Abstract:
A surface emitting photonic device including a substrate; and a waveguide structure on the substrate. The waveguide structure includes an active region along its longitudinal axis and the active region is for generating light. The waveguide structure also has a trench formed therein transverse to the active region and defining a first wall forming an angled facet at one end of the active region, the first wall having a normal that is at a non-parallel angle relative to the longitudinal axis of the waveguide structure. The trench also defines a second wall located opposite the first wall.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/186,730, filed Jun. 12, 2009, and incorporated herein by reference. 
     This application is related to U.S. application Ser. No. 10/958,069, filed Oct. 5, 2004, and entitled “Surface Emitting and Receiving Photonic Device;” U.S. Provisional Application No. 60/512,189, filed Oct. 20, 2003; and, U.S. Provisional Application No. 60/578,289, filed Jun. 10, 2004, the disclosures of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to surface emitting and receiving photonic devices, and more particularly to improved surface emitting laser devices and methods for fabricating them. 
     Semiconductor lasers typically are fabricated by growing the appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an active layer parallel to the substrate surface. The material is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer, and metallic contacts are attached to the semiconductor material. Finally, laser mirror facets typically are formed at the ends of the laser cavity by cleaving the semiconductor material to define edges or ends of the laser optical cavity so that when a bias voltage is applied across the contacts the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. 
     The prior art also discloses processes for forming the mirror facets of semiconductor lasers through etching, allowing lasers to be monolithically integrated with other photonic devices on the same substrate. The formation of total-internal-reflection facets within an optical cavity through the creation of such facets at angles greater than the critical angle for light propagating within the cavity is also known. 
     The use of an etch process to form two total-internal-reflection facets at each end of a linear laser cavity, with each facet being positioned at an angle of 45° with respect to the plane of the active layer, is also described in the prior art. In such devices, light in the cavity may be directed perpendicularly upward at one end of the cavity, resulting in surface emission at one facet, while the facet at the other end of the cavity may be oppositely angled to direct the light perpendicularly downward to, for example, a high reflectivity stack below the laser structure. 
     The prior art also describes devices which combine etched 45° facets with cleaved facets. The resultant devices cannot be tested in a full-wafer and as such suffer from the same deficiencies as cleaved facet devices. Furthermore, they are incompatible with monolithic integration in view of the need for cleaving. Chao, et al., IEEE Photonics Technology Letters, volume 7, pages 836-838, attempted to overcome these short-comings, however, by providing an interrupted waveguide structure, but the resultant device suffered from scatter at each end of the laser cavity. 
     Vertical Cavity Surface Emitting Lasers (VCSELs), have gained popularity over the past several years; however, VCSELs do not allow in-plane monolithic integration of multiple devices and only allow light to exit their surface mirror at perpendicular incidence. A common aspect of these prior surface-emitting devices is that the photons are always emitted from the optical cavity in a direction perpendicular to the plane of the active layers. 
     A laser with low to no ripples in its far-field intensity profile is very desirable, for example, in efficient coupling of the laser beam into an optical fiber. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, the invention features a surface emitting photonic device including: a substrate and a waveguide structure on the substrate. The waveguide structure includes along its longitudinal axis an active region for generating light. The waveguide structure has a trench formed therein transverse to the active region and defining a first wall forming an angled facet at one end of the active region, the first wall having a normal that is at a non-parallel angle relative to the longitudinal axis of the waveguide structure, the trench also defining a second wall located opposite the first wall. 
     Other embodiments include one or more of the following features. The first and second walls of the trench define an opening in waveguide structure that is no greater than one of 8 μm, 4 μm or 1 μm. The waveguide structure is made of a semiconductor material and it has a top surface wherein the first wall is at an angle relative to the top surface of about 44.4°±1°. In general, the facet is internally reflective and angled to cause light generated in the active region to be emitted in a direction that is substantially perpendicular to the substrate. The waveguide structure is made up of multiple layers on a top surface of the substrate and the active region is substantially parallel to that top surface. The device also included electrodes on the waveguide structure and the substrate for receiving a bias voltage to activate the waveguide structure to generate a laser output beam. The device is a ridge laser (e.g. a buried heterostructure laser, a Fabry Perot laser, a distributed feedback laser. The waveguide structure is shaped to form an elongated laser cavity having the first facet at a first end of that cavity and having a second facet at a second end of that cavity. The trench has an etched base that is parallel to the substrate or rounded. The second wall has a normal that is parallel to the longitudinal axis of the waveguide structure. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the described embodiments will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing, and additional objects, features and advantages will be apparent to those of skill in the art from the following detailed description of preferred embodiments thereof, taken with the accompanying drawings, in which: 
         FIG. 1  is a top perspective view of a first embodiment of a surface-emitting laser. 
         FIG. 2  is a side elevation of the laser of  FIG. 1 . 
         FIG. 3  is a top plan view of the laser of  FIG. 1 . 
