Abstract:
A deformable mirror includes a vertical comb actuator having a reflective surface attached thereto. The vertical comb drive includes stationary elements interspersed with moving elements. When a potential difference is provided between these elements, the moving elements are pulled downward, thereby deforming the reflective surface. The vertical comb drive typically includes a plurality of actuators, which are individually electrically addressed. Each actuator may be an array of interspersed elements or a cavity and corresponding tooth. Springs support the moving elements and bias the reflective surface in an original position. The vertical comb drive provides a large stroke and substantially linear voltage-versus-displacement curve throughout the stroke.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) of provisional application U.S. Serial No. 60/041,725 filed on Mar. 27, 1997 which is hereby incorporated by reference in its entirety. 
    
    
     GOVERNMENT RIGHTS 
     The U.S. Government has rights in the present application under Contract BMDO 96-002,003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to adaptive optics. More particularly, it relates to the use of a vertical comb drive to actuate the deformable mirror in an adaptive optical system. 
     2. Description of Related Art 
     Atmospheric turbulence has frustrated astronomers ever since telescopes were invented. Atmospheric turbulence introduces aberrations on the wavefront of visible light, causing stars to twinkle and distant objects to shimmer. Atmospheric turbulence also limits the resolution of microscopes, endoscopes, and other optical imaging systems. 
     Adaptive optics systems can be used to measure and compensate for rapidly fluctuating wavefront distortions which blur the images of objects viewed through the turbulent atmosphere. While many adaptive optics systems are currently being used, they suffer from many shortcomings. Current systems are typically complex, expensive, unreliable and difficult to maintain, thus putting them out of reach of amateur astronomers or commercial users. Current adaptive optics systems are also very heavy and operate at relatively slow speed making them unsuitable for airborne applications where the fluctuating wavefront distortions move past the aperture at a much faster rate than stationary systems. 
     FIG. 1 illustrates a simple adaptive optics application. A micromachine deformable mirror  100 , fabricated as an integrated circuit, is mounted onto a printed circuit board substrate  110 . A wavefront sensing and correction microchip  120  is mounted on the substrate  110  next to the deformable mirror  100 . The printed circuit board contains drive electronics  115  that connect the wavefront sensor chip  120  to the deformable mirror  100  chip. Incident light  130  is reflected off the micromachine deformable mirror  100 . The reflected light  140  then passes through a beam splitter  150 . A portion of the reflected light  140  is redirected onto the wavefront sensor  120  by the beam splitter  150 . 
     The wavefront sensor  120  detects any aberration in the wavefront of the reflected light  140 . Once it has detected the aberration, the wavefront sensor  120  then calculates the conjugate of the aberration. By applying the conjugate of the aberration to the incident light  130  as the light strikes the deformable mirror  100 , the aberration is subtracted from the wavefront, and a corrected wave of light is reflected from the deformable mirror  100 . The wavefront sensor  120  drives the deformable mirror  100  via the drive electronics  115  to correct for the detected aberration. 
     The primary obstacles to wider use of adaptive optics systems are cost, complexity, and reliability. Should reliable, low cost solid state adaptive optic systems become commercially available, many new applications can be envisioned. Laser communications is an example of such a market. Laser communication systems can support the signal bandwidth needed for High Definition Television without the need to run fiber optic cables between a remote mobile unit and a roving camera crew. Current laser communications have limited range due to atmospheric distortions that cause beam bending and scatter energy from the beam. An adaptive optic system capable of correcting for these effects can greatly extend range and improve performance. 
     A low cost adaptive optics system would find immediate applications in optical microscopes. Such a system could produce significantly improved imaging in difficult medical environments improving the performance of conventional and confocal microscopes. A low cost adaptive optics system has potential to enhance the operational performance of all clinical microscopes in use today. 
     A low cost adaptive optics system would also find applications in endoscopy. Here the adaptive optics system can be used for both imaging and transmitting the laser energy for endoscopic surgical procedures. There is considerable medical instrumentation and diagnostic equipment in the market place today. However, higher resolution imaging and more accurate placement of laser energy is needed to improve these instruments. 
