Abstract:
A fiber-optic modulator based on a micromachined grating device which is both polarization independent and achromatic in behavior is described. The device is a two dimensional grating or periodic structure which is symmetric in the X and Y axes. It is comprised of a membrane with holes cut in it that moves downward with the application of a voltage which starts diffracting light. The hole region may have a raised island to provide achromatic behavior.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of and claims the benefit of priority from U.S. application Ser. No. 09/855,873, filed May 14, 2001, which is a continuation-in-part of and claims the benefit of priority from U.S. Pat. No. 6,501,600, issued Dec. 31, 2002, both of which are fully incorporated herein by reference for all purposes.  
         [0002]    U.S. Pat. No. 6,501,600 is a continuation-in-part of and claims the benefit of priority from U.S. Pat. No. 6,169,624, issued Jan. 2, 2001, and also claims the benefit of priority to U.S. Provisional Application No. 60/171,685, filed Dec. 21, 1999, both of which are also fully incorporated herein by reference for all purposes. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates to polarization independent grating modulator. More particularly, the present invention relates to micromachined grating modulators which exhibit polarization independent behavior.  
           [0005]    2. Description of Related Art  
           [0006]    Optical modulators are an important component in optical systems for controlling and modulating light. In particular, for fiber-optic networks, modulators are used for imparting data modulation on the transmitting laser beam and as an electronically controlled variable optical attenuator (VOA) for channel equalization and power control. In fiber-optic networks the state of polarization is unknown and therefore little or no polarization dependence is tolerated from components.  
           [0007]    Bloom et al. (U.S. Pat. No. 5,311,360) demonstrated a micromachined grating modulator comprised of narrow ribbons anchored at the two ends but suspended in the center λ/2 (half wavelength) above the substrate. The ribbons are separated by gaps of the same width. Both ribbon and gap have a reflective coating from which light is reflected in phase and therefore it emulates a minor. By applying a voltage to the ribbons, the electrostatic force moves the ribbon down by λ/4. Now the ribbon and gap are out of phase and all the light is diffracted out in multiple orders. Thus modulation is achieved.  
           [0008]    One limitation of the previous invention is that the height difference between the ribbon and gap leads to poor spectral performance. Bloom et al. (U.S. Pat. No. 5,841,579) improved on this by inventing a flat grating light valve comprised of ribbons of equal width with very little gap between 10 them. In the nominal position, all ribbons are at the same height. By applying a voltage and pulling every other ribbon down, the grating is turned on.  
           [0009]    For fiber-optic applications operating over the bandwidth of erbium doped fiber amplifier (EDFA), the spectral performance of the previous invention is not acceptable especially at high attenuation. Godil et al. (Achromatic optical modulator, patent application Ser. No. 09/372,649, filed Aug. 11, 1999) demonstrated a device with alternate narrow and wide ribbon. By proper choice of the ribbon widths and gap width, spectrally flat attenuation over the EDFA band over a large dynamic range is obtained.  
           [0010]    A limitation of the previous inventions, because of lack of symmetry, is that they are not completely polarization independent. In particular, at high attenuation the polarization dependence is unacceptably high for fiber-optic networks. What is needed is a micromachined modulator which exhibits achromatic and polarization independent behavior.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is directed towards a fiber-optic modulator comprising of an input optical fiber carrying a light beam through a lens onto a micromachined reflective modulator, back through the lens into an output optical fiber. The micromachined modulator is a two dimensional grating or periodic structure which is modulated by the application of a voltage. The two dimensional grating is symmetric in the X and Y axes, and therefore leads to polarization independent behavior. The achromatic modulator invention of Godil (patent application filed August 1999) is also incorporated to give achromatic behavior.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows a fiber-optic modulator comprised of a micromachined grating device of the present invention.  
         [0013]    FIGS.  2 A- 2 B show the plan view and cross-sectional view of the micromachined grating device in the preferred embodiment.  
         [0014]    FIGS.  3 A- 3 B show the plan view and cross-sectional view of the micromachined grating device in the alternate embodiment with square holes and islands.  
         [0015]    FIGS.  4 A- 4 B show the plan view of the micromachined grating device in the alternate embodiment without achromatic compensation.  
