip  160  is coupled to a bench  140  via a pump optical fiber  142 . The pump light  112  from the optical fiber  142  is collimated by a first lens LensA. that is affixed to the bench  140 . The pump light  112  then is transmitted through the dichroic mirror  132  and then focused by a second lens LensB onto the gain substrate  116  of the VCSEL  115 . 
     Preferably, the bench  140 , in turn, is installed in a hermetic package  144  with optical fibers passing through fiber-feedthroughs  146 ,  148  of the package  144 . 
     The dichroic mirror is reflective to longer wavelength of the VCSEL light  134 , emitted by the VCSEL, but transmissive to the pump light  112 ,  124  in the illustrated example. Specifically in the illustrated example, the tunable signal from the VCSEL  115  is reflected by the dichroic mirror  132 , which is affixed to the bench  140 , and directed to a fold mirror  150  which is also affixed to the bench  140  and then to a third lens  152 , which is affixed to the bench  140 . The third lens  152  focuses light into an entrance aperture of an output optical fiber  154 . 
     Even with very effective pump isolation, Faraday or geometric, pumps can be noisy on their own. Amplitude noise, frequency noise, or joint amplitude/frequency noise can be a problem. Diode lasers, the most practical pump source, have natural amplitude and frequency noise driven by spontaneous emission and shaped by relaxation oscillations [ 15 ]. Fabry-Perot diode lasers can have mode hopping noise, Single frequency pumps, such as DFB (distributed feedback lasers), DBR (distributed Bragg reflection lasers), and discrete mode lasers [ 16 , 17 ], can avoid this issue. Volume Bragg grating stabilized lasers are another candidate [ 18 , 19 , 20 ]. 
     Placing the pump in the coherence collapse regime of operation allows control of the pump noise, if not eliminating it. Coherence collapse can be induced by placing a reflector some distance from the laser diode chip to destabilize it in a controlled way [ 21 ]. Often this is done by placing a fiber Bragg grating (FBG)  162  in the laser pigtail  142  [ 22 ]. The FBG  162  limits laser emission to a narrow band of wavelengths and induces coherence collapse which generates randomly phased modes c/(2L) apart, where c is the speed of light and L is the equivalent air distance between the laser chip and the FBG. Beating between these modes creates amplitude noise bands spaced c/(2L) apart in RF frequency. Reducing L, as seen in  FIG. 3 , can shift and spread out the noise to create wide bands of low noise, and can be sufficient to eliminate this source of noise from the detection bandwidth in many OCT applications. There is still noise in a narrow band around DC, but this may be acceptable in many cases. 
       FIG. 2  also shows another geometric isolation strategy. The VCSEL  115  receives the pump beam  112  at an angle. The angle between the incident and reflected pump beams must be greater than the divergence angle of the pump beam. Preferably the angle θ between the center axis of the incoming pump beam  112  and the proximal surface  1165  of the gain substrate  116  is less than  88  degrees, and preferably greater than 75 degrees in the horizontal or vertical planes, or some hybrid plane, which angle is generally dictated by the aperture of the focusing lens LensB in front of the VCSEL. This is achieved by aligning Lens B so that the beam of pump light  112  is offset from the center of Lens B and also offset from the axis of the VCSEL light  134  exiting from the VCSEL  115 . 
     With this configuration, the reflected beam  124  of pump light is now displaced from the incoming beam  112 . In the illustrated embodiment a light absorbing beam block substrate  170  is installed on the bench  140  to intercept the reflected beam  124 . This prevents feedback that will destabilize the pump. 
     Here, a non-normal incidence angle of the incoming pump beam  112  into the VCSEL  115  offsets the reflected beam  124  in space so that it can be blocked by a natural lens aperture or by the beam block  170  intentionally inserted into the package and installed on the bench  140 . In the case of single transverse mode source, fiber or laser, the offset angle of the returning beam can prevent coupling of the returning light, even without a beam block or aperture. These methods prevent light from being feed back into the pump  160  and destabilizing it (making it noisy). 
