Imaging coherent radiometer - Patent Review 4735507

Claims

What is claimed is:

1. An imaging radiometer for detecting and determining location and wavelength of coherent radiation, or coherent absence of radiation, in the presence of non-coherent ambient radiation which comprises, in combination:

an unequal path symmetrical interferometer which divides incoming radiation containing coherent and non-coherent radiation into a first beam and a second beam;

the optical path length difference between the path length traversed through said unequal path symmetrical interferometer by the first beam and the path length traversed through said unequal path symmetrical interferometer by the second beam being substantially greater than the coherence length of the non-coherent radiation, but substantially less than the coherence length of the coherent radiation or coherent absence of radiation;

modulation means to cause a predetermined difference in the optical frequencies between the first beam and the second beam proportional to a reference waveform;

means for forming a recombined beam, which recombined beam consists of the first beam and the second beam after having traversed said unequal path interferometer;

detecting means;

imaging means to image the scene being viewed onto said detecting means;

said detecting means detecting the interference of the first beam with the second beam across the entire wavefront of the recombined beam;

first processing means for processing the interference detected to detect and determine direction of coherent radiation or the coherent absence of radiation; and

second processing means to determine the wavelength or wavelength set of coherent radiation or coherent absence of radiation in a scene being viewed.

2. An apparatus as claimed in claim 1 wherein said modulation means comprises:

fixed plane polarizing means substantially in the optical path of the incoming energy;

a rotating birefringent element disposed in the optical path of the incoming radiation following said fixed plane polarizing means, the two sides of said rotating birefringent element being substantially perpendicular to the optical path of the incoming radiation, substantially parallel to one another and substantially flat;

partially reflecting surfaces disposed on said two sides of said rotating birefringent element; and

means to provide said modulation signal proportional to the rotational velocity of said birefringent element.

3. An apparatus as claimed in claim 1 wherein said modulation means comprises:

a fixed plane polarizer to plane polarize the incoming radiation;

a fixed quarterwave plate oriented to circularly polarize the first beam;

means to change by a predetermined amount the optical frequency of the circularly polarized beam; and

means to plane polarize the frequency modulated circularly polarized beam.

4. An apparatus as claimed in claim 3 wherein said means to change by a predetermined amount the optical frequency of the circularly polarized beam comprises:

a rotating quarterwave plate disposed substantially in the axis of the circularly polarized beam; and

a reflector to cause the frequency modulated circularly polarized beam to pass back through said rotating quarterwave plate; and

means to provide said modulation signal proportional to the rotational velocity of said rotating quarterwave plate.

5. An apparatus as claimed in claim 3 wherein said means to modulate the optical frequency of the circularly polarized beam comprises:

a rotating half-wave plate disposed substantially in the axis of the circularly polarized beam; and

means to provide said modulation signal proportional to the rotational velocity of said rotating half-wave plate.

6. An apparatus as claimed in claim 3 wherein said means to modulate the optical frequency of the circularly polarized beam comprises:

two quarterwave plates, rotating at the same frequency and each disposed substantially in the optical path of the circularly polarized beam; and

means to provide said modulation signal proportional to the rotational velocity of said two quarterwave plates.

7. An apparatus as claimed in claim 3 wherein said means to plane polarize the frequency modulated circularly polarized beam comprises:

a quarterwave plate disposed in substantially the optical axis of the frequency modulated circularly polarized beam.

8. An apparatus as claimed in claim 3 which further comprises:

adjustment means to increase or decrease the optical path of the second beam.

9. An apparatus as claimed in claim 8 wherein said adjustment means comprises:

a substrate which has a first side which is substantially flat and substantially perpendicular to the optical path of the second beam;

a reflective surface disposed on said first side of said substrate; and

means to move said substrate along the optical path of the second beam to increase or decrease the optical path of the second beam.

