Hybrid optical correlator/digital processor for target detection and discrimination

• US 6,603,536 B1
• Filed: 03/22/2002
• Issued: 08/05/2003
• Est. Priority Date: 01/24/2002
• Status: Active Grant

• Current Assignee: Textron Systems Corporation (Textron Inc)
• Original Assignee: Textron Systems Corporation (Textron Inc)
• Inventors: Hasson, Victor, Nordstrom, Gerald A.
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US10/105,081
• Publication Number:
  US 20030137647A1
• Time in Patent Office: 501 Days
• Field of Search: 324/76.37, 324/76.33, 356/5.09, 356/5.15, 356/28.5
• US Class Current: 356/28.5
• CPC Class Codes: G01S 13/587 using optical means
• CPC Class Codes: G01S 17/50 Systems of measurement based on relative movement of target
• CPC Class Codes: G01S 7/481 Constructional features, e.g. arrangements of optical elements
• CPC Class Codes: G01S 7/487 Extracting wanted echo signals , e.g. pulse detection
• CPC Class Codes: G02B 27/46 Systems using spatial filters
• CPC Class Codes: G06E 3/001 Analogue devices in which mathematical operations are carried out with the aid of optical or electro-optical elements optical elements per se G02B; devices consisting of a plurality of solid state components, including light sensitive semiconductor components, formed in or on a common substrate H01L27/14; electro-, magneto- or acousto-optics, non-linear optics G02F1/00; graph reading G06K11/00

Abstract

An optical correlator is used to detect the presence of a target in a laser detection and ranging (LADAR) system by analyzing the return signal, and to provide initial estimates of the targets range and velocity to the LADAR receiver. The optical correlator includes an acoustic optical Bragg cell that deflects an input laser signal using the received and down-converted LADAR signal. An integrating optical detector is disposed to receive the optical outputs of the acoustic optical Bragg cell and an optical processor analyzes and processes the integrating optical detector data. The integrating optical detector integrates the optical outputs of the Bragg cell over time and the integrating optical detector output is sampled over a sampling period so that target detection is uncorrelated to noise. An integrating optical detector data point that exceeds a predetermined threshold is considered to be a valid detected target and the location of the output of the optical correlator on the integrating optical detector is indicative of the Doppler shift and hence the velocity of the detected target. The time of detection of the target is indicative of the range to the target. This allows the range-Doppler-amplitude of the target to be estimated and provided to the receiver to allow for more accurate processing of the receiver data. In addition, the optical correlator can be used to provide whole body Doppler and range estimates of the target and can also inherently averages the speckle data.

Claims

1. A hybrid processor for detecting a target in an intermediate frequency (IF) signal, the hybrid processor comprising:

a correlator RF drive module receiving the IF signal, the correlator RF drive module configured and arranged to up-convert the IF signal into a correlator drive signal having a correlator frequency and to bandlimit the correlator drive signal;

an optical spectrum analyzer/correlator including

a laser source providing a laser beam;

an acousto-optical Bragg cell having a modulator input coupled to the correlator RF drive module and receiving the correlator drive signal therefrom;

collimating optics disposed between the laser source and the acousto optical Bragg cell the collimating optics configured and arranged to collimate the laser beam and to provide a collimated laser beam, the acousto-optical Bragg cell disposed to receive the collimated laser beam;

the acousto-optical Bragg cell responsive to the correlator drive signal by diffracting the collimated laser beam, the acousto-optical Bragg cell providing a plurality of diffracted output light beams and an undiffracted output light beam;

an optical system configured and arranged to receive the plurality of diffracted light beams and to Fourier transform the plurality of diffracted light beams and further configured and arranged to image the plurality of the diffracted light beams onto an image plane;

an integrating optical detector array including a plurality of photodetectors, the optical photodetector array disposed within the image plane and providing a plurality of output signals each corresponding to the output of one of the plurality of photodetectors;

an optical processor coupled to the integrating optical detector array and receiving the plurality of output signals therefrom, the optical processor operative to process the plurality of output signals to detect a target, wherein the optical processor provides a cue signal upon the detection of a target.

2. The hybrid processor of claim 1 wherein the optical processor in processing the received plurality of output signals is operative to compare each of the plurality of output signals to a predetermined threshold, wherein a target is detected when one of the output signals exceeds the predetermined threshold.

3. The hybrid processor of claim 1 wherein the optical processor is further operative to correlate the plurality of output samples modulo the number of the plurality of photodetectors divided by the number of the plurality of diffracted light beams.

4. The hybrid processor of claim 1 wherein the optical processor is further operative to estimate the Doppler shift of the target as a function of the position of the photodetector corresponding to the detected output signal within the plurality of photodetectors.

5. The hybrid processor of claim 1 wherein the optical processor further receives an indicia of the transmission of a pulse, and estimates the target range as a function of the time delay between the transmission to the target detection.