     The offset of the reflected beam  124  is controlled by precise control of the incidence angle θ of the incoming pump beam  112  focused into the VCSEL at the gain substrate  116 . Here the pump  160  and any SOA are external to the integrated hermetic package  144  and connected through optical fibers  154 ,  142 . In other schemes, either the pump or SOA or both could be incorporated into the package  144  and still benefit from use of any of the three forms of geometric isolation. These ideas enable low noise VCSEL pumping without the adoption of bulky, high-cost Faraday isolation. 
       FIG. 3  includes spectrograms showing how VCSEL, amplitude noise can be tailored into wide, low noise bands by using a pump purposefully put into a state of coherence collapse. Coherence collapse is induced by placing a fiber Bragg grating (FBG)  162  into the pigtail  142  of the pump laser  160 . By shortening the fiber length, defining a secondary cavity between the pump chip  160  and FBG  162 , to 0.14 meters, a 700 MHz wide low noise region is created that is wide enough for many OCT applications. Generally, the secondary cavity should be equivalent to about 0.3 meters in fiber or less, or 0.5 meters equivalent air path or less. 
     The FBG  162  in the pump pigtail  142  provides improved operation even when imperfect geometric isolation is present. In this case, the FBG pump in coherence collapse improved stability. It does two things: It changes a popcorn-like noise process to a more smooth Gaussian-like process. Then the short fiber length moves the noise bands out to n×700 Hz. Unfortunately there is still noise near DC, but it is easier to deal with. 
       FIGS. 4A and 4B  show two additional optical layouts or optically-pumped tunable VCSEL swept source modules with various levels of co-package integration. 
       FIG. 4A  shows an optically-pumped tunable VCSEL swept source module with an amplification stage. 
     In more detail, the pump light is received into the hermetic package  144  and onto the bench  140  from a separately packaged pump laser  160 . The pump&#39;s pigtail  142  is received through a feedthrough  146  in the package  144  and its end is secured down onto the bench  140  by a fiber mounting structure FL 1 . The fiber mounting structure FLI is preferably a fiber LIGA fiber holder (LIGA=Lithographic, Galvanoformung, Abformung (in English: Lithography, Electroplating, and Molding)). 
     Coherence collapse pumping is possible in this version by adding a fiber Bragg grating to the pigtail  142 . 
     The pump light is transmitted through the angled WDM filter/dichroic mirror  132 . and transmitted through the tunable mirror of the MEMStunable mirror die  130 . The light is focused onto the proximal surface  116 S of the gain substrate  116  by the Lens B. 
     In terms of coupling the pump light into the gain substrate  116  any of the three previous techniques can be employed. The pump light can be defocused in front of the proximal surface  116 S as shown in  FIG. 1A ; the pump light can be defocused behind the proximal surface  116 S as shown in  FIG. 1B ; or the pump light coupled into the gain substrate at an angle, displaced from the center axis of Lens B as described in connection with  FIG. 2 , 
     The dichroic mirror  132  reflects the VCSEL light  134 , emitted by the VCSEL. The tunable signal from the VCSEL  115  is reflected by the dichroic mirror  132 , which is affixed to the bench  140 , and directed to the fold minor  150  which is also affixed to the bench  140 . 
     The VCSEL light is then directed to pass through an isolator  180  that prevents backreflections. After the isolator, a focusing lens  186  couples the VCSEL light into an SOA  182  mounted to a submount  184 , which in turn is mounted to the bench  140 . At the output side of the SOA  182 , the amplified VCSEL light is focused by an output focusing lens  188  to couple the light into an output fiber pigtail  154  secured to the bench by a second fiber mounting structure FL 2 . 
       FIG. 413  shows another optically-pumped tunable VCSEL swept source module with an amplification stage. It is similar in construction and operation to the embodiment shown in  FIG. 4A , so that explanation applies here. 
     The difference is that the pump  160  is integrated onto the bench  140  and in the package  144 . Specifically, a pump chip  190  is mounted to a pump submount  192 , which in turn is mounted to the bench. 
