WO1994021013A1

WO1994021013A1 - Polarized surface-emitting laser

Info

Publication number: WO1994021013A1
Application number: PCT/US1994/002356
Authority: WO
Inventors: Jack L. Jewell; Greg R. Olbright
Original assignee: Photonics Research Incorporated
Priority date: 1993-03-05
Filing date: 1994-03-04
Publication date: 1994-09-15
Also published as: AU6276294A; US5331654A

Abstract

A vertical-cavity, surface-emitting semiconductor diode laser (10) having a monolithic and planar surface and having lateral anisotropy in order to control the polarization of the emitted beam of light. The diode laser includes a body of a semiconductor material having an active region (24, 26, 28) therein which is adapted to generate radiation and emit the radiation from a surface (37) of the body, and a separate reflecting mirror (18, 32) at opposite sides of the active region with at least one of the mirrors being partially transparent to the generated light to allow the light generated in the active region to be emitted therethrough. The anisotropy may be provided by utilizing anisotropy in the atomic or molecular structure of the materials forming the diode, or by anisotropic patterning or deliberate misalignment in processing of the diode through externally applied forces or fields to control the polarization of the emitted beam.

Description

POLARIZED SURFACE-EMITTING LASER

BACKGROUND OF THE INVENTION

Field of the Invention

This invention was made with U.S. Government support under Contract number F30602-92-C-0091 awarded by Rome Lab, Department of the Air Force. The United States Government has certain rights in this invention.

The present invention relates to a semiconductor vertical-cavity, surface- emitting laser, and, more particularly, to a semiconductor vertical-cavity, surface- emitting laser with monolithic and planar structure and having lateral anisotropy in order to control the polarization of the emitted beam of light.

Description of the Prior Art Semiconductor laser diodes, in general, comprise a body of a semiconductor material having adjacent regions of opposite conductivity type forming a p-n junction therebetween. The body is adapted to generate and emit radiation when an appropriate potential is applied across the p-n junction. Vertical-cavity, surface- emitting lasers (VCSELs) emit radiation in a direction perpendicular to the plane of the p-n junction or substrate rather than parallel to the plane of the p-n junction, as in the case of conventional edge-emitting diode lasers. In contrast to the elliptical and astigmatic beam quality of conventional edge emitting lasers, VCSELs advantageously emit a circular symmetric Gaussian beam. Thus, anamorphic correction of the emitted beam of VCSELs is not required. VCSELs moreover, can readily be made into two-dimensional laser arrays as well as be fabricated in extremely small sizes.

Accordingly, two-dimensional VCSEL arrays have various applications in the fields of optical memory, laser printing and scanning, optical communications, integrated optoelectronic integrated circuits, optical computing, optical interconnection, etc. The circular symmetry of the beams emitted from VCSELs arises partly from the fact that they are usually fabricated in a circularly-symmetric structure. It is also well known that VCSELs fabricated in square or even rectangular shaped structure tend to emit circularly-symmetric beams as long as the device is greater than about 5 micrometers in diameter. A consequence of this circular symmetry is the lack of a preferred polarization direction. VCSEL beams emitting in a single transverse mode are linearly polarized. However, the direction of the polarization is often different from one VCSEL element to another. For optical systems employing arrays of VCSELs, the variation in polarization direction can greatly degrade system characteristics, such as efficiency and beam uniformity. Because of the high Q-factor of VCSEL cavities, a slight anisotropy in the optical characteristics can give slight preference to one polarization direction and thus cause the laser to emit a beam polarized in the preferred direction. The semiconductor material forming the VCSELs is crystalline, and the beams show a statistical preference to be polarized in alignment with one or the other crystal axes. However, the ambiguity in polarization direction remains.

