Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     Adaptive mirrors permit distorted wave fronts to be reformed into undistorted wave fronts. An example of this problem occurs when a plain wave front from a distant star passes through the earth&#39;s atmosphere and is distorted by turbulent layers of air. The heating and cooling of the atmosphere by local weather effects cause these turbulent layers. In general, the further the light travels through the air and the denser the air is, the greater the amplitude of the distortion. Adjusting a mirror surface to match this distortion allows a reflected plain wave front to be observed. The actuator for adjusting the mirror surface to match the wave front distortion must operate very rapidly with response times of one thousandth of a second or less and is called an adaptive actuator. With an adaptive actuator, the adaptive mirrors should perfectly match the distorted wave front laterally and have half the amplitude of the wave front distortion. Another kind of actuator, called an active actuator, corrects for quasi-static surface errors in the mirror. Such errors may arise from inadequate polishing of the mirror, the force of gravity particularly as the mirror is tilted, unequal expansion of the mirror as a result of temperature changes or creep in the mirror surface as a result of internal strains in the mirror. The adaptive mirror should perfectly match these errors.  
         [0002]     The phase of the light depends on the wavelength, so the shorter the wavelength the greater the phase error becomes when expressed in fractions of a wavelength.  
         [0003]     The actuator correction for these faults does not need to have a rapid response time, but should be capable of being set very accurately. The shorter the wavelength, the greater the phase error and the more critical the required correction.  
         [0004]     It is known in the art to use the Fried (freed) coefficient as a statistical measure of the phase error. As the Fried coefficient becomes smaller the distortion becomes greater.  
         [0005]     As the light to be refocused moves from the infrared range to the visible range, the adaptive mirror surfaces which needs to be controlled to a fraction of a wavelength becomes subject to even finer tuning.  
         [0006]     The stiffness of a composite mirror can be calculated by the structural stiffness module. The manner of this calculation may be found in “Development of Lightweight Mirror Elements for the Euro 50 Mirrors,” by Bennett et al. Proceedings of the 2nd Bäckaskog Workshop on Extremely Large Telescopes, Sep. 11-12, 2003, SPIE (in press).  
         [0007]     Many piezoelectric materials are known. They have been made into actuators to move or displace upon application of a predetermined voltage. The voltage causes a piezoelectric substance to expend or contract. For a given voltage a single actuator of piezoelectric material expands in all directions. Anything connected to such a device is displaced or thrown this change in distance. For a given device a set throw range is established. If double the throw distance is needed two identical devices are placed together in electrical series connections or stacked. Applied voltage must be doubled for both actuators to fully respond and give double the throw distance. Lateral movement in such stacks is ignored. Third, fourth and more actuators are added to the stack for greater throw distances.  
         [0008]     For audio devices piezoelectric material is coated on a sheet of metal, such as brass, steel, or other desired material creating the equivalent of a bimetallic strip. In this application the lateral expansion causes the device to bow to a given radius of curvature for a preselected voltage and thickness of the metal sheet and piezoelectric coating. In general, the thinner the greater the amount of curvature or bowing. These devices have been used to generate sound waves as the device bows and flattens.  
         [0009]     1. Description of the Prior Art  
         [0010]     Adaptive mirrors have many limitations including thickness of surface. If too thick the result is being unable to match wave fronts having closely spread irregularities. Other limitations are spacing of actuators to deform the surface and inability of actuators to push with enough force to deform the surface to the shape required. These are interrelated problems. In general actuators have their throw distances or displacement range extended by stacking piezoelectric devices in electrical series connections. As the distance desired increases, the voltage increases. Piezoelectric devices are made of metallic plates with a piezoelectric coating and an electrical lead to each so a voltage can be applied to it. A bimetallic effect can be achieved in this manner to cause the device to bow. Applications of this bowing have been limited to audio equipment to produce sound waves.  
         [0011]     2. Field of the Invention  
         [0012]     This invention relates to a method and device to adjust the active and adaptive actuator settings for an active or adaptive mirror. In particular this invention relates to a device to adjust for gravity induced distortion and manufacturing irregularities in the polished surface as well as atmospherically induced wave front distortion in the incident light beam. This invention permits active problems to be corrected and also allows adaptive problems to be corrected by distorting the adaptive surface to correct for wave front distortion in an incoming wave front.  