         FIG. 4  is a side elevation of a second embodiment of a surface-emitting laser. 
         FIG. 5  is a side elevation of a third embodiment of a surface-emitting laser. 
         FIG. 6  is a side elevation of a fourth embodiment of a surface-emitting laser. 
         FIG. 7  is a top perspective view of a fifth embodiment of a surface-emitting laser. 
         FIG. 8  is a top plan view of a sixth embodiment, combining a surface emitting laser and an area detector. 
         FIG. 9  is a side elevation in partial section of the laser and area detector of  FIG. 8 . 
         FIG. 10  is a top plan view of a seventh embodiment, combining a surface emitting laser and an in-plane detector. 
         FIG. 11  is a side elevation in partial section of the laser and in-plane detector of  FIG. 10 . 
         FIG. 12  is a top perspective view of an eighth embodiment, incorporating multiple surface emitting lasers. 
         FIG. 13  is an enlarged view of the surface emitting regions of the multiple lasers of  FIG. 12 . 
         FIG. 14  is a top plan view of a laser positioned for improved packing density. 
         FIG. 15  is a cross-sectional view of the waveguide structure using angled etching only. 
         FIG. 16  is a cross-sectional view of the waveguide structure using an angled and perpendicular etching. 
         FIG. 17  shows the ideal far-field corresponding to a 1-d waveguide structure for a 1310 nm device having a far-field angle (full-width half maximum of the far-field) of 47°. 
         FIGS. 18(A)  and (B) show a 2-d waveguide structure with a perpendicular etched facet, the far-field corresponding to this structure, and the far-field from  FIG. 17  for comparison. 
         FIGS. 19(A)  and (B) show a 2-d waveguide structure with a 45.0° etched facet, the far-field corresponding to this structure, and the far-field from  FIG. 17  for comparison. 
         FIGS. 20(A)  and (B) show a 2-d waveguide structure with a 45.0° etched facet with an etched base, the far-field corresponding to this structure, and the far-field from  FIG. 17  for comparison. 
         FIG. 21  shows the far-field corresponding to a 2-d waveguide structure with a 44.0° etched facet with an etched base and the far-field from  FIG. 17  for comparison. 
         FIG. 22  shows the far-field corresponding to a 2-d waveguide structure with a 44.4° etched facet with an etched base and the far-field from  FIG. 17  for comparison. 
         FIGS. 23  (A)-(H) show four 2-d waveguide structures with a 45.0° etched facet with an etched base, each with a different width for the angle-etched slit, and far-fields corresponding to these structures with comparisons with the far-field from  FIG. 17 . 
         FIGS. 24(A)  and (B) show a 2-d waveguide structure with a 44.4° etched facet with a width of 1 μm for the angle-etched slit and an etched base, the far-field corresponding to this structure, and the far-field from  FIG. 17  for comparison. 
         FIGS. 25(A)  and (B) shows a 2-d waveguide structure with a 45.0° etched facet with a width of 1 μm for the angle-etched slit and an etched base; a vertical-etched slit with a width of 10 μm and etched base, the far-field corresponding to this structure, and the far-field from  FIG. 17  for comparison. 
         FIG. 26  shows the far-field corresponding to a 2-d waveguide structure with a 44.4° etched facet with a width of 1 μm for the angle-etched slit and an etched base; a vertical-etched slit with a width of 10 μm and etched base, the far-field corresponding to this structure, and the far-field from  FIG. 17  for comparison. 
       FIGS.  27 (A)-(C) show the far-fields corresponding to a 2-d waveguide structures with a 45.0° etched facet with an etched slit widths of 1, 4, and 8 μm for the angle-etched slit and the ideal far-field for the 1-d structure for comparison, where a 1310 nm device has a far-field angle of 35°. 
       FIGS.  28 (A)-(C) show the far-fields corresponding to a 2-d waveguide structures with a 45.0° etched facet with an etched slit widths of 1, 4, and 8 μm for the angle-etched slit and the ideal far-field for the 1-d structure for comparison, where a 1310 nm device has a far-field angle of 25°. 
       FIGS.  29 (A)-(C) show the far-fields corresponding to a 2-d waveguide structures with a 45.0° etched facet with an etched slit widths of 1, 4, and 8 μm for the angle-etched slit and the ideal far-field for the 1-d structure for comparison, where an 830 nm device has a far-field angle of 23°. 
       FIGS.  30 (A)-(F) show three 2-d waveguide structures with a 45.0° etched facet with rounding at the etched base, each with a different width for the angle-etched slit, and far-fields corresponding to these structures with comparisons with the far-field from  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to a more detailed description of an exemplary embodiment, a surface emitting semiconductor laser  10  fabricated on a substrate  12  is illustrated diagrammatically in  FIGS. 1-3 . Although a ridge laser is described, it will be understood that other types of lasers may be fabricated utilizing the features described herein. For example, the laser structure can also be a buried heterostructure (BH) laser. The type of laser can be a Fabry Perot (FP) laser or a distributed feedback (DFB) laser. 
