Field of the Invention

This invention relates to spatial light modulators, and particularly to such modulators which are controlled by a sampling clock and a strobe signal, and which find use in optical signal processing, such as that used for synthetic-aperture radar systems

Background of the Invention

FIGURE 1 is a simplified block diagram of a synthetic aperture radar (SΛR) system synthesized from the prior art In FIGURE 1 , a vehicle (not illustrated) such as an airplane bears a fan-beam transmit-receivc antenna 10 which produces a footprint on the ground being imaged Antenna 10 is coupled to a transmit-receive (TR) device 12 , which allows the same antenna to be used for both transmission and reception TR device 12 is in turn coupled to a transmitter 14 and to a receiver 16 Transmitter 14 produces transmit pulses at a rate and with a duration controlled by a timing generator or pulse source 18, applied over a path 1 a Such pulses typically have a pulse repetition rate in the kilohertz or tens of kilohert/ range, and a duty cycle in the vicinity of 50%, but other characteristics may be used I he transmit pulses are illustrated as a pulse train 210 in FIGURE 2a The pulses from transmitter 14 are coupled through TR device 12, and are transmitted by antenna 12 toward the ground 1 he pulse energy arriving at the ground has an elliptical footprint, as suggested by 310 of FIGURE 3. In FIGURE 3, the vehicle carrying the SΛR antenna, transmitter and receiver is illustrated as a block 312. moving in the direction of arrow 314 with velocity v. The altitude o the aircraft is designated h. and the distance from the sub-aircraft point to the target region is designated XO The range is readily determined, and is illustrated as R0. Those skilled in the art reah e that the footprint 310 moves with time as the vehicle carrying the radar system moves The energy which falls on the footprint of FIGURE 3 is rellected, depending upon the characteristics of the particular portions of the terrain and any targets thereon A portion of the reflected energy is returned to the antenna in the form of complicated modulation of the original pulse, and is illustrated as 212 in FIGURE 2b The modulated pulse information returned to antenna 10 of FIGURE 1 is coupled by TR device 12 to receiver 16, which performs analog signal processing as may be required, such as low-noise amplification, frequency conversion, and the like, and couples the resulting received signal to a digit er, illustrated as 20. Digitizer 20 converts the received signal lo digital form, and applies the resulting digital received signal to a SAR processor illustrated as a dash block 22 SAR processor 22 performs various types of processing foi converting the received signals into a representation ofthe region being imaged by the SAR radar system, and couples those image- representative signals to an image processor illustrated as a block 24, which adjusts the signals for best display, as for example by allowing selection ot appropriate contrast, colors, or the like, or tor actual processing such as for correlation ot the current image with a stored image

Within SAR processor 22 of FIGURE I , the digitized signals are first applied to a first processor (PROC) 30, which is used when high-resolution images are to be generated High resolution is achieved by breaking the received signal samples into sub-sets, and further processing the subsets to generate multiple sets of image data, from which fne detail can be extracted One simple way to divide the signal samples into different sets is to multiplex the l eceived signals sequentially, in the order of reception, to a plurality ol channels, illustrated as channel 1 (C l ) channel N (CN) in FIGURE 1 Each of the sets of samples traverses its own SAR processor 34a, . 34N, and the resulting images are recombined by a second processor 32 The high-resolution image-representative data at the output of processor 22 is coupled to image processor 24, for further adjustment In the event (hat less resolution is needed, a single SAR processor, such as SAR processoi 34a. may be used, in which case processois 30 and 32, and the other illustrated SAR processors 34, may be dispensed with

High-resolution SAR images are not now generated aboard the aircraft which does the SAR sensing Instead, the digitized information from digitizer 20 of FIGURE 1 is transmitted from the sensing aircraft to a ground station, and the computations are made by ground-based computers It would be desirable to be able to include the computers in the aircraft, so that the images could be viewed directly as the signals are received, and to obviate the need lor high-bandwidth communications from aircraft to ground, but the required computations are sufficiently complex so that ground-based computers are more advantageous