         [0013]     This invention further relates to actuators with relatively large throw distances. In particular this invention relates to actuators with short response times and relatively large throw distances at low voltage. This invention also relates to devices that provide tens of micrometers of throw distance without using stacked actuators. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  is cutaway perspective view of the present invention.  
         [0015]      FIG. 2  is an exploded view of the present invention.  
         [0016]      FIG. 3  shows a unique actuator that will work with the present invention.  
         [0017]      FIG. 4  is a diagram showing the accuracy in displacement control for the active actuator part of the subject of the present invention.  
         [0018]      FIG. 5  is a graph of displacement versus micrometer setting for an embodiment of the present invention.  
         [0019]      FIG. 6  is a cross section showing the design of the piezoelectric actuator disks to maximize actuator throw. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]      FIG. 1  is a cutaway perspective view of an active actuator plus an adaptive actuator. A holding plate  10  has a plurality of openings  12  as shown. The number of openings  12  is determined as described below. In each opening  12 , an actuator  14  is mounted. Each actuator  14  is mounted via a differential thread arrangement described further on. Connected to each actuator  14  is a push-pull rod  16  which is in two pieces for the embodiment shown in  FIG. 1 . One piece is mounted above actuator  14  and the other is mounted beneath actuator  14 . Electrical leads  18  permit electrical signals to be sent to actuator  14  which can be any suitable fast response actuator, such as piezoelectric crystals. Actuator  14  is an adaptive optic actuator that rapidly becomes thicker or thinner as desired. In turn this moves push-pull rods  16  back and forth. Its design is shown in  FIG. 4  and will be described further on. One end of the push-pull rod has a mounting  20  which is fastened to the back of an adaptive optic mirror  22 . Push-pull rods  16  and mount  20  may be connected to each other by gluing a threaded foot to mount  20  and threading a small screw both into it and into rod  16 . Other methods of attachment may be used. In a similar fashion push-pull rods  16  may be glued to actuator  14  and mount  20  is glued to the back side of adaptive mirror  22 . The screws keep the actuator function in line with the normal to the object being displaced.  
         [0021]     Holding plate  10  is mounted in case  24  shown in a cutaway view. Screws  26  may be used to keep holding plate  10  rigidly mounted. Other methods of attachment may be used as desired. Adaptive mirror  22  may be glued to case  24  or held in place by being attached to mounts  20 .  
         [0022]     Adaptive mirror  22  has a predetermined coefficient of thermal expansion. To avoid thermal stress to the mirror, case  24  should have the same coefficient of thermal expansion as well as holding plate  10 . An example of suitable material is a cyanate ester composite. Another material that appears suitable is a carbon-silicon carbon material available as CSIC on the commercial market.  
         [0023]     An actuator may be made of a thin metal sheet coated with a piezoelectric coating. This is a form of bimetallic strip. A bimetallic strip is made of two metals having different expansion coefficients. They are often used to make contact when the temperature in a room changes, for example. By allowing a relatively large area of two thin sheets to bend when the dissimilar expansion coefficients expand one sheet relative to the other one gets a tremendous magnification of motion compared to the motion involved in letting one material expand normal to its surface. For example, in a commercial 6.5″ diameter adaptive optic mirror made up of 37 piezoelectric stacks spaced 1.1″ apart the sensitivity of each stack was 55 Å/volt displacement. It required 1.5 kV to move the mirror surface by 8 μm and the bandwidth was DC to 300 Hz. Compare that to the double sandwich described here, where the throw is approximately one μm/volt and a bandwidth of DC to 2.5 kHz. The high voltages required have always been a problem with piezoelectric actuators and voltage-wise the present invention represents a gain of 1 μ/0.0055 μm/volt=182 times greater for the present system. Actuator  14  may be made of one or two of these actuators, say 19 millimeter in diameter, each separated by a 1.6 millimeter thick insulating ring, and mounted back to back to maximize throw distance for minimum voltage. Examples of usable actuators are ones with a throw distance of ±30 micrometers at 60 V applied with a reaction time of less than a millisecond. If only one plate is used, the throw distance is reduced by a factor of 2. If a conventional piezoelectric stack is used, typically the reaction time as compared to the two plate actuator  14  is increased from 0.4 msec to 3.3 msec (milliseconds) nearly an order of magnitude.  