     As is conventional in the fabrication of solid state ridge lasers, the substrate  12  may be formed, for example, of a type III-V compound, or an alloy thereof, which may be suitably doped. The substrate includes a top surface  14  on which is deposited, as by an epitaxial deposition such as Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE), a succession of layers generally indicated at  16  which form an optical cavity  18  that includes an active region  20 . A horizontal cavity semiconductor laser structure such as the optical cavity  18  typically contains upper and lower cladding regions  19  and  19 ′, formed from lower index semiconductor material than the active region  20 , such as InP, adjacent the active region  20 , which may be formed with InAlGaAs-based quantum wells and barriers. A transition layer  21  of InGaAsP is formed on the top surface of cladding region  19 . 
     An angled facet  22  is formed at a first, or emitter end  24  of the cavity  18  by a masking and etching process in which the facet is etched downwardly and inwardly at or near a 45° angle with respect to the surface  14 . This facet is angled to cause light generated in the optical cavity to be emitted in a direction that is essentially perpendicular, or close to perpendicular, to the plane of the active region  20  and to the surface  14 . Stated differently, the normal of the angled facet is about 45° relative to the normal of the surface and about 45° relative to the longitudinal axis of the optical cavity  18 . The emitter end facet  22  is substantially totally internally reflective so that light propagating along the longitudinal axis of the optical cavity  18  is reflected in a direction perpendicular to this axis, and this travels vertically upwardly in the direction of arrow  26 , as viewed in the Figs. 
     At a second, or reflective end of the optical cavity, generally indicated at  28 , an end facet  30  is formed at an angle of 90° with respect to the longitudinal axis of the cavity, and thus substantially perpendicular to the active region  20  of the laser. In addition, a distributed Bragg reflector (DBR) element  32  and a monitoring photo detector (MPD)  34  are formed at end  28 , facet  30  and elements  32  and  34  being formed through masking and etching in known manner. A ridge  36  extending between emitter end  24  and reflective end  28  is formed by masking and etching the optical cavity  18  above the active region  20  to form the ridge-type laser  10 . At the emitter end  24 , the ridge  36  is widened, or tapered outwardly, as at edges  38  and  40 , to provide an open area  41  above the facet  22  to allow the beam  26  to emerge through the top surface  42  of the optical cavity  18  without distortion. 
     The back of the MPD portion  34  which is the left-hand end as viewed in  FIGS. 1-3 , is etched to form an exit facet  44 . A line  45  perpendicular to the surface of facet  44  forms an angle  46  with respect to the longitudinal axis of the optical cavity  18  ( FIG. 3 ) at or near the Brewster angle for the material from which the laser  10  is fabricated, so that facet  44  has zero or near-zero reflectivity for light generated in cavity  18 . Some of the laser light generated in the optical cavity  18  and propagating longitudinally is emitted at facet  30 , passes through the Bragg reflector  32 , and is received by MPD  34  which monitors the operation of the laser. A portion of this light reaches facet  44 , but is dissipated at that facet because of its zero or near-zero reflectivity, and this prevents undesirable back reflection to the laser. 
     A top electrical contact layer  48  on the top surface  42  of the ridge  36  is typically a low bandgap semiconductor, such as InGaAs, that allows ohmic contacts to be formed with a metal layer applied to it. The transition layer  21  typically is a semiconductor having a bandgap that is between that of the upper cladding layer  19  and that of the contact layer  48 , and in some cases may have a variable bandgap. The contact layer and the transition layer may absorb the light generated in the laser. For example, if an optical cavity  18  having the materials described above generates laser light with 1310 nm in wavelength, the InGaAs contact layer  48  will absorb this light after it is reflected upward from the 45° total internal reflection facet illustrated at  22 . Additionally, if the bandgap of the InGaAsP transition layer  21  is smaller than about 0.95 eV, corresponding to a wavelength of 1310 nm, then the transitional layer will also lead to absorption. Removal of any absorbing layers is, therefore, important to the efficient and reliable operation of the laser. This is accomplished, as illustrated in  FIG. 1 , by providing an aperture  52 , in accordance with the first embodiment. On the other hand, if the laser wavelength is 980 nm and the contact layer is GaAs, there is no need to remove the GaAs contact layer, since it is transparent at that wavelength, but if the lasing wavelength is 830 nm, then removal of the GaAs contact layer would be desirable. The aperture  52  is formed in contact layer  48  by a patterning and etching process, with the opening being located at the open area  41  of the ridge at emitter end portion  24 . This aperture allows light to be emitted from the laser cavity, as described above. It is noted that the beam will normally have a circular or elliptical shape. 
