Abstract:
An apparatus for use in automatically correcting distortion in an optical signal includes an optical relay element positioned to receive the optical signal from a remote source and to relay the optical signal; and a photosensor positioned to receive a portion of the optical signal from the optical relay element, and configured to produce an electronic signal that varies with the distortion in the optical signal. Processing circuitry is configured to receive the electronic signal from the photosensor to detect variations in the electronic signal caused by the distortion in the optical signal, and to generate a control signal in response to the variations. An adjustment element is configured to receive the control signal from the processing circuitry and, in response to the control signal, to correct the distortion in the optical signal.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to correcting distortions in optical signals. 
     In many optical systems, including both optical data systems and optical imaging systems, beam stabilization and beam focus are critical for error-free data transmission or ideal image quality. Beam focus is particularly problematic when optical signals must be transmitted over long ranges, e.g., several miles, which are common in geographical imaging and interplanetary communication applications. Likewise, beam stabilization is difficult to achieve when atmospheric or mechanical jitter exists in the optical data or imaging system. Jitter can result both from movement of the signal source and movement of the signal reception system. For example, human movement is a common source of mechanical jitter in a video recording system, such as a handheld camcorder. 
     FIG. 1 shows a common optical signal reception system  15 , such as a telescope for use in interplanetary communications. The reception system  15  includes one or more curved mirrors  16   a,    16   b,  which together focus an incoming optical signal at a focal point  17 . Another mirror  18  is positioned at the focal point  17  to reflect the focused optical signal onto an optical sensor, such as a photodiode  20 , in a receiver. In general, conventional reception systems such as this one preserve and even introduce distortions in the optical signals that result from jitter and lack of focus. 
     SUMMARY OF THE INVENTION 
     Recognition of the above led the inventor to develop an optical signal acquisition system capable of compensating for spatial vibrations in optical signals and automatically focusing the telescope or lens system from which the optical signal is received. 
     In one aspect, the invention relates to automatically correcting distortion, such as a spatial vibration or a lack of focus, in an optical signal, such as an optical data signal. An optical relay element receives the optical signal from a remote source and directs the optical signal toward a specified target. A photosensor receives a portion of the optical signal and produces an electronic signal that varies with the distortion in the optical signal. Processing circuitry receives the electronic signal from the photosensor, detects variations in the electronic signal caused by the distortion in the optical signal, and generates a control signal in response to the variations. An adjustment mechanism receives the control signal from the processing circuitry and, in response to the control signal, corrects the distortion in the optical signal. 
     In some embodiments, the adjustment element is coupled to the optical relay element, and in other embodiments it is coupled to the target. Also, a beam splitter may be positioned before the optical relay element to reflect a portion of the optical signal toward the photosensor. An optical delay element may be positioned before the beam splitter and the optical relay element. A second photosensor may be used to receive a portion of the corrected optical signal and to produce a feedback signal that varies with any distortion that remains in the corrected optical signal. The processing circuitry may be used to receive the feedback signal from the second photosensor, to detect variations in the feedback signal caused by the distortion that remains in the corrected optical signal, and to alter the control signal in response to the detected variations. 
     In other embodiments, a beam splitter may be positioned after the optical relay element to reflect a portion of the corrected optical signal toward the second photosensor. The optical relay element may be a reflective device, such as a mirror. The adjustment element may be used to adjust the position of the reflective device in response to the control signal. The photosensor may include two pairs of position-sensing, Schottky-barrier photodiodes. 
     In another aspect, the invention relates to automatically compensating for spatial vibration of an optical signal. An optical relay element receives the optical signal from a remote source and directs the optical signal toward a specified target. A photosensor receives a portion of the optical signal and produces an electronic signal that varies as the optical signal moves across the photosensor as a result of the spatial vibration. Processing circuitry receives the electronic signal from the photosensor, processes the electronic signal to determine the extent of the spatial vibration, and generates a control signal in response to the spatial vibration. An adjustment mechanism coupled to the optical relay element receives the control signal from the processing circuitry and, in response to the control signal, adjusts the optical relay element to compensate for the spatial vibration in the optical signal. 