         [0024]      FIG. 2  is an exploded view of the setting mechanism. In  FIG. 2 , push-pull rod  16  is shown mounted in actuator holder  28 , which in turn is within differential thread  30  which in turn is within support collar  32 . A two-part adjustment handle having coarse handle  34  and fine handle  36  may be made of brass or other metal or equivalent substance. Likewise both may have a knurled surface  38  or a portion thereof as shown. Texturing is not part of the invention but is known as a convenience for such surfaces. Coarse handle  34  is cylindrical and hollow along its intended axis of rotation. Coarse handle  34  has two portions of differing outer diameters so a surface  40  is formed. Surface  40  has the larger of the two outer diameters and is set to be greater than the diameter of openings  42  in calibration mounting plate  44  as shown in  FIG. 1 . The use of terms coarse and fine is subjective and can be reversed. These two handles serve as a first and as a second adjustment handles.  
         [0025]     Referring back to  FIG. 1 , calibration mounting plate  44  is rigidly mounted in case  24  by screw  46  or any other holding means. A plurality of openings  42  are placed in mounting plate  44  one each which is axially aligned with openings  12  in holding plate  10 . Again matching coefficients of thermal expansion is recommended for the material used to make mounting plate  44 . Extension  48  is the portion of coarse handle  34  that fits through calibration mounting plate  44  until surface  40  is flush with calibration mounting plate  44 . The length of extension  48  is such that it becomes flush to support collar  32  when fully inserted through calibration mounting plate  44 . On the end of extension  48  at least one tab  50  is placed which fits notches  52  on support collar  32 . In  FIG. 2  two tabs  50  are shown. This number may vary. Tabs  50  extend the length of extension  48  and prevent a flush fit unless they fit into matching notches  52  placed in support collar  32 . When tabs  50  are inserted into notches  52 , support collar  32  may by screwed up and down within holding plate  10  by coarse handle  34 . Fine handle  36  has an extension  54  whose outer diameter allows it to fit within the hollow portion of coarse handle  34 . Extension  54  has a smaller outer diameter than the rest of fine handle  36 , which creates a surface  56 , which fits flush to coarse handle  34  when extension  54  is inserted into coarse handle  34 . The length of extension  54  is set so it is flush to actuator holder  28  except for a tab  58  which fits into a corresponding notch on the opposite side of actuator holder  28  from the side in contact holding push-pull rod  16 . When fine control  36  is turned, extension  54  via tab  58  screws actuator holder  28  back and forth within support collar  32 . Support collar  32  may not move because tab  50  of coarse handle  34  do not permit it to turn except when coarse handle  34  is intentionally turned. Likewise, fine control  36  prevents actuator holder  28  from turning when support collar  32  is moved by coarse handle  34 . The depth of tabs  50  and  58  set the limit of adjustment distance that actuator  14  can be moved. For micrometer distance adjustments tab depths of millimeters give a reasonable safety range. Because coarse handle  34  and fine handle  36  slide through openings  42  to make a flush fit without attachment, one calibration setting mechanism may be used to set each actuator  14 .  
         [0026]      FIG. 3  shows an assembled view of  FIG. 2  and an alternate embodiment if loss of adjustment control is a concern. In  FIG. 2  embodiment support collar  32  must be allowed to turn within the opening it is inserted into. If support collar  32  is rigidly fixed in that same opening the technique to adjust may be varied. To fix support collar  32  in a manner so it cannot twist, it may be glued or have a ridge machined onto it to fit a matching slot. Any known method of making a nontwist mount will work. In the prior embodiment, differential thread  30  was moved as support collar  32  was turned. Since support collar  32  is not allowed to turn in this embodiment, for example, a ridge  33  is added to fit a notch  13  as shown in  FIG. 1 . The effect of ridge  33  within notch  13  is to hold support collar  32  so it cannot turn. Differential thread  31  is rigidly mounted to coarse adjustment handle  35 . Within differential thread  31  is threadably inserted fine adjustment handle  37  which includes a threaded end  39  to which is mounted a push-pull rod  16 , not shown. This embodiment allows as much distance to adjust as desired. Because it is rigidly mounted to push-pull rod  16  and actuator  14  this calibration mechanism must have one per each actuator and may cause crowded conditions behind mounting plate  44 .  
