Abstract:
An interferometer comprises a membrane, a substrate, and a support structure. The membrane comprises a first reflector. The substrate comprises a second reflector. The support structure circumferentially couples the membrane to the substrate and orients the first reflector parallel to and facing the second reflector.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of Fabry-Perot interferometers. More particularly, this invention relates to the field of Fabry-Perot interferometers having a micro-electro-mechanical structure. 
   BACKGROUND OF THE INVENTION 
   Charles Fabry and Alfred Perot invented the Fabry-Perot interferometer in the late 1800&#39;s. The Fabry-Perot interferometer includes two glass plates that have been lightly silvered on facing surfaces. The glass plates are arranged parallel to each other so that the lightly silvered surfaces produce an interference cavity defined by a separation distance between the glass plates. If the separation distance is fixed, the Fabry-Perot interferometer is referred to as a Fabry-Perot etalon. 
   In either the Fabry-Perot interferometer or the Fabry-Perot etalon, the interference cavity causes multiple beam interference. The multiple beam interference occurs when first and second partially reflecting surfaces are oriented parallel to each other and illuminated by light. Provided that reflection coefficients for the first and second partially reflecting surfaces are not small, the light reflects between the two partially reflecting surfaces multiple times. This produces a transmitted multiple beam interference for the light exiting the second surface in a forward direction and a reflected multiple beam interference for the light exiting the first surface in a reverse direction. 
   If the Fabry-Perot interferometer is illuminated by a broad light source and the transmitted multiple beam interference is collected by a focusing lens, a circular interference pattern is produced on a screen at a focal length of the focusing lens. The circular interference pattern exhibits bright narrow rings of light separated by larger dark rings. 
   Goossen et al. in “Silicon modulator based on mechanically-active anti-reflection layer with 1 Mbit/sec capability for fiber-in-the-loop applications,”  IEEE Phtonics Technology Letters , Vol. 6, No. 9, September 1994, pp. 1119–1121, teach a mechanical anti-reflection optical switch. The optical switch consists of a SiN x  membrane suspended over a Si substrate. The SiN x  membrane has a square shape and is suspended from corners by arms. The SiN x  layer has a thickness of a quarter wavelength of incident light. A SiN x  index of refraction for the SiN x  layer is a square root of a Si index of refraction for the Si substrate. When an air gap separating the SiN x  membrane from the Si substrate is an even multiple of a quarter wavelength, an antireflection condition exists. When the air gap is an odd multiple of a quarter wavelength of the incident light, a high reflection condition exists. The optical switch is in an off-state when the anti-reflection condition exists and is an on-state when the high reflection condition exists. 
   Fabricating the SiN x  membrane so that the SiN x  index of refraction is the square root of the Si index of refraction is difficult. Further, fabricating the arms and the SiN x  membrane in a reproducible manner so that production devices operate in a similar manner is difficult. Moreover, it is desirable to have an optical switch which is more economical to produce than the optical switch taught by Goossen et al. 
   Miles, in U.S. Pat. No. 5,835,255 issued on Nov. 10, 1998 and entitled, “Visible Spectrum Modulator Arrays,” teaches a micro-fabricated interferometric light modulator. The micro-fabricated interferometric light modulator includes a transparent substrate and a rectangular membrane suspended above the substrate. The transparent substrate includes first and second surfaces, and also includes a transparent film on the second surface. The transparent film is conductive. A mirror, either a metal or dielectric mirror, lies on the transparent film. The membrane is suspended above the mirror by parallel support structures, which support two edges of the rectangular membrane. The membrane is both reflective and conductive. The membrane and the mirror form an interferometric cavity which is modulated by biasing the membrane relative to the transparent film. In operation, the micro-fabricated light modulator modulates light incident upon the first surface of the transparent substrate by interferometrically causing the incident light to exit the first surface or by interferometrically causing the incident light to not exit the first surface. 
   Miles further teaches an alternative micro-fabricated interferometric light modulator in which the membrane is a square membrane. The square membrane is suspended by arms from centers of each of four lengths defining the square membrane. 
   Fabricating the transparent and conducting film of the micro-fabricated light modulators is difficult. Further, keeping a separation distance defining the interferometric cavity of the micro-fabricated light modulators constant across the interferometric cavity is difficult. Additionally, the combination of the rectangular membrane and the parallel support structures gives rise to a tendency for the rectangular membrane to deform cylindrically. The cylindrical deformation of the rectangular membrane reduces the effectiveness of the interferometric cavity. Moreover, it is desirable to have an interferometric light modulator which is less costly to manufacture and which is more reproducible than the micro-fabricated interferometric light modulators taught by Miles. 
   What is needed is an interferometric light modulator which is economical to fabricate, which is more easily reproducible in a production setting, which does not rely on a rectangular membrane supported by parallel support structures, and which does not rely on arms to support a moving surface. 
   SUMMARY OF THE INVENTION 
   The present invention is an interferometer. The interferometer comprises a membrane, a substrate, and a support structure. The membrane comprises a first reflector. The substrate comprises a second reflector. The support structure circumferentially couples the membrane to the substrate and orients the first reflector parallel to and facing the second reflector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  isometrically illustrates the preferred interferometer of the present invention. 
