Abstract:
This disclosure is generally concerned with optical systems that employ guided-mode grating resonant reflector filters (“GMGRF”) to facilitate wavelength and/or polarization selectivity in the optical system. In one example, an optical system is provided that includes first and second tunable detectors. Each of the tunable detectors includes a GMGRF that is tuned to select a corresponding optical wavelength and/or polarization of an optical data channel, such that the optical wavelength and/or polarization associated with the first tunable detector is different from the optical wavelength and/or polarization associated with the second tunable detector. In this way, an array of tunable detectors can be employed to select some or all of the wavelengths and/or polarizations of an optical data signal having a plurality of data channels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to the field of optoelectronic devices, and more particularly to resonant reflectors for use with optoelectronic devices.  
         [0002]     Various forms of optoelectronic devices have been developed and have found widespread use including, for example, semiconductor lasers, semiconductor photodiodes, semiconductor photo detectors, etc. For some of these applications, an optoelectronic emitter such as a semiconductor laser is coupled to an optoelectronic detector (e.g., photodiode or Resonant Cavity Photo Detector) through a fiber optic link or even free space. This configuration can provide a high-speed communication path, which, for many applications, can be extremely beneficial.  
         [0003]     The increased use of all-optical fiber networks as backbones for global communication systems has been based in large part on the extremely wide optical transmission bandwidth provided by optical fiber. This has led to an increased demand for the practical utilization of the optical fiber bandwidth, which can provide, for example, increase communication system user capacity. In the prevailing manner for exploiting optical fiber bandwidth, wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) techniques are used to enable the simultaneous transmission of multiple independent optical data streams, each at a distinct wavelength, on a single optical fiber, with wavelength-selective WDM and WDD control provided for coupling of the multiple data streams with the optical fiber on a wavelength-specific basis. With this capability, a single optical fiber can be configured to simultaneously transmit several optical data streams, e.g., ten optical data streams, that each might not exceed, say, 10 Gb/s, but that together represent an aggregate optical fiber transmission bandwidth of more than, say, 100 Gb/s.  
         [0004]     In order to increase the aggregate transmission bandwidth of an optical fiber, it is generally preferred that the wavelength spacing of simultaneously transmitted optical data streams, or optical data “channels,” be closely packed to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized. The desire for closely-spaced optical transmission channels results in the need for fine wavelength resolution, which complicates the wavelength-selective WDM and WDD operations required for simultaneous transmission of the channels. Like WDM, Polarization Division Multiplexing (PDM) can also be used to extend the bandwidth of some optical data channels.  
       BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION  
       [0005]     In general, exemplary embodiments of the invention are concerned with optical systems that employ guided-mode grating resonant reflector filter (“GMGRF”) to facilitate wavelength and/or polarization selectivity in the optical system. In one implementation, an optical system is provided that includes a first tunable detector and a second tunable detector. Each of the tunable detectors includes a GMGRF that is tuned to select a corresponding optical wavelength and/or polarization of an optical data channel. Typically, the optical wavelength and/or polarization of the optical data channel is different as between the first and second tunable detectors. Thus, in some implementations, an array of tunable detectors is employed to collectively select each of the wavelengths and/or polarizations of an optical data signal having a plurality of data channels.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:  
         [0007]      FIG. 1  is a schematic diagram of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser in accordance with the prior art;  
         [0008]      FIG. 2  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with an illustrative resonant reflector;  
         [0009]      FIG. 3  is a graph showing the reflectivity versus wavelength of the resonant reflector of  FIG. 2 , both with a non-conductive (k=0) waveguide layer and a slightly conductive (k=10 −5 ) waveguide layer;  
         [0010]      FIG. 4  is a graph showing the reflectance versus wavelength of an all non-conductive (k=0) resonant reflector placed adjacent a top mirror that is also non-conductive (k=0);  
         [0011]      FIG. 5  shows the reflectance versus wavelength of an all non-conductive resonant reflector placed adjacent a top mirror that is slightly conductive (k=10 −5 );  
         [0012]      FIG. 6  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with a cladding or buffer layer interposed between the waveguide layer of the resonant reflector and the top DBR mirror;  
         [0013]      FIG. 7  is a graph showing the reflectance versus wavelength of the resonant reflector of  FIG. 6  when the top layers of the top DBR mirror  86  are non-conductive (k=0);  
         [0014]      FIG. 8  is a graph showing the reflectance versus wavelength of the resonant reflector of  FIG. 6  when the top layers of the top DBR mirror  86  are slightly conductive (k=10 −5 );  
         [0015]      FIG. 9  is a schematic cross-sectional side view of a top surface emitting vertical cavity laser similar to that shown in  FIG. 6 , but with a modified grating fill factor;  
         [0016]      FIG. 10  is a schematic cross-sectional side view of a top surface emitting vertical cavity laser similar to that shown in  FIG. 6 , but with a grating film that has a controlled etch depth;  
         [0017]      FIG. 11  is a schematic cross-sectional side view of a vertical cavity surface emitting laser that is formed by bonding a resonant reflector that was prepared on a first substrate to the top mirror of the vertical cavity surface emitting laser that was prepared on a second substrate;  
         [0018]      FIG. 12  is a schematic cross-sectional side view of the vertical cavity surface emitting laser of  FIG. 11  with a microlens positioned on the backside of the substrate that has the resonant reflector formed thereon;  
