Abstract:
The present invention relates to a deformable mirror, specifically a gimbaled deformable mirror for use with wavefront sensors, which mirror separates the tilt correction from the higher order modes (e.g. defocus, spherical, astigmatism, and coma at higher order aberrations, up to the limits of a particular mirror design) in order to use all of the available mirror deformation stroke for correcting the higher order modes. The separation is done by placing the deformable mirror in a gimbaled structure, so that the deformable mirror can be tilted in two independent, orthogonal axes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of and claims the priority of provisional application Ser. No. 62/092,424, filed Dec. 16, 2014. The subject matter of this provisional is incorporated by reference. 
     This invention relates to deformable mirrors, specifically a gimbaled deformable mirror for use with wavefront sensors. 
    
    
     BACKGROUND 
     Deformable mirrors have long been used as the correction element within adaptive optics systems. Such mirrors are typically: solid faceplate structures, where the faceplate is typically a reflective material such as a thin sheet of glass; or MEMS structures where the correction is applied by a plurality of small segmented mirrored surfaces; or membrane mirrors where the reflective surface is fabricated from a flexible polymer film, typically with a nominal radius of curvature bias imparted on the mirror surface. 
     Solid faceplate structures are best suited for high vibration environments or environments where high optical irradiances are encountered. MEMS mirrors offer a great deal of versatility, but are unsuitable for high power environments and are not well matched to some types of wavefront sensors. Membrane mirrors are inexpensive. but unsuitable for high power or high vibration environments. Furthermore, the base curvature in the mirror needs to be considered in the optical design, as base curvature will produce field angle-dependent aberrations. 
     Deformable mirrors function on the principle of having a thin surface that can be deformed to produce the conjugate aberration to the one measured by an associated wavefront sensor and which is incident on the deformable mirror. The deformation is effected by modulating the force applied to the mirror surface by a series of actuators. Most commonly, these are piezo-electric stacks for solid faceplate structures, although different mirror types will take advantage of different physical phenomena to create the localized deformations (e.g. electrostatic forces, bimorphic structures, etc. The actuator patterns may be rectilinear or arranged in other patterns, such as those describing various Zemike modes. 
     However constructed, prior art deformable mirrors suffer from one common limitation: actuator stroke is typically limited to a maximum of approximately 10 microns. Each actuator has a maximum slope and there is often some cross-talk between actuators. The cross-talk results in an inability to get full, independent motion from each actuator. Furthermore, because of the stroke limitations, the amount of aberration that can be corrected is limited. In most atmospheric aberration scenarios, the strengths of the various aberrations form an approximately geometric progression starting with tilt and progressing through the higher order aberrations (e.g. defocus, spherical, astigmatism, and coma). Most importantly, tilt can strongly dominate other aberrations. Thus, the stroke required to correct the tilt can leave little stroke left for correcting the higher order aberrations, as illustrated in  FIGS. 1-3 . 
     Known prior art includes: (1) U.S. Pat. No. 7,638,768, “Laser Wavefront Characterization”, L. J. Otten, et al.; (2) U.S. Pat. No. 8,009,280, “Wavefront Characterization and Correction”. G. R. Erry, et al.; and (3) U.S. Pat. No. 8,322,870, “Fast Steering, Deformable Mirror System and Method for Manufacturing the Same,” Kirk A. Miller. 
     SUMMARY OF THE INVENTION 
     The present invention separates the tilt correction from the higher order modes (e.g. defocus, spherical, astigmatism, and coma at the level of 3 rd  and higher order aberrations, up to the limits of a particular mirror design) in order to use all of the available stroke for correcting the higher order modes. The separation is done by placing the deformable mirror in a gimbaled structure, so that the deformable mirror can be tilted in two independent, orthogonal axes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating examples of two aberrations that may be corrected by a deformable mirror. The x-axis is the normalized radial coordinate; the y-axis is the magnitude of the aberration in microns. 
         FIG. 2  is a graph illustrating the combination of the aberrations shown in  FIG. 1  (solid line) and the effect of limited mirror stroke (dashed line). Again, the x-axis is the normalized radial coordinate; the y-axis is the magnitude of the aberration in microns. 
         FIG. 3  is a graph of the residual wavefront error due to mirror “clipping” as it has insufficient stroke to correct tip/tilt and more complex aberrations. And, again, the x-axis is the normalized radial coordinate; the y-axis is the magnitude of the aberration in microns. 
         FIG. 4  illustrates the basic structure of the deformable mirror and piezoelectric actuator stack array sub-assembly of the present invention. 
         FIG. 5  illustrates the deformable mirror/piezoelectric actuator stack array and gimbal cage which constitutes the deformable mirror assembly of the present invention. 
         FIG. 6  is a sectional view of the apparatus of  FIG. 5  with the mirror deformed and tilted. 
         FIG. 7  is an optical schematic illustrating the use of the gimbaled tip/tilt deformable mirror of the present with distorted grating wavefront sensor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The basic structure of the deformable mirror sub-assembly  11  is shown in  FIG. 4 , including deformable mirror  13  including face  13 A and center point  13 B, base  15  and piezoelectric actuator stack array  17  permanently attached between mirror  13  and base  15 . As illustrated, base  15  includes gimbal cage support arms  19 , including gimbal cage pivots  23 , and piezoelectric stack supports  21 A and B. With this arrangement assembly  11  (including mirror  13 , base  15  and arms  19 ) pivots about axis  25  (the θ (tip) axis). The individual piezoelectric actuators of stack array  17  are bonded to both base  15  and deformable mirror  13 . Alternately, for instance, a magnetic voice coil array (not shown) can be used in place of piezoelectric actuator stack array  17 . 