Polarization of VCSEL beams has been controlled by deliberately introducing anisotropies into VCSEL cavities. Anisotropic diffraction losses were introduced by depositing high refractive index material onto opposing sidewalls of an etched VCSEL, thereby stabilizing the polarization as described in an article by Shimazu et al , entitled "A method of polarization stabilization in surface emitting laser", published in the JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 239(6A), June, 1991, pgs. L1015-L1017. This method which, as described in the article, requires etching through the top mirror, is not effective when the device is larger than about 5 micrometers across, and the device utilizes a hole etched through the entire semiconductor substrate. Another technique, which uses anisotropic strain through a rectangular etch to control polarization has been described in an article by T. Mukaihara et al. , entitled "Stress effect for polarisation control of surface emitting lasers", published in ELECTRONICS LETTERS, Vol. 28(6), March 12, 1992, pgs. 555. Both of the devices described above use a hole etched through the substrate, resulting in a nonplanar device. This greatly complicates the fabrication and greatly reduces the heat dissipation capability of the devices. The need to apply an external stress in the mounting results in a non-monolithic device which is subject to nonuniformity and reliability problems. Yet another approach described by Wipiejewski et al. , present at the European Conference on Optical Communication, September 1992, uses an external cavity containing a polarizer to control the polarization of the emitted beam. All of these prior art approaches to polarization control suffer serious drawbacks through being nonplanar or requiring external apparatus, and are not considered practical.

SUMMARY OF THE INVENTION

The present invention relates to a monolithic and planar vertical-cavity, surface-emitting laser which comprises a body of a semiconductor material having an active region therein which is adapted to generate radiation and emit the radiation from a surface of the body and is structured to give substantial preference for the laser emission to be polarized in one state. Planar, in this case, refers to the structure having no vertical features deeper than about 10 microns. One form of the invention utilizes anisotropy in the atomic or molecular structure of the materials comprising the VCSEL cavity to control the polarization of the emitted beam. Another form of the invention creates anisotropic features through anisotropic patterning or deliberate misalignment in processing of the VCSELs to control the polarization of the emitting beam. Still another form of the invention creates anisotropy on or near the top surfaces of the VCSELs near the end of the processing to control the polarization of the emitted beam. Yet another form of the invention creates anisotropy in the VCSEL cavities through externally applied forces or fields to control the polarization of the emitted beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sectional view of a vertical-cavity, surface-emitting laser in accordance with the present invention; Figure 2 is a top plan view of Figure 1 ; Figures 3 through 6 are views similar to Figure 2 but showing different means for introducing anisotropic current flow;

Figure 7 and 8 are views similar to Figure 2 but showing two more different means for introducing anisotropic current flow; Figure 9 is a schematic view of an array of VCELs all having the same polarization; and

Figure 10 is a schematic view of an array of VCELs having different polarizations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring initially to Figure 1, there is shown a sectional view of a vertical- cavity, surface-emitting laser 10 in accordance with the present invention. Laser 10 comprises a substrate 12 of a semiconductor material, such as GaAs, having opposed major surfaces 14 and 16. On surface 14, hereinafter referred to as the growth surface, the VCSEL structure is placed, starting with first mirror 18. The first mirror 18 is formed of alternating layers 20 and 22 of materials having different indices of refraction. Preferably, the first mirror 18 is a distributed Bragg reflector formed of alternating layers 20 and 22 of high and low index of refraction semiconductor materials which are also electrically conductive. Although first mirror 18 is designed to be reflective, it can also be designed to be partially transparent to light.

On the first mirror 18 is a first clad layer 24 of a semiconductor material of the same conductivity type as the substrate 12. On the first clad layer 24 is a thin active layer 26 containing active material, and on the active layer 24 is a second clad layer 28. The second clad layer 28 is of similar material as the first clad layer 24 but of opposite conductivity type. The active layer 26 may be of a single semiconductor material, nonconductive or of either conductivity type, or may be of a single or multiple quantum well structure or may be of a superlattice structure. The first and second clad layers 24 and 28 are of materials having bandgaps different from that of the active layer 26 so as to confine electrons to the active layer 26. On the second clad layer 28 is a contact layer 30 of a semiconductor material of the same conductivity type as the second clad layer 28.

On the contact layer 30 is a second mirror 32. The second mirror 32 is also formed of alternating layers 34 and 36 of materials having different indices of refraction. Second mirror 32 has a top surface 37. Although the second mirror 32, like the first mirror 18, may be formed of alternating layers 34 and 36 of semiconductor materials, it can also be formed of alternating layers 34 and 36 of dielectric materials. In either case, the materials of the alternating layers 34 and 36 have high and low indices of refraction. Second mirror 32 may also be at least partially metallic. Although the second mirror 32 is designed to be reflective, it may also be designed to be partially transparent to light. A contact 38 is on the contact layer 30 at each side of ar around the second mirror 32, or on top of the second mirror 32. A contact 40 is on the surface 16 of the substrate 12. As will be explained later in detail, anisotropy is introduced into the optical cavity or the active layer 26 so as to control the polarization of the beam of light emitted by the laser 10.

