Abstract:
An optically-pumped semiconductor (OPS) laser includes a multilayer structure including a mirror-structure surmounted by a multilayer semiconductor gain-structure. A membrane mirror is spaced apart from the mirror structure to form a laser resonator between the mirror structure and the membrane mirror, with the gain-structure included in the laser resonant cavity. Pump light is transmitted through the membrane mirror into the gain structure. The membrane mirror includes a grating having parameters selected such that the membrane mirror has different reflectivities at the wavelength of the laser in two orthogonally-oriented planes of polarization. The membrane mirror is axially movable along the resonator axis for varying the resonator length to select the wavelength of the single lasing mode.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to external-cavity, surface-emitting semiconductor lasers. It relates in particular to an external-cavity, surface-emitting semiconductor laser having a resonator formed between a movable polarization selective cavity-mirror and a polarization selective mirror. 
     DISCUSSION OF BACKGROUND ART 
     In optical communications systems, information is transmitted along an optical fiber as a modulated beam of light. In one preferred optical communication arrangement, wavelengths for the light beam are in a range between about 1.513 micrometers (μm) to about 1.565 μm, corresponding to a frequency range from about 196,000 gigahertz (GHz) to about 192,000 GHz. In a scheme referred to as dense wavelength-division-multiplexing (DWDM) the frequency range is partitioned into 40 channels at 100 GHz intervals. A trunk optical fiber may carry up to 40 different beams at 40 different wavelengths, one corresponding to each channel. The different-wavelengths (optical-carrier) beams are generated by InGaAsP edge-emitting diode-lasers, one for each channel. The output of each laser is modulated to encode the information to be transmitted onto the laser-beam provided by the laser. Communications channels are separated from or added to the trunk optical fiber by wavelength-selective couplers. 
     An optical communications system having such close channel spacing would benefit from a laser that could be rapidly and accurately tuned from the wavelength of one channel to the wavelength of another. One suitable laser type for this purpose is an external-cavity, optically-pumped, surface-emitting semiconductor laser (OPS-laser). Such a laser has a resonator (resonant cavity) formed between two mirrors. One of the resonant cavity mirrors is an integral part of a multilayer structure including a semiconductor multilayer, surface-emitting gain-structure. The mirror can be formed from metal, dielectric, or semiconductor layers or combinations thereof. The other resonant cavity mirror is external to and spaced apart from the gain-structure. This mirror is partially transmissive and is used as an output-coupling mirror. The mirror is usually formed from dielectric layers, semiconductor layers or combinations thereof. 
     The emitting wavelength of the laser depends on the materials of the gain-structure and the optical spacing between the first and second mirrors. The gain-structure provides gain only in a limited range of wavelengths. This range is generally referred to as the gain bandwidth. By way of example, an InGaAsP OPS-laser having a nominal emitting wavelength of about 1.550 μm has a gain bandwidth of about 0.035 μm. The second mirror can be made movable for varying the spacing, thereby tuning the laser to vary the emitting wavelength within the gain bandwidth. (See, for example, U.S. Pat. Nos. 5,291,501; 5,572,543 and 6,154,471, incorporated herein by reference.) 
     A preferred arrangement of such a tunable external-cavity semiconductor laser is one in which the nominal spacing between the mirrors is sufficiently small that the separation between possible resonant wavelengths of the cavity is greater than the gain bandwidth. This is often referred to by practitioners of the art as a short-cavity OPS-laser. The cavity length may be about 30 μm for a OPS-laser having a nominal emitting wavelength of about 1.550 μm. The short cavity provides that the laser can emit at only one wavelength within the gain bandwidth for any variation of laser spacing within one half-wavelength of the nominal spacing. Accordingly, no other mode selection device is necessary. Such a laser is also very compact, enabling a number of such lasers to be assembled in a compact array. 
     In longer cavity lasers, a tilt-tunable etalon, a birefringent filter, or a diffraction grating filter is typically used to limit the number of possible oscillating modes. Such devices also cause the laser to oscillate in a plane-polarized mode. An edge-emitting laser inherently has polarized sensitive gain. Plane-polarized operation is advantageous in optical communications systems that include polarization sensitive devices such as Faraday rotators, and multilayer dielectric mirrors used at non-normal incidence. In a short cavity OPS laser there is insufficient space between the external mirror and the gain-structure to include a tilted etalon or a birefringent filter and the laser oscillates in a minimally defined polarization mode, often with only minimal power difference between two eigen polarizations. A laser without a clearly defined polarization is more susceptible to feedback in the orthogonal polarization. This shortcoming needs to be overcome to improve the potential of the short-cavity, external resonator OPS-laser. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention is directed to a mirror for reflecting light at a lasing wavelength and transmitting light at an optical pump light wavelength. The optical pump light wavelength is the shorter of the wavelengths. The mirror includes a peripherally supported membrane. The membrane has a grating on an outer surface thereof. The grating includes a regular array of spaced-apart parallel strips of a first material having a first refractive index. The grating surmounts at least one optical interference layer of uniform thickness. The optical interference layer can be a layer of the first material, or a layer of a second material having a second refractive index different from the first refractive index. The grating strips are characterized as having a width and a height, and the grating has a grating period defined as the distance between adjacent ones of the parallel strips. The grating period is less than the pump light wavelength. The first material, any second material, the grating width and height, the ratio between the grating width and period, and the thickness of the at least one and any other uniform-thickness layers are selected such that the second mirror has a different specular reflectivity for light having the lasing wavelength, normally incident in first and second polarization planes oriented respectively parallel and perpendicular to the grating strips, and such that the mirror has a transmissivity greater than 50% for pump-wavelength light polarized in any one of the polarization planes. 
     The term specular reflectivity (or simply reflectivity) as used herein can be alternatively defined as the zero-order diffraction efficiency, where this term refers to the zero order diffraction efficiency of the grating and any other optical interference layers as a whole. The term “transmissivity” refers to the zero-order or specular transmissivity. Reflectivity, transmissivity and diffraction efficiency are specified herein alternatively as a percentage or a decimal ratio where 1.0 is 100%. 
     Preferably the specular reflectivity for the lasing wavelength is between about 90% and 99% in one of the polarization planes, and is at least about 1% less in the other polarization plane, and the transmissivity of the mirror for pump-wavelength light is greater than 50% (0.50 in decimal notation) in some polarization orientation. 
