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Abstract

A spectral filter is adapted for use in a receiver, for example in a short range submarine laser signal path, wherein the relative orientations of the receiver is such that the signal may appear diffusely or at an unknown point in a wide external field of view around an optical axis of the receiver. A narrow band spectral filter in the receiver has cascaded stages of tunable retarders with includes multi-conjugate stages that tolerate light that is oblique to normal, up to a diverging internal field of view angle up to about 10°. A fisheye lens assembly refracts incoming light from a wider external field of 170° or more and directs the light into the filter over the narrow internal field of view. Calibration and feedback control can be provided to stabilize the discriminated wavelength peak, which remains at the same center wavelength over the span of the internal field of view notwithstanding the difference in retarder thicknesses traversed along paths that are normal to the retarders versus oblique to the optical axis.

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First Claim