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Abstract

Apparatus for detecting and/or determining the wavelength of coherent radiation in the presence of incoherent ambient radiation. The apparatus includes at least three unequal path interferometers with the radiation path length difference in each interferometer being substantially greater than the coherence length of the incoherent radiation, but substantially less than the coherence length of the coherent radiation, the average radiation path length in said three interferometers being different one from the others. The apparatus further includes detectors for detecting the radiation transmitted through each of the interferometers for generating signals corresponding to the radiation leaving each interferometer, and electronic processing elements to detect and determine the wavelength of the coherent radiation. A preferred form of interferometer is a Fabry-Perot etalon having a plurality of regions of different thicknesses. The etalon has a first surface for receiving the radiation and a second surface spaced from the first surface, so that the radiation or optical path difference between the transmitted portions of radiation impinging on the first surface is substantially greater than the coherence length of the incoherent radiation, but substantially less than the coherence length of the coherent radiation.

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