     The top electrode is deposited on contact layer  48  on the laser and MPD, and a second electrode  54  is deposited on a bottom surface  56  of the substrate, so that a bias voltage can be applied across the ridge  36  between the electrodes to produce lasing. A zero or negative bias can also be applied across the MPD to allow it to generate an electrical current based on the light that impinges upon it. Laser light propagating in the optical cavity  18  will be reflected by facet  22  to exit vertically at first end  24 , as indicated by arrow  26 , and some light will exit horizontally, in the plane of the active region  20 , through the facet  30  at second end  28 . Some of the light exiting through facet  30  will be reflected back into the cavity by the DBR reflector  32  and some will pass through reflector  32  to impinge on the front surface  58  of the MPD  34 , where it will be detected. Light which passes through the MPD will be dissipated by facet  44 , as indicated by arrow  60  ( FIG. 3 ) at the back of the MPD. The monolithically fabricated MPD  34  is not limited to monitoring the operation of the laser, as by measuring its intensity in this configuration, for if desired the MPD can also be used as an extremely fast detector to provide feedback to a circuit that drives the laser. 
     A laser cavity can be optimized by using reflectivity modification coatings. In conventional cleaved-facet lasers, one facet may have a high reflectivity coating while the other facet may be coated to lower reflectivity, for example 90% and 10% reflectivity, respectively, so that most of the laser light emerges from the lower reflectivity facet. In short cavities both facets may have high reflectivity to reduce the cavity round-trip loss, but typically one facet will have a lower reflectivity than the other, for example 99.9% and 99.0% nominal reflectivity, respectively, to allow most of the laser light to emerge from the lower reflectivity facet. In a second embodiment, illustrated in  FIG. 4 , the laser  10  is fabricated in the manner described above, with common elements having the same reference numerals. However, in this case a dielectric layer or stack  70  is deposited on the open area  41  at the first end  24  of the ridge  36  so that it modifies the reflectivity that emitted beam  26  experiences. In addition, as illustrated in  FIG. 5 , the facet  30  at the reflector end  28  of the optical cavity  18  may incorporate an optical layer or stack  72  instead of the Bragg reflector  32 . The use of very high reflectivity coatings at both ends of a very short cavity of below around 5 μm can produce single mode behavior due to the large longitudinal mode spacing of a very short cavity. Modifications in reflectivity can be used to optimize the performance of the laser cavity. 
     Instead of having the back end facet  30  of the laser cavity  18  be a vertical facet, that facet can also be etched at a 45-degree angle as illustrated in  FIG. 6 . In this figure, a laser cavity  80 , fabricated as described above, is etched at both ends to provide angled facets  82  and  84 . This type of laser provides horizontal surfaces for corresponding reflective coatings  86  and  88  formed over apertures  90  and  92 , respectively. The illustrated structures can emit light that is perpendicular to the substrate at both the back facet  84  and the front facet  82 , with the apertures being provided to avoid absorption in the contact layer and transition layers. 
     Single longitudinal mode lasers are more desirable than multi-longitudinal mode lasers in many applications. One such application is in data communications where longer reaches of communications are obtained with a single longitudinal mode lasers compared to a multi-longitudinal laser.  FIG. 7  illustrates an embodiment wherein a single longitudinal mode surface emitting semiconductor laser  100  is fabricated on a top surface  112  of a substrate  114 . As described above for laser  10 , a succession of layers  116  forms an optical cavity  118  that includes an active region (not shown) fabricated as described above. An angled facet  122  is formed at a first end  120  through masking and etching downwardly and inwardly at or near a 45° angle with respect to surface  112 . The facet is substantially totally internally reflective so that the laser emits an essentially vertical or close to vertical output beam  126 . At the second end  128  of the optical cavity, a vertical end facet  130 , which is perpendicular to the active layer of the laser, multiple filtering elements  132 , a distributed Bragg reflector (DBR) element  134 , and a monitoring photo detector (MPD)  136  are formed along the optical axis of cavity  118  through masking and etching. An elongated ridge  140  is formed from the cavity  118  by a masking and etching process. 
     At the emitter end  120  of the laser, the ridge  140  is enlarged outwardly, as illustrated by side walls  142  and  144 , to form an open area  145  to allow the beam  126  to be emitted through the surface of the first end without distortion, as described above with respect to  FIG. 1 . At the second end  128 , the back of the MPD portion  136  is etched to form an exit facet  146  which designed to form an angle at or near the Brewster angle for the laser material, so as to have zero or near-zero reflectivity. After passing through filtering elements  132  and DBR element  134 , some of the laser light generated in optical cavity  118  is received by MPD  136 , which then provides a measure of the operation of the laser. Any light that reaches facet  146  is dissipated because of its zero or near-zero reflectivity to prevent undesirable back reflection to the laser. 