10. An apparatus as claimed in claim 1 wherein said modulation means comprises:

a reflective surface disposed substantially in the optical axis of the first beam and substantially perpendicular to the optical axis of the first beam; and

means to linearly alter the length of the first beam path in a cyclic manner.

11. An apparatus as claimed in claim 10 wherein said means to cyclically alter the length of the first beam path comprises:

means to translate said reflective surface in a cyclical fashion and in a direction parallel to the optical axis of the first beam to the extent of .+-..lambda./4.

12. An apparatus as claimed in claim 11 wherein said means to translate said reflective surface in a cyclical fashion comprises:

a signal generator to provide said modulation signal;

a piezoelectric stack connected at one end thereof to the side of said reflective surface opposite the first beam; and

said signal generator applying said modulation signal to said piezoelectric stack to alternately expand and contract said piezoelectric stack causing said reflective surface to cyclically translate along the optical axis of the first beam to the extent of .+-..lambda./4.

13. An apparatus as claimed in claim 10 which further comprises:

adjustment means to increase or decrease the optical path of the second beam.

14. An apparatus as claimed in claim 13 wherein said adjustment means comprises:

a substrate which has a first side which is substantially flat and substantially perpendicular to the optical path of the second beam;

a reflective surface disposed on said first side of said substrate; and

means to move said substrate along the optical path of the second beam to increase or decrease the optical path of the second beam.

15. An apparatus as claimed in claim 1 wherein said modulation means comprises:

a Fabry-Perot etalon including two substantially transparent plates in spatial relation to one another and a partially reflecting surface disposed upon one surface of each of the said two plates; and

means to oscillate the distance between the said two plates to the extent of .+-..lambda./4.

16. An apparatus as claimed in claim 15 wherein said means to oscillate the distance between the said two plates includes:

a signal generator to provide said modulation signal;

one or more piezoelectric cylinders attachedly disposed between the said two plates holding the said two plates substantially in parallel spatial relation to one another;

said signal generator applying said modulation signal to said one or more piezoelectric cylinders causing them to alternately expand and contract further causing the said two plates to cyclically translate with respect to one another to the extent of .+-..lambda./4.

17. An apparatus as claimed in claim 1 wherein said detecting means includes:

plurality of detectors arranged substantially in one plane in a matrix, said one plane being normal to the wavefront of the recombined beam.

18. An apparatus as claimed in claim 17 wherein the said first processing means includes, for each detector in the matrix:

a low frequency amplifier for each detector in the matrix to amplify the detector signal from each detector in the matrix;

a synchronous detector for each detector in the matrix to form a product of the reference waveform and the amplified detector signal;

an integration network for each synchronous detector in the matrix; and

an output tap for each detector channel in the matrix to indicate the presence or absence of coherent radiation or the coherent lack of radiation in the incoming radiation, the position of each output tap corresponding to a position in the scene being viewed.

19. An apparatus as claimed in claim 1 wherein said detecting means includes a single detector.

20. An apparatus as claimed in claim 19 wherein said first processing means includes:

a low frequency amplifier to amplify the detector signal;

a synchronous detector to form a product of the reference waveform and the amplified detector signal;

an integration network; and

an output tap to indicate the presence or absence of coherent radiation or the coherent lack of radiation in the incoming radiation.

21. An apparatus as claimed in claim 1 wherein said detecting means includes a vidicon tube which produces an analog interference signal proportional to the interference of the first beam with the second beam.

22. An apparatus as claimed in claim 21 wherein said first processing means includes:

a video amplifier to amplify the said analog interference signal from the said vidicon;

a frame grabber to convert said analog video interference signal to a digital video interference signal and to store the digital video signals in an X-channel by Y-channel data file where the number of X-channels corresponds to the number of image pixels in the X direction and the number of Y-channels corresponds to the number of image pixels in the Y direction; and

digital computing means to compare the digital interference signal for each pixel channel in the X by Y data file with internally generated signal to detect the presence or absence of coherent radiation or coherent lack of radiation in the incoming radiation and its position in the scene being viewed.