     It should be noted that even with very effective pump isolation, Faraday or geometric, pumps can be noisy on their own. Amplitude noise, frequency noise, or joint amplitude/frequency noise can be a problem. Diode lasers as describe above, the most practical pump source, have natural amplitude and frequency noise driven by spontaneous emission and shaped by relaxation oscillations [ 15 ]. Fabry-Perot diode lasers can have mode hopping noise. Single frequency pumps, such as DFB (distributed feedback lasers), DBR (distributed Bragg reflection lasers), and discrete mode lasers [ 16 , 17 ], can avoid this issue. Volume Bragg grating stabilized lasers are another candidate [ 18 , 19 , 20 ]. 
     Thus, in one implementation, a volume Bragg grating (VBG)  194  added between the pump chip  190  and the dichroic mirror  132 . Specifically, the VBG could be added before lens L 4  in the diverging beam, or after lens L 4  in the collimated beam, as shown. 
     Alternatively, the VBG, appropriately angled, could also be added as an integral part of the WDM dichroic mirror  132 . This last configuration would require fabricating the VBG inside the WDM substrate with an appropriate angle for the non-normal angle of incidence. The VBG could also be fabricated as integral part of the first coupling lens LensA (for example GRIN lens, glass asphere lens) in the optical train  122 , as a means to reduce the optical cavity lengths (allows for wider spacing of the longitudinal modes which is more favorable for wavelength stabilization) and to reduce the overall size of the assembly. 
     There are many packaging configurations that could make use of these ideas for low-noise optical pumping of a tunable VCSEL. The key ideas are (1) geometric pump isolation, (2) single-frequency pumping (with DFB, DBR, discrete mode, or VBG-stabilized lasers), (3) broad band pumping with a SLED, (4) pumping with an FBG stabilized laser in coherence collapse, and (5) pumping with a standard pigtailed laser (without FBG) in coherence collapse due to a small feedback from the VCSEL. 
       FIG. 5  shows one exemplary MEMS tunable VCSEL  115  for inclusion in the optically pumped tunable VCSEL swept source module  100  described above. 
     The MEMS tunable VCSEL  115  comprises the MEMS tunable mirror die or device  130  that is bonded to the optical gain/bottom mirror gain substrate  116 , also known as a. half VCSEL. 
     In more detail, the MEMS tunable mirror  130  comprises handle wafer material  210  that functions as a support. Currently, the handle is made from doped silicon. 
     An optical membrane or device layer  212  is added to the handle wafer material  210 . Typically silicon on isolator (SOI) wafers are used. The membrane structure  214  is formed in this optical membrane layer  212 . In the current implementation, the membrane layer  212  is silicon that is low doped with resistivity &gt;1 ohm-cm, carrier concentration &lt;5×1015 cm-3, to minimize free carrier absorption of the transmitted light. For electrical contact, the membrane layer surface is usually additionally doped with ion implantation. 
     During manufacture, the insulating layer  216  functions as a sacrificial/release layer, which is partially removed to release the membrane structure  214  from the handle wafer material  210 . Then during operation, the remaining portions of the insulating layer  216  provide electrical isolation between the patterned device layer  212  and the handle material  210 . 
     In the current embodiment, the membrane structure  214  comprises a body portion  218 . The optical axis of the device  115  passes concentrically through this body portion  218  and orthogonal to a plane defined by the membrane layer  212 . A diameter of this body portion  218  is preferably 300 to 600 micrometers; currently it is about 500 micrometers. 
     Tethers  220  (four tethers in the illustrated example) are defined by arcuate slots  225  fabricated into the device layer  212 . The tethers  220  extend radially from the body portion  218  to an outer portion  222 , which comprises the ring where the tethers  220  terminate. In the current embodiment, a spiral tether pattern is used. 
     A membrane mirror dot  250  is disposed on body portion  218  of the membrane structure  214 . In some embodiments, the membrane mirror  250  is optically curved to form an optically concave optical element to thereby form a curved mirror laser cavity, In other cases, the membrane mirror  250  is a flat mirror, or even possibly convex. 
     When a curved membrane mirror  250  is desired, this curvature can be created by forming a depression in the body portion  218  and then depositing the material layer or layers that form mirror  250  over that depression. In other examples, the membrane mirror  250  can be deposited with a high amount of compressive material stress that will result in its curvature. 
