Abstract:
A monolithic mirror assembly is disclosed that is manufactured from a silicon on insulator (SOI) substrate comprising two silicon layers separated by an insulator material. One layer or silicon and the insulator layer are partially etched, thus exposing the underlying second layer of silicon which functions as the reflective surface of the mirror. A first layer of electrodes is disposed on the exposed portion of the insulator area above the reflective surface of the mirror. The mirror is directly mounted on a second substrate comprising a second layer of electrodes. The first and second layers of electrodes are used to variably deform the shape of the mirror to compensate for wave front errors in an optical signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related generally to the correction of distortion of optical signals and, in particular, to the use of adaptive optics to correct that distortion.  
       BACKGROUND OF THE INVENTION  
       [0002]     Optical signals are used in many different fields for many different purposes. One particular use of these signals is in optical telecommunication systems designed to transmit voice and data messages. Such telecommunication systems rely on optical components, such as all-optical routers, to process and route data from one point to another. These optical components often use small mirrors or lenses to redirect free-space light beams propagating in one direction to another desired direction. However, the operation of systems using such mirrors to redirect light beams may be hampered by a variety of factors. For example, temperature variations, turbulence or other phenomena may result in a change in the refractive properties of the medium (e.g., the atmosphere) through which the beam passes. Such changes in the refractive properties of the atmosphere between the transmission point and the receiving point may cause certain portions of the beam to move faster than others resulting in the aforementioned distortion. This distortion may cause discrete sections of the wave front to deviate from the orthogonal orientation to the line of travel of the beam as initially transmitted. This deviation may result in significant degradation of the wave front at its destination and, hence, may result in equally significant degradation of the optical signal carried by the light beam.  
         [0003]     Adaptive optics, which are well known in the art, typically use a wave front sensor to measure phase aberrations in an optical system and a deformable mirror or other wave front compensating device to correct these aberrations (i.e., to bring the reflected wave front into phase). Until recently, such deformable mirrors were typically deformed via piezoelectric drivers, or other well-known methods. More recent efforts rely incorporate upon electrostatic actuation where a mirror is deformed by passing a voltage across one or more of a plurality of electrodes located in electrostatic proximity to that mirror. By controlling the attractive force along different portions of the mirror surface, the shape of the mirror may be altered in a known way, thereby at least partially correcting for the wave front distortion.  
       SUMMARY OF THE INVENTION  
       [0004]     The present inventors have recognized that, while prior attempts at using deformable mirrors are advantageous in many regards, they are also disadvantageous in some respects. For example, prior mirrors have insufficient range of deformation to meet the needs of adaptive wave front correction in various applications such as astronomy and vision science. In particular, the method of actuation of such prior mirrors, namely electrostatic attraction to a single underlying electrode plane, allowed only attraction to the electrode plane, and not a corresponding upward motion away from that electrode plane. This feature limited the range of deformation that could be achieved with prior devices. Although some prior mirrors have incorporated a second electrode plane above the mirror in order to allow actuation in both directions, this second electrode was fabricated as a separate entity that could not be located in sufficiently close proximity to the mirror. As a result comparatively little benefit was achieved.  