FIGURE 4 is a simplified block diagram ot an optical SAR processor In FIGURE 4, a spatial light modulator (SLM), described in greater detail in FIGURE 5, receives the digitized return signals from digitizer 20 of FIGURE 1 , and produces a diffraction pattern as described below, which propagates across the length of the SLM The dimensions o the SLM are selected so that the signals applied to one end of the SLM propagate to the other end ot the SLM in one period of the transmitted pulses 210 of FIGURE 2a I hat is, the returned signal resulting from each transmitted pulse exactly fills the length of the SI M Naturally, some margin or leeway must be allowed for SLM control time and tor errors An example of the pattern of refraction across the surface of SI M 410 resulting from one transmitted pulse is illustrated as 412 in FIGURE 4

Spatial light modulator 410 of FIGURE 4 is reflective, as described below A source of light such as a laser 414 in FIGURE 4 produces a beam 416 of coherent light, which is processed by an optical system 418, if necessary, for expanding the beam to the dimensions of the active portion of spatial light modulator 410 I he expanded beam 41 ot light is reflected and directed toward spatial light modulator 410, where it is modulated by the pattern of refraction then existing or propagating on its surface due to the received signal The modulated beam reflected or difracted from the surface of SLM 41 is applied to half-silvered mirror 426, and passes through the mirror to a Fourier transform lens 428 Lens 428 performs a Fourier transformation to produce a Fourier-transformed image at the F T plane 430 A mask, illustrated in exemplary form as 432, lies in the TT plane 430, and allows only a portion of the transformed light portion to pass In effect, mask 432 represents a spatial filter in the Fourier plane I he filtered , transformed light exiting mask 432 is applied to an imaging lens, which images the Fourier

plane onto the surface of a detector 436. which may be an array of photosensors, or preferably a charge- coupled imager device (CCD) or camera An interference pattern is formed at the surface of detector 436 by combining the light from imaging lens 434 with unmodulated light from laser 414 I hc combining is accomplished by a mirror 438 and a light combining half-silvered splitter 440, which couple light beam 5 422 to the surface of ght sensor array 436 The purpose ot the interfering light is to allow phase information to be recovered, which provides an improved image An electrical signal representing the image applied to detector array 436 is generated on an output signal path 442 As so far described, the arrangement of FIGURE 4 includes a single channel, which recovers amplitude or phase information from the received signals, but not both In the arrangement of FIGURE 4, mask 432 is selected, in 0 known fashion, to recover amplitude information. A second channel is provided by a light splitter 450 and a mirror 452, which couple a further beam 454 to a second Fourier transform lens 456 Lens 456 performs a Fourier transform to a Fourier plane 458 A second mask or filter (not illustrated), which is similar to, but not identical with, mask 432, is placed at the second Fourier plane 458, to extract phase information from the received signal The phase mask filters the Fourier-transformed light, and the S resulting image is focussed onto a second light detector array 460 by an imaging lens 462 An interference is provided by a sample of the unmodulated coherent light coupled to the surface of detector 460 by a splitter 464 The arrangement of FIGURE 4 with the inclusion of the second channel is desirable when only a single SAR processing channel is to be used

FIGURF 5 is a simplified representation of an acoustic light modulator which may be used in the 0 arrangement of FIGURE 4 In FIGURE 5, modulator 41 is a lithium niobate crystal 512 with an electπcal-to-acoustic transducer 51 at one end thereof Transducer 1 includes a pair of electrodes to which the received signal is applied, to create an acoustic wave which propagates across the surface of the crystal in the direction of arrow 514 ^' I he acoustic surface wave causes changes in the index of refraction, illustrated as a pattern of transverse lines 16, which propagate across the surface of the crystal Ideally. 5 the acoustic waves are dissipated upon reaching the end of the crystal, so as not to reflect and interact with the pattern of the index of refraction across the surface The pattern of index of refraction represents the amplitude of the received signal for a period of time Unlortunately, the acoustic modulator is sub)ect to a number of imperfections, including "spreading" of the surface wave into the bulk crystal, as suggested by the "frequency and spatial dispersion" arrow 18, and reflections from the end of the crystal The 0 length of the crystal is selected in coniunction with the pulse repetition rate of the transmitted pulses, so that the time required for an acoustic wave to propagate across the crystal is equal to the duration of a transmitted pulse Thus, as the acoustic wave propagates across the surface of modulator 410 of FIGURE 5. there is an instant at which the entire surface ofthe crystal is "filled" with a pattern of indices of refraction which arise from the returned signals caused by one transmitted pulse At that instant, laser 5 light 414 of FIGURE 4 is pulsed ON, to illuminate the surface of modulator 410, and to thereby generate the modulated light pattern required for the desired Fourier processing, illustrated in FIGURE 5 by arrows 522, representing different targets