         [0019]      FIG. 13  is a schematic cross-sectional side view of an illustrative monolithic substrate having a RCPD, a VCSEL and a MSM;  
         [0020]      FIG. 14  is a schematic diagram of a number of illustrative optoelectronic emitters that have GMRGF filters for data/telecommunication, processing, switching, etc., including embodiments conducive for wavelength division multiplexing, polarization division multiplexing, and space division multiplexing;  
         [0021]      FIG. 15  is a graph showing reflectance versus wavelength for the three illustrative GMRGF filters of  FIG. 14  that have a common polarization direction;  
         [0022]      FIG. 16  is a schematic diagram showing an illustrative array of four VCSELs having a common epitaxial structure including a bottom mirror, an active region, a top mirror and a top GMGRF, where the grating period of each GMGRF is different for each VCSEL;  
         [0023]      FIG. 17  is a schematic diagram showing an illustrative WDMIVDD application using the array of four VCSELs of  FIG. 16 ;  
         [0024]     FIG. WDM/WDD is a schematic diagram showing an illustrative GMGRF filter in accordance with the present invention;  
         [0025]      FIG. 19  is a graph showing the relationship between grating period (Λ) and the resonant wavelength for the GMGRF shown in  FIG. 18 ; and  
         [0026]      FIG. 20  is a graph showing the relationship between the grating height  420 /core depth  418  and the resonant wavelength for the GMGRF shown in  FIG. 18 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]      FIG. 1  is a schematic illustration of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser  10  in accordance with the prior art. Formed on an n-doped gallium arsenide (GaAs) substrate  14  is a n-contact  12 . Substrate  14  is doped with impurities of a first type (i.e., n type). An n-type mirror stack  16  is formed on substrate  14 . Formed on stack  16  is a spacer  18 . Spacer  18  has a bottom confinement layer  20  and a top confinement layer  24  surrounding active region  22 . A p-type mirror stack  26  is formed on top confinement layer  24 . A p-metal layer  28  is formed on stack  26 . The emission region may have a passivation layer  30 .  
         [0028]     Isolation region  29  restricts the area of the current flow  27  through the active region. Region  29  may be formed by deep H+ ion implantation. While a deep H+ implant is provided as an illustration, it is contemplated that any type of current and field confinement may be used, including for example, gain-guided, oxide-confinement, or any other means. The diameter “g” may be set to provide the desired active area, and thus the gain aperture of the VCSEL  10 . Further, the diameter “g” may be set by the desired resistance of the p-type mirror stack  26 , particularly through the non-conductive region  29 . Thus, non-conductive region  29  performs the gain guiding function. The diameter “g” is typically limited by fabrication limitations, such as lateral straggle during the implantation step.  
         [0029]     Spacer  18  may contain a bulk or quantum-well active region disposed between mirror stacks  16  and  26 . Quantum-well active region  22  may have alternating layers of aluminum gallium arsenide (AlGaAs) barrier layers and GaAs quantum-well layers. InGaAs quantum wells may also be used in the active region, particularly where an emission wavelength (e.g., λ=980 nm) is desired where GaAs is transparent. Stacks  16  and  26  are distributed Bragg reflector (DBR) stacks, and may include periodic layers of doped AlGaAs and aluminum arsenide (AlAs). The AlGaAs of stack  16  is doped with the same type of impurity as substrate  14  (e.g., n type), and the AlGaAs of stack  26  is doped with the other kind of impurity (e.g., p type).  
         [0030]     Metal contact layers  12  and  28  are ohmic contacts that allow appropriate electrical biasing of laser diode  10 . When laser diode  10  is forward biased with a more positive voltage on contact  28  than on contact  12 , active region  22  emits light  31  which passes through stack  26 .  
         [0031]     A typical near IR VCSEL requires high reflectivity (&gt;99%). Thus, an all-semiconductor DBR typically requires 20-40 mirror periods with a thickness of 2-4 μm. As such, the epi-structure required for a complete VCSEL, including both top and bottom DBR mirrors surrounding an active spacer region typically includes over 200 layers having a thickness in excess of 7-8 μm.  
         [0032]     As discussed in U.S. patent application Ser. No. ______, entitled “Resonant Reflector For Improved Optoelectronic Device Performance And Enhanced Applicability”, a hybrid mirror structure may be used to reduce the overall mirror thickness.  FIG. 2  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with a hybrid top mirror including a resonant reflector  52  and a distributed Bragg reflector  56 . This device combines the anomalous filtering properties of guided mode resonance in a dielectric waveguide grating with the reflective properties of a conventional DBR mirror.  
         [0033]     The hybrid mirror structure includes, for example, a resonant reflector  52  and a DBR mirror  56 . Although not explicitly shown, it is contemplated that the bottom mirror may also include a resonant reflector structure, if desired. It is known that a dielectric resonant reflector  52  is highly reflective on resonance, and may be more reflective than a corresponding DBR type mirror at the same wavelength. Thus, by using a hybrid approach, it is contemplated that the number of DBR mirror periods needed for a given reflectance may be reduced.  
         [0034]     It is known that lateral straggle effects during ion implantation of the gain guiding region  62  through the DBR mirrors often limits the lateral dimension  64  of the active region to ≧10 μm. This directly impacts the minimum achievable threshold current, single mode operation, and indirectly impacts the speed of the VCSEL. By incorporating a resonant reflector into the top mirror, equivalent or superior reflectance properties in a structure five to ten times thinner may be achieved. This may translate into an ion implant that is more controllable, which may reduce the volume of the active region. A smaller active region may reduce the operating current and power of the device, improve planarity and thus the monolithic integrability of the VCSEL with electronics and smart pixels, and may provide a controllable single mode and single polarization emission with increased modal control.  
