Abstract:
An A liquid-crystal adaptive optics actuator comprising a two-dimensional array of pixels ( 14 ), wherein each pixel ( 14 ) is connected to a control circuit by means of a control line signal path ( 16, 20 ) that comprises an electrical interconnection ( 16 ) and a meandering resistor ( 20 ), each resistor having a resistance value selected to equalize the RC time constant of each control line signal path associated to each pixel. Each control line is thus capable of carrying one or more control signals and the control line signal path is configured such that all the pixels respond to the control signals with a uniform response time.

Description:
GOVERNMENT RIGHTS 
     This work was supported by the Department of Defense under Contract No. FA8650-05-C-7211. The Government has certain rights in this invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage of PCT application PCT/US2013/042638 filed in the English language on May 24, 2013, and entitled “A DAPTIVE  O PTIC  H AVING  M EANDER  R ESISTORS ,” which application is hereby incorporated herein by reference. 
     FIELD 
     The structures and techniques described herein relate to optical transmit-receive systems and more particularly to free-space laser/optical transmit-receive systems. 
     BACKGROUND 
     As is also known in the art, an adaptive optic (AO) actuator provides means to correct a phase front on a pixel by pixel level. 
     As is also known, conventional AO actuators operate as so-called “reflective-mode” devices and are typically implemented via deformable mirrors or MEMS mirrors. With exception of liquid crystal cells, all known technologies for realizing an AO are inherently limited to reflective-mode operation. 
     Use of reflective-mode AO devices often results in unnecessarily complicated optical layouts. Furthermore, reflective-mode AO actuators are generally larger and heavier than desired for many applications. Additionally, such reflective-mode AO actuators are not as fast as desired, do not handle phase fonts with phase discontinuities do not have sufficient spatial resolution, and do not handle high levels of optical power. 
     Also, all mechanically based AOs suffer interactuator modulation, whereby the setting of one pixel affects the setting of adjacent pixels. This prevents such AOs from correcting wavefronts with discontinuous phase, as is common in atmospheres with high levels of turbulence. MEMS-based devices (e.g. such a those manufactured by Boston MicroMachines) offer the smallest known interactuator coupling of about 13%. 
     Prior-art transmissive AOs based on liquid crystal technologies, which alleviate a number of the difficulties with mechanical AOs, are known but suffer from low bandwidth and also variable response time from pixel to pixel. 
     It would, therefore, be desirable to provide an AO actuator that is compact, lightweight, high speed or at least having pixel speeds constant across the aperture, and high power in the preferred transmission-mode embodiment, and which works well with discontinuous phase fronts. 
     SUMMARY 
     In accordance with the concepts, systems, components and techniques described herein, an adaptive optic actuator includes a two-dimensional array of pixels with each of the pixels comprising a meander-resistor with the layout of each meander-resistor selected to provide a uniform time constants to all pixels across the aperture. 
     With this particular arrangement, an adaptive optic having a specially designed electrode layout resulting in uniform time constants to all pixels across the aperture is provided. 
     In accordance with the concepts, systems, components and techniques described herein, an adaptive optic actuator includes a two-dimensional array of pixels with each of the pixels being furnished with a resistor having a resistance value selected to equalize an RC rise time to that pixel. 
     In accordance with the concepts, systems, components and techniques described herein, an adaptive optic actuator includes an array of pixels each of which is provided from a liquid crystal cell comprising: a superstrate having an inner surface; a substrate having an inner surface opposed to the surface of the superstrate; said substrate and superstrate having electrically conductive structures formed thereon and disposed as electrodes which permit different voltages to he applied to each pixel; an electrical signal path, capable of carrying one or more control signals, coupled to each pixel in said array of pixels, wherein each electrode signal path is provided having a path length and resistance such that a substantially uniform time constant is provided to all pixels across said array of pixels. 
     An adaptive optic actuator comprising a two-dimensional array of pixels each of the pixels having an associated control line signal path electrically coupled thereto with each control line signal path being furnished with a resistor having a resistance value selected to equalize an RC rise time to the associated pixel. With this arrangement, each control line is capable of carrying one or more control signals, and the control line signal path is configured such that a uniform time constant is provided to all pixels across the array of pixels. 
     In one embodiment, the pixels are provided having a square cross-sectional shape to better support the intended use of an AO with square beams. 
     As noted above in some embodiments, an electrode layout which results in uniform time constants to all pixels across the aperture is used. In an optical application in which square beams are used, the pixels are square to better support the intended. use of the AO with square beams. It should, of course, be appreciated that the concepts, systems and techniques described herein are not limited to square beams and that any beam shape may be used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of the circuits and techniques described herein, may be more fully understood from the following description of the drawings in which: 
         FIGS. 1 and 2  are a series of plan views of the electrically active substrate of an adaptive optic (AO) actuator; 
         FIGS. 3 and 4  are a series of plan views of a portion of the AO actuator shown in  FIGS. 1 and 2 ; 
         FIG. 4A  is an expanded view of a portion of  FIG. 4 ; and 
         FIG. 4B  is an expanded view of as portion of  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1-4B  in which like elements are provided, having like reference designations throughout the several views, an adaptive optic (AO) comprises a voltage-addressable, transmission-mode, liquid-crystal (LC) cell, as is generally known in the art, having substrates  10  patterned into an array of independent pixels generally denoted  14  to support two-dimensional (2-D) addressing for use as an AO actuator. In addition to each pixel being denoted with reference numeral  14 , it should be noted that each individual pixel in  FIGS. 1-4B  is also provided having a unique alpha-numeric designation (e.g. A 1 -A 4 ; B 1  -B 12 ; C 1  -C 20 , D 1  -D 28 ; E 1  -E 36 , F 1  -F 44 , G 1  -G 40 ). 