The frequency content of the returned signals may extend above the tens ot megahertz, and even into the hundreds of megahertz The ability to process signals with such a bandwidth is one ofthe advantages of the optical signal processing described in conjunction with FIGURE 4 As a result o the very large bandwidth of the processing, the shortest-duration pieces of information existing on the surface of modulator 410 of FIGURE 5 are a fraction of the duration ofthe highest frequency being processed If the highest frequency being processed is 100 MHz., the duration of one cycle at the highest frequency is ten nanoseconds, and the shortest-duration sample must be no longer than five nanoseconds Thus, in order to fully process the returned signal appearing on the surface of acoustic modulator 41 of FIGURE 5, the duration of the light pulse must not exceed five nanoseconds for a useful bandwidth of 1 0 MHz Of course, the actual duration ofthe pulse oflight depends upon the bandwidth A coherent light pulse of such short duration is difficult to produce, requiring very specialized laser sources, and also difficult to process, because of the losses inherent in the optical components, which reduces the amount o light available at the detectors 436 and 460 of FIGURE 4 Unfortunately, it is not possible to keep the laser light source 414 ON for a longer period of time than that allowed by the bandwidth limitation mentioned above in order to allow more photons to be integrated by each photosensor of the detectors, because longer-duration light pulses allow the pattern on the surface of the light modulator to change during the pulse These changes occur because the acoustic waves continue to propagate during illumination, with the result that the image becomes "blurred" When the pulses of light are very short, the resolution is high, but the small amount of light results in poor signal-to-noise ratio (SNR), and when the pulses of light are longer, the signal-to-noise is improved, but the image resolution suffers Improved SAR processors are desired

Summary ot the Invention A light modulator according to an aspect ot the invention includes a line array of light modulator elements, each of which includes an input signal port, tor controlling light falling thereon under the influence of a signal voltage applied to the input signal port of the modulator element A tapped delay line has an input port coupled to a source of electrical signal to be displayed, and also includes a plurality of taps, for generating electrical signals at each of the taps which represent the signals applied to the input port of the delay line, delayed by a particular time delay The intertap delay may be equal for all of the taps A coupling arrangement includes a strobe input port, and is coupicd to the taps of the delay line and to the input signal ports of the array of light modulator elements, for simultaneously sampling the signal occurring at all ofthe taps under the control ofa strobe signal applied to the strobe input port This generates an array of a plurality of sampled signals equal to the number of taps The coupling arrangement applies the array of sampled signals to the input signal ports ot the line array of modulator elements, and holds the sampled value until a later strobe signal is applied to the strobe input port A source of strobe signals is coupled to the strobe input port of the coupling arrangement, for periodically applying the strobe signal to the coupling aπangement, tor causing electrical signals applied to the input port of the tapped delay line to be displayed by the lme array of light modulator elements I he delay line

may be an analog delay line, or it may be digital If analog, it may be a conventional electromagnetic transmission line, or an acoustic transmission line, or it may be a time-sampled bucket-brigade charge- coupled device If digital, it may be an array or a cascade of shift registers, which may have a depth of two or more bits In one embodiment of the invention, the delay line is a six-bit-deep cascade of shift registers clocked by a high-speed clock The light modulator may be of any type, in one embodiment of the invention it is a solid-state array of Fabry-Perot cavity modulators, in which the active or voltage- variable portion comprises quantum wells According to an aspect of the invention, the coupling arrangement includes a plurality of temporary storage or sample-and-hold arrangements or circuits, one coupled to each tap ofthe delay line In the case of an analog signal, the sample-and-hold circuits may be capacitive, and in the case of a digital signal, the sample-and-hold circuits may be storage registers

When the stored signal is in multibit digital form, the coupling arrangement further includes a digital-to- analog converter, for generating an analog signal from the stored multibit signal for apphcation to the modulator element