23. An apparatus as claimed in claim 1 wherein said second processing means comprises an axis-crossing period counter.

24. An apparatus as claimed in claim 1 wherein said second processing means comprises a cross-correlator.

25. An imaging radiometer for detecting and determining location and wavelength of coherent radiation, or coherent absence of radiation, in the presence of non-coherent ambient radiation which comprises, in combination:

an unequal path interferometer which divides incoming radiation containing coherent and non-coherent radiation into a first beam and a second beam path;

the optical path length difference between the path length traversed through said unequal path interferometer by the first beam and the path length traversed through said unequal path interferometer by the second beam being substantially greater than the coherence length of the non-coherent radiation, but substantially less than the coherence length of the coherent radiation or coherent absence of radiation;

fixed plane polarizing means substantially in the optical path of the incoming energy;

a rotating birefringent element disposed in the optical path of the incoming radiation following said fixed plane polarizing means, the two sides of said rotating birefringent element being substantially perpendicular to the optical path of the incoming radiation, substantially parallel to one another and substantially flat;

partially reflecting surfaces disposed on said two sides of said rotating birefringent element; and

means to provide a reference waveform proportional to the rotational velocity of said birefringent element;

means for forming a recombined beam, which recombined beam consists of the first beam and the second beam after having traversed said unequal path interferometer;

detecting means;

imaging means to image the scene being viewed onto said detecting means;

said detecting means detecting the interference of the first beam with the second beam across the entire wavefront of the recombined beam;

first processing means for processing the interference detected to detect and determine direction of coherent radiation or the coherent absence of radiation; and

second processing means to determine the wavelength or wavelength set of coherent radiation or coherent absence of radiation in a scene being viewed.

26. An imaging radiometer for detecting and determining location and wavelength of coherent radiation, or coherent absence of radiation, in the presence of non-coherent ambient radiation which comprises, in combination:

an unequal path interferometer which divides incoming radiation containing coherent and non-coherent radiation into a first beam and a second beam path;

the optical path length difference between the path length traversed through said unequal path interferometer by the first beam and the path length traversed through said unequal path interferometer by the second beam being substantially greater than the coherence length of the non-coherent radiation, but substantially less than the coherence length of the coherent radiation or coherent absence of radiation;

a fixed plane polarizer to plane polarize the incoming radiation;

a fixed quarterwave plate to circularly polarize the first beam;

a rotating quarterwave plate disposed substantially in the axis of the circularly polarized beam; and

a reflector to cause the frequency modulated circularly polarized beam to pass back through said rotating quarterwave plate; and

means to provide a refence waveform proportional to the rotational velocity of said rotating quarterwave plate;

a quarterwave plate disposed in substantially the optical axis of the frequency modulated circularly polarized beam;

means for forming a recombined beam, which recombined beam consists of the first beam and the second beam after having traversed said unequal path interferometer;

detecting means;

imaging means to image the scene being viewed onto said detecting means;

said detecting means detecting the interference of the first beam with the second beam across the entire wavefront of the recombined beam;

first processing means for processing the interference detected to detect and determine direction of coherent radiation or the coherent absence of radiation; and

second processing means to determine the wavelength or wavelength set of coherent radiation or coherent absence of radiation in a scene being viewed.

27. An apparatus as claimed in claim 26 wherein said means to modulate the optical frequency of the circularly polarized beam comprises:

a rotating half-wave plate disposed substantially in the axis of the circularly polarized beam; and

means to provide said modulation signal proportional to the rotational velocity of said rotating half-wave plate.

28. An apparatus as claimed in claim 26 wherein said means to modulate the optical frequency of the circularly polarized beam comprises:

two quarterwave plates, rotating at the same frequency and each disposed substantially in the optical path of the circularly polarized beam; and

means to provide said modulation signal proportional to the rotational velocity of said two quarterwave plates.