     The at least one optical interference layer may be a layer of the first material, i.e., the material of the grating strips. Alternatively, the at least one optical interference layer is a layer of a material having a refractive index different from that of the grating strips. 
     In one preferred embodiment of the inventive grating-membrane mirror there is only one interference layer having the same refractive index as that of the grating strips The grating strips and mirror in this embodiment may be considered as a single grating comprising the spaced apart strips and a uniform thickness portion. The specular reflectivity for laser light in either of the polarization orientations is greater than would be provided by the optical interference layer (uniform thickness portion of the grating) in the absence of the grating strips. In another preferred embodiment of the inventive grating membrane mirror a grating including a uniform thickness portion surmounts four other optical interference layers. The first-wavelength specular reflectivity in either of the polarization orientations is less than would be provided by the uniform thickness portion of the grating and the four optical interference layers in the absence of the grating strips. In calculated examples of each embodiment, a lasing wavelength reflectivity of about 98% for wavelengths between about 1.530 and 1.565 μm is achieved together with greater than 70% transmissivity for 0.980 μm plane-polarized (TM) pump light. Reflectivity for TE polarized light was less than 45% between about 1.530 and 1.565 
     In another aspect of the present invention, the above-described grating membrane mirror provides an output-coupling mirror in a tunable OPS-laser. The OPS-laser includes a multilayer structure including a first mirror surmounted by a multilayer semiconductor gain-structure. The gain-structure has a gain-bandwidth including the lasing wavelength, and is energized by pump light having the pump-light wavelength. The inventive grating membrane mirror is peripherally supported by one surface of a semiconductor substrate over an aperture therein and is electrically isolated from the substrate. The mirror is spaced apart from the first mirror to form a laser resonator. The laser resonator has a longitudinal axis and the gain-structure is included in the laser resonator. Pump light is directed into the gain-structure through the grating membrane mirror. The optical spacing between the first mirror and the grating membrane mirror is selected such that the laser resonator supports only a single lasing mode, the wavelength of which is within the gain-bandwidth of the gain-structure. 
     At least one layer of the grating membrane mirror has an electrically conductive portion and means for making electrical contact with that electrically conductive portion. A electrical contact is provided a surface of the substrate opposite the surface supporting the grating membrane mirror. Applying an electrical potential between the electrically conductive portion of the grating membrane mirror layer and the electric contact on the substrate causes a central portion of the grating membrane mirror to move in a direction parallel to said longitudinal axis of said laser resonant cavity for selecting the wavelength of said lasing mode. 
     The first material, any second material, the grating width and height, the ratio between the grating width and period, and the thickness of the at least one and any other uniform-thickness layers are selected such that the second mirror has a sufficiently different reflectivity in the polarization planes oriented parallel and perpendicular to the grating strips, for light having the oscillating wave wavelength, that the lasing mode is plane polarized in the plane for which the reflectivity is highest, and such that the grating membrane mirror has a transmissivity greater than about 50% (0.50 in decimal terms) at said pump light wavelength in any polarization plane orientation. 
     As noted above the central optical telecommunications wavelength range extends from 1.535 μm to 1.565 μm. The gain-bandwidth of any particular semiconductor gain structure having a nominal emitting wavelength in this range is about 0.035 μm. In a telecommunications system there can be a single laser in accordance with the present invention tunable over a gain bandwidth of about 0.035 μm about the nominal wavelength. In all calculated examples of embodiments of the present invention discussed hereinbelow, a tuning wavelength range between 1.530 and 1.565 μm is assumed. This is done simply to assist in comparing performance of the various examples and should not be construed as limiting the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention. 
     FIG. 1 is an elevation view, partly in cross-section, schematically illustrating one preferred embodiment of a tunable external-cavity OPS-laser including an external cavity mirror in the form of an electrostatically-movable, sub-wavelength-grating membrane mirror in accordance with the present invention, the grating comprising a regular array of spaced-apart, parallel, dielectric strips. 
     FIG. 2 is a fragmentary three-dimensional view schematically illustrating detail of grating strips and spaces and optional interference layers in the membrane mirror of FIG.  3 . 
     FIG. 3 is a cross-section view schematically illustrating important dimensions, values, interfaces and polarization orientations in a grating similar to the grating of FIG. 2 but supported on a substrate. 
     FIGS. 4A and 4B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency with grating depth and grating period for normally incident light in respectively TE and TM orientations in the grating of FIG. 3 having strips of refractive index of about 3.5 on a substrate having the same refractive index, with the light incident on the grating from air. 
     FIGS. 5A and 5B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency with grating depth and grating period for normally incident light in respectively TE and TM orientations in the grating of FIG. 3 having strips of refractive index of about 3.5 on a substrate having a refractive index of 2.5, with the light incident on the grating from air. 
     FIGS. 6A and 6B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency with grating depth and grating period for normally incident light in respectively TE and TM orientations in the grating of FIG. 3 having strips of refractive index of about 1.5 on a substrate having a refractive index of 2.5, with the light incident on the grating from air. 
     FIGS. 7A and 7B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency with grating depth and grating period for normally incident light in respectively TE and TM orientations for the grating of FIG. 3 having strips of refractive index of about 3.5, with the light incident on the grating from air and with the grating suspended in air, i.e., having air as a substrate. 
     FIG. 8 is a contour graph schematically illustrating calculated variation of the absolute value of the difference in zero-order reflected diffraction efficiency between TE and TM polarizations with grating depth and grating period in the grating of FIGS. 7A and 7B. 
     FIG. 9 is a graph schematically illustrating reflected diffraction efficiency as a function of wavelength for TE and TM polarizations for one pair of grating depth and grating period parameters in the grating of FIGS. 7A and 7B. 
     FIG. 10 is a graph schematically illustrating reflected diffraction efficiency as a function of wavelength for TE and TM polarizations for another pair of grating depth and grating period parameters in the grating of FIGS. 7A and 7B. 
     FIG. 11 schematically illustrates one embodiment of a grating, membrane mirror in accordance with the present invention including grating strips of a first material on an interference-thickness membrane-portion of a second material. 
     FIG. 12 schematically illustrates another embodiment of a membrane, grating mirror in accordance with the present invention including grating strips of a first material on an interference-thickness membrane-portion of the same material. 
     FIG. 13 is a graph schematically illustrating calculated zero-order diffraction efficiency for TM polarization as a function of wavelength and thickness of the membrane portion of a grating, membrane mirror in accordance with the present invention, in the configuration of FIG.  11 . 