       FIG. 2  illustrates a first and second multilayer reflector and a substrate of the preferred interferometer of the present invention. 
       FIG. 3  graphically illustrates a transmitted wavelength of the preferred interferometer of the present invention. 
       FIG. 4  graphically illustrates a transmitted wavelengths of the preferred interferometer of the present invention over a telecommunications C band. 
       FIG. 5  illustrates a WDM (wavelength division multiplex) channel monitor employing the preferred interferometer of the present invention. 
       FIG. 6  illustrates a first alternative interferometer of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The preferred interferometer of the present invention is illustrated in  FIG. 1 . The preferred interferometer  10  comprises a membrane  12 , a plurality of posts  14 , and a substrate  16 . The membrane  12  comprises a first reflector  18  and a flexible annular member  20 . Preferably, the membrane  12  further comprises release slots  21 . Alternatively, the membrane  12  does not include the release slots  21 . The substrate  16  preferably comprises a second reflector  22 , a transparent bulk material  24 , and an anti-reflective coating  26 . Alternatively, the substrate  16  does not include the anti-reflective coating  26 . 
   In the preferred interferometer  10 , the membrane  12  couples to the substrate  16  via the plurality of posts  14  and a membrane extension  28 . The plurality of posts  14  and the membrane extension  28  hold the membrane  12  in bi-axial tension. The bi-axial tension in the membrane  10  maintains the first reflector  18  parallel to the second reflector  22 . Preferably, the posts  14 , the flexible annular member  20 , the membrane extension  28  comprise a resilient material. Alternatively, only the flexible annular member  20  comprises the resilient material. Preferably, the resilient material comprises Si 3 N 4 . Alternatively, the resilient material comprises another material with resilient properties. 
   An advantage of the preferred interferometer  10  is that the biaxial tension in the membrane  10  results in the first reflector having a highly flat surface, which promotes parallelism of the first and second reflectors,  18  and  28 . 
   The first reflector  18  includes a first conducting layer. An electrical conductor  32  couples to the first conducting layer, which provides an electrical biasing path for the first conducting layer. The second reflector  28  includes a second conducting layer. In the preferred interferometer  10 , the first and second reflectors,  18  and  28 , form an interferometric cavity  30 . The interferometric cavity  30  is adjusted by electrically biasing the first conducting layer relative to the second conducting layer. This causes the first reflector  18  to move relative to the second reflector  22  adjusting a cavity length for the interferometric cavity  30 . 
   The preferred interferometer  10  transmits light when the cavity length is an integral multiple of a half wavelength of the light. Otherwise the preferred interferometer  10  reflects light. If first, second, and third light wavelengths, λ 1 , λ 2 , and λ 3 , are incident upon the preferred interferometer  10  and the cavity length is an integral multiple of only the second light wavelength λ 2 , the second wavelength λ 2  transmits and the first and third light wavelegnths, λ 1  and λ 3 , reflect. 
   A partial cross-section of the preferred interferometer  10  is further illustrated in  FIG. 2 . The partial cross section  40  comprises the first reflector  18  and the substrate  16 . The substrate  16  comprises the second reflector  22 , the transparent bulk material  24 , and the anti-reflective coating  26 . The first reflector  18  and the second reflector  22  form the interferometric cavity  30 . 
   Preferably, the preferred interferometer  10  operates over telecom C and L bands, which comprises light of wavelength within the range of 1,520 to 1,620 nm. Alternatively, the preferred interferometer operates over a different light wavelength band. 
   It will be readily apparent to one skilled in the art that the preferred interferometer is appropriate for operation over wavelength bands other than the telecom C and L bands. 
   For operation over an infra-red telecommunications band, the first reflector  18  preferably comprises a first multilayer reflector. The first multilayer reflector preferably comprises first and second encapsulating layers,  42  and  43 , of Si 3 N 4  and alternating layers of low refractive index material  44  comprising SiO 2  and high refractive index material  46  comprising poly-Si (polycrystalline Si). Preferably, the first multilayer reflector comprises four pairs of the alternating layers plus an extra layer of the low refractive index material  44 . Alternatively, the first multilayer reflector comprises more or less than four pairs of the alternating layers. 
   For operation over the infra-red telecommunications band, the second reflector  22  preferably comprises a second multilayer reflector. The second multilayer preferably comprises a third encapsulating layer  48  and the alternating layers of the low refractive index material  44  comprising SiO 2  and the high refractive index material  46  comprising poly-Si. Preferably, the second multilayer reflector comprises four pairs of the alternating layers plus an extra layer of the low refractive index material  44 . Alternatively, the second multilayer reflector comprises more or less than four pairs of the alternating layers. 
   The first, second, and third encapsulating layers,  42 ,  43  and  48 , and the alternating layers of the low refractive index material  44  and high index of refraction material  46  preferably comprise quarter wavelength films. Alternatively, the alternating layers comprise a range of thicknesses about a quarter wavelength, which broadens an operational wavelength band. The quarter wavelength films have optical path lengths of a quarter wavelength of an intermediate wavelength within the infra-red telecommunications band. Choosing the intermediate wavelength as 1,550 nm gives the qurater wavelength as 387.5 nm. Layer thicknesses are determined by dividing the quarter wavelength by the refractive index. Table 1 provides the refractive indexes and the layer thicknesses for Si 3 N 4 , SiO 2 , and poly-Si. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Material 
               Refractive index 
               Layer Thickness 
             