In the operation of the laser 10, a current is passed through the active layer 26 between the contacts 38 and 40. Current is confined substantially to flow within an active outer boundary 42 by a nonconductive region 44 in the second clad layer 28. The nonconductive region 44 is preferably formed by a deep ion implantation. The contact 38 on the contact layer 30 and around the second mirror 32 forms an optical aperture of the laser 10 having an aperture outer boundary 46. From the contact 38 current is prevented from flowing away from the active layer by a nonconductive region 48 which extends through the contact layer 30. The nonconductive region 48 has an injection outer boundary 50 and is preferably formed by a shallow ion implantation. Alternatively, the nonconductive region 48 can be formed by etching away that portion of the contact layer 30.

The injected current generates light in the active layer 26. The light is reflected back and forth between the first and second mirrors 18 and 32 along cavity axis 52 to achieve lasing of the light. Since at least one of the mirrors 18 and 32, for example, the second mirror 32, is also partially transparent, some of the lasing light will pass out of the laser 10 through the mirror 32. The light intensity in the cavity forms a standing wave pattern 54 having peaks 56 and troughs 58 in the intensity. Lasing action occurs when the optical gain per round trip through the active layer exceeds the round trip loss of the optical cavity. By introducing anisotropy into the optical cavity or the active layer, the gain to loss ratio can depend upon the polarization state of the light, and the polarization of the emitted beam can be controlled.

Anisotropy in the laser cavity can be introduced into the materials comprising the laser. There are many ways in which material anisotropy in the optical cavity can give preference to one polarization state. If the material is birefringent, i.e., having different refractive indices for light polarized in different orientations, the cavity resonance will occur at different wavelengths for the two polarizations. One of these wavelengths will have greater overall gain in the cavity and will therefore be preferred. Thus, one polarization state will be referred and will dominate the lasing action. Crystalline materials, such as the dielectric's quartz and calcium fluoride are birefringent, as are the semiconductors gallium arsenide, indium phosphide and the numerous combinations of semiconductors containing gallium, aluminum, indium, arsenide, phosphide and other elements. In all these cases, it is important to orient the material such that the refractive index for light polarized in one first direction perpendicular to optical axis 52 is different for light polarized perpendicular to optical axis 52 and the first direction. Anisotropic gain in the active material can cause the gain to be higher for one polarization state than the other. Examples of such active materials are quantum wires and anisotropic quantum dots, both are typically of semiconductors, such as gallium arsenide. Absorption losses can also be anisotropic in a material. A familiar example is the polymer polarizing sheets which, due to having long molecules aligned with one another, absorbs strongly for one polarization than another. Materials having similar properties can be introduced into the laser cavity to cause the anisotropic losses in the cavity, giving preference to one polarization, and therefore controlling the polarization of the laser emission.

One means for introducing material anisotropy into the cavity is to orient the substrate growth face 14 at an oblique angle with respect to the major crystal axis as shown in Figure 1. Crystal axis 60 and 62 are shown to be oblique to the growth face 14. A consequence of this orientation is illustrated in Figure 2. Figure 2 is a top plane view of the laser 10 from the side of the second mirror 32. Figure 2 shows the outer active boundary 42, the outer aperture boundary 46, and the outer injection boundary 50. Transverse cavity axes 65 and 66 are perpendicular to cavity axis 52. Light which is linearly polarized along transverse cavity axis 66 has its polarization aligned more closely with crystal axis 62 than does light which is linearly polarized along transverse cavity axis 64. Thus, the optical properties of the cavity are anisotropic, one polarization state will be preferred over other, and the polarization of the laser emission can be controlled.

Another means for introducing material anisotropy into the cavity is to pattern the semiconductor wafer, with or without semiconductor material grown on top of the substrate, with anisotropic features, followed by epitaxial overgrowth of semiconductor material having a difference lattice constant than the underlying material. The different lattice constant results in strain in the overgrown material. The anisotropic patterning makes this strain anisotropic. Therefore, the optical properties of the overgrown material will be amsotropic. The anisotropy can manifest itself in the refractive index, absorption, or gain properties of the overgrown material, or combinations thereof. An example of this technique is to grow the structure of semiconductor laser 10 up through first mirror 18. The growth is then stopped and features having, for example, dimensions 7x10 microns, are patterned and etched.