     FIG. 14 is a graph schematically illustrating calculated zero-order diffraction efficiency for TE and TM polarization as a function of wavelength and thickness of the membrane portion of a grating membrane mirror in accordance with the present invention, in the configuration of FIG.  12 . 
     FIG. 15 is a graph schematically illustrating calculated zero-order diffraction efficiency for TE and TM polarization as a function of wavelength of a grating, membrane mirror in accordance with the present invention, in the configuration of FIG. 12, with grating parameters and membrane portion thickness selected to provide zero-order reflection diffraction efficiency for TM polarization of about 0.98 between wavelengths of 1.530 and 1.565 μm and zero-order transmissivity greater than 0.80 at a wavelength of about 0.98 μm. 
     FIG. 16 is a graph schematically illustrating detail of calculated zero-order reflection diffraction efficiency for TE and TM polarization as a function of wavelength between wavelengths of 1.52 and 1.57 μm in the grating membrane mirror of FIG.  15 . 
     FIG. 17 is a graph schematically illustrating detail of calculated zero-order transmission efficiency for TE, TM, and average polarization as a function of wavelength between wavelengths of 0.94 and 1.02 μm in the grating membrane mirror of FIG.  15 . 
     FIG. 18 is a graph schematically illustrating detail of calculated zero-order transmission efficiency for TE “P” polarization at 20 degrees light incidence as a function of wavelength between wavelengths of 1.26 and 1.34 μm in the grating, membrane mirror of FIG.  15 . 
     FIG. 19 is a cross-section view schematically illustrating yet another embodiment of a grating membrane mirror in accordance with the present invention including two uniform-thickness layers and a grating layer including a uniform-thickness portion. 
     FIG. 20 is a cross-section view schematically illustrating still another embodiment of a grating membrane mirror in accordance with the present invention including two uniform-thickness layers and with grating strips formed on one of the uniform-thickness layers and having a refractive index different from that of the uniform-thickness layer on which it is formed. 
     FIG. 21 is a cross-section view schematically illustrating a further embodiment of a grating membrane mirror in accordance with the present invention including four uniform-thickness layers and a grating layer including a uniform-thickness portion. 
     FIG. 22 is a graph schematically illustrating calculated zero-order diffraction efficiency for TE and TM polarization as a function of wavelength in one example of the grating membrane mirror of FIG. 21 arranged to provide zero-order diffraction efficiency of about 0.98 at wavelengths between about 1.53 and 1.565 μm and transmissivity greater than 0.70 for light having a wavelength of about 0.980 μm and at 20.0 degrees incidence. 
     FIG. 23 is a graph schematically illustrating detail of calculated zero-order diffraction efficiency for TE and TM polarization as a function of wavelength in one example of the grating membrane mirror of FIG. 22 at wavelengths between about 1.50 and 1.62 μm. 
     FIG. 24 is a graph schematically illustrating detail of calculated zero-order transmissivity at 20 degrees incidence for TE and TM polarization as a function of wavelength in one example of the grating membrane mirror of FIG. 22 at wavelengths between about 0.90 and 1.10 μm. 
     FIGS. 25A-K are cross-section views schematically illustrating steps in one preferred method of constructing the example of the grating membrane mirror of FIGS.  21 - 24 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like features are designated by like reference numerals, FIG. 1 schematically illustrates a preferred embodiment  30  of a tunable, optically pumped, semiconductor (OPS) laser including a grating membrane mirror  31  in accordance with the present invention. Laser  30  includes a multilayer structure (OPS-structure)  32  including a mirror structure  34  and a semiconductor multilayer gain-structure  36 . Mirror structure  34  may include dielectric or semiconductor layers only, or one or more dielectric or semiconductor layers in combination with a metal layer. OPS-structure  32  is preferably in thermal contact with a substrate or heat sink  38 . 
     Gain-structure  36  is a surface-emitting gain-structure comprising a plurality of active (optical-gain-providing) layers (not shown) spaced apart by optical-pump-light-absorbing spacer layers (not shown). As OPS-structures are well known in the art to which the present invention pertains, a more detailed description of OPS-structure  32  is not presented herein. (See for example, U.S. Pat. Nos. 5,991,318 and 6,285,702, assigned to the same assignee as herein and incorporated by reference.) 
     Continuing with reference to FIG.  1  and additionally to FIG. 2, mirror  31  is peripherally supported on a semiconductor substrate  44 . Mirror  31  includes a grating portion  46  including a plurality of parallel strips  48  separated by spaces  50 . Strips  48 , here, have a rectangular cross-section, but this should not be considered as limiting the present invention. Grating  46  optionally includes a uniform-thickness portion  46 A of the same material as strips  48 . Mirror  31  optionally includes one or more uniform-thickness layers in addition to grating  46 . Two such layers  52  and  54  are depicted in FIG.  2 . If mirror  31  includes at least one such additional layer, uniform portion  46 A of grating  46  may be omitted and strips  48  formed directly on the additional layer. The material of strips  48  uniform thickness portion  46 A, and any other layers, may be a dielectric material or a semiconductor material. 
     Mirror  31  has an electrically conductive portion (not shown) and means for making electrical contact with this electrically conductive portion. Mirror  31  and the electrically conductive portion thereof are electrically isolated from substrate  44 . An annular, metal contact-layer  56  is deposited on a surface  44 A of substrate  44 . A DC potential is established between substrate  44  and mirror  31  via leads  58  and  60 . This applied potential provides an electrostatic driving field for moving mirror  31 . The magnitude of the field is dependent, inter alia, on the applied potential and the dimension of a gap  62  between surface  44 B of substrate  44  and mirror  31 . 
     Mirrors  31  and  34  define a resonator  64  of the laser. For the above-exemplified active-layer material providing gain at wavelengths between about 1.515 μm and 1.565 μm, the optical spacing between the mirrors should be nominally about 30 μm or less. This spacing provides that the laser can lase at only a single oscillation mode of the resonator, along a resonator axis  65 . The optical spacing between mirror structure  24  and mirror  42  includes the optical thickness of gain-structure  36 . Varying the electrostatic driving potential applied to mirror  31  moves the mirror axially on axis  65 , thereby varying the exact spacing between mirrors  31  and  34  and, consequently, the wavelength of the single oscillating mode of laser  30 . 
