Method and arrangement for compensating for frequency jitter in a laser radar system by utilizing double-sideband chirped modulator/demodulator system

[0002] 1. Field of the Invention

[0003] This invention relates generally to an improvement in laser radar systems and more particularly, but not by way of limitation, to a method and arrangement for compensating for frequency jitter in a laser radar system by using a double-sideband chirped modulator/demodulator system.

[0004] 2. Description of the Prior Art

[0005] Because of its resistance to jamming and interference from outside sources, and its superior range and angular accuracy, light has replaced electromagnetic energy in many applications including communications and measuring systems. Laser rangefinder techniques have been successfully shown to determine the ranges of targets at distances up to five miles with accuracy of 2 to 10 meters. As with conventional radar systems, laser systems may be classified into two basic categories: (1) direct, or incoherent detection, and (2) heterodyne or coherent detection.

[0006] The theory of the former dictates that the best signal-to-noise ratio is provided when the transmitted energy is concentrated into the shortest possible pulse. This yields a good range measuring and resolution capability. On the other hand, coherent detection requires highest possible average transmitted power for the best signal-to-noise ratio, irrespective of pulsed width. Also, accurate target radial velocity measurements can be obtained in the latter case.

[0007] It has long been known that radar resolution and accuracy are functions of the signal bandwidth, being AM or FM in nature, regardless of the transmitter waveform. Thus, a continuous power, or long pulse mode of operation heterodyne system, may also yield good range measuring and resolution capability when using a wideband signal. One technique for modulating a wideband signal is to frequency modulate the long pulse, and this has been termed a “chirped” signal and the associated receiver includes matched delay vs. frequency components to compress the return pulses. A more complex receiver is required to extract the wide band information available from this type of reflected signal, as opposed to the direct detection system. These receivers, as used in conventional radar systems, are designated as matched-filter signal processing receivers. The advantages of such signal processing techniques in radars are:

[0008] 1. More efficient use of the average power available to the transmitter.

[0009] 2. Increased system accuracy, both in the ranging and velocity measurements.

[0010] 3. Reduction of jamming vulnerability.

[0011] Also in the past, a laser range finder using heterodyne detection and chirp pulse compression has been described. U.S. Pat. Nos. 4,662,741 and 4,666,295 describe such a system as a laser range finder using a wideband signal consisting of a linear FM chirp pulse of relative long duration. The matched filter at the receiver end is a Surface Acoustic Wave (SAW) device, which compresses the relative long FM chirp pulse into a narrow one (of the same bandwidth), from which the range and velocity information may be extracted. The duration of the compressed pulse is approximately the inverse of the bandwidth of the original signal. Thus, as the amount of frequency that is chirped increases, so does the resolution of the range and velocity measurements.

[0012] However, current radar systems which utilize single-sideband linear FM chirp modulation to provide target range measurements are susceptible to frequency jitter effects which are small random shifts in the return signal frequency produced by target/platform motion, laser frequency instabilities, or rapid beam scanning in the presence of diffuse target-induced speckle. Speckle relates to the problem of signal fluctuations of an imaging laser beam when reflected from an extended rough target surface. The dynamic speckle generated by the scan mechanism introduces additional frequency broadening which directly effects range precision of the system. Physically the signal fluctuations are caused by random constructive and destructive interference of wavelets from various scatterers on the target surface as the transmitted beam moves across the target surface during the finite period of receiver signal integration of each data pixel. In a radar system that uses a linear frequency modulation (FM) chirp followed by pulse compression at the receiver, the transmitted pulse width is normally a few microseconds during which the laser beam moves a substantial distance on the target surface. Substantial speckle phase variations are introduced in the received signal due to the relatively long dwell time of a single pixel. This effect becomes the dominant determinate of range precision for a high speed scanning system. Thus, it is apparent that a need exists for frequency jitter compensation in laser radar systems which utilize linear FM chirp modulation.

[0013] In accordance with the present invention, the range precision for long transmitter pulses such as for a linear FM chirp waveform is materially improved by transmitting an up and down linear chirp pair simultaneously and using a proper demodulation process. Thus, the linear portion of the speckle phase variations is automatically compensated and the range precision of the radar system is significantly improved.