     A low cost adaptive optic system could also find application in ophthalmology and optometry. In this application a small low power laser device is used to create an artificial guide star on the retina of the eye. The spot of light reflected off the retina forms a source to drive a wavefront sensor of an adaptive optic system. The kind of retinal camera that stands to benefit even more from adaptive optics is the confocal scanning laser ophthalmoscope (CSLO). Many clinical applications require the ability to optically section the retina in depth. Such sectioning can be achieved with confocal imaging in principle but current CSLOs do not tap the potential of confocal imaging because the retinal image quality is too poor. A CSLO equipped with adaptive optics could reap the full benefit of confocal imaging, improving the transverse resolution of current instruments by a factor of three and the axial resolution by a factor of ten. 
     Typical adaptive optics systems are composed of at least three core elements: (1) a wavefront sensor to detect optical aberrations, (2) electronic circuitry to compute a correction, and (3) a deformable mirror to apply the correction. The deformable mirror is a critical component of an adaptive optics system. It is used to apply the correction to the distorted wavefront. In current technology, the deformable mirror is also the most expensive component of the adaptive optical system. In order to realize a low cost adaptive optical system, a low cost deformable mirror must be developed. The current art presents three alternative technologies for deformable mirrors: liquid crystals, stacked piezoelectrics, and Micro-Electro-Mechanical Systems (“MEMS”). 
     Phase Modulating Liquid Crystal Display (LCD), devices offer low weight, low cost, and low power alternatives to large opto-mechanical devices. Also, cost effective bulk manufacturing methods currently exist for these devices. However, current LCD devices suffer from limited fill factor, limited bandwidth, and inadequate dynamic range. Bandwidth limitations will preclude the use of LCD&#39;s for airborne or missile applications. 
     Stacked piezoelectrics (“SPZT”) utilize a new generation of piezoelectric technology that costs less and features the best advantages of actuators made from piezoelectric (“PZT”) or lead manganese niobate (“PM”) technologies. However, current SPZT devices suffer from high current operation, significant actuator nonuniformity, relatively high power dissipation, and moderate hysteresis effect. Moreover, these devices are relatively expensive when compared to liquid crystals or MEMS devices. 
     The recent advent of MEMS technology offers an alternative for the construction of cost effective mechanical mirror actuators. The technology is based upon the well established fabrication methods used to manufacture integrated circuits. Micromachining promises major improvements in overall performance and reduction of cost. The MEMS deformable mirrors have shown the lowest fabrication cost, lowest power consumption, lowest mass, lowest volume, elimination of hysteresis, elimination of polarization effects best power on, and lowest non-uniformity of any current deformable mirror technology. An additional advantage of this technology is that integrated electronic circuits can be fabricated directly on the same substrate as the micromirror. This allows for significant system simplification in that mirror drive electronics can be fabricated directly on the mirror substrate. MEMS technology offers light weight components, thus making it entirely suitable for ground based, airborne, and even hand held wavefront correction applications. 
     However, prior MEMS deformable mirror designs have been based on the electrostatic parallel plate capacitor actuators. This method of actuation has been successful but produces actuators with limited stroke, e.g., less than one micron, and a nonlinear voltage-versus-displacement curve. The nonlinear curve results from the 1/r dependence of the attractive force on gap distance. Recently, new MEMS actuator concepts have been demonstrated which use electrostatic comb drives to produce large stroke and linear voltage verses displacement curves. Unfortunately the current designs do not produce the vertical displacements necessary for use in an optical system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a low cost deformable mirror for use in an adaptive optics system which overcomes the problems noted above. It is further an object of the present invention to provide a deformable mirror with a high stroke and substantially linear voltage-versus-displacement curve. 
     These and other objects of the present invention may be realized by providing a deformable mirror by combining a vertical comb drive with a reflective surface. The deformable mirror may also include a spring for biasing the vertical comb drive to maintain the reflective surface in an original position absent application of a voltage to the vertical comb drive. 