The structure up through the contact layer 30 is then overgrown. In this example, most of the materials could be aluminum gallium arsenide having various relative concentrations of aluminum and gallium grown on a gallium arsenide substrate. The active material 26 in this case could be indium gallium arsenide, whose lattice constant does not match to that of aluminum gallium arsenide. Active material 26 would than be anisotropically strained, giving rise to anisotropic refract index, absorption and gain, thereby causing the semiconductor laser 10 to emit light polarized in one direction.

Still another means for introducing material anisotropy into the cavity is to create anisotropic stress, and therefore temporary anisotropic strain, in the substrate during deposition of any of the layers of the semiconductor laser 10. When the stress is removed, the substrate will return to its original flat shape, thereby causing anisotropic strain in materials which were deposited on the substrate when it was in the stressed state. Such materials will therefore be anisotropically strained and will cause the polarization of semiconductor laser 10 to be polarized in one direction as described above. An example of this process is to fasten the partially fabricated semiconductor laser to a mount having a slight cylindrical shape, and forcing the substrate to have a slightly cylindrical shaped. After depositing the upper mirror 32, the structure is removed from the mount so that it relaxes to its original flat shape.

This returning to the flat shape introduces compressive strain in upper mirror 32 only along the direction that had been curved during deposition. The layers 36 and 37 in the upper mirror 32 will therefore be anisotropic and cause the emission of the semiconductor laser 10 to be polarized as described above.

In the examples described above, the emitted light polarization is usually linear. For the cases of anisotropic gain or absorption, the linear polarization is oriented such that the gain to loss ratio is maximized. For anisotropy in refractive index, the linear polarization direction is generally along one of the crystal axes, the preferred axis having a cavity resonance which maximizes the gain to loss ratio as described earlier.

Anisotropic current flow can also be introduced by anisotropic fabrication of the VCSEL to control its polarization. Figures 3-6 show several anisotropic configurations, each of which can be easily accomplished by appropriate patterning of the photolithographic masks used in the fabrication of the laser 10. In Figure 3, injection outer boundary 50 has been made elongated. In Figure 4, active outer boundary 42 has been made elongated. In Figure 5, aperture outer boundary 46 has been made elongated. In Figure 6, injection outer boundary 50 is not centered with respect to the active outer boundary 42, or the aperture outer boundary 46. There are several consequences of anisotropic current flow. Heating of the device due to electrical resistance will produce an anisotropic, for example cylindrical, thermal lensing effect in the cavity. The electrical fields in the cavity produced by the voltage applied to the contact can change the optical properties of the materials making them amsotropic in their optical properties. Anisotropy in the active outer boundary 42, as shown in Figure 4, can also cause anisotropic losses in the cavity, especially if ion implantation is employed.

Referring now to Figure 7, there is shown another approach for introducing anisotropy into a VCSEL even if all materials used are isotropic and the VCSEL apertures are isotropic. In addition to the outer boundaries previously defined, there is shown an additional anisotropic structure 68 which causes anisotropic losses in the optical cavity. Losses are increased for one polarization direction compared to the other by means including, but not limited to, increased absorption, increased scattering, increased diffraction and decreased reflection. Anisotropic structure 68 can also be fabricated to display birefringence. In general, the structure 68 can comprise a material which is isotropic on an atomic scale, but which has been patterned anisotropically. For example, structure 68 can be a grating having grating lines 70. The grating can be in the surface of one of the semiconductor layers of the laser 10, one of the layers (either semiconductor or dielectric) of either of the mirrors 18 and 32, or a separate metal layer, for example gold or tungsten, formed in or on the laser 10. For example, anisotropic stmcture 68 could be formed by etching into a semiconductor layer of the stmcture and having on top of it another semiconductor or a dielectric. If etching is the method of forming anisotropic stmcture 68, the overlying material is also likely to be anisotropic. When anisotropic stmcture 68 is etched into the top surface 37 of the semiconductor laser 10, the overlying material is most likely air, or it could be a liquid, or it could be a solid material, such as an epoxy. Light polarized linearly along the grating lines 70 will be absorbed more strongly than light polarized perpendicular to the lines 70. The grating shown in Figure 7 would therefore emit light polarized linearly in the vertical direction. The gratings most effective when the widths of the lines 70 are small compared to an optical wavelength and the lengths are substantially longer than the widths. For emission at visible or near infra-red wavelengths, which are most desirable, the grating line widths are most effective if they are extremely small, thereby making fabrication somewhat difficult to achieve. A consequence of larger widths is increase absorption loss for polarization in the low-loss direction. The effect of the anisotropic stmcture 68 can be controlled by appropriate longitudinal placement in the optical cavity. Referring again to Figure 1, particularly to the intensity distribution 54, it is seen that the effect of the grating can be maximized or minimized by placing it in the peaks 56 or troughs 5i8 of the intensity distribution 54 respectively. Furthermore, placing anisotropic stmcture 68 farther from the active region 26 will incur smaller effects than placing it closer to the active region 26. Thus, a very lossy grating is most advantageously located in an intensity trough 58 as far as possible from the active region 26.