6. The hybrid processor of claim 1 wherein the optical processor includes:

a timing and control module operative to provide a plurality of control signals;

an accumulator coupled to the the integrating optical detector array and receiving the plurality of output signals therefrom, the accumulator configured and arranged to accumulate the plurality of output signals over a predetermined period of time;

a optical processor memory coupled to the accumulator and configured and arranged to store the plurality of accumulated values wherein each of the plurality of accumulated values corresponds to a respective photodetector;

a threshold detector coupled to the optical processor memory, the threshold detector configured and arranged to compare each of the plurality of accumulated values to a predetermined threshold, wherein in the event that one of the plurality of accumulated values exceeds the predetermined threshold a threshold signal is provided to the timing and control module;

the timing and control module responsive to the threshold signal by providing a cue signal output.

7. The hybrid processor of claim 6 wherein the timing and control module is further responsive to threshold signal by estimating the Doppler shift of the detected target as a function of the position of the photodetector corresponding to the detected output signal within the plurality of photodetectors.

8. The hybrid processor of claim 6 wherein the timing and control module is further responsive to the threshold signal and further receives an indicia of the transmission of a pulse, and estimates the target range as a function of the time delay between the transmission to the target detection.

9. The hybrid processor of claim 1 wherein the optical processor is further operative to estimate the Doppler shift of the target, and to estimate the range of the target.

10. The hybrid processor of claim 9 further including:

a first sample module configured and arranged to sample the IF signal and provide a plurality of sampled IF signals;

an analog to digital converter coupled to the first sample module and configured and arranged to digitize the plurality of sampled IF signals into a plurality of digitized IF data;

a first in first out buffer coupled to the analog to digital converter and configured and arranged to store a predetermined amount of the digitized IF data and dumping excess data when overfilled;

a digital signal processor coupled to the optical processor and to the first in first out buffer;

the first in first out buffer responsive to the cue signal by not accepting new digitized IF data and providing the stored data to the digital signal processor;

the digital signal processor responsive to the cue signal, the estimate of the Doppler shift of the target, and the estimate of the range of the target by processing the stored data provided by the first in first out buffer to determine at least one target characteristic.

26. The hybrid processor of claim 1 wherein the optical processor is further operative to estimate the Doppler shift of the target as a function of the position of the photodetector corresponding to the detected output signal within the plurality of photodetectors.

11. A method for instantaneous all-range cueing, said method comprising the steps of:

providing an intermediate frequency (IF) signal;

up-converting and bandlimiting the IF signal into a correlator drive signal having a correlator frequency;

applying the correlator drive signal to an acousto-optical Bragg cell;

applying a collimated laser beam to the acousto-optical Bragg cell;

detecting a plurality of diffracted light beams from the acousto-optical Bragg cell;

comparing the detected plurality of diffracted light beams with a predetermined threshold;

in the event that one of the detected plurality of diffracted light beams exceeds the predetermined threshold, providing a cue signal indicative of a detected target.

12. The method of claim 11 further including the step of estimating the Doppler shift of the detected target.

13. The method of claim 11 further including the step of estimating the range of the detected target.

14. The method of claim 11, further comprising the step of receiving the cue signal in a digital signal processor.

15. A method of cueing for signal processing, said method comprising the steps of:

receiving a detected intermediate frequency (IF) signal;

converting said IF signal to a first radio frequency (RF) signal and a second RF signal;

up-converting said first RF signal, thereby producing an up-converted signal;

causing said up-converted signal to propagate through an acousto-optic cell;

directing a probe laser signal through said acousto-optic cell;

detecting one or more diffracted output signals from said acousto-optic cell;

producing a cueing signal in response to said detecting one or more diffracted output signals;

determining one or more characteristics of an object from said second RF signal in response to said cueing signal.

16. The method of claim 15 further comprising the step of:

converting a wide bandwidth optical signal pulse to said IF signal in a heterodyne detector.

17. The method of claim 16 further comprising the step of producing said wide-bandwidth optical signal pulse from a mode-locked laser.

18. The method of claim 15 wherein said step of converting said IF signal is up-converting said IF signal.

19. The method of claim 18, said step of converting said IF signal further comprising the step of filtering said IF signal, thereby producing a filtered signal.

20. The method of claim 19 wherein said step of filtering is match-filtering.

21. The method of claim 19 further comprising the step of down-converting said filtered signal and thereby producing a baseband signal.

22. The method of claim 21 further comprising resolving said baseband signal into an in-phase components and an in-quadrature component.