         [0016]    FIGS.  5 A- 5 H show a process for fabricating the micromachined grating device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 shows the fiber-optic micromachined modulator  100  comprised of input fiber  110  and output fiber  112  held in a double bored ferrule  120 . Light from the input fiber  110  is collimated by lens  130 , impinges on the micromachined device  200 , reflects and is focused into the output fiber  112 . By applying a voltage to the device  200 , light is diffracted in a two-dimensional pattern and the through light in the output fiber is reduced. Thus modulation and attenuation function is achieved.  
         [0018]    It is desirable to achieve the modulation function in an achromatic and polarization independent way. The device  200  which accomplishes this is shown in FIGS. 2A, 2B with a plan view and cross-sectional view respectively. The device is comprised of round islands  230  of height h and a membrane  210  which is anchored  205  all around with round holes  220  cut in it. The ring region  225  is formed between the island and the membrane. Release holes  240  in the membrane, facilitate the release or etch of the sacrificial layer under the membrane.  
         [0019]    Device  200  is periodic in X and Y with a period A which is typically in the 20 to 200 micron range. The device is symmetrical in X and Y, and therefore leads to polarization independent behavior. The island  230  has a height h which is mλ/2, where m is an integer and λ is the wavelength of light. Typically m is 3 and for λ=1.55 μm, h is 2.32 μm. The island maybe made of silicon, poly silicon, oxide, silicon nitride or it may be silicon covered with oxide or nitride. The top surface of the membrane  210  is nominally coplanar with the islands. The membrane is anchored down to the substrate  250  at discrete anchor points  205 . The design of the anchor may be more elaborate for a more rigid anchoring. The substrate  250  may be a silicon wafer, quartz wafer, glass plate, or any other suitable material. The membrane film is tensile which keeps it suspended. The membrane may be silicon nitride, poly silicon, oxide, aluminum, or some other suitable material. The holes  220  in the membrane are larger than the islands. The whole device is covered with a blanket evaporation of aluminum or gold. For h=2.32 μm, light reflected from the ring region  225  between the island and the membrane is 6π out of phase with respect to the island and the membrane. Therefore the device looks like a mirror in this state which is the on state for the modulator. When a voltage is applied to the membrane, electrostatic force moves the membrane downwards and the device starts diffracting light in a two-dimensional pattern. To achieve full extinction, when the membrane is moved λ/4, it is necessary that the membrane area be equal to the area of the island and the ring region  225 . In addition, the invention of Godil (Achromatic optical modulator, filed August 1999) teaches that to obtain achromatic behavior the area of the ring region should be 1/(2m) of the membrane area. For this particular case, it is ⅙ th .  
         [0020]    Another variation of the device  200 ′ is to have square islands and square holes in the membrane as shown in FIGS. 3A, 3B. Now the device does not require release holes and is easier to layout. All other considerations and explanations apply equally here as described in the previous paragraph. Other island and hole shapes are also possible.  
         [0021]    Another variation of the device, if achromatic behavior is not important, is not to have the islands as shown in FIGS. 4A, 4B. The device is now simpler with one reduced processing/masking step. To achieve full extinction, the area of the membrane  410  should be equal to the area of the holes  430  in the membrane. Anchors  405  are similarly designed and release holes  440  serve the same function. The top surface of the membrane is mλ/2 above the substrate, where m is typically 3 or 4.  
         [0022]    Process and device fabrication of the preferred embodiment shown in FIG. 2 is now described. The process flow is shown in FIGS.  5 A- 5 H starting with a silicon wafer  250 . The first lithography mask defines the islands  230  which emerge after the silicon is etched down 2.32 μm with RIE (reactive ion etching) as shown in FIG. 5B. This is followed by growing a thin thermal oxide  235  in the range of 200-600 angstroms. LPCVD polysilicon or amorphous silicon  245  is deposited next as the sacrificial layer. It is important that the poly or amorphous silicon be optically smooth. The polysilicon is patterned and etched down to the oxide to define the anchors  205  as shown in FIG. 5E. Sacrificial layer  245  may be PSG (phospho-silicate glass) or some other oxide, which is removed using hydrofluoric acid. Sacrificial layer  245  may also be a polymer, which is removed using an oxygen plasma etch. This is followed by depositing LPCVD silicon nitride  255  as the mechanical layer. The silicon nitride may be stoichiometric or silicon rich. The silicon nitride is defined and etched after patterning the photoresist  265 . Xenon difluoride etch is used to remove the polysilicon or amorphous silicon sacrificial layer. Finally the photoresist  265  is removed with an oxygen plasma etch followed by a blanket aluminum or gold evaporation.  
         [0023]    The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art.