         [0005]     Therefore, the present inventors have invented an integrated, monolithic deformable mirror structure that solves this problem. In accordance with one embodiment of the present invention, a mirror assembly is manufactured from a silicon on insulator (SOI) substrate comprising two silicon layers separated by an insulator material. One layer or silicon and the insulator layer are partially etched, thus exposing the underlying second layer of silicon which functions as the reflective surface of the mirror. A first, transparent layer of electrodes is attached to the exposed portion of the insulator area above the reflective surface of the mirror. An integrated, monolithic mirror structure is formed by bonding the mirror to a second chip (also referred to herein as a support structure) which includes a second layer of electrodes. The first and second layers of electrodes are used to variably deform the shape of the mirror to compensate for wave front errors in an optical signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]      FIG. 1A  shows a bulk silicon on insulator such as is useful in accordance with the principles of the present invention;  
         [0007]      FIGS. 1B and 1C  show one embodiment of etching the substrate of  FIG. 1A  to form a small mirror in accordance with the principles of the present invention;  
         [0008]      FIG. 2  shows a side view and a top view of the mirror of  FIG. 1C ;  
         [0009]      FIG. 3A  shows an illustrative substrate useful in accordance with the principles of the present invention for supporting the mirror of  FIG. 2 ;  
         [0010]      FIG. 3B  shows one embodiment illustrating how the substrate of  FIG. 3B  may be etched and patterned with a first layer of electrodes in accordance with the principles of the present invention;  
         [0011]      FIGS. 4A and 4B  show the mirror of  FIG. 2  disposed on the support of  FIG. 3B  thus forming an integrated mirror structure in accordance with one embodiment of the present invention; and  
         [0012]      FIG. 5  shows an integrated monolithic mirror whereby, by applying voltages to electrodes in proximity to the reflective surface of the mirror, the mirror may be adaptively deformed to correct for wavefront distortion of a light beam. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     As previously discussed, wave front distortion may result when any changes to the refractive properties of the transmitting medium are encountered along the line of travel of a light beam. These changes may cause discrete sections of the wave front of the beam to deviate from their transmitted, orthogonal orientation to the line of travel of the beam. The result is a distortion of the image of the wave front when it reaches its destination, which may be for example a mirror, a focal plane of a telescope, an optical wave front sensor (e.g., a curvature wave front sensor or a Shack-Hartman wave front sensor), or any other destination. By way of example, in optical communications systems, distortion may result in significant degradation of the communications signal or even the total loss of communications.  
         [0014]      FIG. 1A  shows substrate  100  useful for forming a small reflective mirror such as that used to correct for wave front distortion in an optical communications system. Substrate  100  is, for example, a silicon-on-insulator (SOI) substrate having, illustratively, silicon layer  101  disposed on layer  102 , which is, illustratively, an insulating oxide layer (e.g., silicon dioxide). Layer  102  is, in turn, disposed on layer  103  which is, illustratively, a thin layer (e.g., a layer of 1 micron or thinner) of material such as silicon nitride, single crystal silicon, polysilicon, polyimide, or other known materials.  FIGS. 1B and 1C  show how substrate  100  may be etched to form a micro mirror. Specifically, referring to  FIG. 1B , an area of layer  101 , represented by length  104 , is etched using well-known etching techniques to expose a portion of the underlying insulator layer  102 . Next, as shown in  FIG. 1C , a portion of layer  102 , illustratively that portion represented by length  105 , is etched away to expose layer  103 . Length  105  is, for example, shorter than length  104  so that, once etching is complete, supporting steps  106  of layer  102  remain exposed. Accordingly, the structure of  FIG. 1C  is a mirror structure having reflective surface  107 . Thus, for example, a light beam traveling in direction  108  is, upon reaching surface  107 , reflected from surface  107  and redirected in illustrative direction  109 . A metallic coating (e.g., aluminum) may be formed on surface  107  to enhance this reflectivity.  
         [0015]      FIG. 2  shows a top view of the mirror structure of  FIG. 1C . Illustratively, referring to  FIG. 2 , the etching described above has exposed circular areas  106  and  107  respectively of layers  102  and  103  of substrate  100 . One skilled in the art will recognize that tension of mirror surface  107  is maintained due to the support to the mirror surface provided by the remaining portions of layers  101  and  102 . Illustratively, radius  201 , which is the outer radius of area  107  and the inner radius of area  106  is 5 mm. Similarly, radius  202 , which is the outer radius of area  106 , is illustratively 5.5 mm. One skilled in the art will recognize that many different shapes and sizes of areas may be etched using well known techniques to form mirrors useful in accordance with the principles of the present invention.  