Another way to control the effect of anisotropic stmcture 68 is by dilution. Again using the example of a grating, the spacing to width ratio of the grating lines 70 can be increased to decrease the grating's effect. Furthermore, the lengths of the grating lines 70 can be reduced so as not to extend across the entire optical cavity, as it does in Figure 7. Yet another means for controlling the effect of anisotropic stmcture 68 is to vary the depth of its features along the dimension cavity axis 52.

Referring to Figure 8, there is shown a highly developed anisotropic stmcture 72 which, for illustrative purposes, uses a grating stmcture. Here the grating lines 74 are short compared to the aperture outer boundary 46. In Figure 8, the other boundaries referred to previously are not shown in order to better illustrate -li¬

the distribution of the anisotropic stmcture 72. It is seem that the effect of anisotropic stmcture can be made as large or as small as desired by varying the density of distribution of the grating lines 74. The distribution of grating lines 74 in Figure 8 is advantageously chosen to produce smaller losses in the center of the optical cavity were light intensity for the lowest order transverse mode is highest.

Thus, this kind of grating structure not only gives preference to one polarization, but it can also give preference for one or another transverse mode.

The ability to vary the effect of anisotropic structures 68 and 72 through several means allows the stmcture to be optimized for manufacturability or for other desirable features. One particular example is to place the anisotropic stmcture 68 or

72 on the top surface 37 of the mirror 32. This location allows the VCSEL to be fabricated through its entire process by normal means, and only then introducing the anisotropy. One desirable configuration is for anisotropic stmcture 68 or 72 to comprise narrow "trenches" etched into top surface 37. The "trenches" would cause greater loss to occur for one polarization than for the other and thereby control the polarization of the emitted beam. On the other hand, if anisotropic stmcture 68 or 72 is easily destroyed by environmental factors, as in a metallic grating, it may be preferable to locate it only near top surface 37, allowing the overlying material to protect it.

The need for polarization control is greatest when multiple lasers are used in an array. In most cases the desire is for all polarizations to be the same. Figure 9 illustrates this case for laser array 72, in which all lasers 74, having linear polarization oriented vertically, as indicated by the arrows. In this case all beams will propagate through an optical system (not shown) with nominally identical transmission/reflection characteristics. Such systems are usually appropriate for applications such as optical memory or laser printing. In other applications, such as optical interconnections between electronic chips or boards, it may be desirable to have the polarizations vary over the array. This variation can take numerous forms. One example is shown in Figure 10 for laser array 72, where each 2x2 subarray has tow lasers 76 and 78 polarized vertically, one laser 80 polarized horizontally, and one laser 82 polarized at 45 degrees as indicated by the arrows. In this example, the vertically polarized beams will be transmitted to one destination, the horizontally polarized beams to another destination, and the 45 degree beams split to be transmitted to both destinations.

It is to be appreciated and understood that the specific embodiments of the invention are merely illustrative of the general principles of the invention. Various modifications may be made upon the preferred embodiments described consistent with the principles set forth. For example, the laser stmcture and electrical injection schemes can both take on a variety of forms, differing substantially from the stmcture illustrated in Figure 1 and still utilize the invention. Both electrical contacts can be on the top side of the substrate, or the upper contact can be on top of the upper mirror. The upper contact can also contribute to the reflectivity of the upper mirror. In this case the aperture inner boundary, as defined, could be zero and the aperture inner boundary would need re-definition. Etching processes could replace part or all of the implantations shown. Lifting off of the VCSEL stmcture from the original substrate does not affect the invention even for cases where substrate orientation is employed because that substrate crystal orientation is preserved in the crystal orientation of the VCSEL stmcture. Any of the techniques for controlling polarization can be combined. The scope of the invention is indicated by the appended claims rather than by the foregoing description.