     In yet another aspect, the invention relates to automatically focusing an optical signal. An optical relay element receives the optical signal from a remote source and focuses the optical signal on a specified target. A photosensor receives a portion of the optical signal and produces an electronic signal that varies with changes in the focus of the optical signal. Processing circuitry receives the electronic signal from the photosensor, processes the electronic signal to determine whether the focus of the optical signal can be improved, and generates a control signal if the focus can be improved. An adjustment mechanism coupled to the optical relay element receives the control signal from the processing circuitry and, in response to the control signal, adjusts the optical relay element to improve the focus of the optical signal. 
     The invention is useful in a wide variety of applications, including interplanetary communications and video recording systems. For example, optical signals from a space probe several million miles from Earth can be stabilized and focused with nanometer precision, e.g., to a beam size of 0.005 mm 2  at the detector, which allows data communication rates of 1 Gbit/sec or greater. Likewise, optical signals in an optical recording system or an optical microscope can be focused and stabilized automatically with nanometer precision. All of this may be accomplished with a simple, passive detection device that requires no applied bias or power source, and therefore that introduces virtually no noise to the signal acquisition environment. 
     Other embodiments and advantages will become apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a conventional optical signal acquisition system. 
     FIG. 2 is a perspective view of a photosensor for use in an improved optical signal acquisition system. 
     FIGS. 3A and 3B are graphs illustrating output from the photosensor versus position of the optical signal on the semiconductor surface of the photosensor. 
     FIG. 4 is a schematic diagram of an optical signal acquisition system capable of compensating for spatial vibrations in a received optical signal. 
     FIG. 5 is a flow chart for the operation of a computer in the optical signal acquisition system of FIG.  4 . 
     FIG. 6 is a schematic diagram of an alternative optical signal acquisition system capable of compensating for spatial vibrations in a received optical signal. 
     FIG. 7 is a schematic diagram of an optical signal acquisition system capable of automatically focusing the telescope or lens system from which the optical signal is received. 
     FIGS. 8A and 8B together are a flow chart for the operation of a computer in the optical signal acquisition system of FIG.  7 . 
     FIG. 9 is a schematic diagram of a computer system that may be used to carry out the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a photosensor  25  that is suited for use in an optical signal acquisition system embodying the invention. The photosensor  25  is a high-resolution, displacement-type or position-type photosensor having two pairs of Schottky-barrier contacts  30   a-b,    32   a-b  comprising Schottky photodiodes. The contacts  30   a-b,    32   a-b  in each pair enclose a two-dimensional photosensitive area exposing the photosensor&#39;s semiconductor surface  34 . The exposed portion of the semiconductor surface has dimensions of approximately 1×1 mm 2 , and perhaps as great as 10×10 mm 2 . As the optical signal impinges upon the semiconductor surface  34 , the contact pairs  30   a-b,    32   a-b  produce two short-circuit current or open-circuit voltage signals, I(x) and I(y), the amplitudes of which indicate the relative position of the optical signal between the contacts in the pair. Therefore, each contact pair  30   a-b,    32   a-b  indicates the position of the light beam in one of two orthogonal dimensions, defined by an x-axis  33   a  and a y-axis  33   b.    
     The photosensor  25  also may include a conductive backplane  36  on its rear surface  35 . The conductive backplane  36  may be formed from a conductive material, e.g., a conductive metal such as gold or silver, coated onto the rear surface  35  of the photosensor  35 . As the optical signal strikes the semiconductor surface  34  of the photosensor  25 , electric potential forms between each of the Schottky-barrier contacts  30   a-b,    32   a-b  and the conductive backplane  36 . The total potential between the four contacts and the backplane can be measured, either as an open-circuit voltage or a short-circuit current, and used to normalize the output signals I(x) and I(y) produced by the photosensor. 
     FIG. 3A shows the relationship between the amplitude of the signal I(x) and the position at which the optical signal strikes the semiconductor surface of the photosensor. The signal I(x) varies approximately linearly with the position x of the optical signal between the contacts in the pair  30   a,    30   b  lying along the x-axis  33   a.  At x=−X, the signal I(x) has a value of approximately +A; at x=+X, the signal has a value of approximately −A; and between the two ends, at x=0, the signal has a value of approximately zero. 
