Abstract:
An assembly used in a deformable mirror comprises a support having at least one opening therein. A thin optical substrate making up the mirror is provided and has a light reflective first surface and an opposite back surface. A coupling is provided for controllably movably coupling the back surface of the optical substrate and the support to one another. The coupling includes at least one shaped memory metal cylindrical part operatively connecting the optical substrate and the support.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to U.S. application Ser. Nos. 08/293,787, now U.S. Pat. No. 5,535,043 entitled, “DEFORMABLE MIRROR WITH REMOVABLE ACTUATORS; 08/965,440, now U.S. Pat. No. 6,011,639, entitled MONOLITHIC DEFORMABLE MIRROR ASSEMBLY; 08/992,022 now U.S. Pat. No. 6,084,332, entitled, HIGH ACTUATOR DENSITY DEFORMABLE MIRROR; 08/982,920, now U.S. Pat. No. 5,940,203, entitled, HUGH ENERGY BURST DEFORMABLE MIRROR; 09/071,510, now U.S. Pat. No. 5,917,644, entitled INTEGRAL HIGH ENERGY BUTTON DEFORMABLE MIRROR, and U.S. Pat. No. 5,745,278 entitled INNOVATIVE DEFORMABLE MIRROR ACTUATOR CONFIGURATION. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to deformable mirrors that are used in atmospheric correction systems as well as in high and low energy laser beam train lines. The invention can also be applied commercially anywhere there is a need for low cost, easily assembled and removable actuator arrays. 
     BACKGROUND OF THE INVENTION 
     The invention is a configuration of a deformable mirror that permits easy actuator replacement. What is unique is the use of shaped-memory alloys to achieve this in a minimum space. This characteristic makes the invention applicable to high spatial frequency deformable mirrors such as are currently used in atmospheric correcting system (SAAO) and airborne laser weapon programs where the actuators are 7 mm apart. 
     As discussed in U.S. Pat. No. 5,745,278, deformable mirrors primarily intended for use as a beam train optic require frequency response, reliability, and low cost of manufacture as critical to the design, performance and usefulness. Thus the invention has particular usefulness in deformable mirrors that are used in adaptive optical systems. This would include low and high energy beam train configuration; that is both atmospheric correcting systems (SAAO) as well as laser weapon systems. 
     A deformable mirror which is used as a downstream optic in conjunction with a substantially larger optical system which is disposed upstream thereof must possess a high degree of sensitivity with respect to its ability to make highly minute adjustments to the reflective surface of the mirror. This is because such optical downstream mirrors represent the upstream optics in miniature. Such mirrors have a diameter in the range of five to fifteen inches, but for the larger sizes use on the order of about one thousand separate piezoelectric actuators to effect such adjustments. As can be expected, the nearly one thousand piezoelectric actuators used are highly miniaturized and make assembly and disassembly of the approximately one thousand piezoelectric actuators and the deformable mirror, when necessary, painstakingly tedious. 
     Current state-of-the art deformable mirrors such as SAAO must be satisfied with degraded performance if any of its 941 actuators fail since they are very difficult to replace. This replacement includes a complete replacement of the optical faceplate, a very difficult process. It is known to mechanically decouple the nose of the actuator from the back of the face plate, such as by cutting it with a wire saw. Other techniques involve melting epoxy joints with the application of heat to remove actuators. Heretofore, however, there are no known methods that utilize shaped-memory alloys, either with cooling or heating for replacing individual actuators. 
     Thus an object of the invention is to permit easy replacement of actuators in deformable mirrors thus maintaining their required performance characteristics without degradation with reasonable cost and schedule impacts. 
     Accordingly, it is an object of the present invention to provide deformable mirror actuation using individual piezoelectric actuators connected via shaped-memory alloys to the reaction plate thereby permitting disassembly inherent in systems utilizing such actuators. 
     Still a further objective of the invention is to provide a stress and strain free connection between the mirror and the supporting structure. 
     Yet still a further object of the invention is to provide a method of disassembly and reassembly that does not require high or low temperatures (below −60° F.). 
     Yet still a further object of the invention is to provide a means of in situ actuator replacement. 
     SUMMARY OF THE INVENTION 
     The invention resides in an assembly comprising a support having at least one opening therein. A thin optical substrate is provided and has a light reflective first surface and an opposite back surface. A coupling means is provided for controllably movably coupling the back surface of the optical substrate and the support to one another. The coupling means includes at least one shaped memory metal cylindrical part operatively connecting the optical substrate and the support. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially fragmentary vertical section view of the front end of the actuator stem and mirror connection as well as a vertical sectional view of the back end through an actuator and the structure to which it is mounted. 
     FIG. 2 is a flow chart of the process by which the front end actuator stem and the mirror mount are connected. 
     FIG. 3 is a flow chart of the process by which the actuator stem and the back end sleeve are connected. 