     Laser  20  is optically pumped by light  70  from a pump laser (not shown). For the above-exemplified active-layer and spacer-layer materials of gain-structure  26 , the pump-laser is preferably a diode-laser providing pump-light at a wavelength of either about 0.980 μm or about 1.30 μm. Pump-light  70  is directed, here, at an angle α to resonator axis  65 . Substrate  44  has a circular aperture  72  extending therethrough. Pump-light  70  enters laser  30  via aperture  72  in substrate  44 , through grating membrane mirror  31 , and is absorbed in gain-structure  36 . It should be noted here that the above discussed arrangement for directing pump light into laser  30  through mirror  31  is but one of a number of such possible arrangements and should not be construed as limiting the present invention. Other options include passing the pump light through the mirror coaxially along axis  65 . 
     A primary purpose of including grating  46  in the inventive membrane mirror is to provide that the mirror has sufficiently different reflectivity, in polarization planes perpendicular to each other, for normally incident light, that the laser will oscillate (lase) in only that polarization plane in which the reflectivity is the higher. Depending on factors including the gain provided by gain-structure  36  and the power in pump light  70 , a reflectivity difference of as little as one percent (1%) between the polarization planes may be sufficient. This reflectivity difference, at least, can be provided in the inventive membrane mirror. 
     It has also been determined that given certain conditions, a grating of a material having a refractive index greater than about 1.6 can provide a significantly higher reflectivity in at least one plane of polarization than could be achieved in a free-standing layer of the same material having a uniform optical thickness of one-quarter wave or some odd multiple thereof. It has also been determined that the grating can be caused to optically interfere with a number (one or more) of uniform-thickness layers to provide a spectral response of the mirror with spectral features more closely spaced and with wider variation than would be possible were the grating layer replaced by a uniform layer of the same material and depth as the grating strips. 
     It is believed that the grating surface, considered as a crenellated or corrugated interface between a medium having the refractive index of the above-discussed grating strips  48  and a medium from which light is incident, would not provide, in itself, a zero-order diffraction efficiency higher than the Fresnel reflection for an interface between the same media. It is believed that this is true for any refractive index difference across the corrugated interface for any dimension, spacing or shape of strips and grooves  48  and  50 , and for any polarization orientation. 
     It is believed that in order to achieve the enhanced reflectivity, grating strips and grooves must be formed on a substrate having a substantially different refractive index from that of the grating strips. Alternatively the strips and grooves of the grating may be formed (as depicted in FIG. 2) in a layer having a uniform portion  46 A as discussed above, with a medium, layer or substrate of a material having a substantially different refractive index interfacing with the plane face of the layer (in the example of FIG. 2, this is layer  52 ). 
     Continuing now with a more detailed description of above conditions and constraints for the configuration of grating  46 , FIG. 3 depicts a grating layer  46  in optical contact with a substrate  51 . Grating  46 , here, comprises grating strips  48  in direct optical contact with a substrate  51 . Light is incident on the grating at normal incidence from a lossless medium having a refractive index no, which in examples discussed below is 1.0, i.e., the refractive index of air or vacuum. Substrate  51  is assumed to be of effectively infinite extent as far as optical interference is concerned, i.e., any incident light passing through the grating into the substrate is not reflected back. Grating strips  48  have a refractive index n 1 , here, greater than n 0 . The substrate has a refractive index n s . If value of n s  were 1.0, grating  46  would consist effectively of a parallel array of strips suspended in air. If value of n s  were equal to n 1 , grating  46  would consist effectively of a crenellated or corrugated interface between a medium having a refractive index n s  and a medium having a refractive index n 0 , i.e., there would be no other interface to provide optical interference. 
     Important dimensions of the grating are the depth of spaces (height of strips) D, the grating period P, the strip width S and the spacing or groove width G, and a ratio S:P referred to hereinafter as the duty cycle (DC) of the grating. D is referred to hereinafter as the grating depth. In all examples and embodiments discussed herein, it is important that grating period P is less than the shortest wavelength to be reflected or transmitted by the grating. The grating may be referred to as a sub-wavelength grating for this reason. Providing that P is less than one wavelength is important in providing that light is reflectively diffracted or transmitted in only the zero-order. 
     It is assumed in all examples and embodiments discussed herein that DC is 0.50. This should not be considered as limiting the present invention. Varying DC can be expected to vary spectral properties of a grating membrane mirror in accordance with the present invention. Varying DC is particularly effective in varying polarization separation if variation of other parameters is constrained by other design requirements. 
     Important values in the grating are the zero-order diffraction efficiency 0R (specular reflectivity) and the zero-order transmissivity 0T (specular transmissivity). Value 0R is specified herein at a hypothetical plane coincident with the top of grating strips  48 . Transmissivity 0T is specified herein as a value immediately below the substrate surface and will be absorbed or transmitted by the substrate depending on the absorption coefficient of the substrate. The values of 0R and 0T will be discussed with reference to light polarized with its electric field vector either parallel (TE polarization) or perpendicular (TM polarization) to grating strips and spaces  48  and  50 . 
     FIGS. 4A and 4B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency 0R as a function of grating depth D and grating period P for normally incident light having a wavelength of 1.550 μm, in respectively TE and TM orientations. In this and similar contour graphs discussed hereinbelow, the contours are contours of equal zero-order diffraction efficiency (specular reflectivity) and are labeled with the appropriate value. In this example, grating strips  48  are assumed to have a refractive index n 1  of about 3.5. The refractive index n s  of substrate  74  is assumed to be identical with n 1 . Accordingly, grating  46 , here, is simply a corrugated interface between a medium having a refractive index of 3.5 and a medium having a refractive index of 1.0. Incident light passes through the grating but is not reflected back from any interface below the grating. 
     In the TE orientation (FIG.  4 A), 0R has a relatively small variation over a parameter space bounded by P between about 0.5 and 0.8, and D between about 0.2 and 0.8. In the TM orientation (FIG. 4B) 0R is generally relatively low (less than 0.2) over most of the parameter space of FIG.  4 B. The maximum value of 0R in any polarization orientation is about 0.306. This is about the same value as the Fresnel reflectivity of an interface between a medium having a refractive index 1.0 and a medium having a refractive index of 3.5. In the example FIGS. 4A and B there is no enhancement of reflection by the grating. There is, however, a difference between 0R in the TE and TM polarizations over most of the parameter the space. 