             
                 
                 
             
           
           
             
                 
               Si 3 N 4   
               2.00 
               193.8 nm 
             
             
                 
               SiO 2   
               1.44 
               269.1    
             
             
                 
               Poly-Si 
               3.63 
               106.7    
             
             
                 
                 
             
           
        
       
     
   
   For operation over the infra-red telecommunications band, the transparent bulk material preferably comprises Si. Alternatively, the transparent bulk material comprises SiO 2 . The anti-reflective coating  26  preferably comprises a quarter wavelength film of Si 3 N 4 . Alternatively, the anti-reflective coating  26  comprises a quarter wavelength film of another suitable optical coating material. 
   Si 3 N 4  has low optical absorption for wavelengths below 4,350 nm. Poly-Si and single crystal Si have low optical absorption for wavelengths above 850 nm. SiO 2  has low optical absorption over a wavelength range from 159 to 7,700 nm. 
   Referring to  FIGS. 1 and 2 , the preferred interferometer  10  is fabricated using semiconductor processing techniques of film deposition and etching. Fabrication begins with a Si wafer, which forms the transparent bulk material  24 . The second reflector  22  is deposited on the Si wafer by depositing the layers of the second multilayer reflector. Next, a sacrificial layer of poly-Si is deposited onto the second reflector. Alternatively, the sacrificial layer comprises a material other than poly-Si such as SiO 2 . (Note that in a later step, the sacrificial layer will be etched away through the release holes  21 , hence it is called the “sacrificial” layer.) 
   Following deposition of the sacrificial layer, the sacrificial layer is etched to form an inverse of the plurality of posts  14  and to form edges of the sacrificial layer where the membrane extension  28  will couple to the substrate  16 . A first layer of Si 3 N 4  is then deposited over the sacrificial layer forming a first layer of the membrane, a first layer of the plurality of posts, and a first encapsulating layer  42  of the first multilayer reflector. Following this, the alternating layers of the low refractive index material  44  and the high refractive index material  46  are deposited on a center region of the membrane  12 . A second layer of Si 3 N 4  is deposited over the first layer of Si 3 N 4  and over the alternating layers of the first multilayer reflector, which completes fabrication of the membrane  12 , the plurality of posts  14 , and the first reflector  18 . The release slots  21  are then etched through the first and second layers of Si 3 N 4  to the sacrificial layer. Preferably, XeF 2  gas is then used to etch the sacrificial layer through the release slots  21 . The XeF 2  gas etches the sacrificial layer to completion, which releases the membrane  12  and forms the interferometric cavity  30 . Alternatively, another selective etchant is used to etch the sacrificial layer. 
   An advantage of employing the sacrificial layer in the fabrication of the preferred interferometer  10  is that, because the sacrificial layer is formed with a uniform thickness, the sacrificial layer assures parallelism of the first and second reflectors,  18  and  28 . 
   It will be readily apparent to one skilled in the art that, since the release slots  21  function to provide access to the sacrificial layer for the XeF 2  gas, the release slots  21  can be replaced by other access entries to the sacrificial layer such as release holes in the membrane extension  28 . 
