1. Field of the Invention
This invention relates to deformable mirrors which have replaceable, electrically operated actuators for deforming the mirror's reflecting surface. The disclosed invention is a method and apparatus for installing and adjusting the actuators to control the surface shape of the mirror's reflecting surface.
2. Summary of the Prior Art
It is well known in the prior art to use deformable mirrors to correct optical signals for aberrations introduced into a wavefront due to the transmission of an optical signal through a distorting medium, e.g. the earth's atmosphere. Such deformable mirrors may be used, for example, to correct aberrations introduced into laser signals or optical images imaged by a telescope as they pass through the earth's atmosphere, or to introduce known aberrations into a laser signal prior to its transmission through the earth's atmosphere. Examples of deformable mirrors which may be used to perform the foregoing function appear in U.S. Pat. Nos. 3,904,274 and 4,657,358 which are owned by the assignee of the present invention and the teaching's of which are incorporated herein by reference. Mirrors of the foregoing type employ a plurality of actuators, for example piezoelectric actuators, which are independently operable to selectively deform areas of a deformable reflecting surface to correct known anomalies contained in an impinging optical signal's wavefront. Signals for driving the actuators to correct the aberrated wavefront may be provided by a wavefront sensing and correction system such as that described in U.S. Pat. No. 3,923,400, the teachings of which are also incorporated herein by reference.
The use of wavefront control systems, for example deformable mirrors, in large aperture optical systems mandates the use of a large diameter mirror to achieve effective wavefront control. Additionally, such a system should ideally also contain a large number of closely spaced actuators to permit adjustments to be made at locations closely spaced together across the deformable mirror's reflecting surface. To accommodate the foregoing requirements, a deformable mirror will frequently require the use of a large number of actuators to achieve effective control of the mirror's reflecting surface.
As noted in U.S. Pat. No. 4,657,358, actuators for use in controlling a deformable mirror may be manufactured from stacks of electrodistortive material, for example lead magnesium niobate (PMN) or lead zirconate titanate (PZT). The electrodistortive material is interleaved in the stack with alternating layers of electrical conductors to permit the application of an electrical signal to the material.
Prior known deformable mirrors which utilize stacked electrodistortive material to provide control of the mirror's reflecting surface have been constructed with the actuators rigidly fastened to the structure supporting the mirror's reflecting surface, for example by cement or a frit bond. Such construction has not provided for the ease of repair of a deformable mirror in the event of a failure of one or more actuators. In the event of the failure of an actuator due, for example, to the mechanical failure of an actuator or to the failure of electrical contacts between one or more layers of electrodistortive material, the components of the mirror had to be carefully disassembled to effect the removal and replacement or repair of the failed actuator. Such a disassembly procedure is time consuming, frequently difficult and often impossible to effect due to the spacing between adjacent actuators and the need to prevent damaging good actuators during the repair procedure. Consequently, there is a need for a deformable mirror which utilizes electrically operated actuators which may be easily replaced without damaging other actuators during the replacement procedure. A related requirement is for a deformable mirror having replaceable actuators which may be easily adjusted during the replacement procedure to control their influence on the deformation of a deformable mirror's reflecting surface.
In a related patent application owned by the owner of the present patent application and the teachings of which are incorporated herein by reference, a construction for a deformable mirror is disclosed in which the mirror's electrodistortive actuators may be easily removed from the mirror's base or support structure for purposes of calibrating and/or replacing one or more of the mirror's actuators. Each actuator may include a threaded portion which may be received in a threaded aperture in the base. The position of each actuator with respect to the mirror's deformable reflecting surface may be adjusted by screwing the actuator into its threaded aperture until the top of the actuator just contacts the structure supporting the reflecting surface without deflecting the surface. Further adjustment of the position of each actuator with respect to the mirror's reflecting surface may be used to control the initial (or rest) shape of the mirror's reflecting surface.
While a mirror constructed utilizing replaceable actuators as described above has the advantage of permitting the replacement and/or alignment of one or more of the mirror's actuators, it suffers from the requirement that each actuator must be precisely adjusted within its respective aperture to insure that the mirror's reflecting surface does not contain any deformations caused by improperly positioned actuators. Consequently, the actuator adjustment procedure is tedious, and must be carried out when the mirror is initially assembled and each time that an actuator is replaced. The actuator alignment procedure is both time consuming and requires careful measurement of the mirror's reflecting surface to insure a uniform reflecting surface so that there is preferably less than 1/10 wave difference at the frequency of the light to be reflected from the reflecting surface across the mirror's reflecting surface.