     The membrane mirror dot  250  is preferably a reflecting dielectric mirror stack. In some examples, it is a dichroic mirror-filter that provides a defined reflectivity, such as between 1 and 10%, to the wavelengths of laser light generated in the VCSEL  115 , whereas the optical dot  250  is transmissive to wavelengths of the pump light  112  that are used to optically pump the active region in the half VCSEL device  116 . 
     In the illustrated embodiment, three metal pads  234  are deposited on the proximal side of the membrane device  110 . These are used to solder or thermocompression bond, for example, the half VCSEL device  116  onto the proximal face of the membrane device  130 . The top pad also provides an electrical connection to the half VCSEL device  116 . 
     Also provided are three wire bondpads  334 A,  334 B, and  334 C. The left VCSEL electrode wire bond pad  334 A is used to provide an electrical connection to the metal pads  234 . On the other hand, the right membrane wire bond pad  234 B is used to provide an electrical connection to the membrane layer  212  and thus the membrane structure  214 . Finally, the handle wire bond pad  334 C is used to provide an electrical connection to the handle wafer material  210 . 
     The half VCSEL device  116  generally comprises an antireflective coating  414 , which is optional, and an active region  418 , which preferably has a single or multiple quantum well structure. The cap layer can be used between the antireflective coating  414 , if present, and the active region  418 . The cap layer protects the active region from the surface/interface effects at the interface to the AR coating and/or air. The back mirror  416  of the laser cavity is defined by a distributed Bragg reflector (DEM) mirror. Finally, a VCSEL spacer  415 , such as GaAS, functions as a substrate and mechanical support. 
     The material system of the active region  418  of the VCSEL device  116  is selected based on the desired spectral operating range. Common material systems are based on III-V semiconductor materials, including binary materials, such as GaN, GaAs, InP, GaSb, InAs, as well as ternary, quaternary, and pentenary alloys, such as InGaN, InAlGaN, InGaP, AlGaAs, InGaAs, GaInNAs, GaInNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb, InGaSb, InAsSb, and InGaAsSb. Collectively, these material systems support operating wavelengths from about 400 nanometers (nm) to 2000 nm, including longer wavelength ranges extending into multiple micrometer wavelengths. Semiconductor quantum well and quantum dot gain regions are typically used to obtain especially wide gain and spectral emission bandwidths. 
     In the preferred embodiment, the polarization of the light generated by the MEMS tunable VCSEL  115  is preferably controlled and at least stabilized. In general, this class of devices has a cylindrical resonator that emits linearly polarized light. Typically, the light is polarized along the crystal directions with one of those directions typically being stronger than the other. At the same time, the direction of polarization can change with laser current or pumping levels, and the behaviors often exhibit hysteresis. 
     Different approaches can be taken to control the polarization. In one embodiment, polarization selective mirrors are used. In another example, non-cylindrical resonators are used. In still a further embodiment, asymmetrical current injection is used when electrical pumping is employed. In still other examples, the active region substrate includes trenches or materials layers, which result in an asymmetric stress, strain, heat flux or optical energy distribution, are used in order to stabilize the polarization along a specified stable polarization axis. In still a further example, asymmetric mechanical stress is applied to the VCSEL device  116 . 
     Defining the other end of the laser cavity is the rear mirror  416  that is formed in the half VCSEL device  116 . In one example, this is a layer adjacent to the active region  418  that creates the refractive index discontinuity that provides for a portion of the light to be reflected back into the cavity, such as between one and 10%. In other examples, the rear mirror  116  is a high reflecting layer that reflects over 90% of the light back into the laser cavity. 
     In still other examples, the rear VCSEL distributed Bragg reflector (DBR) minor  416  is a dichroic mirror-filter that provides a defined reflectivity, such as between 1 and 100%, to the wavelengths of laser light generated in the laser  115 , whereas the rear mirror  116  is transmissive to wavelengths of light that are used to optically pump the active region in the VCSEL device  116 , thus allowing the VCSEL device  112  to function as an input port of pump light. 
       FIG. 6  schematically shows the MEMS tunable VCSEL  100  in cross-section along A-A to show a proximal-side electrostatic cavity and a distal-side electrostatic cavity  224 . 