An optical processor according to the invention includes a modulator as described above, and includes the light modulator itself, which in turn includes (a) a line array of light modulator elements, each of which includes an input signal port, for controlling light falling on the modulator elements under the influence of signal voltage applied to the input signal port of each modulator clement (b) a tapped delay line including an input port coupled to a source of electπcal signal to be displayed, and also including a plurality of taps, for generating electrical signals at each of the taps which represent the signals applied to the input port ofthe delay line, delayed by a sampling time delay, and (c) a coupling arrangement including a strobe input port, which coupling arrangement is coupled to the taps of the delay line and to the input signal ports ot the array oflight modulator elements, for simultaneously sampling the signal occurring at all ofthe taps under the control of a strobe signal applied to the strobe input port of the coupling arrangement, to thereby generate a plurality of sampled signals, one for each tap I he sampling strobe signal is very short, and results in a sample ofthe received signal pattern which existed on the tapped delay line at the instant that the delay line was "full" of the received signals arising from a single transmitted pulse The sampled signals are held in storage, and are continuously applied to the input signal ports ofthe line array of modulator elements until a later strobe signal is applied to the strobe input port The optical processor further includes a coherent light source for producing coherent light under the control of an ON-OFF control signal applied to an ON-OFF control port The optical processor further includes an arrangement for illuminating the modulator with the coherent light when the coherent light source is ON, as a result of which the modulator modulates the coherent light to produce modulated light, whether reflected or transmitted An arrangement is provided for transforming the modulated light to produce transformed light, for spatially filtering the transformed light to produce filtered transformed light, and imaging the filtered transformed light to produce imaged light In one embodiment of the invention, the transforming arrangement is a Fourier lens arrangement The imaged light is combined with the coherent light to produce an output interference pattern at a light detector, which may be a CCD line-array imager When the delay line is clocked at a sampling rate, the optical processor may include

an ON-OFF control arrangement coupled to the port of the coherent light source, for maintaining the light source in an ON condition for a processing period exceeding one the sampling period, and preferably for maintaining the light source ON with a duty cycle of greater than 50%

A synthetic aperture radar system according to an aspect of the invention includes a transmitter for transmitting pulses at a rate which defines a pulse-to-pulse interval A receiver receives returns from targets which reflect the pulses, for generating received signals An optical processor corresponding to the processor described above is provided, including (a) a line array of light modulator elements, each of the light modulator elements including an input signal port, for controlling light falling thereon under the influence of signal voltage applied to the input signal port of the modulator element; (b) a tapped delay line including an input port coupled to the receiver, and also including a plurality of taps, for generating electrical signals at each of the taps which represent the received signals applied to the input port of the delay line, delayed by a sampling time delay established by the inter-tap delay, and (c) a coupling arrangement including a strobe input port, which coupling arrangement is coupled to the taps of the delav line and to the input signal ports ofthe array of light modulator elements, for simultaneously sampling the signal occurring at all ofthe taps under the control of a strobe signal applied to the strobe input port of the coupling arrangement, to thereby generate a plurality ot sampled signals, and for applying the sampled signals to the input signal ports of the line array of modulator elements, and holding the value until a later strobe signal is applied to the strobe input port The processor further includes a coherent light source for producing coherent light under the control of a signal applied to an ON-OFF control port An arrangement is provided for illuminating the modulator with the coherent light during those times when the coherent light source is ON, as a result of which the modulator modulates the coherent light to produce modulated light A further arrangement operates on the modulated light for transforming the modulated light to produce transformed light, and for spatially filtering the transformed light to produce filtered transformed light, and for imaging the filtered transformed light to produce imaged light, and for combining the imaged light with the coherent light to produce an output interlerence pattern A source of sampling strobe signals is coupled to the strobe input port of the coupling arrangement, tor applying the sampling strobe signal to the coupling arrangement at the pulse rate, so that the sampled signals are optically displayed for more than 50% of the transmitted pulse interval, for causing signals applied to the input port of the tapped delay line to be displayed by the line array of light modulator elements

Brief Description of the Drawing FIGURE 1 is a simplified block diagram of a synthetic aperture radar system which is made up of prior-art elements but which does nol necessarily represent the prior art.