     The vertical comb drive may include a first array of stationary elements and a second array of moving elements correspondingly interspersed with the first array. The reflective surface would be attached to the second array. A layer covering tops of elements of the second array may be provided. A spring may be used to suspend the first array relative to the second array, preferably via the layer. The stationary elements and the movable elements may be circular or planar. The layer may be attached directly to the reflective surface or with a post which is preferably in a center of the layer. Voltage may be applied to each stationary element of the first array individually or each moving element of the second array individually. 
     The vertical comb drive may include an array of vertical comb actuators. Each actuator of the array may be individually addressed to provide a voltage thereto. Springs for individually suspending each of the second arrays of each actuator in the array may be provided. Preferably, an anchor for supporting the springs is also provided. 
     The vertical comb drive may include a plurality of cavities and teeth interdigitally mounted with said cavities, the reflective surface being attached to the teeth. A top layer may be provided between the teeth and the reflective surface. A conductor for individually connecting each tooth of the teeth to a voltage source may also be provided. 
     These and other objects of the present invention may also be realized in accordance with a method of deforming a mirror including attaching the mirror to a vertical comb actuator and applying a voltage to the vertical comb actuator. The vertical comb drive may include an array of vertical comb actuators and the applying individually applies voltage to the vertical comb actuators. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a schematic of a simple adaptive optics system; 
     FIG. 2 is a perspective cross-sectional view of a single deformable mirror element having a vertical comb drive of the present invention; 
     FIG. 3 is a perspective cross-sectional view of an alternative embodiment of the present invention; 
     FIG. 4A is a side cross-sectional view of another alternative embodiment of the present invention; 
     FIG. 4B is a bottom view of the configuration shown in FIG. 4A; and 
     FIG. 5 is a cross section of two adjacent actuators in accordance with the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As noted above, typical MEMS have been based on electrostatic parallel plate capacitor actuators. The separation between the plate determines the amount of stroke available, but an increase in the separation increase the voltage required to deform the mirror. Thus, an increase in the stroke invariably increases the voltage. Further, since this separation between the plates is changing throughout the stroke, the response of the deformable mirror to the applied voltage is nonlinear. Electrostatic comb drives have been demonstrated to produce a large stroke and linear operation, but do not produce the vertical displacement required for use in deformable mirror applications. 
     Therefore, in accordance with the present invention, a vertical comb mirror actuator (VCAM) having a set of interdigitized shells for actuating a deformable mirror is provided. The VCAM includes a set or array of stator shells and a corresponding set or array of movable slider shells. The mirror membrane is attached to the slider shells. Introducing a voltage potential between the stator shells and the slider shells pulls the slider shells down, deforming the thin membrane mirror. The vertical displacement is the result of a force created by the edge capacitance between the slider shells and the stator shells. The force generated by the comb drives is independent of the relative position of the two electrode plates and is linear while the plates overlap. This essentially eliminates the non-linear voltage-verses-displacement curve so problematic in the prior art. Unlike the parallel plate design, the gap between these elements remains constant throughout the stroke. In addition to the substantially linear operation of the VCAM, the VCAM can also provide a high stroke, e.g., 20 μm, with relatively low voltages, e.g., 35-100 volts. 
     The actual design of the shells may vary in accordance with a desired application. The shape of the shells is typically designed to maximize surface area between the shells. The other design parameters of the shells include the width of the shells, the width of the gap between the shells, and the number of shells. The length of the shells is determined by the required stroke. For example, when the actuator is to have a stroke of greater than six microns, the typical requirement to correct for atmospheric distortions, applying a maximum of two hundred volts, and a frequency response of 10 kHz, a working design has a shell width of ten microns, an inter-shell gap of five microns and seven slider shells. 