     FIGS. 5A and 5B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency 0R as a function of grating depth D and grating period P for normally incident light having a wavelength of 1.550 μm, in respectively TE and TM orientations. Here, grating strips  48  are assumed to have a refractive index n 1  of about 3.5. The refractive index n s  of substrate  51  is assumed to be 2.5. Incident light passes through the grating, and a portion of that light not reflected by the grating is reflected from the interface between the grating and the substrate. 
     The maximum value of 0R in any polarization orientation is about 0.625. This value occurs for TM polarization, and is higher than the Fresnel reflectivity of an interface between a medium having a refractive index 1.0 and a medium having a refractive index of 3.5, and higher than the reflectivity of a quarter-wave thick layer having a refractive index 3.5. The maximum reflectivity for TE polarization is about 0.461. This is higher than the Fresnel reflectivity of an interface between a medium having a refractive index 1.0 and a medium having a refractive index of 3.5, but lower than the reflectivity of a quarter-wave thick layer having a refractive index 3.5 (about 0.51). An air suspended uniform-thickness layer having a quarter-wave optical thickness of any material provides the highest reflectivity that can be obtained at normal incidence in a single uniform thickness layer of that material. In the example FIGS. 5A and 5B the grating can be considered as enhancing reflection for TM polarized light only. 
     FIGS. 6A and 6B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency 0R as a function of grating depth D and grating period P for normally incident light having a wavelength of 1.550 μm, in respectively TE and TM orientations. Here, grating strips  48  are assumed to have a refractive index n s  of about 3.5. The refractive index n s  of substrate  74  is assumed to be 1.5. Again, incident light passes through the grating, and a portion of that light not reflected by the grating is reflected from the interface between the grating and the substrate. The reflectivity from the substrate-grating interface is higher in this example than in the example of FIGS. 5A and 5B. The maximum value of 0R in both polarization orientations is greater than 0.99. In the example FIGS. 6A and 6B the grating can be considered as substantially enhancing reflection for both TE and TM polarized light. From FIG. 6B it can be seen that substantial enhancement of reflectivity for TM polarization can be achieved in wide area of the total parameter space. 
     FIGS. 7A and 7B are contour graphs schematically illustrating calculated variation of zero-order reflected diffraction efficiency 0R as a function of grating depth D and grating period P for normally incident light having a wavelength of 1.550 μm, in respectively TE and TM orientations. Here, grating strips  48  are assumed to have a refractive index n 1  of about 3.5. The refractive index n s  of substrate  74  is assumed to be 1.0. This represents the grating strips as simply being suspended in a plane array, in air, an arrangement that may be difficult to achieve in practice. 
     The maximum value of 0R occurs for TM polarization, and is in excess of 0.9998. In the example FIGS. 7A and 7B the grating can be considered as substantially enhancing reflection for both TE and TM polarized light. It can be seen from FIG. 7B that substantial enhancement of reflectivity for TM polarization can be achieved in wide area of the total parameter space. 
     FIG. 8 is a contour graph schematically illustrating variation of the absolute value of the difference in zero-order reflected diffraction efficiency between TE and TM polarizations with grating depth and grating period in the grating of FIGS. 7A and 7B. It can be seen this difference exceeds 0.9 only in relatively small areas in the total parameter space. The largest of these areas occurs around a grating period of about 0.7 μm and a grating depth of about 0.5 micrometers. 
     At this point in the discussion, analysis has concentrated on determining optimum arrangements for providing grating-enhanced reflectivity and polarization separation at one wavelength, here 1.550 μm. As a mirror in accordance with the present invention is required to transmit pump light at 0.980 μm or 1.30 μm, it is important to examine the grating effects at a range of wavelengths including these pump-light wavelengths. A discussion of the wavelength (spectral) response of one configuration of the hypothetical “air-suspended” grating of FIGS. 7A and 7B is set forth below 
     FIG. 9 is a graph schematically illustrates reflected zero-order diffraction efficiency as a function of wavelength for TE and TM polarizations for a grating depth D of 0.5 μm and a grating period of 0.75 μm in the grating of FIGS. 7A and 7B. These grating parameters are selected to provide maximum difference in polarization at a wavelength of 1.550 μm. There is a broad, spectrally flat reflection response for TM polarization (solid curve) at wavelengths longer than 1.550 μm. Diffraction efficiency in this region varies between about 0.98 and 0.99. This is a useful value range for mirror  31  of FIG.  1 . Unfortunately, most of this range falls outside a desired tuning range of between 1.530 and 1.565 μm. There is a broad reflection minimum between about 1.28 μm and 1.38 μm. This would allow adequate transmission at normal incidence for 1.30-μm TM-polarized pump light. Transmission at 0.980 μm in either polarization plane is less than optimum. 
     FIG. 10 is a graph schematically illustrating reflected diffraction efficiency as a function of wavelength for TE and TM polarizations for a grating depth D of 0.45 micrometers and a grating period P of 0.65 micrometers in the grating of FIGS. 7A and 7B. These grating parameters are selected to provide maximum reflectivity around a wavelength of 1.550 μm, regardless of polarization separation. The broad, spectrally flat reflection response for TM polarization (solid curve) is centered about 1.550 μm. Diffraction efficiency in the 1.530 μm to 1.565 μm region varies between about 0.998 and 0.999. This is higher than would be optimum for mirror  31  of FIG. 1 in most cases. Reflection at 1.30 μm in both polarization planes is sufficiently high that transmission would be less than adequate. Reflection at 0.980 μm in either polarization orientation is sufficient to provide adequate transmission. 
     FIG. 11 schematically illustrates an embodiment  31 A of a grating membrane mirror in accordance with the present invention, wherein the grating  46  comprises grating strips  48  supported on a surface  45  of a membrane portion  46 A. Grating parameters are specified as discussed above with reference to FIG.  3 . Membrane portion  46 A has a uniform thickness U. Zero-order reflection diffraction efficiency is specified at a plane coincident with the top of grating strips  48 . Zero-order transmissivity is specified at surface  47  of membrane portion  46 A. 
     It is assumed, here, that grating membrane  46  is suspended in air. As such, the refractive index n 0  on both sides of the membrane grating is assumed to be 1.0. Grating strips  48  have refractive index n 1 , which, in all examples discussed below, is assumed to be the refractive index of polysilicon. This is about 3.502 at a wavelength of 1.550 μm. The refractive index n 2  of membrane portion  46 A is assumed to be less than the refractive index of the grating strips, but, of course, is greater than that of air. Accordingly, a portion of light passing through membrane portion  46 A and incident on surface  47  thereof will be reflected from the surface as indicated by arrow R F . The magnitude of the reflection will be determined by the Fresnel reflection at surface. This, of course, will be higher, the higher the value of refractive index n 2 . 