     An optical port  240  through handle wafer material  210  has generally inward sloping sidewalls  244  that end in the port opening  246 . As a result, looking through the distal side of the handle wafer  210 , the body portion  218  of the membrane structure  214  is observed. The port is preferably concentric with the membrane mirror dot  250 . Further, the backside of the body portion  218  is coated with a membrane backside AR coating  119  in some examples. This AR. coating  119  is used to facilitate the coupling of pump light  112  into the laser cavity and/or the coupling of laser light  134  out of the cavity 
     The thickness of insulating layer  216  defines the electrostatic cavity length of the distal-side electrostatic cavity  224 . Presently, the insulating layer  216  is between 3.0 and 6.0 μm thick. It is a general rule of thumb, that electrostatic elements can be tuned over no greater than one third the distance of the electrostatic cavity. As result, the body portion  218 , and thus the mirror optical coating  230  can be deflected between 1 and 3 μm in the distal direction (i.e., away from the VCSEL device  112 ), in one embodiment. 
     Also shown are details concerning how the half VCSEL device  116  is bonded to the membrane device  130 . The MEMS device bond pads  234  bond to VCSEL proximal-side electrostatic cavity electrode metal  422 . These metal layers are electrically isolated. Specifically, the MEMS device bond pads  234  are separated from the membrane layer  212  by MFMS device bond pad isolation oxide  236 ; the VCSEL proximal-side electrostatic cavity electrode metal  422  is isolated from the remainder of the VCSEL device by the VCSEL isolation oxide layer  128 . Neither of the VCSEL proximal-side electrostatic cavity electrode metal  422  nor the VCSEL isolation oxide layer  128  interfere with the optical operation since they do not extend into the region of the free-space portion  252  of the laser&#39;s optical cavity. 
     The distal-side electrostatic cavity  224  and the proximal-side electrostatic cavity  226  are located on either side of the membrane structure  214 . Specifically, the distal-side electrostatic cavity  224  is created between the handle wafer material  210  and the membrane structure  214 , which is the suspended portion of the membrane layer  212 . A voltage potential between the handle wafer material  210  and the membrane layer  212  will generate an electrostatic attraction between the layers and pull the membrane structure  214  toward the handle wafer material  210 . on the other hand, the proximal-side electrostatic cavity  226  is created between the membrane structure  214  and the VCSEL proximal-side electrostatic cavity electrode metal  422 . A voltage potential between the membrane layer  212  and the VCSEL proximal-side electrostatic cavity electrode metal  422  will generate an electrostatic attraction between the layers and pull the membrane structure  214  toward the VCSEL device  112 . 
     In general, the size of the proximal-side electrostatic cavity  226  measured along the device&#39;s optical axis is defined by the bond metal thickness, thickness of VCSEL proximal-side electrostatic cavity electrode metal  422  and MEMS device bond pads  234  along with the thicknesses VCSEL isolation oxide layer  128  and MEMS device bond pad isolation oxide  236 . 
     The minimum oxide thickness is determined by the required voltage isolation. Oxide break down is nominally 1000V/micrometer. So, for 200V isolation that would be  2000 A, which is preferably doubled for margin. So the thickness of layers VCSEL isolation oxide layer  128  and MEMS device bond pad isolation oxide  236  is greater than 4000 A. 
     The current metal bond thickness is 6000 A (each layer) with approx. 3000 A compression during bonding. Based on this, the minimum size of the proximal-side electrostatic cavity  226  is 0.85 micrometers. 
     At this minimum electrostatic gap point, a zero optical gap results when the membrane mirror dot  250  is 1.7 micrometers thick. 
     To increase the optical gap, the thickness of the VCSEL isolation oxide layer  128  can be increased without effecting the operation of the cavity. 
     It should be noted that in the defocus methods of pump isolation discussed with respect to  FIGS. 1A and 1B , the defocusing is with respect to the surface  116 S of the gain substrate or half VCSEL  116 . The location of this internal surface is best shown in  FIG. 6 , 
     In a similar vein, pump beam  112  angle θ is measured with respect to the surface  116 S, shown in  FIG. 6 . 
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     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.