     As illustrated in  FIG. 5 , the deformable mirror sub-assembly  11  is then installed into gimbal cage (or ring)  31 , with pivots  23  received in pivot supports  33  for rotation about axis  25 . Ring  31  includes pivots  35  which, in turn, are received in pivot supports (not shown) in support ring  37 . With this arrangement, ring  31  pivots about axis  39  (the θy (tilt) axis) relative to support ring  37 . And, assembly  11  (including mirror  13 , base  15  and arms  19 ) pivots about axis rotate about axis  25  (the θx (tip) axis). In combination with a wavefront sensor, support ring  37  is secured to the associated supporting structure by fasteners (not shown), which are received in fastener openings  37 A. θy piezoelectric stack  41  and θx piezoelectric stack  43 , all as shown in  FIG. 5 , provide the tip (θx) and tilt (θ y ) capabilities. Piezoelectric stack  41  is captured or adhered between cavity  22 A in support  21 A and a cavity (not shown) in support ring  37 . Similarly, stack  43  is captured between cavity  22 B in support  21 B and a cavity (also not shown) in support ring  37 . 
     There are a number of important considerations to effecting the rotation about mirror face  13 A in order to correct higher order aberrations First, the gimbal must be designed so that pivot point  13 B, which is the intersection of the θx, θy and Z axes (as shown in  FIG. 6 ) is the nominal location of the center of deformable mirror  13 . Failure to do so means that the mapping between the associated wavefront sensor (not shown) and the mirror is corrupted as the mirror pivots. This mapping error corrupts the deformable mirror influence function and creates a poorly responding or unstable system. Second, the gimballing is effected using orthogonal axes, e.g. orthogonal axis  25  (θx) and  39  (θy) in the nominal plane of mirror surface  13 A. This is important because the cross-talk created by non-orthogonal axes makes efficient control of the deformable mirror difficult. Third, even though the present invention allows for the compensation of larger aberrations, particularly tilt, it is important to understand that in any imaging application, where the light incident on the deformable mirror is coming from a variety of field points, optimal performance comes from keeping the deformable mirror as close to normal to the optical (Z) axis as possible, as large tilt angles, even when properly gimbaled, also create the mapping error previously discussed. It should be obvious to the experienced practitioner in the art that the separation of the tilt from the rest of the deformable mirror as illustrated by the apparatus of  FIGS. 4-6 , is not the same as making the tilt function a fast steering mirror. Fast steering mirrors compensate for system motion, typically driven by a control loop controlled by a gyroscope or some similar motion sensor, to take out platform motion and are well suited to compensate for relatively severe platform vibration. In contrast, the present invention is driven by the output from a wavefront sensor and is inherently suited to smaller corrections. In a sufficiently stable environment, the present invention can replace a fast steering mirror. In a high vibration environment, however, an ideal system would enjoy the advantages of both a fast steering mirror and the enhanced, fine correction produced by the present invention. 
     Most wavefront correction systems, such as those using a Shack-Hartmann sensor, require separate methods of measuring tip/tilt and higher order aberrations. However, the distorted grating wavefront sensor (such as disclosed in U.S. Pat. No. 7,638,768) is able to measure both tip/tilt and higher order aberrations with a single sensor. The combination of the gimbaled deformable mirror disclosed herein and the distorted grating wavefront sensor therefore offers the novel solution of a single sensor and single deformable mirror, compared with conventional systems which require two sensors and two mirrors (one for tip/tilt and one for higher order aberrations). Such a single sensor and single deformable mirror can be extremely compact and robust. 
     With reference to  FIG. 7 , adaptive optics system  51  includes the gimbaled tip/tilt deformable mirror of the present invention  53  (e.g., assembly  11 , gimbal cage  31 , support ring  37 ), beam splitter  55 , imaging camera  57 , re-imaging system  59  and distorted grating wavefront sensor  61 . As illustrated, AO system  51  is a closed loop configuration, where the wavefront sensor  61  follows gimbaled tip/tilt deformable mirror assembly  53  in the optical system. Alternately, the gimbaled tip/tilt deformable mirror assembly may be used in an open loop configuration, where such assembly follows the wavefront sensor. 
     The ability to use a single detector and single deformable mirror greatly simplifies the electronic and/or software control loop required for optical correction as there is no possibility of crosstalk between different sensors and mirrors because, in contrast to the present invention, if two sensors were used to drive a combined tip/tilt and higher order mirror, or conversely a single detector were to drive separate mirrors, it is likely that the two separate control loops required would “fight” each other, wherein one system would try to correct small systematic errors from the other leading to instability. In conventional systems with two independent control loops (one sensor driving a tip/tilt mirror and another sensor driving a higher order deformable mirror) crosstalk between the systems is almost inevitable and requires complex processing to ensure stable control. The combination of the distorted grating wavefront sensor and the gimbaled tip/tilt and higher order deformable mirror of the present invention eliminates these issues and provides inherently stable control. 
     Whereas the drawings and accompanying description have shown and described the preferred embodiments of the present invention, it should be apparent to those skilled in the art that various changes may be made in the forms and uses of the inventions without affecting the scope thereof.