     FIG. 3  graphically illustrates intensities of first, second, and third VDM (wavelength division multiplex) channels,  50 ,  52 , and  54 , on a channel spacing  55  of 0.2 nm and also illustrates an interferometer transmission  56  of the preferred interferometer  10  configured for the telecom C band. The interferometer transmission  56  has a maximum transmission at 1544.5 nm, which is the wavelength of the second WDM channel  52 , and has a full width half maximum  58  of 0.0300 nm. This is accomplished by adjusting the cavity length of the preferred interferometer  10  to an integral multiple of half of 1544.5 nm. Thus, directing the first, second, and third WDM channels,  50 ,  52 , and  54 , onto the preferred interferometer  10  with the cavity length adjusted to integral multiple of half of 1544.5 nm causes the first and third WDM channels,  50  and  54 , to reflect from the preferred interferometer  10  and also causes the second WDM channel  52  to transmit through the preferred interferometer  10 . 
     FIG. 4  graphically illustrates the interferometer transmission  56  of the preferred interferometer  10  of the present invention tuned over the telecom C band. The preferred interferometer  10  is tuned by adjusting the cavity length of the interferometric cavity  30 . The interferometric cavity  30  preferably has a non-deflected cavity length of 3,126 nm. With a 0.0 nm relative deflection of the first reflector  18 , the interferometer transmission  56  occurs at 1,563.0 nm. With a 37.0 nm relative deflection  60  of the first reflector  18  towards the second reflector  22 , the interferometric transmission occurs at 1,544.5 nm. With a 74.0 nm relative deflection  62  of the first reflector  18  towards the second reflector  22 , the interferometric transmission occurs at 1,526.0 nm. 
   It will be readily apparent to one skilled in the art that the non-deflected cavity length of 3,126 nm can be replaced by a longer or shorter cavity length. If the non-deflected cavity length is replaced by a significantly longer cavity length, a larger relative deflection is needed to tune the preferred interferometer  10  to a specific wavelength. For example, if the non-deflected cavity length is 6,252 nm, twice the 37.0 nm relative deflection  60  of the first reflector towards the second reflector is needed to move the interferometric transmission to 1,544.5 nm. If the non-deflected cavity length is replaced by a significantly shorter cavity length, a smaller relative deflection is needed to tune the preferred interferometer to the specific wavelength. For example, if the non-deflected cavity length is 1,563 nm, half the 37.0 nm relative deflection  60  is needed to tune the preferred interferometer to 1,544.5 nm. 
   A WDM channel monitor employing the preferred interferometer  10  of the present invention is illustrated in  FIG. 5 . The WDM channel monitor  70  comprises an input optical fiber  72 , a first collimating lens  74 , the preferred interferometer  10 , a second collimating lens  76 , and an output optical fiber  78 . The first ball lens  74  couples the input optical fiber  72  to the preferred interferometer  10 . The second ball lens  76  couples the preferred interferometer  10  to the output optical fiber  78 . Preferably, the output optical fiber  78  couples to a photodetector (not shown), which is coupled to electronics (not shown). Alternatively, the output optical fiber  78  couples to downstream optical network components. 
   In operation of the WDM channel monitor  70 , the first reflector  18  is resonated to cause an interferometric transmission to sweep across a wavelength band. The photodetector detects the interferometric transmission and outputs a photodetector signal to the electronics. The electronics process the photodetector signal to provide individual channel power for WDM channels across the wavelength band. 
   A first alternative interferometer of the present invention is illustrated in  FIG. 6 . The first alternative interferometer  80  comprises an alternative membrane  82 , a spacer layer  84 , and an alternative substrate  86 . The alternative membrane  82  comprises a third reflector  86 . The alternative substrate comprises a fourth reflector  88  and a transparent bulk material  90 . In the alternative interferometer  80 , the third and fourth reflectors,  86  and  88 , form an alternative interferometric cavity  92 . In the alternative interferometer  80 , the spacer layer  84  couples the alternative membrane  82  to the substrate  86 . 
   It will be readily apparent to one skilled in the art that other various modifications may be made to the embodiments without departing from the spirit and scope of the invention as defined by the appended claims.