     It is important that the thickness U of the membrane portion is sufficiently uniform that reflected light R F  can optically interfere with light at the grating strips. This provides that the thickness U of grating portion  46 A can be adjusted to vary spectral properties of grating  46  as a whole. 
     The thickness uniformity requirements for providing optical interference are well known to those skilled in the art to which the present invention pertains. Such uniformity is readily achievable in practice by forming membrane portion  46 A by a layer deposition process such as sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), or the like. A detailed description of fabrication of grating membrane mirrors in accordance with the present invention is presented further hereinbelow. 
     FIG. 12 schematically illustrates another embodiment  31 B of a grating membrane mirror in accordance with the present invention. This embodiment is similar to the grating membrane mirror of FIG. 11 with an exception that uniform thickness portion  46 A of grating  46  is formed of the same material as that of grating strips  48 . Accordingly the uniform thickness portion has no upper surface in the mechanical sense. Thickness U of portion  46 A, here, is measured between lower surface  47  and a virtual plane  45 A coincident with the base of grating strips  48 . 
     FIG. 13 is a graph schematically illustrating calculated zero-order diffraction efficiency 0R for TM polarization as a function of wavelength and thickness of the membrane portion of an example of the grating, membrane mirror of FIG. 11, wherein grating strips  48  are assumed to be fabricated from polysilicon and membrane portion  46 A is assumed to be fabricated from CVD deposited silicon dioxide (SiO 2 ) having a refractive index of about 1.42 at a wavelength of about 1.550 μm. Grating depth D is 0.45 μm and grating period P is 0.65 μm. Diffraction Efficiency as a function of wavelength has the same general form, regardless of whether uniform thickness or membrane portion  46 A of the mirror has a thickness of one-quarter, three eighths, one-half, or three-quarters of wavelength at a wavelength of 1.550 μm. The value of the diffraction efficiency at 1.550 μm for any of the exemplified thicknesses U is greater than 0.998, which, as noted above, is higher than optimum for a mirror in accordance with the present invention. Although not shown in FIG. 11, it has been determined that calculated zero-order diffraction efficiency for TM polarization as a function of wavelength is also insignificantly affected by variation in U. 
     FIG. 14 is a graph schematically illustrating calculated zero-order diffraction efficiency 0R for TE and TM polarization as a function of wavelength and thickness of the membrane portion of a grating membrane mirror in accordance with the present invention, in the configuration of FIG.  12 . Grating depth D is 0.45 μm and grating period P is 0.65 μm. Refractive index n 1  is assumed to have a value of about 3.5 at a wavelength of about 1.550 μm. Here, there are significant variations in 0R as a function of wavelength for both TE and TM polarization. 
     Comparing FIG.  14  and FIG. 10, for example, it can be seen that the broad high diffraction efficiency (reflection) band centered around a wavelength 1.550 μm in FIG. 10 (where U is zero) is shifted significantly to shorter wavelengths when U is selected such that membrane portion  46 A has an optical thickness of one-quarter wavelength at 1.550 μm. The solid curve of FIG. 14 indicates that this band is shifted such that it is centered around a wavelength of about 1.38 μm. The 1.550 μm wavelength is outside of the high diffraction efficiency region. Increasing the optical thickness of the membrane portion of the grating to three-quarters of a wavelength at a wavelength of 1.550 μm causes a reduction of diffraction efficiency in the broad, high diffraction efficiency region. 
     This sensitivity of the grating configuration of FIG. 12, not only to variations in grating period P, but to the optical thickness of the membrane portion indicates that some combination of these variables may provide in a membrane grating formed from only one material a mirror that has a reflectivity less than between about 0.97 and 0.99 at 1.550 μm (or some other lasing wavelength) and a high transmission (about 75 percent or greater) at some other wavelength for allowing transmission of pump light through the mirror. As noted above, one suitable reflectivity value for mirror  31  in the laser of FIG. 1 is about 0.98 over a range of wavelengths from about 1.530 μm to about 1.550 μm. Pump light may have a wavelength of 0.980 μm or about 1.30 μm. 
     In one set of design calculations for such a mirror, a grating, membrane mirror in the configuration of FIG. 12 was assumed to be made of polysilicon having a refractive index of about 3.5 at a wavelength of about 1.550 μm. Grating depth D and grating period P were initially set at 0.45 and 0.65 respectively. Membrane optical thickness U was initially set at 0.66 μm, about three half-wavelengths optical thickness at a wavelength of about 1.550 μm. The particular membrane thickness was selected among other reasons as that which would provide mechanical integrity in the membrane. The calculated diffraction efficiency for TM polarization as a function of wavelength showed a broad, shallow modulated, high diffraction efficiency region extending from a wavelength of about 1.40 μm to a wavelength of about 1.62 μm. A portion of this range had the desired diffraction efficiency 0R about 0.98 near the mid point of the range. Next, a contour plot was calculated having D and P as parameters to determine adjustments necessary to center the 0.98 diffraction efficiency band over the 1.55-μm wavelength. This involved adjusting D from 0.45 to 0.46. Following this adjustment, a contour plot having U and P as parameters was calculated to determine an adjustment to thickness U that would provide a minimum diffraction efficiency (maximum transmissivity) at a wavelength of 0.980 μm. U was adjusted from 0.66 μm to 0.675 nm. This maximized the 0.980 μm transmission and set the diffraction efficiency around 1.550 μm to about 0.982. 
     FIG. 15 is a graph schematically illustrating calculated zero-order diffraction efficiency for TE and TM polarization as a function of wavelength for the grating membrane mirror having the adjusted D, P and U parameters in a wavelength range between 0.9 μm and 1.8 μm. The low TM diffraction efficiency at 0.980 μm is clearly visible. FIG. 16 is a graph schematically illustrating detail of calculated zero-order reflection diffraction efficiency for TE polarization (dotted curve) and TM polarization (solid curve) as a function of wavelength between wavelengths of 1.52 and 1.57 μm of the grating, membrane mirror of FIG.  15 . This shows excellent polarization separation throughout the range and a minimal variation of the TM diffraction efficiency over the desired tuning range of 1.53 μm and 1.565 μm. 