     The actuator also preferably includes a spring for mounting or suspending the slider shells, biasing the slider shells in a neutral position when no voltage is applied to the stator shells, and preventing horizontal, i.e., side-to-side movement, of the slider shells. Preferably, a spring is positioned at each of the four corners of the actuator element and is attached at one end to a wall provided next to the actuator and at the other end either to the mirror itself or to the slider shells. Thus, the dimensions of the spring also affect the actuator design. A working spring design for the design parameters noted above is a folded spring having two legs, each leg having a length of one hundred twenty-five microns, a width of twenty microns, a spacing between the legs being ten microns and the space between the last leg and the mirror or surface adjoining the slider shells being five microns, and the spring having a spring constant of 12.4 μN/μm. 
     When providing an array of multi-element actuators, the larger the spacing between the actuators, the less force is needed to move one actuator with respect to adjacent actuators. For the above stroke or throw of six microns, and delta motion between adjacent actuators of one micron, the actuators should be spaced by approximately five hundred microns. The number of actuators in the array is application dependent, and is a design criteria to be considered by those skilled in the art. 
     The following configurations illustrate different VCAMs in accordance with the present invention. Preferably, the deformable mirror is a thin continuous membrane of reflective material spanning each of the actuator elements in the array. The deformable mirror can also be a segmented design, but the continuous membrane design was found to have superior optical characteristics. The deformable mirror may be made of, for example, silicon nitride coated with metal to reflect at a desired wavelength or electroplated metal. The more precise the deformations need to be, the more flexible the mirror material should be. 
     The combs or shells may be made by plating nickel on a silicon substrate using a photoresist to define the combs and then planarizing the nickel by lapping the structure, thereby forming the combs with a multi-layer electro-forming. Alternatively, the combs may be made in silicon with a trench refill process alone for use with segmented mirror or using two wafers and anodic wafer bonding for use with continuous mirrors. 
     FIG. 2 is a cross-section perspective view of a single deformable mirror actuator element  200  of the present invention. The preferred embodiment consists of two sets of interdigitized circular shells, a set of stator shells  210  and a set of movable slider shells  220 . Preferably, a layer  230  overlies the top of the slider shells  220  so that movement of any of the slider shells  220  affects the layer  230 . The use of the layer  230  allows for a continuous profile to be realized by the deformable mirror  250 , as opposed to the step functions which would result if the slider shells  220  were directly attached to the deformable mirror  250 . 
     The layer  230  is attached by a vertical post  240  to the thin membrane mirror  250 , thereby attaching the continuous membrane deformable mirror  250  to the actuator  200 . Preferably, the post is in the center of the layer  230  and the mirror  250 . The stiffness of the post  240  will determine how much of the movement of the slider shells  220  is translated to the mirror  250 . The position and number of posts may be varied to apply the deformation to different regions of the mirror  250 . For example, if tilt is a parameter to be controlled, four posts, one at each corner of the actuator may be provided. Typically for a complex shape, the single post is used and the surface area of the mirror region effected by movement of the actuator attached by that post smoothly abuts adjacent mirror regions effected by adjacent actuators. 
     Applying an activation voltage to the actuator element  200  via the stator shells  210  pulls the slider shells  220  down, thereby deforming the thin membrane mirror  250 . The stator shells  210  may be individually addressed. When the activation voltage is removed, the small attached springs  260  pull the slider shells  220  back into position, thereby reforming the thin membrane mirror  250 . In practice, an array of actuator elements  200  would be fabricated on a single base substrate  270 . The actuator elements  200  would be individually electrically addressed to form a complete adaptive optics deformable membrane mirror  250  by appropriately patterning the substrate  270 , or by providing individual conductors for each shell. 
     FIG. 3 is a perspective cross-section of an alternative embodiment of a single actuator  300  embodying the present invention. The actuator element  300  of this embodiment provides stator shells  310 , slider shells  320  integral to a slider top or layer  330 , a post  340  connecting the slider shells  320  to the deformable mirror membrane  350 , springs  360  to return the deformable mirror  350  to its initial position, and a substrate  370  upon which the actuator resides. In FIG. 3, an example of a wall  380  to which the spring  360  is attached is shown. The view in FIG. 3 is a cut away, as the wall  380  would be beneath the mirror  350 . 