FIGURE 2a is an amplitude-time plot of a train of transmit pulses, and FIGURE 2b is an amplitude-time plot of the modulation which is superposed upon the transmitted pulses by the terrain in the footprint onto which it is directed, FIGURE 2c illustrates display rate or pulse-rate sampling pulses, and FIGURE 2d illustrates light-ON pulses which begin just after each pulse of FIGURE 2c, and end just before the following pulse of FIGURE 2c,

FIGURE 3 is a simplified representation of the transmitted energy and its relationship to the target being detected,

FIGURE 4 is a simplified block diagram of an optical SAR processor which may be used in the arrangement of FIGURF 1 , FIGURE 5 is a simplified representation of an acoustic surface-wave optical modulator which may be used in the arrangement of FIGURE 4; and

FIGURE 6 is a simplified block-diagram representation of an optical modulator according to an aspect of the invention

Description of the Invention

In FIGURE 6, an optical modulator 610 includes a line array 612 of optical modulator pixels, some of which are designated 612a, 612b, 612N, which are similar to those described in allowed patent application serial number 08/109,550, filed 8/20/93 in the name of Worchesky et al As described therein, the modulator is reflective, and each modulator pixel includes a voltage-controllable Fabry-Perot etalon/quantum-well arrangement, and each modulator pixel is driven by an electronic circuit located behind the light modulator Thus, each modulator pixel of array 622 in the arrangement ot FIGIJRI 6 is controlled by an analog voltage individually applied to the pixel over an electrical conductor array 614, including electrical conductors 614a, 614b, . .614N As illustrated in FIGURE 6, received electrical analog signal or data is applied over a path 616 to the input end of a tapped delay line 618, which may take the form of a simple electromagnetic transmission line with physical taps along its length, or an acoustic delay line As an alternative, tapped delay line 618 may take the form of a linear or line array of solid-state "bucket brigade" charge-coupled stages, controlled by a sampling clock signal applied over a sampling clock path 620 from a source 640 In a preferred embodiment of the invention, the transmission line is in the form of an array of shift registers which has a depth of six bits, so that a six-bit digital representation of the received signal amplitude is coupled through the delay line Regardless of the exact form of the tapped delay line 18, delayed samples are available at each tap 622a, 622b. 622N, of an array 622 of taps, with the signals which appear on mutually adjacent taps separated by the inter-tap duration The inter-tap duration is, of course, the sampling clock interval in the case ot a shift register type tapped delay line I he number of taps, and therefore the number of stages in delay line 618, depends upon the desired resolution, and on the pulse-to-pulse period or interval of the SAR radar system

Each tap 622a, 622b, 622N of the tapped delay line 618 couples the signal appearing at the tap to a short-term memory 624a, 624b, 624N of an array 624 of short-term memories Each short- term memory of array 624 samples the signal at its tap under the control of a display-rate sampling clock, which is applied simultaneously by a path 626 to all ot the short-term memories of array 624 The display-rate sampling pulse is very short, on the same order of duration as the high-speed sampling clock signal applied to delay line 618 over sample clock path 620 1 he display-rate pulses are illustrated as 230 of FIGURE 2c. and occur about concurrently with the end of transmitted pulses 210 The short-term memory array in the case of a transmission-line type delay line carrying an analog signal, mav take the

form of an array of capacitive sample-and-hold circuits, and in the case of a digital shift-register type tapped delay line carrying multibit digital signals, may take the form of an array of multibit storage registers, one multibit storage register at each tap. However implemented, the short-term memory 622a. 622b, 622N samples the tap signal at each display clock cycle, and makes the sample available on its output signal path 628a, 628b, 628N of an array 628 of sampled signal ports l hus, a representation is stored in the array of short-term memories ofthe pattern of signal which, at the display-rate sampling instant, appears on the delay line The high-speed clock signal applied over path 620 to tapped delay line 618 may have a frequency of, for example, 100 MFIz , while the display-rate clock signal may be a pulse having a duration of less than 10 nanoseconds, recurring at Ihe display refresh rate of, for example, every ten milliseconds (a refresh rate of 100 Hz.)