     FIG. 17 is a graph schematically illustrating detail of calculated zero-order transmission efficiency for TE, TM, and average polarization as a function of wavelength between wavelengths of 0.94 and 1.02 μm in the grating, membrane mirror of FIG.  15 . Here, although the grating has been configured for peak TM polarization transmission at pump-light wavelength of 0.980 μm at normal incidence, transmission for TE polarization is comparable. This would allow the use of unpolarized light as pump light. This is advantageous inasmuch as light from the diode-laser arrays typically used to provide such light is typically delivered along an optical fiber, and is unpolarized. 
     FIG. 18 is a graph schematically illustrating detail of calculated zero-order transmission efficiency for TE “P” polarization at 20 degrees light incidence as a function of wavelength between wavelengths of 1.26 and 1.34 μm in the grating, membrane mirror of FIG.  15 . At this angle of incidence transmissivity for 1.30 μm pump light is peaked at greater than 0.84. 
     It should be noted, here, that in a conventional multilayer dielectric mirror used at non-normal incidence, and accordingly polarization sensitive, polarization orientation could be specified only with respect to the plane of incidence. Polarization orientation is conventionally designated “P” or “S” depending on whether the electric field vector is respectively parallel of perpendicular to the plane of incidence. In a grating membrane mirror in accordance with the present invention, there are “P” and “S” orientations for conditions where the plane of incidence is parallel or perpendicular to the grating strips, and the spectral response of the mirror in either orientation may be different. 
     FIG. 19 schematically illustrates an embodiment of the inventive grating membrane mirror  31 C including two uniform-thickness optical interference layers  52  and  54  and a grating  46  including a uniform-thickness (optical interference) portion  46 A and grating strips  48  spaced apart by grooves  50 . Grating  46 , as discussed above, is preferably made from a high refractive index material such as polysilicon. Layer  52  has a lower refractive index than that of grating  46 . Layer  54  is preferably made from the same material as grating  46 . 
     In one preferred method of forming grating  46 , sufficient material is deposited to form a uniform-thickness layer having a thickness equal to the sum of the desired thickness of portion  46 A and the desired depth or height of grating strips  48 . A mask including slit-like apertures having the desired width of the grating strips is then lithographically formed on the layer. The layer and the mask are reactively ion-etched until grooves  50  and strips  48  having the desired depth are provided, thereby forming grating  46 . Anything that remains of the mask is chemically removed from the layer to leave the finished grating. Layers  48  may also be formed by depositing only sufficient material to form a layer having the desired thickness of uniform thickness portion  46 A of the grating; lithographically forming mask including slit-like apertures having the desired width of the grating strips; depositing sufficient material on the mask and into the slits to form grating strips  48 ; then chemically removing the mask and any material thereon to leave grating strips  48  on uniform thickness portion  46 A of the finished grating. 
     FIG. 20 schematically illustrates another embodiment  31 D of the inventive grating membrane mirror  31  including two uniform-thickness optical interference layers  52  and  54  and a grating  46  including only grating strips  48  spaced apart by grooves  50 . Grating strips  48 , as discussed above, are preferably made from a high refractive index material such as polysilicon. Layer  52  has a lower refractive index than that of the grating strips. Layer  54  is preferably made from the same material as the grating strips. 
     The use of multiple optical interference layers in combination with the grating is particularly useful in cases where the inventive grating membrane mirror is required to have specific reflection or transmission properties at two or more different wavelengths but is not required to have a high degree of polarization separation at any one of the wavelengths. If a high reflectivity, for example about 0.95 or greater, is desired at one wavelength, most practical combinations of layers in mirrors  31 C and  31 D will not provide such a reflectivity value through optical interference alone, particularly if the layers are configured to provide another value of reflectivity or transmission at another wavelength. The grating will be required to enhance the reflectivity of the layers to provide the desired high reflectivity value. The multiple uniform thickness layers will, reduce the amount of enhancement necessary compared with above-discussed single layer gratings wherein the grating is primarily responsible for providing a high reflectivity. This will allow more flexibility in the choice of grating parameters, which, in turn can provide that the grating parameters may also be selected to contribute to providing desired optical properties at other wavelengths. 
     It is emphasized that a grating membrane mirror in accordance with the present invention is intended for use a resonator mirror in a laser, and that the primary objective of the grating portion of the mirror is to provide sufficiently different zero-order diffraction efficiency (specular reflection) at a lasing wavelength for TE and TM polarizations that the laser will only lase in that polarization that has the higher reflectivity. As noted above, this reflectivity difference may be as little as about 1.0% (0.01 in decimal notation). Because of this it is possible to consider embodiments of the inventive grating mirror including multiple optical interference layers in which the grating does not provide any reflection enhancement, and may even contribute to reducing reflectivity provided by the multiple interference layers. 
     FIG. 21 is a cross-section view schematically illustrating an embodiment  31 E of a grating membrane mirror in accordance with the present invention including four uniform-thickness dielectric layers  52 ,  54 ,  55 , and  57  and a grating layer  46  including a uniform-thickness portion  46 A. Grating  46  and layers  54  and  57  are formed from a relatively high refractive index material such as polysilicon. Layers  52  and  55  have a lower refractive index than that of layers  54  and  57  and preferably include a material such as silicon dioxide (SiO 2 ). 
     FIGS. 22 and 23 are graphs schematically illustrating calculated zero-order diffraction efficiency for TE (dotted curve) and TM (solid curve) polarization as a function of wavelength in one example of grating membrane mirror  31 E of FIG. 21 arranged to provide reflection of about 0.98 at wavelengths between about 1.53 and 1.565 μm and transmission greater than 0.70 for light having a wavelength of about 0.980 μm and at 20.0 degrees incidence. Grating  46  and layers  54  and  57  are assumed to be of polysilicon, and layers  52  and  55  are assumed to be layers of SiO 2 . Uniform thickness portion  46 A of grating  46  has a thickness of 0.094 μm. Grating strips  48  have a depth D of 0.220 μm. The grating period is 0.575 μm with a duty cycle of 0.5. Layer  52  has a physical thickness of 0.287 μm; layer  54  has a physical thickness of 0.146 μm; layer  55  has a physical thickness of 0.300 μm and layer  57  has a physical thickness of 0.116 μm. 