         [0035]     It is recognized that the hybrid approach of  FIG. 2  is compatible with alternate existing gain-guiding techniques including etched pillars (with or without planarization and/or regrowth), lateral oxidation, selective growth, etc. By decreasing the overall thickness of the VCSEL mirrors, the resonant reflector may improve the processibility and performance of the alternate current guiding approaches. While ion implantation is provided as an illustration, it is contemplated that any type of current and field confinement may be used, including for example, gain-guided, oxide-confinement, or any other means.  
         [0036]     The resonant reflector  52  of  FIG. 2  includes a three layer waveguide-grating structure suitable for use in a near IR VCSEL. The three-layer stack may be designed to function both as an anti-reflection (AR) coating near the emission wavelength for the VCSEL-structure substrate and independently as a guided-mode resonant reflector waveguide-grating (i.e. a guided-mode grating resonant reflector filter or GMGRF).  
         [0037]     The three layers of resonant reflector  52  may form an anti-reflective region, which provides little reflectance for at least a predetermined range of wavelengths including a resonant wavelength. The grating multilayer waveguide structure shown at  52  causes the structure to become substantially more reflective, at least at the resonant wavelength.  
         [0038]     Alternatively, the three-layer stack  52  may be designed to function both as a high-reflectivity coating for the VCSEL-structure substrate and independently as a guided-mode resonant reflector waveguide-grating. In this embodiment, the three-layer structure  52  forms a highly-reflective mirror region which provides reflectance for at least a predetermined range of wavelengths including a resonant wavelength (e.g., near 980 nm). The overall reflectance of the top mirror, including layers  66  and  68 , may be less than that required for lasing. This may be accomplished by, for example, reducing the number of mirror periods in the top DBR mirror  56 . Grating layer  58  causes the guided mode resonant reflector structure  52  to become substantially more reflective at least near the resonant wavelength. In either case, the number of DBR mirror layers beneath the resonant reflector  52  may be reduced relative to the conventional VCSEL construction shown in  FIG. 1 .  
         [0039]     Resonance is achieved in the resonance reflector  52  by matching the first-diffraction order wave vector of the grating  58  to the propagating mode of the waveguide  66 . Since the latter depends on polarization, the reflectance is inherently polarization-selective. The resonant wavelength is determined primarily by the grating period  60 , and the bandwidth is determined primarily by the modulation of the refractive index and fill factor of the grating  58 .  
         [0040]      FIG. 3  is a graph showing reflectance curves for a resonant reflector assuming two values for the imaginary component of the refractive index (k=0 and 10 −5 ) in any one layer of the waveguide-grating structure, and k=0 in the other two layers. In this example, the top layer, middle layer and bottom layer are formed from Indium Tin Oxide (ITO), GaAs, and AlGaAs, respectively. The refraction indices for the top, middle and bottom layers are 1.96, 3.5 and 3.24, respectively, and the thicknesses of the top, middle and bottom layers are preferably λ/4, 3λ/4 and λ/4, respectively. For this measurement, the layers are placed on a substrate with an effective reflective index of 3.2. This structure is simulated to exhibit one transverse Electric (TE) mode resonance (with a polarization parallel to the grating), no perpendicular resonance and a low out of resonant reflectance near 10 −6 .  
         [0041]     The imaginary component “k” of the refractive index is related to optical absorption and electrical conductivity of the resonant reflector. The case k=10 −5 , which roughly corresponds to the minimum conductivity required to inject current through the resonant reflector, produces about 5 percent absorption. The same three layers, all with k=0, indicating a dielectric resonant reflector, produces theoretically 100 percent reflectance.  
         [0042]     This graph illustrates the extreme sensitivity of the resonant reflector  52  to absorption, or more generally, to loss of any kind. Thus, to maximize the reflectance provided by the resonant reflector, the absorption (e.g. k=0) for each of the layers  58 ,  66  and  68  should be near zero. This means that the conductivity of the resonant reflector should also be zero (e.g., non-conductive).  
         [0043]     Despite the advantages of using a resonant reflector in conjunction with a DBR mirror stack, it has been found that the reflectivity of the resonant reflector can be limited if not properly isolated from the DBR mirror stack.  FIG. 4  is a graph showing the reflectance versus wavelength of an all non-conductive (k=0) resonant reflector placed adjacent a top mirror that is also non-conductive (k=0). The reflectance curve has a narrow bandwidth, and reaches about 100% reflectivity at the resonant wavelength. In contrast,  FIG. 5  shows the reflectance versus wavelength of an all non-conductive resonant reflector placed adjacent a top mirror that is slightly conductive (k=10 −5 ). As can be seen, having an adjacent top mirror that is slightly conductive significantly degrades the performance of the resonant reflector. Too much energy in the guided-mode in the waveguide overlaps into the lossy, conductive DBR films of the optoelectronic device.  
         [0044]     To overcome this and other difficulties, the present invention contemplates isolating the resonant reflector from adjacent conducting layers. Isolation is preferably accomplished by providing a non-conductive (e.g. dielectric) buffer or cladding layer between the resonant reflector and the adjacent conducting layer of the optoelectronic device. The non-conductive cladding or buffer layer is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide of the resonant reflector, to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the adjacent conductive layer of the optoelectronic device. In a preferred embodiment, the waveguide is formed from a dielectric that has a higher refractive index than the refractive index of the buffer or cladding layer, and also higher than the average refractive index of the grating. The thickness of the waveguide preferably depends on the refractive index difference between the waveguide and the buffer or cladding layer.  