Claims

WHAT IS CLAIMED IS:

1. A monolithic vertical-cavity, surface emitting diode laser comprising: a body of a semiconductor material having opposed surfaces and an active region therein which is adapted to generate light and emit the light from one of the surfaces thereof; a separate reflecting mirror at opposite sides of the active region forming an optical cavity which includes said active region and reflecting mirrors, at least one of the mirrors being partially transparent to the generated light to allow the light generated in the active region to be emitted therethrough; and characterized in that the laser is substantially planar by having all vertical projections or holes less than 10 microns deep, and that the laser is stmcture to give substantial preference for laser emission to be polarized in one predetermined state.

2. The diode laser of claim 1 , further comprising at least one anisotropic material within said cavity oriented to give substantial preference for laser emission to be polarized in one state.

3. The diode laser of claim 2 in which the anisotropic material is a semiconductor.

4. The diode laser of claim 3 further comprising a substrate having a growth face and in which the anisotropy in the semiconductor material results from the substrate growth face being oriented at an oblique angle with respect to any of the major crystal axes of the substrate material.

5. The diode laser of claim 3 in which the active region has anisotropic gain.

6. The diode laser of claim 1 in which the optical losses in the cavity are anisotropic.

7. The diode laser of claim 1 in which the optical thickness of the cavity is anisotropic.

8. The diode laser of claim 1 in which the substrate or a layer above the substrate is patterned to produce anisotropy in at least one material in the laser stmcture.

9. The diode laser of claim 1 in which the substrate is anisotropically strained during deposition of at least part of the laser stmcture.

10. The diode laser of claim 1 in which the polarization state is linear.

11. The diode laser of claim 1 characterized in that the active region is bounded by an active outer boundary, and further comprising: means for injecting electrical current from an electrical contact from a region having an injection outer boundary into the active region; an optical aperture having an aperture outer boundary; characterized in that the boundaries are structured to give substantial preference for laser emission to be polarized in one state.

12. The diode laser of claim 11 in which the injection outer boundary is anisotropic.

13. The diode laser of claim 11 in which the active outer boundary is anisotropic.

14. The diode laser of claim 11 in which the aperture outer boundary is anisotropic.

15. The diode laser of claim 11 in which at least one of the boundaries is substantially not centered with respect to at least one other boundary.

16. The diode laser of claim 1 characterized in that the optical cavity has a cavity axis and at least one transverse optical mode, and further comprising at least one anisotropically structured material to give substantial preference for laser emission to be polarized in one orientation.

17. The diode laser of claim 16 in which the anisotropically structured material has first and second dimensions, both dimensions oriented substantially perpendicular to the cavity axis.

18. The diode laser of claim 17 in which the first dimension is smaller than an optical wavelength.

19. The diode laser of claim 17 in which the second dimension is larger than an optical wavelength.

20. The diode laser of claim 17 in which the light in the cavity forms a standing wave intensity pattern having peaks and troughs.

21. The diode laser of claim 20 in which the anisotropically stmcture material is placed in the vicinity of a trough of the standing wave intensity pattern.

22. The diode laser of claim 20 in which the anisotropically structured material is placed in the vicinity of a peak of the standing wave intensity pattern.

23. The diode laser of claim 16 in which the anisotropically stracmred material is a metal, a dielectric, a semiconductor, a liquid, a gas, or any combination thereof.

24. The diode laser of claim 16 in which the anisotropically structured material is furthermore patterned to give preference to one transverse mode of the cavity.

25. The diode laser of claim 24 in which the transverse mode is the lowest order transverse mode.

26. The diode laser of claim 1 further comprising an array of the lasers in which at least two lasers are structured to give substantial preference for their emitted beams to be polarized in predetermined polarization states.

27. The diode laser of claim 26 characterized in that at least two lasers in the array emit beams in the same predetermined polarization state.

28. The diode laser of claim 26 characterized in that at least two lasers in the array emit beams in different predetermined polarization states.