29. An imaging radiometer for detecting and determining location and wavelength of coherent radiation, or coherent absence of radiation, in the presence of non-coherent ambient radiation which comprises, in combination:

an unequal path interferometer which divides incoming radiation containing coherent and non-coherent radiation into a first beam and a second beam path;

the optical path length difference between the path length traversed through said unequal path interferometer by the first beam and the path length traversed through said unequal path interferometer by the second beam being substantially greater than the coherence length of the non-coherent radiation, but substantially less than the coherence length of the coherent radiation or coherent absence of radiation;

a reflective surface disposed substantially in the optical axis of the first beam and substantially perpendicular to the optical axis of the first beam; and

a signal generator to provide a reference waveform;

a piezoelectric stack connected at one end thereof to the side of said reflective surcvace opposite the first beam;

said signal generator applying said reference waveform to said piezoelectric stack to alternately expand and contract said piezoelectric stack causing said reflective surface to cyclically translate along the optical axis of the first beam to the extent of .+-..lambda./4

means for forming a recombined beam, which recombined beam consists of the first beam and the second beam after having traversed said unequal path interferometer;

detecting means;

imaging means to image the scene being viewed onto said detecting means;

said detecting means detecting the interference of the first beam with the second beam across the entire wavefront of the recombined beam;

first processing means for processing the interference detected to detect and determine direction of coherent radiation or the coherent absence of radiation; and

second processing means to determine the wavelength or wavelength set of coherent radiation or coherent absence of radiation in a scene being viewed.

30. An imaging radiometer for detecting and determining location and wavelength of coherent radiation, or coherent absence of radiation, in the presence of non-coherent ambient radiation which comprises, in combination:

an unequal path interferometer which divides incoming radiation containing coherent and non-coherent radiation into a first beam and a second beam path;

the optical path length difference between the path length traversed through said unequal path interferometer by the first beam and the path length traversed through said unequal path interferometer by the second beam being substantially greater than the coherence length of the non-coherent radiation, but substantially less than the coherence length of the coherent radiation or coherent absence of radiation;

a Fabry-Perot etalon including two substantially transparent plates in spatial relation to one another and a partially rflecting surface disposed upon one surface of each of the said two plates; and

a signal generator to provide a reference waveform;

one or more piezoelectric cylinders attachedly disposed between the said two plates holding the said two plates substantially in parallel spatial relation to one another;

said signal generator applying said reference waveform to said piezoelectric stack to alternately expand and contract said piezoelectric stack causing said reflective surface to cyclically translate along the optical axis of the first beam to the extent of .+-..lambda./4

means for forming a recombined beam, which recombined beam consists of the first beam and the second beam after having traversed said unequal path interferometer;

detecting means;

imaging means to image the scene being viewed onto said detecting means;

said detecting means detecting the interference of the first beam with the second beam across the entire wavefront of the recombined beam;

first processing means for processing the interference detected to detect and determine direction of coherent radiation or the coherent absence of radiation; and

second processing means to determine the wavelength or wavelength set of coherent radiation or coherent absence of radiation in a scene being viewed.

31. A coherent energy modulator which comprises:

an unequal path symmetrical interferometer which divides incoming radiation containing coherent and non-coherent radiation into a first beam path and a second beam path through which a first beam and a second beam, respectively, travel and then recombine;

the optical path length difference between the first beam path and the second beam path being substantially greater than the coherence length of the non-coherent radiation, but substantially less than the coherence length of the coherent radiation;

fixed plane polarizing means substantially in the optical path of the incoming energy;

a rotating birefringent element disposed in said first beam path following said fixed plane polarizing means, the two sides of said rotating birefringent element being substantially perpendicular to the optical path of the incoming radiation, substantially parallel to one another and substantially flat;

partially reflecting surfaces disposed on said two sides of said rotating birefringent element; and

means to detect the rotational frequency of said birefringent element.