         [0045]      FIG. 6  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with a cladding or buffer layer  80  interposed between the waveguide layer  82  of the resonant reflector  84  and the top DBR mirror  86 . As indicated above, the cladding or buffer layer  80  is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide of the resonant reflector, to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering an adjacent conductive layer of the optoelectronic device.  
         [0046]     In the illustrative embodiment, the grating layer  90  is SiO 2  with an index of refraction of about 1.484 and a thickness of 0.340 μm. The waveguide layer  82  may be GaAs with an index of refraction of 3.523 and a thickness of 0.280 μm. Alternatively, the waveguide may be a ternary compound such as Al x Ga 1-x As, with x close to one, or a high refractive index dielectric such as TiO2, ZrO2, HfO2, or Si3N4. The thickness of the waveguide preferably depends on the refractive index difference between the waveguide and the buffer or cladding layer. The cladding or buffer layer  80  in the illustrative embodiment is AlO, with an index of refraction of 1.6 and a thickness of 0.766 μm. Finally, the top DBR mirror layer  92  may be AlGaAs with an index of refraction of 3.418 and a thickness of 0.072 μm. In this embodiment, the cladding or buffer layer  80  has an increased thickness and a reduced index of refraction relative to the embodiment shown in  FIG. 2 , both of which help prevent energy in the evanescent tail of the guided mode in the waveguide layer  82  from entering the top DBR mirror layer  92 . It is contemplated however, that similar results may be achieved by either increasing the thickness or reducing the index of refraction of the cladding or buffer layer  80 , if desired.  
         [0047]     As indicated above, the cladding or buffer layer  80  may be AlO, which has a relatively low refractive index. In one method, this can be accomplished by initially forming the cladding or buffer layer  80  with AlGaAs, with a relatively high concentration of aluminum (e.g. &gt;95%). AlGaAs has a relatively high index of refraction. Then, the waveguide layer  82  and grating layer  90  are provided. The cladding or buffer layer  80 , waveguide layer  82  and grating  90  may then be removed around the periphery of the desired optical cavity. Contacts  93  may then be deposited on the exposed top mirror  86  to provide electrical contact to the top mirror. Then, the device may be subject to an oxidizing environment, which oxidizes the AlGaAs material of the cladding or buffer layer  80 , resulting in AlO which has a relatively low refractive index. The AlGaAs material is preferably oxidized laterally in from the exposed edges of the cladding or buffer layer  80 .  
         [0048]      FIG. 7  is a graph showing the reflectance versus wavelength of the resonant reflector of  FIG. 6  when the top layers of the top DBR mirror  86  are non-conductive (k=0). The reflectance curve has a narrow bandwidth (0.00975 nm), and theoretically reaches 100% reflectivity at the resonant wavelength.  FIG. 8  is a graph showing the reflectance versus wavelength of the resonant reflector of  FIG. 6  when the top layers of the top DBR mirror  86  are slightly conductive (k=10 −5 ). As can be seen, the reflectance curve still has a narrow bandwidth (0.0097 nm), and theoretically reaches 100% reflectivity at the resonant wavelength. Therefore, and unlike  FIG. 5 , there is little or no degradation in the observed reflectance of the resonant reflector, even when placed adjacent a conductive layer.  
         [0049]      FIG. 9  is a schematic cross-sectional side view of a top surface emitting vertical cavity laser similar to that shown in  FIG. 6 , but with a modified grating fill factor. The grating fill factor is defined as the grating spacing  102  divided by the grating period  100 . The resonant wavelength of a resonant reflector is often determined by the grating period  100 , and the spectral bandwidth is often determined by the modulation of the refractive index and fill factor of the grating.  
         [0050]     When the grating is formed from an oxide such as SiO 2 , the modulation of the refractive index is related to the difference between the dielectric constants of the grating material and the material that fills the spaces between the grating elements, divided by the average dielectric constant across the grating. The average dielectric constant across the grating can be changed by varying the fill factor of the grating. For example, and assuming a constant grating period, the grating fill factor can be increased by reducing the width of each grating element. A limitation of achieving a desired spectral bandwidth of a resonant reflector by altering the grating fill factor is that the design rules of many manufacturing processes limit the minimum width of the grating elements. Thus, to achieve some spectral bandwidths, the design rules may have to be pushed, which may reduce the manufacturing yield for the devices.  FIG. 10  is a schematic cross-sectional side view of a top surface emitting vertical cavity laser similar to that shown in  FIG. 6 , but with a grating film that has a controlled etch depth to control the spectral bandwidth of the resonant reflector. Like above, a grating film  120  is provided and subsequently etched to form two or more spaced grating regions  122   a - 122   c  separated by one or more spaced etched regions  124   a - 124   b . However, rather than etching all the way through the grating film  120  to achieve a desired grating fill factor, and thus a desired spectral bandwidth, the depth of the etch is controlled. By controlling the depth of the etch, a desired average dielectric constant across the grating can be achieved. A benefit of this approach is that the grating width and grating spacing may be optimized to the design rules of the manufacturing process, and the etch depth can be controlled to achieve the desired spectral bandwidth. For example, a fill factor of about 50% is preferred. This may increase the producibility and yield of the resonant reflector.  