     In the exemplary embodiment of  FIGS. 1-4B , the adaptive optic is provided having a plurality of pixels having various shapes (squares and triangles) and arranged in columns and rows. Each of the pixels is coupled to a corresponding one of an output of a control circuit (not shown in  FIGS. 1-4B ). It should be noted that control circuits may be disposed on a substrate of the LC, via “flip-chip” or other chip-on-glass assembly technique, or else the control circuits may he “off-glass”. The control circuits are coupled to pixels  14  via electrical signal paths which coupled control signals to conductors disposed on the substrate. Electrical transitions  16  and compensating resistors  20  (also referred to as an RC balancing resistors) form a portion of such signal paths to the pixels  14 . Such signal paths may also include, for example, a flex cable coupled to a controller or other signal source (not visible in  FIG. 1 ). It should be noted that each pixel  14  is individually addressable via the leads which are very narrow and are not clearly shown on the figures, but which run from each compensating resistor to its corresponding pixel being routed in the narrow gaps between pixels. Thus the control circuit(s) are capable of providing one or more control signals to each of the pixels of the AO. 
     Each signal path coupled between the flex circuit and pixels includes an RC balancing resistor, generally denoted  20 . In preferred embodiments the resistors are provided having a meander-resistor layout ( FIG. 4B ). The path lengths in the meander-resistor layout are selected so as to result in a substantially uniform speed in controlling pixels across the array. That is, the time it takes for any pixel in the AO pixel array to respond to a control signal is substantially the same regardless of pixel location within the AO. Since these response times are equal, any effects (such as decrease of RMS voltage arising from delay and attenuation between the voltage source and the pixel) will be equalized across all pixels, enabling compensation for such voltage decrease in the calibration tables already needed for any liquid-crystal device. 
     Referring now to  FIGS. 3-4B , as noted above, the AO includes a specially designed electrode layout which results in uniform time constants to all pixels across the aperture. As most clearly visible in  FIG. 4B , a meander-resistor layout is used to achieve the desired uniform time constants. Each pixel has a capacitance which is known in advance, given its area and the device thickness as well as the properties of the liquid crystal, and thus resistors may be designed which make the product of R i C i  the same, where i runs over for all pixels. Normally one would determine the resistance and capacitance of the pixel with the largest product (the “slowest” pixel), e.g., one of the pixels in the center of the aperture, whose connection to the edge of the aperture is longest and hence most resistive. For this pixel, here denumerated pixel no.  1 , we have some value of R 1 C 1 . The meander resistor chosen for this pixel would be of minimal resistance, preferably zero resistance, i.e., would be absent from the substrate layout. For each of the other pixels, Whose intrinsic R i0 C i  (comprising the pixel capacitance C i  and the resistance R i0  of the connection from the edge of the aperture to the pixel without adding a compensating resistor) is therefore smaller than R 1 C 1 , a compensating resistor is included in series with the connection. Thus, all pixels are “slowed down” such that they all have substantially the same RC response time. 
     The above approach enables one to compensate for the effect of voltage attenuation in the RC networks in a uniform manner for all pixels, enabling use of a single calibration table for all pixels, which is a highly desirable feature for obtaining accurate phase control. Compensating resistors  20  may be disposed in the superstrate glue line (i.e. a space into which sealant or “glue” is disposed to form sidewalls and couple a superstrate a substrate) and resistors  20  are coupled via connecting elements  16  which provide a transition to leadouts (and eventually to flex circuits  18   a ,  18   b  and controllers) outside of the superstate. As clearly visible in  FIGS. 3 and 4 , a total of eight meander-resistor layout cell types are needed for pixels in one quadrant of the AO 
     As noted above, an electrode layout which results in uniform time constants to all pixels across the aperture is used. In an optical application in which square beams are used, the pixels are square to better support the intended use of the AO with square beams. It should, of course, be appreciated that the concepts, systems and techniques described herein are not limited to square beams and that any beam shape may be used. Likewise, an AO with a different pixel geometry than square may employ compensation resistors designed according to this teaching. For example, a hexagonal close-packed array, as is known in the art, a useful geometry for an AO. The feed lines for the pixels are of variable length, with longer ones for the central pixels, so the compensation resistor technique here taught is applicable for equalizing the response time and hence enabling more convenient drive voltage circuitry having the same calibration table for all pixels. 
     Having described one or more preferred embodiments of the circuits, techniques and concepts described herein, it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these circuits, techniques and concepts may be used. Accordingly, it is submitted that that the scope of the patent should not be limited to the described embodiments, but rather, should be limited only by the spirit and scope of the appended claims.