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US 5,245,408 AGranted
Electro-optic coherent light detector
AssigneeThe United States of America as represented by the Director, National Security Agency
InventorJonathan D. Cohen
Life of this patent
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Abstract
An apparatus is described which detects the presence or absence of coherent light and provides an estimate of the coherent light's wavelength. The apparatus employs a common-path "polarization interferometer" in which the two linear polarization paths act as interferometer legs. Electro-optic modulation is used to effect periodic differential path length changes in the interferometer. The apparatus performs synchronous time-integrating detection on the light emerging from the interferometer to measure coherent contributions in the presence of obscuring incoherent light.
Claims14
An electro-optic coherent light detector, comprising:
- (a) means for propagating light along a propagation axis x;
- (b) a periodic electrical modulation signal source;
- (c) a polarization interferometer disposed along said propagation axis x, said interferometer comprising:
- (i) an optical polarizer;
- (ii) an optical analyzer;
- and
- (iii) a birefringent modulator positioned between said polarizer and said analyzer and responsive to said modulation signal, said birefringent modulator having two mutually orthogonal axes y and z orthogonal to x and having a first optical path along axis x for light propagating in the x direction with polarization parallel to the z axis and a second optical path along axis x for light propagating in the x direction with polarization parallel to the y axis, the difference between the lengths of said first and second paths exceeding the coherence length of incoherent light and varying in accordance with said modulation signal;
- and
- (d) means for detecting intensity components of light from said interferometer which are synchronous with said modulation signal.
6 more elements
An electro-optic coherent light detector, comprising:
- (a) means for propagating light along a propagation axis x;
- (b) a periodic electrical modulation signal source;
- (c) a polarization interferometer disposed along said propagation axis x, said interferometer comprising:
- (i) an optical polarizer;
- (ii) an optical analyzer;
- and
- (iii) a birefringent modulator positioned between said polarizer and said analyzer and responsive to said modulation signal, said birefringent modulator having two mutually orthogonal axes y and z orthogonal to x and having a first optical path along axis x for light propagating in the x direction with polarization parallel to the z axis and a second optical path along axis x for light propagating in the x direction with polarization parallel to the y axis, the difference between the lengths of said first and second paths exceeding the coherence length of incoherent light and varying in accordance with said modulation signal;
- (d) means for collecting light and directing it through said interferometer;
- (e) means for detecting intensity components of light from said interferometer which are synchronous with said modulation signal, said detecting means further comprising means for generating an electrical detection signal, said electrical detection signal generation means comprising means for measuring the harmonics of said modulation signal, and
- (f) focusing means for receiving light from said interferometer and directing it to said detection means.
7 more elements
An electro-optic coherent light detector, comprising:
- (a) means for propagating light along a propagation axis x;
- (b) a periodic electrical modulation signal source;
- (c) a polarization interferometer disposed along said propagation axis x, said interferometer comprising:
- (i) an optical polarizer;
- (ii) an optical analyzer;
- and
- (iii) a birefringent modulator positioned between said polarizer and said analyzer and responsive to said modulation signal, said birefringent modulator having two mutually orthogonal axes y and z orthogonal to x and having a first optical path along axis x for light propagating in the x direction with polarization parallel to the z axis and a second optical path along axis x for light propagating in the x direction with polarization parallel to the y axis, the difference between the lengths of said first and second paths exceeding the coherence length of incoherent light and varying in accordance with said modulation signal;
- and
- (d) means for detecting intensity components of light from said interferometer which are synchronous with said modulation signal, said detection means comprising a two-dimensional detector array and further comprising means for forming the two-dimensional image of the subject light in the plane of said two-dimensional detector.
6 more elements
Dependencies are inferred from the claim text.