     After the etching steps described above, a top electrical contact layer (not shown) such as that described with respect to  FIG. 1  is formed on the top surface of the ridge and on the MPD, and this layer is patterned so as to provide an opening  148  in the contact layer in the open area  145 . This opening is located over the facet  122  at end portion  120  to permit light generated in the laser cavity to be emitted in a circular or elliptical shape, as beam  126 . 
     A second electrical contact layer (not shown) is deposited on the bottom surface of the substrate, so that a bias voltage can be applied across the ridge to produce lasing and a zero or negative bias can be applied across the MPD to allow it to generate an electrical current based on the light that impinges upon it. The laser light so produced in the optical cavity will exit vertically at first end  120 , as indicated by arrow  126 , and longitudinally at second end  128 , where some light will be transmitted through the facet  130 , through filters  132 , and through the DBR element  134 , and will impinge on the front end  150  of the MPD  136  to be detected by the MPD and then dissipated at the back facet  146  of the MPD. 
     As is the case with the device of  FIGS. 1-3 , the single longitudinal mode device  100  of  FIG. 7  can have a dielectric layer or stack (not shown) deposited at the first emitter end  120  of the ridge, in the manner illustrated in  FIG. 4 , so that it modifies the reflectivity of the emitter end. 
     Although single DBR elements  32  and  134  are illustrated in the embodiments of  FIGS. 1 and 7 , respectively, it will be understood that multiple DBR elements could also be used to obtain higher reflectivity at the second ends  28  and  128 , respectively. The DBR elements can take the form of element  32  in  FIG. 1  where the DBR is not patterned during the ridge etch so that it does not acquire the ridge configuration, or can take the form of element  134  in  FIG. 5  where the element includes the ridge shape. Furthermore, it will be understood that the DRB element(s) can be replaced by dielectric reflectivity modification layer or stack. 
     In modern systems, it is highly desirable to have a transmitter of light and a detector of light side-by-side on a single substrate, or chip. Having such a combination is even more desirable if the devices are made out of the same material. Accordingly, in the embodiment illustrated in  FIGS. 8 and 9 , a surface emitting, or vertically emitting, laser  158 , which may be a laser such as the laser  10  of  FIG. 1 , is combined with a detector  160  to provide both a light emitter and a light detector on a common substrate, such as the substrate  12  of  FIG. 1 . The surface-emitting laser  158  is similar to that of  FIG. 1  for purposes of illustration, and common features are similarly numbered, but it will be apparent that variations of the surface emitter can be used.  FIG. 9  is a cross-sectional view taken along line  9 - 9  of  FIG. 8  to illustrate the structure of detector  160 . For the sake of clarity, in  FIG. 9  the detector is shown to have a height smaller than the laser, but this is not a requirement. 
     Area detector  160  is located adjacent to surface emitting laser  158 , as illustrated, and is fabricated from the same layers  16  as were deposited on the substrate to form the optical cavity. The detector is masked and etched in these layers during the masking and etching steps used for forming the second end  28  of the laser, which steps include formation of the vertical end facet  30  (which is perpendicular to the active layer of the laser), the distributed Bragg reflector (DBR) element  32 , and the monitoring photo detector (MPD)  34 . 
     The area detector  160 , in the illustrated configuration, may be generally rectangular with a top surface  162  that receives an impinging beam  164  within a detection area  166 , and uses the same active layer  20  as the one used in the laser  10 . A top electrical contact  168  is applied on the top surface  162  of the detector, while leaving the area of detection  166  free of this contact. A bottom contact  170  is also applied to the back of the substrate  12  and a negative or zero bias is applied between the top and the bottom contacts  168  and  170  to allow an incoming beam  164  to be detected by the detector. 
     In another embodiment, illustrated in  FIGS. 10 and 11 , a surface-emitting laser  176 , which may be similar to laser  10  of  FIG. 1  for purposes of illustration, is combined with an in-plane detector  180  on a substrate  178 . Features in common with the surface-emitting laser  10  of  FIGS. 1-3  are similarly numbered, with  FIG. 11  being a cross-section taken along lines  11 - 11  of  FIG. 10 . For clarity, the detector  180  is shown to have a height smaller than the laser  176  in  FIG. 11 . 
     The in-plane detector  180  is located adjacent and generally parallel to the surface-emitting laser  176 . Detector  180  incorporates an elongated body portion  182  having a longitudinal axis that is illustrated as being parallel to the axis of the optical cavity  18  of laser  10 ; however, it will be understood that these axes need not be parallel. The detector body is fabricated in the deposited layers  16  from which the laser optical cavity is formed, using the same masking and etching steps. A reflective input facet  184  is formed at a first, input end  186  of the detector, with facet  184  being etched at or near a 45° angle with respect to the surface of substrate  178  during the formation of facet  22  on laser  10 . The body portion  182  and a back facet  188  are formed during the masking and etching steps used to form the second, or reflector, end  28 , the vertical end facet  30 , the distributed Bragg reflector (DBR) element  32 , and the monitoring photo detector (MPD)  34  of laser  176 . Although the detector back facet  188  is shown as being perpendicular to the plane of the active layer  20  of the deposited material, it will be understood that this facet can be etched at an angle other than the perpendicular. 