         [0051]      FIG. 11  is a schematic cross-sectional side view of a vertical cavity surface emitting laser that is formed by bonding a resonant reflector  132  that is prepared on a first substrate  130  to a top mirror  134  of a vertical cavity surface emitting laser that is prepared on a second substrate. In accordance with this embodiment, a resonant reflector is formed on a front side  131  of a first substrate  130 . This includes forming at least a waveguide  136  and a grating  138 , as shown. Then, at least a portion of an optoelectronic device, such as a vertical cavity surface emitting laser or resonant cavity photodetector, is prepared on a front side of a second substrate. In  FIG. 11 , this includes a bottom DBR mirror, an active region, a top DBR mirror  134 , and one or more contacts  140 .  
         [0052]     Thereafter, the front side of the first substrate  130  is bonded to the front side of the second substrate to complete the optoelectronic device. The first substrate  130  may be bonded to the second substrate using an optical epoxy  144 , and preferably a non-conductive optical epoxy. The optical epoxy is preferably sufficiently thick, or has a sufficiently low refractive index relative to the refractive index of the waveguide  136  of the resonant reflector  132 , so that the energy from the evanescent wave vector in the waveguide  136  is substantially prevented from entering the optoelectronic device on the first substrate. A anti-reflective coating  148  may be applied to the backside of the first substrate  130  as shown.  
         [0053]     It is recognized that the relative position of the waveguide  136  and grating  138  may be changed. For example, and as shown in FIG. I  1 , the grating may be positioned more toward the front side of the first substrate than the waveguide. Alternatively, however, the waveguide may be positioned more toward the front side of the first substrate than the grating, if desired.  
         [0054]      FIG. 12  is a schematic cross-sectional side view of the vertical cavity surface-emitting laser of  FIG. 11  with a microlens  150  positioned on the backside of the first substrate  130 . For top emitting devices, a microlens such as a collimating microlens may be formed on the backside of the first substrate  130 . For back emitting devices, a collimating microlens may be formed on the backside of the substrate that carries the bottom mirror, the active region and the top mirror of the optoelectronic device. In either case, the collimating microlens  150  is preferably placed in registration with the output of the optoelectronic device as shown.  
         [0055]     It is contemplated that a number of optoelectronic devices may be formed on a common substrate, as shown in  FIG. 13 . One application for such a configuration is a monolithic transceiver that includes one or more light emitting devices  190  and one or more light receiving devices  192  and  194 . In this illustrative embodiment, both the light emitting and light receiving devices are formed on a common substrate (not shown). In one example, a bottom mirror is first formed on the common substrate. The bottom mirror may serve as the bottom mirror for more than one of the optoelectronic devices  190 ,  192  and  194 , and is preferably a DBR mirror stack that is doped to be at least partially conductive. An active region  200  is then formed on the bottom mirror, followed by a top mirror  202 . Like the bottom mirror, the top mirror  202  is preferably a DBR mirror stack, and is doped to be the opposite conductivity type of the bottom mirror. The active region  200  may include cladding layers  204  on either side of the active region  200  to help focus the light energy and current in the active region.  
         [0056]     A deep H+ ion implant, as shown at  206   a - 206   c , may provide gain guide apertures for selected optoelectronic devices, and may further electrically isolate adjacent devices from one another. While a deep H+ implant is provided as an illustration, it is contemplated that any type of current and field confinement may be used, including for example, gain-guided, oxide-confinement, or any other means. Contacts  208   a - 208   d  may be provided on the top mirror  202  and on the bottom surface of the common substrate to provide electrical contact to each of the optoelectronic devices.  
         [0057]     Next, a cladding or buffer layer  210  may be provided above the top mirror  202 . A resonant reflector may then be provided on top of the cladding or buffer layer  210 . The resonant reflector may include a waveguide  212  and a grating film  214 . For some optoelectronic devices, such as top emitting devices  190 , the grating film  214  may be etched to form a grating, as shown. The grating may substantially increase the reflectivity of the resonant reflector in those regions. For other optoelectronic devices, such as top receiving devices  192 , the grating film may either include a different grating structure (e.g., wider spectral bandwidth), or remain non-etched as shown. This may reduce the reflectivity of the resonant reflector, thereby allowing light to more easily enter the optical cavity. For yet other optoelectronic devices, such as Metal-Semiconductor-Metal (MSM) receiving devices  194 , the grating film may be removed altogether, and a metal grid  214   a - 214   c  may be formed on the waveguide layer  212  or cladding or buffer layer  210 , as desired.  
         [0058]     To isolate the resonant reflector from the optoelectronic devices, and in particular the conductive top mirror  202 , the cladding or buffer layer  210  may be sufficiently thick to substantially prevent energy in the evanescent tail of the guided mode in the waveguide  212  from entering the top mirror  202 . Alternatively, or in addition, the cladding or buffer layer  210  may be formed from a material that has a sufficiently low refractive index relative to the refractive index of the waveguide  212  to substantially prevent energy in the evanescent tail of the guided mode in the waveguide  212  from entering the top mirror  202 .  
         [0059]     Implementation of the described resonant reflector optoelectronic structures will permit polarization, emission wavelength and mode control. These structures and properties can be designed and fabricated using techniques such as lithography or holography, and may not be subject to growth thickness variations alone. The above techniques can be applied to produce, for example, VCSELs with high power single-mode/polarization emission from apertures exceeding a few microns in diameter. Furthermore, wavelength and/or polarization variation across a chip, array or wafer can be used for spatially varied wavelength/polarization division multiplexing, multi-wavelength spectroscopy, etc.  