[0014] An arrangement for accomplishing the above involves applying a double-sideband linear chirped waveform to a laser transmitter beam for frequency modulation accordingly. One sideband is a frequency up-chirp and the other is a frequency down-chirp. The modulation of transmitter may be by a suitable electro-optic device, such as a Bragg cell. The two chirped sidebands are preferably generated by employing up- and down-chirp surface acoustic wave (SAW) dispersive filters or by ramping two voltage controlled oscillators (VCO's). The reflected return beam is detected ,after mixing with a local oscillator laser beam, and separated into two channels corresponding to each sideband and are then demodulated. Demodulation is preferably accomplished by using SAW dispersive delay filters to generate the compressed pulses.

[0015] Each of the demodulated pulses will be affected by frequency jitter, as described above, except that a frequency shift which advances the pulse in one channel will delay the pulse in the other channel by exactly the same amount. The compressed pulses are then input into suitable standard peak detection circuits for range-to-pixel measurements. Frequency jitter compensation is obtained by averaging the peak-detection readings from the two channels for each pixel. In addition, simultaneous, single pixel doppler information is extracted by taking the difference in the readings for the two channels.

[0016] Other features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description constructed in accordance with the accompanying drawings and wherein:

[0017] FIG. 1 illustrates a preferred embodiment of an arrangement for compensation for frequency jitter in a laser radar system which uses a double sideband chirped modulator/demodulator system that is constructed in accordance with the principles of the present invention;

[0018] FIG. 2 is a simplified schematic which illustrates a second embodiment of the invention;

[0019] FIG. 3 is a simplified schematic which illustrates in greater detail the arrangement for demodulating the signal, as combined with the local oscillator beam, which is received from the target;

[0020] FIG. 4 is a waveform that illustrates the effect of constant frequency shift on the up-chirp signal;

[0021] FIG. 5 is a waveform that illustrates the effect of constant frequency shift on the down-chirp signal;

[0022] FIGS. 6A and 6B illustrates the ramp-up input and ramp-down input for generating the up- and down-chirp signals in the arrangement of FIG. 2;

[0023] FIGS. 7A and 7B illustrate the VCO outputs for generating the up- and down-chirp signals in the arrangement of FIG. 2;

[0024] FIG. 8 is a waveform which illustrates the combining of the up- and down-chirp signal channels. This waveform is a double sideband chirped waveform and represents the envelop of a linear AM chirp modulation waveform; and

[0025] FIG. 9 represents the speckle phase effect on the compressed pulses for an up- and down-chirp pair whereby the true range of a target may be determined.

[0026] Referring now to the drawings in detail and in particular to FIG. 1, the reference character 10 generally designates a frequency jitter compensating laser radar system constructed in accordance with a presently preferred embodiment of the present invention. The radar system 10 includes a transmitter laser 12 which emits an optical laser carrier beam 14 at a pre-established center frequency, in the range of 70 to 200 MHz. The light beam 14 is preferably vertically polarized.

[0027] A means is provided to frequency modulate the optical carrier beam 14 to provide a simultaneously up- and down-chirped frequency modulated transmitter light beam. In the illustrated preferred embodiment of the invention seen in FIG. 1, this means includes a suitable elctro-optic device 16 designed for FM modulation. A suitable electro-optic device would be a Pockel's cell having a (1, 1, 1) cut CdTs crystal such as made by the II-VI Inc. company of Saxonbury, Pa. The crystal is cut so that the (1, 1, 1) crystal axis is along the optical axis and is electroded for driving along two transverse orthogonal directions. The two driving voltages are out of phase by 90 degrees and are of amplitude sufficient to produce a half-wave relative phase change between the orthogonal directions. The means for producing the two driving voltages are shown as 16 and 18 which are suitably electroded to the Pockel's cell 16. Preferably, these means to generate the two simultaneously up- and down-chirped sidebands employ up- and down-chirp SAW dispersive filters which were built by General Dynamics, Electronic Division, the construction of which is well known in the art.

[0028] In the illustrated configuration, the electro-acoustic device 16 acts as a spinning half-wave plate rotating at an angular velocity, Wr, equal to the driving frequency. The output 22 from the electro-acoustic device 16 is directed to a suitable quarter-wave plated 24 with fast axis of 45 degrees. The output 26 from the quarter-wave plate contains two components. The vertically polarized component is down shifted by a frequency of 2 Wr whiled the horizontally polarized component is upshifted by 2 Wr. Frequency modulation of the driving voltages is directly converted into frequency modulation of the optical carrier.