     The in-plane detector  180  includes a top surface region  200  for receiving an impinging light beam  202  to be detected ( FIG. 11 ), at the same active layer  20  as the one used in the laser. A top electrically conductive contact  204  is applied on the top surface  206  of the detector  180 , with an aperture being formed in the contact in the area of detection  200 , so that the impinging light is not blocked. A bottom electrically conductive contact  208  is applied to the back of the substrate  12  in the region of the detector, and a negative or zero bias is applied between the top and the bottom contacts. An incoming beam  202  enters the detector through its top surface in the region  200 , and is reflected by internally reflective facet  184  to be directed longitudinally along the axis of the detector active layer  20 , as illustrated by arrow  210 , for detection in known manner. 
     The reflectivity of areas  166  ( FIG. 8) and 200  ( FIG. 10 ) can be modified by depositing a dielectric layer or stack on these areas to provide antireflection surfaces for incoming beams  164  and  202 , respectively. This would allow more efficient collection of the light by the detector. 
     It will be understood that multiple lasers and/or detectors such as those described above can be fabricated on a single substrate in the form of an array, to thereby enable applications such as parallel optical interconnects, wavelength selectivity, and the like. For example, multiple lasers of different wavelengths such as the array  218  illustrated in  FIGS. 12 and 13  can be provided on the same chip or substrate, and can be positioned to direct their outputs into a single output medium such as, for example, a fiber. Thus, the array  218  of lasers may be configured to extend radially from a common center or hub  219  with four lasers  220 ,  222 ,  224  and  226  of the kind illustrated at  100  in  FIG. 7  being positioned on a common substrate  228  in such a way that their respective output ends,  230 ,  232 ,  234  and  236  are clustered in close proximity to one another and around a central axis  240 , with the second ends of the lasers extending radially outwardly from the hub. The output beams from the lasers are emitted vertically upwardly, in a direction perpendicular or close to perpendicular to the surface of substrate  228  and parallel to axis  240 . By providing each of the four lasers with a different bandgap, each laser produces an output beam having a different wavelength, so that the array  218  produces an output along axis  240  of a selected wavelength or combination of wavelengths that may then be directed to a common output device such as an optical fiber  242 . Although four lasers are illustrated, it will be understood that this is for purposes of illustration, and that other numbers of lasers may be used. The bandgaps of each laser may be selected through a process such as impurity-free vacancy diffusion or regrowth, with such techniques being well known in the field. 
     The output ends  230 ,  232 ,  234  and  236  of the four lasers each include an angled facet, and these are formed in the same masking step, but with four separate etching steps. A slight deviation from a 45° angle etch in each of the etching steps can be used to guide the four beams slightly away from the perpendicular so that they impinge on the centrally located object, such as the fiber  242 . The back facets, filtering elements, and the MPDs for the four lasers are formed through a common masking and etching step. Finally, the ridge structure is formed through masking and etching, and the devices are metallized on the top and the bottom surfaces to provide electrical contacts, as described above. 
     The radial array  218  of the lasers is possible because the CAIBE (chemically assisted ion beam etching) process that is used in fabricating the lasers provides a uniform etch that does not depend on the crystallographic planes of the semiconductor crystal. This allows surface-emitting lasers to be positioned in any desired configuration on the substrate, as illustrated in  FIG. 12  and as further illustrated in  FIG. 14 , wherein a semiconductor laser  250  is positioned diagonally on a rectangular substrate  252 . Conventional methods, using cleaving to form facets for example, do not permit such positioning. 
       FIG. 15  illustrates a cross-section of a waveguide structure  300 , such as a laser, with an angled slit  302  having an etched base  311  etched into the waveguide. The waveguide structure  300  comprises a substrate  304 , a lower cladding  306 , an active region that forms the core  308 , an upper cladding layer  310 , and a contact layer  312 . These layers are epitaxially deposited as described above. The angled etched slit  302  forms the angled etch facet  314  at the waveguide with an angle HA with respect to the longitudinal axis of the waveguide, as illustrated in  FIG. 15 . The width of this angled slit is W 1 . The process used to form the structure of  FIG. 15  requires an angled etch that can be performed in CAIBE. 
       FIG. 16  illustrates a cross-section of a waveguide structure  320 , such as a laser, with an angled slit  322  as well as a vertical slit  324  etched into the waveguide. The waveguide structure comprises a substrate  326 , a lower cladding  328 , an active region that forms the core  330 , an upper cladding layer  332 , and a contact layer  334 . These layers are epitaxially deposited as described above. The angled etched slit  322  forms the angled etch facet  336  at the waveguide with an angle HA with respect to the longitudinal axis of the waveguide, as illustrated in  FIG. 16 . The width of the angled and vertical slits are W 2  and W 3 , respectively. The process used to form the structure of  FIG. 16  requires an angled etch as well as a vertical etch, both of which can be performed using CAIBE. 