     Likewise, FIG. 3B shows that the signal I(y) varies approximately linearly with the position y of the optical signal between the contacts in the pair  32   a,    32   b  lying along the y-axis  33   b.  At y=−Y, the signal I(y) has a value of approximately +A; at y=+Y, the signal has a value of approximately −A; and between the two ends, at y=0, the signal has a value of approximately zero. 
     The following U.S. patent and publications, all of which are incorporated by reference, describe the structure, fabrication, and operation of suitable photosensors: (1) U.S. Pat. No. 4,987,461, issued Jan. 22, 1991; (2) S. D. O&#39;Connor &amp; S. F. Soares, “Picometer displacement tracking of an optical beam in a silicon Schottky barrier sensor,” Electronics Letters, Vol. 30, No. 22, Oct. 27, 1994; (3) K. A. M. Scott et al., “A High resolution Si position sensor,” Applied Physics Letters, Vol. 62, No. 24, Jun. 14, 1993; (4) S. F. Soares, “Photoconductive Gain in a Schottky Barrier Photodiode,” Japanese Journal of Applied Physics, Vol. 31, Part 1, No. 2A, February 1992; and (5) B. W. Mullins et al., “A Simple High-Speed Si Schottky Photodiode,” IEEE Photonics Technology Letters, Vol. 4, No. 4, April 1991. The sensors described in these publications are relatively simple and inexpensive to manufacture, costing as little as pennies per unit. The semiconductor surface  34  of the photosensor may be coated with a transmitting “passivation” layer, e.g., a layer of oxide or nitride film, to protect the surface from damage that might otherwise occur from repetitive use. 
     FIG. 4 shows an optical signal acquisition system  40  that compensates for spatial variations in received optical signals. A telescope  42 , e.g., the earth-bound telescope discussed above, focuses the optical signal to a predetermined diameter, e.g., approximately one wavelength. An optical conductor  45 , such as a fiber optical cable, and one or more mirrors  44 ,  46  may be positioned as needed to direct the optical signal from the telescope to the signal acquisition system  40 . 
     The signal acquisition system  40  includes two beam splitters  48 ,  50  that redirect some of the light in the optical signal toward two two-dimensional, high-resolution photosensors  52 ,  54 , which are of the type shown in FIG.  2 . Each of the photosensors may be packaged, e.g., on a critically damped accelerometer. Image intensifier crystals  53 ,  55  may be positioned between the beam splitters  48 ,  50  and the photosensors  52 ,  54  to intensify the optical image before it reaches the photosensors. 
     An optical delay element  56  and an adjustable mirror  58  are positioned between the beam splitters  48 ,  50 . The optical delay element  56  compensates for electronic signal processing times. One or more adjustment elements, e.g., piezoceramic actuators  60 ,  62 , are coupled to the mirror  58  to adjust the mirror&#39;s position in response to spatial vibrations in the optical signal. Signal processing circuitry, e.g., a digital acquisition (DAQ) system  66  and a digital computer  68 , detects the spatial vibrations in the optical signal and controls the piezoceramic actuators  60 ,  62  to compensate accordingly. Another photosensor, e.g., a standard high-speed photodiode  64 , receives the corrected optical signal and delivers it to an appropriate signal processing system for data acquisition. 
     In operation, the optical signal enters the signal acquisition system  40  and encounters the first beam splitter  48 , which directs a portion, typically between 1% and 5%, of the signal toward the first 2-D photosensor  52 . The first image intensifier crystal  53  amplifies the intensity of this portion of the optical signal, by producing many, e.g., ten photons for each photon that strikes the crystal  53 . The intensified optical signal then strikes the semiconductor surface of the first photosensor  52 . 
     As discussed above, the photosensor  52  produces two short-circuit current or open-circuit voltage signals, the magnitudes of which indicate the position of the optical signal on the semiconductor surface of the photosensor  52 . The DAQ system  66 , which may include a very low-noise, high-gain amplifier coupled with either an analog-to-digital (A/D) converter or a sensitive voltmeter, receives the signals from the photosensor  52 , amplifies and digitally samples the signals, and then provides the digital samples to the computer  68 . The computer  68  executes a program, as described below, to detect the motion of the optical signal across the semiconductor surface of the photosensor  52  and to generate control signals governing the operation of the piezoceramic actuators  60 ,  62 . 