[0029] The frequency modulated transmitter laser beam is then directed toward a target for scanning thereof for each pixel location. The scanning of the target may generate dynamic speckle in the return beam 28 reflected from the target as previously described. The return beam 28 is then subjected to suitable demodulation. The return beam 28 is directed to a suitable thin firm polarizer 30 which separates the beam 28 into two channels corresponding to each sideband. The polarizer 30 permits the passage of a horizontally polarized beam 32 representing the channel for the up-chirp while it redirects a vertically polarized light beam 34 which represents the channel for the down-chirp. A suitable thin film polarizer is made by Laser Power Optics of Del Mar, Calif.

[0030] The horizontally polarized laser beam 32 may be turned by a suitable mirror 36 and combined with a local oscillator beam 38 having horizontal polarization emitted by a suitable local oscillator laser 40 at a suitable beam splitter device 40 to provide a combined beam 42. The combined beam 42 is incident upon a suitable detector 44 which converts the combined beam 42 into an electrical signal on lead 46 representative of the up-chirped channel. The lead 46 is connected to a suitable signal pre-processor 48. The output of the pre-processor appears on lead 50 which is connected to a suitable up-chirp compressor 52 which is preferably a suitable up-chirp surface acoustic wave (SAW) dispersive filter as is known in the art. The output of the up-chirp compressor 52 appears as a compressed electrical pulse on output lead 54 for further processing and analysis in the radar system.

[0031] The horizontally polarized laser local oscillator beam 38 is incident on a suitable quarter-wave plate 56 to provide a vertically polarized beam 58 which is direct to a suitable beam splitter 60 where it is combined with beam 34 to provide a combined beam 62 which is received by a suitable detector 64.

[0032] A detector 64 which converts the combined beam 62 into an electrical signal on lead 66 which is representative of the down-chirp channel. The lead 66 is connected to a suitable pre-processor 68 which is connected to a suitable down-chirp compressor 70 by lead 72. As before the down-chirp compressor 72 is preferably a suitable down-chirp surface acoustic wave (SAW) dispersive filter. The output of the down-chirp compressor 72 appears as a compressed electrical pulse on output lead 54 for further processing and analysis in the radar system.

[0033] Referring now to FIG. 3, a typical demodulation process for further utilization of the compressed up-chirp pulse and down-chirp pulse will be described. The up-chirp electrical signal that appears on input 50 to the up-chirp pulse compressing circuit 50 will then appear as a compressed pulse on output 54. Similarly, the down-chirp electrical signal that appears on input 70 to the down-chirp compression circuit 72 will appear as compressed down-chirp pulse on output 74. Frequency jitter compensation will then be obtained by averaging the peak-detection readings from the two channels for each pixel. In addition, single pixel doppler information may be extracted by taking the difference in readings of the two channels. Accordingly, the output lead 54 is coupled to the envelope detection circuit 76 which is connected to the band pass filtering circuit 78 which in turn is connected to a suitable peak detection circuit 80 that develops an output pulse on lead 82. Lead 82 serves as the input for the up-chirp channel to a suitable pulse offset compensation 84. Similarly, the output lead 74 for the down-chirp channel is connected through similar envelop detection 86, bandpass filtering 88, and peak detection circuitry 90 to provide a down-chirp pulse channel output on lead 92 connected to the pulse offset compensation circuit 92.

[0034] Since any frequency jitter experienced will affect the demodulated pulses as previously described, a frequency shift will advance one pulse in one channel will delay the pulse in another channel by exactly the same amount. Thus, the output pulse that appears on the output 94 of the pulse offset compensation 84 will have compensated for any frequency jitter thereby significantly improving system range precision in a frequency jitter-limited laser radar system.

[0035] Reference will now be made to certain Figures which illustrate the waveforms involved in implementation of the present invention. I have found that speckle phase variations are dominated by the linear term. These linear phase variations correspond to frequency shifts which, after pulse compression, are converted to fixed offsets at the time of arrival. As seen in FIGS. 4 and 5, for a given frequency shift these offsets advance or lag behind the ideal time of arrival depending on whether an up-chirp or a down-chirp signal is transmitted. If the magnitudes of the two offsets from the up-chirp and down-chirp channels are identical but opposite in direction, their effects may be compensated and the true time of arrival, i.e., center of the two pulses, may be determined. This concept is realized in this present invention by transmitting up- and down-chirp pairs simultaneously. Accordingly, the laser beam sweeps across the target surface producing identical phase variations for each pulse pair.