T he output of the array 624 of short-term memories of FIGURE 6 is an array of received signals, representing the pattern of received signals which appeared on the tapped delay line at the moment at which the display-rate sampling clock signal occurred ^' I he pattern of received signals is coupled to an array 630 of output coupling devices 630a, 630b, 630N, which couple the array of received signals to the array 612 oi modulators In the case of analog signals sampled by an array 624 of capacitive sample- and-hold circuits, the output coupling may be simply an electπcal conductor, and in the case of multibit digital signals stored in digital-register type short-term memories, the output coupling array may be an array of digital-to-analog converters. However implemented, the elements of array 630 couple the sampled received signals from short-term memories 624a, 624b, . . 624N to the corresponding ones 630a, 630b, 630N of the output coupling arrangements ^' I he coupling arrangements 630a, 630b. 630N, in turn, couple analog signals to their corresponding optical modulator elements 612a, 612b, 612N

In operation of the arrangement of FIGURE 6, the received signals are applied to tapped delay line 618. and propagate therethrough At a particular instant, namely the display sampling instant established by a pulse 230 of FIGURE 2c. the pattern of received signals then appearing on the tapped delay line 618 of FIGURE 6 is sampled, and the samples are stored in the short-term memory array 624. and arc held therein until the next subsequent display-rate sampling signal T he stored received signals are applied through their respective coupling elements of coupling element array 630 to the optical modulator elements, with the result that the particular pattern of received signals, which appeared on tapped delay line 618 at the moment o the display-rate sample pulse, is transferred to the optical modulator elements 612, and are held there until the next succeeding display-rate sample pulse Thus. the optical pattern resides in the modulator semi-permanently, notwithstanding the continuous passage of received signal through the tapped delay line, and the continuous change in the pattern of received signal in the delay line ln this context, semipermanent means held until the next following display-rate sampling pulse The application of the received signals to the array of modulators converts the time sequence of received signals into a spatial sequence of optical modulation

The semipermanence of the pattern of the received signal at the outputs of the short-term memories 624a, 624b, , 624N of array 624 of short-term memories makes it possible to improve the optical signal processing described in conjunction with FIGURE 4 Referring once again to FIGURE 4. it

will be recalled that one of the problems which resulted from the use ofthe acoustic optical modulator of FIGURE 5 was that the desired pattern of the received signal was transitory, existing in the modulator for only a fraction of a cycle at the highest design frequency of operation. As a result, the optical processing could take place only at that instant, and the laser was therefore pulsed for only a short time This short time, in turn, required complex laser controls, and resulted in poor signal-to-noise ratio at the output of the processor According to an aspect of the invention, the arrangement of FIGURE 6, when used in an optical processor such as that of FIGURE 4, has the major advantage that the received signal pattern available for optical processing is semipermanent, rather than transitory Since the pattern of received signal is stored in the short-term memories 624, it is available to the optical modulator elements for the full duration of the pulse-to-pulse interval ofthe radar system I his availability, in turn, means that the coherent light source can remain illuminated for a period of time much greater than in the arrangement of FIGURE 4, and the signal-to-noise ratio of the resulting processed optical signal is much improved Referring now to FIGURE 2c, display-rate pulses 230 are timed to occur about coincident with the lagging or trailing edges of transmit pulses 210 of FIGURE 2a, and are applied to short-term memories 624 of FIGURE 4 by way of signal path 626 Pulses 230 have a very short duration, as mentioned above, which may be on the order of less than ten nanoseconds, for causing the short-term memories to sample the signal then appearing on the tapped delay line Pulses 240 of FIGURE 2d represent the coherent light ON pulses which are applied over signal path 19b of FIGURE 4 to illuminate the coherent light source 414, to implement the optical processing, and are timed to begin slightly after the end of a display-rate pulse 230, and to end just before the next following display-rate pulse 230 By contrast, the duration of the light pulse which would be appropriate for use with an acoustic modulator, such as that of FIGURE 5, in an optical processor such as that of FIGURE 4, would be approximately the same as the duration of pulses 230 of FIGURE 2c, which is much smaller than the duration of pulses 240

Other embodiments of the invention will be apparent to those skilled in the art For example, digital signals may have any number of bits rather than the six bits described The light modulator may be transmissive rather than reflective because it is a line array and not a two-dimensional array The light modulator elements may be fabricated in any form, and need not use Fabry-perot etalons or quantum wells in the modulator elements