     After passing through the first beam splitter  48 , the optical signal encounters the optical delay element  56 , which delays the optical signal by a predetermined amount, e.g., 1 μs, while the signal processing circuitry performs its calculations. The computer  68  attempts to meter delivery of the control signals to the piezoceramic actuators  60 ,  62  so that the actuators adjust the position of the mirror  58  very precisely to compensate for the motion of the optical signal. 
     The second beam splitter  50  directs a portion of the corrected optical signal through the second image intensifier crystal  55  and onto the second 2-D photosensor  54 . Like the first photosensor  52 , the second photosensor  54  produces two short-circuit current or open-circuit voltage signals indicating the position of the optical signal on the photosensor&#39;s semiconductor surface. These signals provide feedback indicating how precisely the computer&#39;s control algorithm is tracking the spatial vibrations in the optical signal. The computer  68  uses the signals from the second photosensor  54  to adjust the piezoceramic control signals accordingly. The second beam splitter  50  and the second 2-D photosensor  54  may be eliminated from the signal acquisition system if beam-correction feedback is not needed. 
     FIG. 5 is a flow chart for operation of the computer&#39;s vibration-tracking program. The computer first measures the magnitudes of the output signals from the first photosensor (step  150 ) and uses the measured amplitudes to determine the location, in Cartesian coordinates, of the optical signal on the photosensor&#39;s semiconductor surface (step  152 ). The computer uses the coordinates to determine a calibration factor for the adjustable mirror (step  154 ). The computer then measures the magnitudes of the output signals from the second photosensor (step  156 ), determines the coordinates of the optical signal on the second photosensor (step  158 ), and uses these coordinates to determine a calibration adjustment factor (step  160 ). The computer uses the calibration adjustment factor to adjust the calibration factor in response to over-compensation or under-compensation of the mirror (step  162 ). The computer then generates a control signal based on the adjusted calibration factor and delivers the control signal to the piezoceramic actuators (step  164 ), which, in response, adjust the position of the mirror. 
     FIG. 6 shows an alternative configuration for the signal acquisition system  40 , which includes a non-adjustable mirror  70  and a standard high-speed photodiode  72  mounted on an adjustable stage  74 . As before, the first beam splitter  48  directs a portion of the optical signal toward the first 2-D photosensor  52 . The DAQ system  66  and the computer  68  use the signals from the photosensor  52  to generate control signals that govern the operation of one or more adjustment elements, piezoceramic actuators  76 ,  78 , coupled to the adjustable stage  74 . The computer  68  attempts to meter its delivery of the control signals so that the actuators  76 ,  78  adjust the position of the photodiode  72  to track the motion of the optical signal precisely. 
     FIG. 7 shows how the optical signal acquisition system  40  and the telescope  42  may be configured for automatic focusing of the telescope  42 . In this configuration, a beam splitter  80  directs a portion of the optical signal onto a two-dimensional, high-resolution photosensor  82 , of the type shown in FIG.  2 . The short-circuit current or open-circuit voltage signals produced by the photosensor  82  indicate the power density of the signal as it strikes the photosensor&#39;s semiconductor surface. In particular, the optical signal has maximum power density when it is most tightly focused by the telescope  42 . The signals produced by the photosensor  82  are proportional to the power density of the signal, with maximum values at peak power density. As the telescope&#39;s focus becomes less sharp, the power density of the optical signal decreases, and the magnitudes of the photosensor&#39;s output signals decrease. 
     The DAQ system  66  receives the signals produced by the photosensor  82 , amplifies and digitally samples the signals, and delivers the samples to the computer  68 . The computer  68  executes a program, as described below, to determine the power density of the optical signal as it strikes the semiconductor surface of the photosensor  52  and to generate a control signal that will govern the operation of an adjustment element, such as a stepper motor  84 , coupled to a mirror  87  nominally positioned at the telescope&#39;s focal point  89 . The computer  68  focuses the telescope automatically by activating the stepper motor  84 , and therefore adjusting the position of the mirror  87 , in a manner that gives the optical signal its highest possible power density at the semiconductor surface of the photosensor  82 . 