       FIG. 17  shows the far-field corresponding to a one-dimensional solution in RSoft to an exemplary InP-based 1310 nm emitting epitaxial laser structure and will be referred to as the “ideal” far-field.  FIG. 17  shows the case of a 1310 nm laser structure that has a 47° far-field, where the far-field angle is defined as the full-width half-maximum (FWHM) of the far-field. This epitaxial structure is based the following layers on an InP substrate: 0.5 μm n-InP (this layer and the substrate acting as the lower cladding); 0.18 μm AlGalnAs lower graded region; an active region containing fourteen 6 nm thick compressively strained AlGalnAs quantum wells, each sandwiched by 10 nm tensile strained AlGalnAs barriers; 0.18 μm AlGalnAs upper graded region; 1.65 μm thick p-InP upper cladding; and highly p-doped InGaAs cap layer. 
       FIG. 18(A)  shows the refractive index profile of a vertically etched facet  410  in the InP-based 1310 nm emitting epitaxial laser structure. Simulations including all the layers of the epitaxial laser structure show that the structure can be approximated well by using a core  402  that has a refractive index or index of about 3.415 and a thickness of 0.31 μm, the upper and lower cladding layers  404  and  400  that have an index of about 3.2, and air has an index of approximately 1.0. A two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 18(B)  with a solid line. The far-field from  FIG. 17  is shown in  FIG. 18(B)  with a dashed line for comparison. Although there is a slight change in shape, there is no beam pointing or ripples in the far-field pattern. The beam pointing is defined as the angle at which the maximum intensity point in the far-field deviates from 0° and here both far-fields show a maximum intensity at 0°, and, hence, no beam pointing. 
       FIG. 19(A)  shows the index profile of an etched facet  412  with HA=45° in the InP-based 1310 nm emitting epitaxial laser structure having lower and upper cladding layers  400  and  404  and a core  402 . The index values are the same as those presented in connection with the device shown in  FIG. 18(A) . A two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 19(B)  with a solid line. The far-field from  FIG. 17  is shown in  FIG. 19(B)  with a dashed line for comparison. The far-field pattern shows a beam pointing of around −2.5° and small ripples have appears in the far-field pattern. 
       FIG. 20(A)  shows the index profile of a more realistic etched facet  412  with HA=45° in the InP-based 1310 nm emitting epitaxial laser structure having lower and upper cladding layers  400  and  404 , and a core  402 . The index values are the same as those presented in connection with the device shown in  FIG. 18(A) . This structure now shows the etched based  414  for the etched facet  412 . A two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 20(B)  with a solid line. The far-field from  FIG. 17  is shown in  FIG. 20(B)  with a dashed line for comparison. The far-field pattern shows a beam pointing of around −5°. The ripples in the far-field have increased over that of the solid curve in  FIG. 19(B)  and the far-field pattern is more distorted over the ideal far-field pattern. 
     To decrease the beam pointing in the far-field, the angle HA was reduced from 45.0° to 44.0° in the structure of  FIG. 20(A) . The two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 21  with a solid line. The far-field from  FIG. 17  is shown in  FIG. 21  with a dashed line for comparison. The far-field pattern shows almost zero beam pointing, although the intensity in the far-field is not evenly distributed about 0°. 
     To balance the intensity about the 0°, the angle HA was changed from 45.0° to 44.4° in the structure of  FIG. 20(A) . The two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 22  with a solid line. The far-field from  FIG. 17  is shown in  FIG. 22  with a dashed line for comparison. The far-field pattern shows slight beam pointing, although the intensity in the far-field is now more evenly distributed about 0°. However, the angle HA of the slit with respect to the substrate and a longitudinal axis of the waveguide in the range of 44.4°±1° give rise to reasonable beam pointing. 
       FIGS. 23(A) , (C), (E) and (G) show four 2-d waveguide structures with angled etched facets; etched bases; and HA=45.0°, each with a different width for the angle-etched slit. This is the type of structure illustrated in  FIG. 15 . Two-dimensional solution for each of the four structures were obtained through simulations using RSoft Fullwave software and resulted in the far-fields shown in  FIGS. 23(B) , (D), (F) and (H) with a solid line. The far-field from  FIG. 17  is shown  FIGS. 23(B) , (D), (F) and (H) with a dashed line for comparison. The far-field pattern corresponding to the 1μm slit shows no noticeable ripples and a beam pointing angle of around −5°. The far-fields corresponding to 2, 4, and 16 μm slits show increasing ripples with slit width. 