         [0016]      FIG. 3  shows a support substrate  300  which is, illustratively, a substrate having a first layer  301 , such as, illustratively, a layer of polyimide material, disposed on a second layer  302 , which is, for example, a layer of silicon in a printed circuit board. A support structure is created by, illustratively, etching away a portion of layer  301  to expose a portion of layer  302 , resulting in the illustrative etched supporting structure shown in  FIG. 3B . In that figure, an area of layer  301  represented by cross section length  304  is etched away exposing, for example, a circular area of layer  302  having a diameter of length  304 . A layer  303  of electrodes, such as, illustratively, electrodes useful in adaptive optics applications, is then patterned onto the exposed portion of layer  302  using methods well known to one skilled in the art.  
         [0017]      FIG. 4  shows how a monolithic integrated mirror structure useful for correcting wavefront distortion in a light beam may be formed. Specifically,  FIG. 4A  shows how mirror structure  100  of  FIG. 2  is, illustratively, lowered in direction  401  until it is brought into contact with the support structure  300  of  FIG. 3B  and attached to the illustrative polyimide material  305  of support structure  300 . One skilled in the art will recognize that this attachment may be achieved by, for example, flip chip bonding. Thus, the integrated structure  402  is characterized by a reflective layer  107  supported by supporting structure  305  and held in tension by etched layers  101  and  102 . Illustratively, a layer  403  of transparent electrodes is disposed such that it is suspended on the remaining exposed portion  106  of insulating layer  102  above the reflective surface  107  of the mirror and in the path of an incoming light beam.  
         [0018]      FIG. 5  shows how, in operations, the structure of  FIG. 4B  may be used to correct for wave front distortion of the light beam  501 . Referring to  FIG. 5 , a light beam  501  propagating toward reflective surface  107  of structure  402  is split by beam splitter  510  such that part of the beam is directed toward illustrative wave front sensor  511 . Wave front sensor  511  is, for example, a well-known Shack-Hartmann wave front sensor, which is used to detect the presence and magnitude of wave front distortion in light beam  501 . An example of such a wave front sensor, as used in a free space optical communications system, is described in the co-pending U.S. Patent Application titled “Method and Apparatus for the Correction of Optical Signal Wave Front Distortion Within a Free-Space Optical Communications System,” having Ser. No. 09/896804, filed Jun. 29, 2001, which is hereby incorporated in its entirety herein.  
         [0019]     As is well known to one skilled in the art of adaptive optics, wave front information collected by the wave front sensor  511  is forwarded to, for example, illustrative computer  508 , that, if necessary, determines an appropriate shape of the reflective surface  107  of the mirror that would correct for the wave front distortion once the light beam is incident upon and reflected from reflective surface  107 . Computer  508  then generates signals that are forwarded via lead  512  to controller  513  which generates one or more voltages to be passed over one or more electrodes in proximity to mirror reflective surface  107 , such as electrodes  506  and  507 , in electrode layers  403  and  303 . As one skilled in the art will fully appreciate that, when, for example, a voltage V 0  is passed over electrode  506  and a voltage V 1  is passed over electrode  507 , electrostatic forces are generated between the electrodes and the mirror. These forces cause, for example, a discrete portion of the mirror to be attracted in direction  502  toward electrode  506  and a discrete portion of the mirror to be attracted in direction  503  toward electrode  507 . Generally, within limits, the greater the magnitude of the voltage passed across a particular electrode, the greater the force applied to the mirror and, hence, the greater the displacement of the reflective surface  107  of the mirror relative to its nominal position when no such voltage is applied. Accordingly, by applying a plurality of voltages to a plurality of electrodes in layers  403  and  303 , reflective surface  107  of the mirror structure  402  can be deformed in a relatively complex manner in order to compensate for complex, distorted wave fronts. The result is that, upon being reflected, the wave front distortion of the light beam  501  is reduced.  
         [0020]     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are within its spirit and scope. Furthermore, all examples and conditional language recited herein are intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. Diagrams herein represent conceptual views of mirrors and light beams. Diagrams of optical components are not necessarily shown to scale but are, instead, merely representative of possible physical arrangements of such components.