32. A coherent energy modulator which comprises:

an unequal path interferometer which divides incoming radiation containing coherent and incoherent radiation into a first beam path and a second beam path through which a first beam and a second beam, respectively, travel and then recombine;

the optical path length difference between the first beam path and the second beam path being substantially greater than the coherence length of the incoherent radiation, but substantially less than the coherence length of the coherent radiation;

a fixed plane polarizer to plane polarize the incoming radiation;

a fixed quarterwave plate to circularly polarize the first beam;

means to modulate the optical frequency of the beam of circularly polarized light; and

means to plane polarize the frequency modulated circularly polarized beam of light.

33. An apparatus as claimed in claim 32 wherein said means to modulate the optical frequency of the beam of circularly polarized light comprises:

a rotating quarterwave plate disposed substantially in the axis of the beam of circularly polarized light; and

a retro-reflector to cause the beam of frequency modulated circularly polarized light to pass back through said rotating quarterwave plate.

34. An apparatus as claimed in claim 32 wherein said means to modulate the optical frequency of the beam of circularly polarized light comprises:

a rotating half-wave plate disposed substantially in the axis of the beam of circularly polarized light.

35. An apparatus as claimed in claim 32 wherein said means to modulate the optical frequency of the beam of circularly polarized light comprises:

two quarterwave plates, rotating at the same frequency and each disposed substantially in the optical path of the beam of circularly polarized light.

36. An apparatus as claimed in claim 32 wherein said means to plane polarize the frequency modulated beam of circularly polarized light beam comprises:

a quarterwave plate disposed in substantially the optical axis of the frequency modulated beam of circularly polarized light.

BACKGROUND OF THE INVENTION

The present invention relates to a device for detecting the presence of coherent radiation or the coherent lack of radiation in the presence of non-coherent background radiation. More particularly, the present invention relates to the use of an imaging optical radiometer to make such detections and determine the direction and wavelength of such radiation or such lack of radiation.

Imaging optical radiometers, constructed in accordance with the concept of this invention are adapted, among other possible uses for use in detecting and determining the wavelength of coherent radiation, or coherent absence of radiation. In addition, it is adapted to determine the direction of arrival of the source of the coherent radiation or coherent lack of radiation and indicate such position in a display of the field of view.

Such a device can find application in detecting specific gas clouds, oil and mineral exploration and detection through Fraunhofer line discrimination techniques and intelligence surveillance.

Conventional laser receivers use a narrow-band optical filter or diffraction gratings in combination with a photodetector, bandpass amplifier and thresholded peak detector to detect the presence of coherent radiation. This approach has two disadvantages: one, the laser wavelength must be known and two, the video bandwidth required to pass nanosecond pulses also passes a lot of detector and/or background photon noise. The coherent radiometer approach has a broad spectral response and a noise integration time limited only by the available observation time.

The prior art is evidenced by U.S. Pat. Nos. 3,824,018 to R. Crane Jr. and 4,309,108 to E. Seibert, both of which are assigned to the same assignee as the present application. The aforementioned patents disclose the use of Fabry-Perot etalon interferometers.

While the prior art devices detect presence, wavelength and direction of arrival of coherent radiation from a single source, our contribution is to do so for all coherent sources within a scene, resulting in an imaging coherent radiometer with longer integration times for sensitivity enhancement, to also do so for the coherent absence of radiation, and for other advantages which will become apparent as the description proceeds.

SUMMARY OF THE INVENTION

The present application is related to U.S. patent application Ser. No. 884,695, filed on July 11, 1986 entitled "Fabry-Perot Scanning and Nutating Imaging Coherent Radiometer" which is assigned to the same assignee as the present invention and filed on even date therewith.

The present invention contemplates the provision of a new and improved apparatus to detect the presence, wavelength and direction of arrival of coherent radiation or the coherent absence of radiation in the presence of non-coherent ambient radiation.