         [0027]      FIG. 4  shows an actuator  14  with push-pull rod  16  mounted between actuator holder  28  and adaptive mirror  52 . Push-pull rod  16  may be glued on or otherwise rigidly fixed to both actuator holder  28  and adaptive mirror  52 . As shown, actuator holder  28  has threads  60  and a notch  62 , which fits tab  58  previously shown.  
         [0028]     In the preferred embodiment push-pull rod  16  is split into two segments separated by piezoelectrical plates  64  connected at the edges. As shown two piezoelectrical plates  64  are mounted back to back to a buffer material  66 . Piezoelectric plates  64  may be commercially available models such as KBT-33-RB-2CN, KBT-33-RB-2S, KBT-XXRB-2SC/N, or KBS-35DA-3A, all offered by Kyocera. In general a piezoelectric plate is a metal plate such as brass, stainless steel or so formed with a piezoelectric coating. Electrical leads are connected to each in the known fashion. When a voltage is applied to them the piezoelectric plate expands laterally and bows causing the displacement to increase. The displacement resulting from the bowing is much greater than the vertical expansion normal to the plate surface as described above. Also the larger the area of the metal plate that is coated, the more the displacement because a bigger surface is warping. This means that in addition to greater voltages greater areas per actuator can be required. The present invention provided 30 μm vertical displacement for low voltage of the metal plate, which is about 0.004 inch thick, and the diameter of the plate is no greater than the influence function of the faceplate of the mirror to be adapted. Plates  64  may be glued to material  66  and to push-pull rod  16  segments as shown. Any glue that does not shrink as it dries is appropriate. Glues that shrink will warp the thin optical surface as they dry and are inappropriate for this design. Buffer material  66  ideally may be a silicon rubber ring between plates  58 . Use of a rubber ring will work for material  66  with the added advantage of allowing space for plates  58  to bow inward towards each other. Electrical leads  18  are connected to a voltage source as desired to cause plates  58  to expand or contract. A cross section of this preferred embodiment is shown in  FIG. 6 .  
         [0029]     For high quality control of adaptive optic mirrors, the final polished surface may still have a surface contour that effects performance. Such contours can be observed via interference fringes and the local areas that are too high or too low identified. Installation of the mirror surface and connection of the plurality of actuators  14  to the backside of adaptive optic mirror  52  may also induce surface distortions. By systematically inserting the coarse and fine handles through calibration mounting plate  44  each actuator  14  may be screwed in the direction needed to level adaptive optic mirror  52  surface at that location. Once the surface distortion is removed using active optics, the applied voltage to each actuator  14  will, if mirror surface  52  is thin enough to have a short influence function, distort adaptive optic mirror surface  52  in the direction and amount of displacement necessary to correct for an incoming distorted wave front.  
         [0030]     Various combinations of threads may be used between coarse control and fine control. To date the best combination of threads has been found to be 1/2-20 SAE and a M8-1.25 metric screw. The matching threads should be as long as possible without binding and the threads themselves as deep as possible to provide maximum contact. An example is to use 1/2-20 SAE threads, which have a half-inch diameter, for this purpose. The thick heavy rod also helps to keep the screws from being bent internally, which introduces a systematic error into the measurements.  
         [0031]      FIG. 5  shows the surface displacement in micrometers, μm, for a turn of eight graduation on a fine adjustment micrometer, which has 100 graduations per turn. This data is for the above combination of threads.  
         [0032]      FIG. 6  is a cross section of the preferred actuator for the present invention. By having plates  64  in parallel with the piezoelectric layers facing each other and separated, the throw distance is twice the bowing of a single plate. In  FIG. 6 , each plate  64  is free to bend like a bimetallic strip and much further throw distance is achieved than if only the vertical expansion of the plate has an effect, as in a conventional piezoelectric stack. For the same voltage, the throw distance increases by a factor of 2 compared to that of a single plate and 182 times that achieved per volt for a conventional piezoelectric stack. Throws of 140 μm have been achieved at higher voltage, but normally 30 μm is all that is needed for an adaptive optic application. In practice 30 μm throw distances have been achieved at 30 volts. It is to be noted that the device works with a single plate  64  as the actuator. Use of two electrically in parallel doubles the effect without increasing the voltage. The size of the throw distance depends on the thickness of the actuator materials and the surface area. The thicker the less bending and the greater the surface area the larger the bending. For the application shown, surface area should be comparable to the influence factor of the mirror surface.

Technology Category: 3