         [0060]      FIG. 14  is a schematic diagram of a number of illustrative optoelectronic emitters that have a guided-mode resonant reflector waveguide-grating (i.e. a guided-mode grating resonant reflector filter or GMGRF) for data/telecommunication, processing, switching, etc., including embodiments conducive for wavelength division multiplexing, polarization division multiplexing, space division multiplexing, etc. The illustrative optoelectronic emitters are shown at  300 ,  302 ,  304  and  306 , respectively, and in the illustrative embodiment are monolithically formed on a common substrate  308 . Each optoelectronic emitter includes a GMRGF filter integrated with or adjacent to the top mirror of the emitter. In one embodiment, the optoelectronic emitters are VCSEL devices.  
         [0061]     The GMGRF of each emitter includes a waveguide and a grating. The various gratings are shown using parallel lines in  FIG. 14 , with each grating having different characteristics. For example, a first optoelectronic emitter  300  includes a grating that extends in a horizontal direction and has a first grating period. The grating direction helps determine the polarization direction of the light that is resonantly reflected by the GMGRF. The grating period may help determine the resonant wavelength of the GMGRF. Thus, the output emission may be inherently wavelength selective.  
         [0062]     A second optoelectronic emitter is shown at  302 . Optoelectronic emitter  302  includes a grating that also extends in a horizontal direction, but has a second grating period. Thus, the second optoelectronic emitter  302  may produce light that is polarized in the same direction as the first optoelectronic emitter  300 . However, the narrower grating period may produce a shorter wavelength than the first optoelectronic emitter  300 .  
         [0063]     A third optoelectronic emitter is shown at  304 . Optoelectronic emitter  304  includes a grating that also extends in a horizontal direction, but has a third grating period. Thus, the third optoelectronic emitter  304  produces light that is polarized in the same direction as the first and second optoelectronic emitters. However, the larger grating period may produce a longer wavelength than the first and second optoelectronic emitters.  
         [0064]     Finally, a fourth optoelectronic emitter is shown at  306 . Optoelectronic emitter  306  includes a grating that also extends in a perpendicular direction to that of the other optoelectronic emitters  300 ,  302  and  304 . Thus, the fourth optoelectronic emitter  306  produces light that is polarized in a direction that is perpendicular to that of the other optoelectronic emitters  300 ,  302  and  304 .  
         [0065]     As can readily be seen, changing the period and/or direction of the grating may allow designers to control lithographically both the operating wavelength and polarization direction of the corresponding optoelectronic emitter devices. This may allow for SDM/WDM/PDM architectures having distributed wavelength and polarization modes. If the admission of light having any polarization is desired, the GMGRF may be fabricated with two crossed gratings aligned orthogonally with each other (e.g. bi-gratings).  
         [0066]     As indicated above, the use of a GMGRF reflector can reduce the number of top DBR mirror periods when compared to an all-epitaxial DBR VCSEL or RCPD device. Reducing the number of DBR mirror periods can reduce the overall thickness of the device, which can lead to greater amenability to integration. Increased planarity and the utilization of standard semiconductor planar batch fabrication processes may also help improve producibility. The capability to readily control the wavelength, angular and bandwidth properties of the GMGRF reflector can provide greater flexibility in the construction of multi-element integrated circuits.  
         [0067]      FIG. 15  is a graph showing reflectance versus wavelength for the three GMRGF filters  300 ,  302  and  304  of  FIG. 14 . Curves  310 ,  312  and  314  correspond to the GMGRF reflectance versus wavelength for optoelectronic devices  300 ,  302  and  304 , respectively. In the illustrative embodiment, the grating period (Λ) of the first optoelectronic device  300  is 480 nm, which in the example shown, results in a peak reflectance at a wavelength of about 843 nm. The grating period (Λ) of the second optoelectronic device  302  is 460 nm, which in the example shown, results in a peak reflectance at a wavelength of about 811 nm. Finally, the grating period (Λ) of the third optoelectronic device  304  is 500 nm, which in the example shown, results in a peak reflectance at a wavelength of about 882 nm. The reflectance versus wavelength of the fourth optoelectronic device  306  is not shown in  FIG. 15 .  
         [0068]     As described above, the number of DBR mirror periods of the top mirror of a VCSEL or RCPD device may be reduced, preferably so that resonance cannot readily be established without the additional reflectance provided by the GMGRF. In the illustrative embodiment, and referring to  FIG. 15 , the reflectance provided the GMGRF varies from near zero to near 100%, depending on the wavelength. Therefore, unique wavelength and/or polarization selectively may be provided for each optoelectronic device  300 ,  302 ,  304  and  306  by selecting appropriate parameters for the corresponding GMGRF. In this configuration, the resonant wavelength of the GMGRF may help determine, to a large extent, the cavity resonance wavelength and/or polarization of the corresponding optoelectronic device.  
         [0069]      FIG. 16  is a schematic diagram showing an illustrative array of four VCSEL devices  320 ,  322 ,  324  and  326  fabricated as a common epitaxial structure. The common epitaxial structure includes a bottom mirror  330 , an active region  332 , and a top mirror  334 . A top GMGRF is integrated with or provided adjacent to the top mirror  334 , and in the embodiment shown, the grating period (Λ) of each GMGRF is different for each VCSEL device. As can be seen, the number of top DBR mirror periods is less than the number of bottom DBR mirror periods. As indicated above, the number of top DBR mirror periods is preferably reduced so that the lasing threshold cannot readily be established without the additional reflectance provided by the corresponding GMGRF.  