This apparatus takes the form of an imaging coherent radiometer which includes collecting optics to increase the radiation collection aperture and substantially collimate the incoming radiation. The collimated beam then enters an unequal path interferometer which divides incoming radiation, containing coherent and non-coherent radiation, into a first beam path and a second beam path through which a first beam and a second beam, respectively, travel. The optical path length difference (OPD) between the first beam path and the second beam path is greater than the coherence length of the non-coherent radiation, but less than the coherence length of the coherent radiation or coherent absence of radiation.

Modulation means in the first beam path modulates the optical frequency of the first beam. The first beam and the second beam then recombine after traversing their respective beam paths. Means are then provided to detect the interference between the first and second beams. The interference so detected is processed to determine the existence, direction and wavelength of coherent radiation or the coherent lack of radiation.

Generally, the modulation means increases or decreases the optical frequency of the first beam of radiation. When the first beam, now with an increased or decreased frequency, combines with the second beam a beat frequency is produced. This beat frequency is at the same frequency as the optical modulation frequency. With the OPD chosen as previously described, only the components of coherent radiation or coherent lack of radiation in the incoming radiation will interfere upon being recombined. Therefore, only these coherent components will produce a beat frequency which can be detected.

Following the modulation means are means for detecting the beat signals produced and means for generating signals which can later be visually or electronically interpreted for coherent radiation source detection, direction and/or wavelength.

The wavelength of the incoming coherent radiation can be determined by incorporating into the detecting means an axis crossing frequency counter to generate a numerical count inversely proportional to the input wavelength or period discrimination means to compare the period of the modulation signal to that of the modulator control waveform.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception on which the disclosure is based may readily be utilized as a basis for designing other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent structures as do not depart from the spirit and scope of the invention.

Specific embodiments of the invention have been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a coherent energy radiometer in accordance with the present invention;

FIG. 2 is a graphical representation of the reference and signal waveforms for the detector apparatus of FIG. 1;

FIG. 3 is a diagram illustrating the geometry of a Fabry-Perot etalon;

FIG. 4 is a diagram illustrating the geometry of a rotating birefringent Fabry-Perot etalon modulator;

FIG. 5 is a diagram illustrating the geometry of a modulator incorporating a Michelson interferometer with an optical frequency shifter in one arm;

FIG. 6 is a diagram illustrating the geometry of a modulator incorporating a Michelson interferometer with a vibrating end mirror;

FIG. 7 is a diagram illustrating the geometry of a modulator consisting of a Michelson interferometer with one adjustable end mirror and an axially scanned end mirror;

FIG. 8 is a diagram illustrating the geometry of a modulator incorporating a Fabry-Perot etalon interferometer with a vibrating reflector;

FIG. 9 shows an axis-crossing period counter adapted for use in the coherent energy radiometer of FIG. 1;

FIG. 10 shows a cross-correlator adapted for use in the coherent energy radiometer of FIG. 1;

FIG. 11 is a schematic illustration of an imaging coherent radiometer constructed in accordance with the concepts of this invention as shown in FIG. 1;

FIG. 12 is a schematic illustration of an imaging coherent radiometer utilizing a vidicon detector, computer analysis and video display output;

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the basic components of a coherent radiometer. Collecting optics 2 serve to increase the radiation collection aperture and substantially collimate the incoming radiation. Collimated radiation, designated "CR", then enters into an interferometer/modulator 3.

It can be understood by those skilled in the art that the collecting optics 2 are only needed when the source of radiation is in the near field, i.e., within a few thousand feet of the coherent radiometer. When the source is at a distance large optics, such as a telescope, are required to increase the collecting aperture.

Interferometers may be categorized in a number of ways, three of which are method of beam separation, the optical path length difference (OPD) of the two beams, and interferometer symmetry. As used in this context the term symmetrical and asymmetrical refer to the number of beam reflections in the two optical paths of the interferometer. If both paths have either an even or odd number of reflections the interferometer is called symmetrical. If one path has an even number and one an odd number of reflections, the interferometer is called asymmetrical. The present invention utilizes only symmetrical interferometers.