     FIGS. 8A and 8B together are a flow chart for operation of the computer&#39;s autofocus program. The program begins by generating a control signal that instructs the stepper motor  84  to place the mirror  87  at a predetermined initial position (step  200 ). The program then reads the digital data provided by the DAQ system (step  202 ), which indicates the magnitude of the optical signal&#39;s power density. The program stores the power density value in a dedicated storage area, e.g., at a predetermined location in a memory device, along with information indicating the present position of the mirror (step  204 ). The program then generates a control signal instructing the motor to step the mirror toward the primary mirror  85  by a predetermined distance (step  206 ) and again reads the data provided by the DAQ system (step  208 ). The program compares the newly read data and the data stored in memory to determine whether the present power density value is greater than the stored value (step  210 ). If the present value is greater, the program replaces the data stored in memory with the newly read data and with information indicating the present position of the secondary mirror (step  212 ). The program then again steps the mirror toward the primary mirror (step  206 ), reads the data provided by the DAQ system (step  208 ), and determines whether the present power density value is greater than the stored value (step  210 ). The program continues in this manner until the mirror has stepped to a position that produces a power density value equal to or less than the stored value. 
     If the program determines that the present power density value is not greater than the stored value, the program steps the mirror back to the position stored in memory (step  214 ), then steps the mirror away from the primary mirror by the predetermined distance (step  216 ). The program then reads the power density value provided by the DAQ system (step  218 ) and determines whether the present value is greater than the value stored in memory (step  220 ). If so, the program replaces the stored data with the present power density value and with information indicating the present position of the mirror (step  222 ). The program then again steps the mirror away from the primary mirror (step  216 ), reads the power density value from the DAQ system (step  218 ), and determines whether the present value is greater than the stored value (step  220 ). The program continues in this manner until the mirror has stepped to a position that produces a power density value equal to or less than the stored value. When this happens, the program steps the mirror back to the position stored in memory (step  224 ), which is treated as the optimal focus position. The computer may execute the autofocus program at predetermined time intervals or upon receiving an instruction from the user to do so. 
     FIG. 9 shows a programmable computer  300  that may be used to carry out the invention. The computer  300  may include, among other things, a processor  304 , a random access memory (RAM)  306 , a non-volatile memory  308  (e.g., a writable read-only memory such as a flash ROM), a hard drive controller  310 , a video controller  312 , a display controller  313 , and an input/output (I/O) controller  314 , all coupled by a processor (CPU) bus  316 . The computer  300  may be preprogrammed, e.g., in ROM, or it may be programmed by loading an executable program  302  from another source, such as a hard disk  318 , a floppy disk, a CD-ROM or another computer. 
     The hard drive controller  310  is coupled to the hard disk  318  and is used to deliver information, such as the executable program  302 , from the hard disk  318  to the processor bus  316 . The I/O controller  314  is coupled by means of an I/O bus  320  to an I/O interface  322 . The I/O interface  322  receives and transmits data in analog or digital form over communication links such as a serial link, local area network, wireless link, or parallel link. Also typically coupled to the I/O bus  320  are a display  324 , a keyboard  326 , a pointing device such as a mouse  328 , a network interface card (NIC)  330 , and a modem  332  for connecting the computer  300  to another computer or to a computer network, e.g., to an Internet service provider (ISP) or an on-line service provider (OSP). Alternatively, separate connections (i.e., separate buses) may be used for some of the components connected to the I/O bus  320 , including the I/O interface  322 , the display  324  and the keyboard  326 . 
     While the optical image acquisition and optical recording systems above have been described to include a programmable computer, the signal processing circuitry may be implemented in many ways, including in digital electronic circuitry or in computer hardware, firmware, software, or in combinations of them. Apparatus embodying the invention may be implemented, in part, in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor; and methods embodying the invention may be performed by a computer processor executing instructions organized, e.g., into program modules to carry out the invention by operating on input data and generating output. Suitable processors include, e.g., both general and special purpose microprocessors. Generally, a processor receives instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions include all forms of non-volatile memory, including, e.g., semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM. Any of the foregoing technologies may be supplemented by or incorporated in specially-designed ASICs (application-specific integrated circuits). 
     Other embodiments are within the scope of the following claims. For example, the vibration-correction and autofocus techniques described above may be implemented in other types of optical signal processing systems, including video cameras, optical microscopes, binoculars, telescopes, and night vision systems.