     FIG. 24 is a graph schematically illustrating detail of zero-order transmissivity for TE (“P” polarized) and TM polarization at 20 degrees incidence as a function of wavelength at wavelengths between about 0.90 and 1.10 μm. There is a broad transmission region between wavelengths of about 0.94 and 1.06 μm for TE polarization only. 
     The following differences between grating membrane mirror  31 E and above discussed embodiments  31 A and  31 B (wherein there is only a grating  46 ) should be noted. The grating depth is substantially less; the difference in diffraction efficiency at wavelengths between about 1.53 and 1.565 μm is substantially less; and the TE diffraction efficiency is higher than the TM diffraction efficiency at wavelengths between about 1.53 and 1.565 μm, which is the opposite in above discussed examples wherein there is only a grating  46 . Referring in particular to FIG. 23, it can be seen that the zero-order diffraction efficiency for both TE and TM polarizations is less than the reflectivity (dashed curve) would be would be if grating strips  48  were removed from the grating. In mirror  31 E, the grating reduces rather than enhances the reflectivity that would be provided by the uniform thickness layers and the uniform thickness portion of the grating alone. 
     One advantage of the above-discussed example of mirror  31 E compared with embodiments including two or less uniform thickness layers is that the transmission at 0.980 μm occurs in a relatively broad transmission band. In other above discussed embodiments the transmission band including 0.98 μm is considerably narrower (compare FIGS.  24  and FIG.  17 ). In the narrower transmission region there can be considerable enhancement of intrinsic absorption in the layer materials. This could be a problem in applications where high pump power is required. Another advantage is that the grating depth is smaller compared with the total thickness of the mirror than in above discussed examples. This will provide that there is a lesser difference between the stiffness of the mirror parallel and perpendicular to the grating strips, resulting in lesser deformation of the membrane in flexure. Clearly, however, these advantages are achieved at the expense of a more complicated structure and a more costly manufacturing process. 
     A description of one preferred manufacturing process for mirror  31 E is set forth below with reference to FIGS. 25A-K. The method involves a number of lithographic, layer deposition and etching steps. Each drawing depicts a stage in the fabrication process. Lithographic steps are not depicted in the drawings in order to minimize the number of drawings required for the description. Lithographic steps necessary to perform the various operations will be evident to those skilled in the art to which the present invention pertains. 
     In the preferred method, substrate  44  is formed from a 100-millimeter diameter &lt;100&gt; silicon wafer having a thickness of about 300 μm. Layers  80  and  82  of silicon dioxide are grown by plasma enhanced chemical vapor deposition (PECVD) on surfaces  44 B and  44 A respectively of substrate  44  (see FIG.  25 A). Layer  80  preferably has a thickness of about 2.0 μm and layer  82  has a thickness of about 1.0 μm. 
     Next, layer  84  of silicon (polysilicon) having a thickness of about 0.116 μm is grown by a low-pressure CVD (LPCVD) on layer  80  (see FIG.  25 B). A corresponding layer  86  is grown on layer  82 . An ion implantation step is performed in these areas to provide electrically conductive areas  84 C in layer  84  (see FIG.  25 C). 
     Following the ion implantation step, layer  88  of low stress silicon dioxide having a thickness of 0.300 μm is grown on layer  84  (see FIG.  25 D). Annular apertures  90  are etched through the layer to conductive area  84 C of underlying silicon layer  84 . 
     Referring now to FIG. 25E, a polysilicon layer  92  having a thickness of 0.146 μm is grown on layer  88 . This layer fills apertures  90 . Another layer  94  of low-stress silicon dioxide having a thickness of 0.287 μm is then grown on polysilicon layer  92  (see FIG.  25 F). Circular apertures  96  are etched through this layer to the underlying polysilicon layer  92  above doughnut-shaped apertures  90 . 
     Next, a layer  98  of polysilicon having a thickness of about 0.094 μm is deposited on layer  94  by LPCVD (see FIG.  25 G). Material of the layer partially fills apertures  96 . Following deposition of layer  98 , a dry-etching step is performed to provide silicon-lined apertures  100  extending through conductive portion  84 C of layer  84  (see FIG.  25 H). The dry-etching step also removes bottom layer  86 . 
     Referring to FIG. 25I grating strips  48  having a height of 0.220 μm and being separated by spaces  50  having are formed on layer  98 . The grating period is 0.575 μm and the duty cycle is 0.50. In the finished mirror layer  98  provides uniform thickness portion  46 A of the grating  46 . It should be noted here that in alternative method of providing grating strips layer  98  could initially be made 0.317 μm thick and spaces  50  and strips  48  formed by reactive ion etching as discussed above. 
     Referring next to FIG. 25J, undercut-etching to free a portion of mirror  31  from substrate  44  is performed. This begins with plasma etching aperture  72  (of FIG. 1) in oxide layer  82 . Aperture  72  is then extended through substrate  44  by induction coupled plasma (ICP) etching. The ICP etching is terminated at oxide layer  80 . Chemical etching is then performed to remove a sufficient portion of layer  80  to free a central portion of mirror  31  such that mirror becomes a membrane mirror supported at its periphery, via the remains of layer  80 , on substrate  44 . Removing the oxide layer defines the gap  62  (of FIG. 1) between mirror  44  and surface  44 B of substrate  44 . At this point, what remains of oxide layer  82  is also removed from substrate  44 . Metallization is provided in apertures  100  and on surface  44 A of substrate  44  (see FIG. 25K) to provide electrical contacts  104  and  46  respectively. Leads  58  and  60  of FIG. 1 are attached to contacts  104  and  46  respectively. It should be noted that layers  94 ,  92 ,  88 , and  84  correspond to layers  52 ,  54 ,  55 , and  57  of FIG.  24 . Layer  98  corresponds to uniform  46 A of grating  46  as noted above. 
     From the description of membrane steps provided above, one skilled in the art could readily determine steps necessary to fabricate a grating membrane mirror including fewer layers. A grating membrane mirror in accordance with the present invention having only a grating  46 , could be fabricated using about eight less lithographic and layer deposition steps than are described above. 
     In the above-presented description of the present invention grating  46  is considered to be a square wave grating profile. This should not be construed as limiting the present invention. Grating  46  may have other profiles without departing from the spirit and scope of the present invention, including, for example a sinusoidal profile. In a sinusoidal profile depth D would be the peak to valley height of the sinusoid. Strip width would be measured at 50% of the peak-to-valley depth. The grating parameters could be determined using parametric contouring in the same manner as discussed above with reference to “square-wave” gratings. 
     The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.