23. A hybrid processor for detecting in an intermediate frequency (IF) signal a target illuminated by a laser, the hybrid processor comprising:

a correlator RF drive module including;

an optical receiver for receiving a reflected laser signal from said object and providing as an output an electrical signal;

a first mixer receiving said electrical signal and operative to mix said electrical signal with a first mixing signal to provide as an output a first mixed signal;

a matched filter receiving said first mixed signal and operative to filter said first mixed signal to provide a first filtered signal; and

a second mixer receiving said first filtered signal and operative to mix said first filtered signal with a second mixing signal to provide as an output an IF signal;

an optical spectrum analyzer/correlator including;

a laser source providing a laser beam;

an acousto-optical Bragg cell having a modulator input coupled to the correlator RF drive module and receiving the correlator drive signal therefrom;

collimating optics disposed between the laser source and the acousto optical Bragg cell the collimating optics configured and arranged to collimate the laser beam and to provide a collimated laser beam, the acousto-optical Bragg cell disposed to receive the collimated laser beam;

the acousto-optical Bragg cell responsive to the IF signal by diffracting the collimated laser beam, the acousto-optical Bragg cell providing a plurality of diffracted output light beams and an undiffracted output light beam;

an optical system configured and arranged to receive the plurality of diffracted light beams and to Fourier transform the plurality of diffracted light beams and further configured and arranged to image the plurality of the diffracted light beams onto an image plane;

an integrating optical detector array including a plurality of photodetectors, the optical photodetector array disposed within the image plane and providing a plurality of output signals each corresponding to the output of one of the plurality of photodetectors;

an optical processor coupled to the integrating optical detector array and receiving the plurality of output signals therefrom, the optical processor operative to process the plurality of output signals to detect a target.

24. The hybrid processor of claim 23 wherein the optical processor in processing the received plurality of output signals is operative to compare each of the plurality of output signals to a predetermined threshold, wherein a target is detected when one of the output signals exceeds the predetermined threshold.

25. The hybrid processor of claim 23 wherein the optical processor is further operative to correlate the plurality of output samples modulo the number of the plurality of photodetectors divided by the number of the plurality of diffracted light beams.

27. The hybrid processor of claim 23 wherein the optical processor further receives an indicia of the transmission of a pulse, and estimates the target range as a function of the time delay between the transmission to the target detection.

28. The hybrid processor of claim 23 wherein the optical processor includes:

a timing and control module operative to provide a plurality of control signals;

an accumulator coupled to the the integrating optical detector array and receiving the plurality of output signals therefrom, the accumulator configured and arranged to accumulate the plurality of output signals over a predetermined period of time;

a optical processor memory coupled to the accumulator and configured and arranged to store the plurality of accumulated values wherein each of the plurality of accumulated values corresponds to a respective photodetector;

a threshold detector coupled to the optical processor memory, the threshold detector configured and arranged to compare each of the plurality of accumulated values to a predetermined threshold, wherein in the event that one of the plurality of accumulated values exceeds the predetermined threshold a threshold signal is provided to the timing and control module;

the timing and control module responsive to the threshold signal by providing a cue signal output.

29. The hybrid processor of claim 28 wherein the timing and control module is further responsive to threshold signal by estimating the Doppler shift of the detected target as a function of the position of the photodetector corresponding to the detected output signal within the plurality of photodetectors.

30. The hybrid processor of claim 28 wherein the timing and control module is further responsive to the threshold signal and further receives an indicia of the transmission of a pulse, and estimates the target range as a function of the time delay between the transmission to the target detection.

31. The hybrid processor of claim 23 wherein the optical processor provides a cue signal upon the detection of a target and the optical processor is further operative to estimate the Doppler shift of the target, and to estimate the range of the target.

32. The hybrid processor of claim 31 further including:

a first sample module configured and arranged to sample the IF signal and provide a plurality of sampled IF signals;

an analog to digital converter coupled to the first sample module and configured and arranged to digitize the plurality of sampled IF signals into a plurality of digitized IF data;

a first in first out buffer coupled to the analog to digital converter and configured and arranged to store a predetermined amount of the digitized IF data and dumping excess data when overfilled;

a digital signal processor coupled to the optical processor and to the first in first out buffer;

the first in first out buffer responsive to the cue signal by not accepting new digitized IF data and providing the stored data to the digital signal processor;

the digital signal processor responsive to the cue signal, the estimate of the Doppler shift of the target, and the estimate of the range of the target by processing the stored data provided by the first in first out buffer to determine at least one target characteristic.

33. The hybrid processor of claim 32 wherein the sample module includes:

an I/Q processor;

a power splitter coupled to the output of said second mixer and operative to split the IF signal into first and second output IF signals and operative to coupled said first IF signal to said optical spectrum analyzer/correlator and said second IF signal to said I/Q processor operative to provide a plurality of sampled in-phase signals and quadrature phase signals to said analog to digital converter.

34. The hybrid processor of claim 31 wherein the matched filter has a bandwidth that is a function the Doppler shift of the target.