       FIG. 24(A)  shows a 2-d waveguide structure with an angled etched facet; an etched base; and HA=44.4°, with a width of 1 μm for the angled-etched slit. This is also the type of structure illustrated in  FIG. 15 . The two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 24(B)  with a solid line. The far-field from  FIG. 17  is shown  FIG. 24(B)  with a dashed line for comparison. The far-field doesn&#39;t have any noticeable ripples and a beam pointing angle at or around 0°. 
       FIG. 25(A)  shows a 2-d waveguide structure with an angled etched facet; an etched base; and HA=45.0°. The structure is of the type illustrated in  FIG. 16  where both an angled etch and a vertical etch are used to create the structure. The angled etch is performed first and the width is 5 μm for the angled-etched slit. The vertical etched is performed next and the width is 10 μm for the vertical-etched slit. The two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 25(B)  with a solid line. The far-field from  FIG. 17  is shown  FIG. 25(B)  with a dashed line for comparison. The far-field has some ripples and a beam pointing angle is around −4°. 
     To decrease the beam pointing in the far-field, the angle HA was reduced from 45.0° to 44.0° in the structure of  FIG. 25(A) . The two-dimensional solution was obtained for this structure through simulation using RSoft Fullwave software and resulted in the far-field shown in  FIG. 26  with a solid line. The far-field from  FIG. 17  is shown in  FIG. 26  with a dashed line for comparison. The far-field pattern shows slight beam pointing and the intensity in the far-field is evenly distributed about 0°. 
     By using the 1310 nm emitting epitaxial structure with the following layers on an InP substrate: 0.5 μm n-InP; 0.105 μm AlGalnAs lower graded region; an active region containing five 6 nm thick compressively strained AlGalnAs quantum wells, each sandwiched by 10 nm tensile strained AlGalnAs barriers; 0.105 μm AlGalnAs upper graded region; 1.65 μm thick p-InP upper cladding; and highly p-doped InGaAs cap layer, the far-field was angle to 35°. This is shown as the ideal far-field, corresponding to the 1-d structure, in  FIG. 27 . Simulations including all the layers of the epitaxial laser structure show that the structure can be approximated well by using a core with an index of about 3.325 and a thickness of 0.34 μm, and upper and lower cladding layers of an index of about 3.2. Further reduction in the far-field angle to 25° is obtained by using the 1310 nm emitting epitaxial structure with the following layers on an InP substrate: 0.5 μm n-InP; 0.045 μm AlGalnAs lower graded region; an active region containing three 6 nm thick compressively strained AlGalnAs quantum wells, each sandwiched by 10 nm tensile strained AlGalnAs barriers; 0.045 μm AlGalnAs upper graded region; 2.45 μm thick p-InP upper cladding; and highly p-doped InGaAs cap layer, as shown in  FIG. 28  for the ideal far-field, corresponding to the 1-d structure. Simulations including all the layers of the epitaxial laser structure show that the structure can be approximated well by using a core with an index of about 3.207 and a thickness of 0.34 μm, and upper and lower cladding layers of an index of about 3.2. Comparing  FIGS. 23 ,  27 , and  28 , the 2-d far-field shows less ripples and structure for smaller far-field angles for a given slit width. 
       FIG. 29  shows the case of an 830 nm emitting epitaxial structure is based the following layers on a GaAs substrate: 2.0 μm n-Al 0.4 Ga 0.6 As lower cladding; 0.1 μm Al 0.25 Ga 0.75 As to Al 0.4 Ga 0.6 As lower graded region; an active region containing a single 7.5 nm thick GaAs quantum well; 0.1 μm Al 0.4 Ga 0.6 As to Al 0.25 Ga 0.75 As upper graded region; 2.0 μm thick p-Al 0.4 Ga 0.6 As upper cladding; and highly p-doped GaAs cap layer, with a far-field angle of 23° for the corresponding 1-d structure. Simulations including all the layers of the epitaxial laser structure show that the structure can be approximated well by using a core with an index of about 3.41223 and a thickness of 0.18 μm, and upper and lower cladding layers of an index of about 3.34. The 2-d far-field has small structure and ripples even at 8 μm slit. 
     In some cases, the etched base of a slit may be rounded, as shown in the index profiles for the 2-d waveguide structures in  FIG. 30 , based on the same epitaxial structure used for  FIG. 23 . The dashed lines show the 2-d far-fields for various slit widths are also shown in  FIG. 30  and shows minimal differences with the far-field shapes observed in  FIG. 23 . 
     Far-field measurements from fabricated devices show results consistent with the simulations. 
     Generally, ripples are undesirable in the far-field pattern for most applications. The control in the beam pointing angle is important in many applications. Some applications benefit from a beam pointing angle since it allows coupling to the fiber with elimination of reflection from the fiber from coupling back into the fiber. However, in other situations zero beam pointing gives the best results. 
     Although the present invention has been illustrated in terms of various embodiments, it will be understood that variations and modifications may be made without departing from the true spirit and scope thereof as set out in the following claims.