[0036] Referring now to FIGS. 7A, 7B and 8, it will be seen that when an up-chirp and down-chirp having general waveforms as seen in FIGS. 7A and 7B, are simultaneously generated and combined in a double-sideband chirped waveform, as seen in FIG. 8, results. This waveform could also be termed a linear AM chirp modulation waveform. The transmitter laser beam when modulated by the electro-optic device 16 of FIG. 1 will be frequency modulated accordingly so that the radar beam which scans the target will be thus modulated so that the return beam may be demodulated in accordance with the present invention to provide frequency jitter compensation.

[0037] The pulses that appear on the inputs 82 and 92 to the pulse offset compensation circuit 92 of FIG. 3 are shown added together in FIG. 3 to determine the true range of the target. As seen the speckle phase effect on the compressed up- and down-chirp pulses will cause compensation offsets.

[0038] While FIG. 1 illustrated an embodiment of the invention which utilized an electro-optic device to frequency modulate the transmitter laser beam, the invention would also contemplate the use of an acoustic-optic device to modulate the carrier laser beam. FIG. 2 illustrates an alternative embodiment of the invention which has been so constructed. In FIG. 2, another embodiment 96 utilizes ramp-up and ramp-down generators 98 and 100, as made by Wavetek of San Diego, Calif. to generate simultaneously the ramp-up and ramp-down pulses seen in FIGS. 6A and 6B. These ramp-up and ramp-down pulses are simultaneously applied to the inputs of suitable voltage controlled oscillators (VCO's). Such VCO's are made by Vectron Labs. Inc. of Norwalk, Conn. The up-chirp and down-chirp signals, as seen in FIGS. 7A and 7B appear respectively on inputs 106 and 108 of a suitable combiner 110 to provide a double-sideband chirped waveform seen in FIG. 8. The output of the combiner 110 is suitably amplified by amplifier 112, the output of which is applied to a suitable acoustic-optic device 114 which also receives incident light from a transmitter laser 116 which emits a laser beam 118 which is incident to the acousto-optic device 114. The acousto-optic device 114 may be a Bragg cell which is well known in the art and which frequency modulates the laser light beam in accordance with the electrical signal seen in FIG. 8 to provide a frequency modulated laser beam 120 which is transmitted through a suitable transmit/receive section 122 to scan a target T with a modulated laser pulse for each pixel location. A typical Bragg cell is Model AOM 100 made by Phillips Research Labs., Redhill, Surrey, England.

[0039] The return beam 127 which is reflected from the target and which may have frequency jitter induced by target speckle is directed by transmit/receive section 122 to a suitable detector 124. A local oscillator laser 126 emits a continuous wave local oscillator beam 128 that has a center frequency offset from the center frequency of the carrier beam 118 by a predetermined amount. The heterodyne detector 124 detects the combined beam of the local oscillator 126 and the return beam and creates an electrical signal on output 130 that is then pre-processed by a suitable pre-processor 132 before being outputted to splitter 134 where it is split into an up-chirp channel signal on output 136 and a corresponding down-chirp channel signal on output 138. The up-chirp signal on output 138 is then compressed by a suitable up-chirp compressor circuit 140 while the down-chirp signal is similarly compressed by a suitable down-chirp compressor 142. Suitable chirp compressors are typically surface acoustic wave (SAW) filters such as made by Anderson Laboratories of Bloomfield, Conn.

[0040] The present invention as achieved a bandwidth of 25 Mhz and it is believed that a bandwidth of over 100 Mhz is easily achievable with present technology. The preferred chirp slope for the present implementation is 5.6E-3 Mhz/nsec while using a center frequency of 87 Mhz for the carrier laser beam.

[0041] In summary, a method and arrangement for frequency jitter compensation for a laser radar range system has been described. The invention has shown how a pair of up- and down-chirp pulses are simultaneously generated and are used to frequency modulate a laser transmitter beam. A frequency modulated laser pulse scans a target and the return pulse for a pixel location may contain target induced speckle. Demodulation of the pulse provides frequency jitter compensation by the frequency offset compensation provided by the pair of compressed up- and down-chirp pulses which are then used for further processing and analysis.

[0042] Changes may be made in the combination and arrangement of parts or elements as heretofore set forth in the specification and shown in the drawings. As evidenced by the differences between the embodiments of the invention of FIGS. 1 and 2 in which electro-optic and acoustic-optic devices were illustrated as frequency modulating the transmitter laser beam and several methods of generating the up- and down-chirp signals, it is to be understood that changes may be made in the precise embodiment disclosed without departing from the spirit and scope of the invention as defined in the following claims.