     FIGS. 4 a - 4   f  show a method for attaching the actuator to the supporting structure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shaped-memory metal is used because of the significant dimensional changes that it can undergo with reasonable temperature changes −60° F. cooling medium is readily commercially available. The key issue is that even though the dimensions of the parts used in this invention are small, sufficient interference fits and dimensional changes can be achieved that result in mechanical “clamping” and releasing capability. This “clamping” force must be capable of acting against the force the actuator provides to the back of the mirror without permitting relative movement to the parts. Typical materials such as aluminum change dimension with temperature 13 to 14 parts per million per degree F. Shaped memory metal subjected to two to three hundred degrees F. temperature change will undergo a change in crystalline state that results in dimensional changes in the order of 2 to 4 parts per hundred. This is a significant dimensional change such that forces are introduced that are sufficient to prevent movement between parts. 
     As shown in FIG. 1, the invention shows an actuator  10  connected at its back end to the reaction structure  12 , and on the front to the back side  14  of a deformable mirror  16 . A connection is made between the actuator  10  front end FE and the rear face of the mirror  16 . That is, the actuator front end FE is coupled to the back side of the mirror by a cylindrical compression fitting  20  of shaped memory metal simultaneously compressing the cylinder fitting  20  onto a mount  22  formed on the back of the mirror and onto a corresponding cylindrical structure  24  formed on the nose  26  of the actuator stem. This connection produces an axially holding frictional force capable of transmitting the axial load of the actuator to the mirror. 
     Referring for the moment to FIG.  2  and to the method of forming the connection of the invention, it should be seen that the principle of operation of this coupling involves the application of a cold medium, such as liquid nitrogen, to enlarge the fitting  20  resulting in undoing the mechanical connection to the mirror. The shaped alloy metal remembers that at low temperature it is larger in diameter. Restoring the coupling to room temperature restores the grip. Specifically, in step  30  the fitting  20 ′″ is fabricated at room temperature. Then the fitting  20 ″ is cooled to change its crystalline structure (step  32 ). At the cooled temperature, the fitting  20 ′ is mechanically expanded to enlarge its overall diameter (step  34 ). The fitting  20 ′ is then installed over coaligned tubes  35 ,  35 ′ (step  36 ). Thereafter, the fitting  20  is allowed to return to room temperature whereupon the fitting shrinks over the tubes  35 ,  35 ′ (step  40 ). 
     As seen in FIG. 1, the actuator  10  is received within an opening  42  formed in the reaction structure  12 . The actuator is comprised of a sleeve  50  which is mounted into the opening  42  in accordance with a further feature of the invention and further has an internal confine  54  which receives an actuator stem  52 . The back end BE of the actuator  10  facing the reaction structure  12  has several different requirements compared to the front end FE. That is, the actuator  10  has a slot  44  which includes leads  40  which connect to electronic components. An epoxy pinning  58  is provided between the stem  52  and the internal confine  54  of the sleeve to ensure a moment (stress and strain) free connection. The use of the available volume must be maximized because of the close spacing of the actuators. 
     In further accordance with the invention, it should be seen that the invention resides in a method by which the sleeve  50  is connected and disconnected to the reaction plate  12  as best seen in FIG.  3 . Specifically, in step  60  the sleeve  50 ′″ is fabricated at room temperature (Step  60 ). Then the sleeve  50 ″ is cooled to change its crystalline structure (step  62 ). At the cooled temperature, the fitting  50 ′ is mechanically reduced to decrease its overall diameter (step  64 ). The sleeve  50 ′ is then installed into the opening  42  in the reaction plate  12  (step  66 ). Thereafter, the sleeve  50  is allowed to return to room temperature whereupon the sleeve expands into the opening  42  (step  68 ). Moreover, where necessary, at step  70 , it is further possible cool the sleeve  50  after being mounted within the opening  42  in the reaction structure plate  12 . Note that the shape-memory alloy works in both directions in this application, (i.e., expanding or contracting with cold). 
     Referring now to FIGS. 4 a - 4   f,  it should be seen that the method of connecting an actuator  10  to the reaction plate  12  begins as illustrated in FIG. 4 a  with providing the mirror  16  so that its back  14  with the mount  22  faces the reaction plate  12 . Thereafter, as seen in FIG. 4 b,  the actuator stem  52  is inserted within the opening  50  and is aligned with the mount  22  and the fitting  20  is then disposed over the ends of the stem  52  and the mount  22  in the manner set forth above regarding FIG.  2 . As seen in FIG. 4 c,  the sleeve  50 ′ in its cooled state is then inserted within the opening  50  about the stem  52  and allowed to warm to room temperatures seen in FIG. 4 d.  The epoxy joint  58  is next formed to affix the stem  52  to the sleeve  50 . In the event that the actuator  10  needs to be removed from the assembly, the sleeve  50  and the fitting  20  are cooled as set forth above to effect removal. It is noted that temperature changes do not generally adversely effect adjacent parts and that no forces are transmitted to the mirror during assembly or disassembly. A new actuator assembly and sleeve could be installed in the reversed manner. 
     In summary, every actuator is installed to the back of the mirror using the coupling described in FIG.  2 . Using the principle shown in FIG. 3, a shaped memory metal sleeve is installed in the reaction plate at every actuator location. Each sleeve does not touch the actuator stem running through it. A gap exists there between and is filled with epoxy through the central hole and bonds the actuator, through the sleeve, to the reaction plate. The sleeve is configured so that upon cooling it shrinks in diameter and releasably secures the actuator to the reaction plate. Undoing the joint permits the entire actuator assembly to be removed through the opening in the reaction plate.