     FIG. 4A is a cross section view of yet another alternative embodiment of an actuator embodying the present invention. Unlike the configurations shown in FIGS. 2 and 3, in which each actuator has multiple elements, i.e., more than two shells, and the array of these distinct multiple element actuator, the configuration in FIGS. 4A and 4B is a continuous array of two element actuators. In particular, the base  470  of the actuator  400  is made of a conductive material. In this conductive material there are holes or cavities or trenches  410  which are either drilled, etched, or electroformed or created by any suitable fabrication technique. Inside these holes, there is another conductive material in the form of teeth  420  which does not touch the sides of the holes, but is electrically isolated from the base material. These teeth are mounted on another structural base  430  that keeps the teeth  420  rigidly in place within the holes  410  in the base  400 . This base  430  can be either electrically conductive or insulating, but does need to provide a surface for the electrical conductor  440  which connects each tooth  420  to a voltage source. 
     When a voltage is applied between the teeth  420  and the actuator base  400 , an electrostatic force is generated pulling the teeth  420  into the holes  410 . Note in this embodiment that the voltage is applied to individually addressed teeth  420  which are the moving elements of the actuator  400 , rather than to the stationary elements as in the previous embodiments. If it is desired that this vertical motion be applied to an optical mirror surface  450 , that surface is applied to the tooth base material  430  by some suitable application method such as sputtering or vapor deposition. As in FIGS. 2 and 3, springs attached outside of the actuator region are provided for supporting the teeth  420  above the cavities  410 . 
     FIG. 4A illustrates the same embodiment as FIG. 4B, but it is a top view instead of a cross section view. As can be seen from the perspective shown in FIG. 4B, the teeth can be circular, although the teeth can be any shape, e.g., rectangular, octagonal, etc., to match the needs of any particular application or fabrication method. Further, while the surface in FIG. 4B is circular, it may be any desired shape, including rectangular. 
     The configuration of FIGS. 4A and 4B is more useful for effecting bulk parameters, such as displacement plus tilt in two orthogonal directions, while the configurations in FIGS. 2 and 3 are more useful for effecting complex shapes on the mirror surface, as explained below. 
     FIG. 5 is a cross-section of two adjacent actuators  500  attached to the same continuous membrane mirror  550 . The actuator  500  may have the configuration of actuator  200  shown in FIG. 2 or that of the actuator  300  shown in FIG.  3 . In practice, there will be hundreds of evenly spaced actuators  500  for one continuous membrane mirror  550 . 
     The actuators  500  include stator  510  and sliders  520 . The sliders  520  are integrated with a slider top  530 . Posts  540  attach the slider  520  via the slider top  530  to the deformable mirror membrane  550 . The posts  540  are thus evenly spaced on the under side of the membrane  550  and are centered on the actuator  500  and the portion of the mirror  550  the actuator  500  is to effect. Each actuator  500  and post  540  will pull on the mirror  550  with the strength of the actuator force that is controlled by the amplitude of the applied voltage. 
     If only one actuator  500  is actuated, it will cause a dimple in the membrane  500  as shown in FIG.  5 . The stiffness or rigidity of the membrane  550  itself prevents the membrane from being deflected with sharp edges at the deflection point of the posts  540 . Instead, the result is the gradually curved dimple as shown in FIG. 5, allowing the variation in the surface from one actuator to the next to be smooth. The actuators  500  are all addressed separately, so that when multiple actuators are actuated each with different forces, very complex surface shapes can be produced. 
     In addition, springs  560  return the deformable mirror  550  to its initial position as shown in the actuator on the left in FIG.  5 . The actuator  500  resides on a substrate  570 . An anchor or wall  580 , for each actuator  500 , serves to support the spring  560 . The actuator on the right is in the deflected position with the springs  560  bent and the membrane mirror  550  deformed. This deflection is accomplished by applying a voltage between the stator  510  and the slider  530 , thereby resulting in an attractive force between them, pulling the slider  530  down as shown on the right relative to the initial position shown on the left. 
     Many variations in the design of the comb itself may be realized in accordance with the present invention. The density of the actuators is also application specific. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.