The OPD of an interferometer used in the present invention is carefully chosen. Coherent radiation, such as that produced by laser light, may be characterized by its unique coherent properties: spatial, spectral, temporal and polarization. The temporal coherence property is described in terms of coherence length and is the property used in the present invention to distinguish coherent radiation from non-coherent radiation. This is because it is specific to laser radiation and unique relative to a natural background or foreground radiation in that laser radiation has a long coherence length relative to non-coherent radiation. In addition, the coherence length signature of laser radiation is not distorted by natural propagation effects. The interferometer OPD is selected so that it is longer than the coherence length of the incoherent background or foreground radiation and shorter than the coherence length of coherent radiation. The result is that the non-coherent radiation will be substantially unmodulated leaving only the coherent laser energy modulated at the interferometer output. As will later be shown modulation is accomplished by producing a linear change in the OPD of the interferometer proportional to a modulation signal carried by a line 13 from a modulation waveform generator 10. By changing the OPD in a linear manner, the coherent radiation that passes through the modulated leg of the interferometer will be at a different frequency than that which passes through the other leg. When the beams traversing the two legs are recombined the resulting waveform has a frequency equal to the average of the frequencies of the two beams. The amplitude of the recombined beam will vary in a cyclical manner at a frequency equal to the optical modulation frequency. This cyclical variation in amplitude is the "beat" signal which is indicative of the presence of coherent radiation or the coherent absence of radiation.

Referring to FIG. 2 this phenomenon is illustrated quite clearly. The top wave pattern 157 shows the two interfering optical waveforms. The modulated wave is shown as "M" and, due to frequency modulation, is at a slightly higher frequency than the unmodulated wave, designated as "U". The lower wave pattern 158 illustrates the result of combining waveform "M" with waveform "U". The resultant waveform 160 has a frequency which is the average of the frequency of waveform "M" and waveform "U". The amplitude varies in a cyclical manner which repeats with a frequency equal to the modulation frequency which is equal to the frequency of waveform "M", less the frequency of waveform "U". The frequency of this "beat" or varying amplitude shown for illustration by the envelope 159, is directly related to the modulation signal carried by line 13 from the modulation waveform generator 10 of FIG. 1.

Returning now to FIG. 1, the recombined beam, designated "RB", emanating from the interferometer/modulator 3 falls incident on the detector 4. An imaging lens, not shown, may be used to image the recombined beam, "RB", onto the detector 4 which is small so as to give high angular selectivity. Following the detector 4 is a conventional low-frequency amplifier 5 designed to pass the modulation signal generated at the detector 4 output and reject any spurious background frequencies from non-coherent radiation.

The synchronous detector 6 provides a signal indicative of whether or not there is coherent radiation in the incoming radiation `R`. The synchronous detector 6 forms a product of the signal from the amplifier 5 times a reference waveform. As described hereinbelow, the reference waveform for the interferometer/modulators 3 shown in FIGS. 4 and 5 comes directly from the modulation waveform generator 10 through a line 12, shown as dashed, to the synchronous detector 6. The interferometer/modulators 3 shown in FIGS. 6, 7 and 8 generate a reference waveform which is carried by a line 11, shown as dashed, to the synchronous detector 6.

A conventional synchronous detector forms a product from two inputs, a signal A sin.omega.(t+.delta.) and a reference B sin.omega.t, where:

A and B=signal and reference amplitudes

.delta.=unknown phase of signal relative to the reference

.omega.=2.pi.f, where f=frequency

t=time

2.pi.f.delta.=phase difference, in radians, between signal and reference

Conventionally B is made large compared to A; then the output is proportional to the unknown phase and A.

When the signal and reference are out of phase by 2.pi.f.delta.=.pi. radians, then the product is zero and the signal would go undetected. To overcome this problem the synchronous detector 6 forms a second product:

signal.times.reference from A sin .omega.(t