         [0070]     The VCSEL devices  320 ,  322 ,  324  and  326  may be processed with topside electrical contacts with the GMGRF films positioned in the emitting aperture of each VCSEL. To tune the GMGRF, and in one example, a 1 nm increase in the grating period (Λ) may yield about a 1.8 nm increase in the resonant wavelength. Thus, if the desired resonant wavelengths were 842 nm, 847 nm, 852 nm, and 857 nm, respectively, then the grating periods for the four VCSEL devices  320 ,  322 ,  324  and  326  may be approximately 477.2 nm, 480 nm, 482.8 nm, and 485.8 nm.  
         [0071]      FIG. 17  is a schematic diagram showing an illustrative WDM/WDD/PDM application using the array of VCSEL devices of  FIG. 16 . Optical Emitter-A  350  corresponds to VCSEL  320 , which produces a first wavelength λ 1  as shown at  351 . Optical Emitter-B  352  corresponds to VCSEL  322 , which produces a second wavelength λ 2  as shown at  353 . Optical Emitter-C  354  corresponds to VCSEL  324 , which produces a third wavelength λ 3  as shown at  355 . Finally, Optical Emitter-D  356  corresponds to VCSEL  326 , which produces a fourth wavelength λ 4  as shown at  357 . The various wavelengths  351 ,  353 ,  355  and  357  are provided to a common optical receiver element by block  360 . The common optical receiver element may be, for example, a common optical fiber  362 , as shown.  
         [0072]     In order to increase the aggregate transmission bandwidth of an optical fiber, it is generally preferred that the wavelength spacing of simultaneously transmitted optical data streams, or optical data “channels,” be closely packed, to accommodate a larger number of data channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized. Because the wavelength of each VCSEL can be tightly controlled lithographically, a relatively large number of optical data channels can be accommodated. In some cases, the wavelength of adjacent optical channels may be 5 nm or less. Also, and as described above, the polarization direction of the VCSEL devices can be lithographically controlled, which may allow polarization Division Multiplexing (PDM) and/or WDM and PDM multiplexing. This can even further extend the bandwidth of some optical data channels.  
         [0073]     In the illustrative embodiment, one or more optoelectronic receivers  370 ,  372 ,  374  and  376  are provided. In one embodiment, the light transmitted by the optical fiber  362  is provided to each of the optoelectronic receivers  370 ,  372 ,  374  and  376 . Each of the optoelectronic receivers  370 ,  372 ,  374  and  376  may be tuned to select the wavelength of one (or more) of the optical data channels. For example, and in one illustrative embodiment, each of the optoelectronic receives  370 ,  372 ,  374  and  376  is a RCPD device, with a GMGRF resonator that is tuned to a wavelength of a desired optical data channel. Each RCPD may be similar to the VCSEL devices  320 ,  322 ,  324  and  326  discussed above with respect to  FIG. 16 , but may be operated in a reverse bias mode. In some embodiments, the effective reflectivity of the top mirror (including the GMGRF) may be reduced and the lateral dimensions of the resonant cavity may be increased relative to a VCSEL device, which may increase the amount of light that is allowed to enter by the optical cavity.  
         [0074]     Alternatively, or in addition, selected wavelengths may be directed to an optical receiver by an optical filter, optical splitter, or the like. In this embodiment, the optical receiver may be a wide band optical receiver, as the wavelength selectivity is provided by the optical filter, optical splitter, or the like, rather than the optical receiver itself.  
         [0075]      FIG. 18  is a schematic diagram showing an illustrative GMGRF filter  400  in accordance with the present invention. As described above, the GMGRF filter  400  may be incorporated into or positioned adjacent the top and/or bottom mirror of a VCSEL or RCPD device. The resonant wavelength of the GMGRF  400  may determine, at least to a large extent, the cavity resonance of the VCSEL or RCPD device.  
         [0076]     In one illustrative embodiment, the GMGRF  400  includes a buffer layer  402  interposed between a core layer  404  and the top DBR mirror layer  406 . The buffer layer  402  also serves as a clad layer. As indicated above, the buffer layer  402  is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the core layer  404  of the resonant reflector, to substantially prevent energy in the evanescent tail of the guided mode in the core layer  404  from entering an adjacent conductive layer of the optoelectronic device.  
         [0077]     In the illustrative embodiment, an upper clad layer  410  is provided over a grating etched into the core layer  404 . The grating elements  412  of the grating preferably have a grating period  414 , and the core layer  404  preferably has a core depth  418  between adjacent grating elements  412 . The overall core thickness at the grating elements  412  is shown at  416 . In the illustrative embodiment, the core layer  404  is TiO2 with an index of refraction of about 2.41, and has a core depth of about 0.175 μm. Alternatively, it is contemplated that the core layer  404  may be, for example, GaAs or some other relatively high refractive index dielectric such as ZrO2, HfO2, or Si3N4. The thickness of the core layer  404  preferably is dependent on the refractive index difference between the core layer  404  and the buffer layer  402 .  
         [0078]     In the illustrative embodiment, the upper clad layer  410  is SiO 2  with an index of refraction of about 1.48, and having a clad depth  417  of about 0.285 μm. The upper clad layer  410  extends down between the grating elements  412  of the core layer  404 , as shown. Thus, the upper clad layer  410  has an overall clad layer thickness between grating elements  412  that equals the clad depth  417  plus the grating height  420 , or in this case about 0.495 μm. The buffer layer  402  in the illustrative embodiment is SiO2, with an index of refraction of 1.48 and a thickness of 0.285 μm. The top layer of the DBR mirror  406  may be, for example, AlGaAs with an index of refraction of 3.2. In this embodiment, and as described above, the buffer layer  402  preferably has an increased thickness and/or a reduced index of refraction, both of which help prevent energy in the evanescent tail of the guided mode in the core layer  404  from entering the top DBR mirror  406 .  
         [0079]     The resonant wavelength of the GMGRF  400  can be set in a reliable manner by appropriately selecting GMGRF parameters, such as the grating period (Λ)  414 , core depth  418  versus grating height  420 , grating direction, etc. More specifically, and in one illustrative embodiment, in an array of VCSEL devices having a common epitaxial structure (bottom mirror, active region, top mirror including a GMGRF), the lasing wavelength of each VCSEL in the array can be prescribed individually by changing the GMGRF parameters that affect the resonant wavelength. Most conveniently, this can be done by changing the grating period (Λ)  414  while keeping all other parameters fixed, or by increasing the grating height  420  while maintaining a substantially constant core thickness  416  (and hence reducing the core depth  418  of the core layer  404  by a corresponding amount), while keeping the grating period (Λ)  414  fixed. However, it is contemplated that these and/or other GMGRF parameters maybe changed in any suitable manner to achieve the desired resonant wavelength for each VCSEL in the array.  
         [0080]      FIG. 19  is a graph showing the relationship between the grating period (Λ)  414  and the resonant wavelength for the GMGRF shown in  FIG. 18 . In this example, the DBR epilayers in the top mirror of a VCSEL are represented as a single layer having an average refractive index of 3.2. The upper clad layer  410  and buffer layer  402  have optical thicknesses of a half-wave at approximately 850 nm (about 0.285 μm). The grating elements  412  provide modulation in the dielectric constant of the core layer  404 , and enforces coupling of a normally-incident planewave via the first diffractive order of the grating into a guided-mode supported by the core layer  404 .  
         [0081]     The resonant wavelength of this GMGRF structure can be determined approximately by using the mode-matching condition: 
 
β=2π/Λ  Equation (1)
 
 where Λ is the grating period  414  and β is the eigenvalue (propagation wavenumber) of the guided mode excited. 
 
         [0083]     The structure may be modeled as a homogenized multilayer planar waveguide by replacing the grating layer with a homongeneous film having an effective refractive index (n eff ) determined by the Rytov effective medium expression, which in a first-order approximation reads: 
 
&lt; n&gt;   2   =FF*n   hi   2 +(1 −FF )* n   lo   2  for TE polarization  Equation (2) 
 
OR
 
&lt; n&gt;   −2   =FF*n   hi   −2 +(1 −FF )* n   lo   2  for TM polarization  Equation (3)
 
 where FF equals the grating fill factor defined with respect to the high index material. 
 
         [0085]     A desired resonant wavelength λ 1  and polarization (TE or TM) are then selected. The film thicknesses are preferably chosen on the basis of out-of-band optical reflectance, throughput, and core isolation from the substrate. The grating may be modeled as its homogenized equivalent thin film, and the structure may be analyzed as a multi-layer planar waveguide. This analysis may yield the guided mode eigenvalues β for the waveguide, and one (usually the fundamental mode with largest β) is chosen. The grating period which excites this mode is then given approximately by equation (1) above. Illustrated results for the GMGRF  400  of  FIG. 18  are shown graphically in  FIG. 19 .  
         [0086]      FIG. 20  is a graph showing the relationship between the grating height  420 /core depth  418  and the resonant wavelength for the GMGRF shown in  FIG. 18 . As indicated above, the resonant wavelength of the GMGRF  400  may be controlled by increasing the grating height  420  while maintaining a substantially constant core thickness  416  (and hence reducing the core depth  418  of the core layer  404  by a corresponding amount), while keeping the grating period (Λ)  414  fixed. Illustrated results for the GMGRF  400  of  FIG. 18  are shown graphically in  FIG. 20  for four combinations of grating height  420  and core depth  418 , while leaving the overall core thickness  416  and grating period  414  constant.  
         [0087]     Regardless of the method used to choose GMGRF parameters to achieve a desired resonant wavelength, the structure can be analyzed with a grating solver to estimate more precisely the GMGRF resonant wavelength and the Fabry-Perot resonance of the combined top-mirror assembly with the rest of the VCSEL or RCPD structure. If desired, minor adjustments in the GMGRF parameters can then be made to yield a desired resonant wavelength.  
         [0088]     It is recognized that the wavelength selectivity capability of such GMGRF filters has applicability in display applications. As the grating itself may determine the wavelength of operation, and fabrication is done lithographically, laterally-displaced wavelength dependent emitters can be formed. Such a structure may also serve as a quasi-tunable laser source. Wavelength tunable VCSELs and detectors, as described above, may also find use in spectroscopic and sensing applications.  
         [0089]     The improved performance coupled with the capability to control polarization can also lend itself to applications in polarization-sensitive optical read/write applications. Included are various forms of CD, DVD, and holographic storage applications. Laser printing heads may also benefit. The performance advantage, and use of thinner top and/or bottom mirrors becomes even more paramount when extending VCSELs into the visible wavelengths, where typical all-epitaxial DBRs become prohibitively thick and may require twice as many layers.  
         [0090]     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached.