Birefringent spectral filter with wide field of view and associated communications method and apparatus

A communications system, comprising:

• a source of a light signal having at least one wavelength related aspect;
• a receiver including an optical wavelength filter with a plurality of tunable birefringent retarders in at least one stage with at least one polarizer mounted in a housing to define a light propagation path along an optical axis, wherein the retarders comprise stacked plates normal to the axis, rotationally oriented relative to one another and relative to the polarizer so as to discriminate for a discrimination wavelength, the receiver comprising at least one lens operable to refract light incident on an input aperture so as to propagate through the retarders and the polarizer to a sensor;
• wherein at least one of the source and the receiver has at least one of an indefinite location, an indefinite orientation and a scattered diffusion relative to the other of the source and the receiver, such that the light signal is incident on the receiver over a predetermined external field of view;
• wherein the lens is configured to reduce the external field of view to an angular span substantially encompassing the retarders and the polarizer, wherein the angular span includes a range of propagation angles from parallel to the optical axis up to at least ±7.5° oblique to the axis;
• and,
• wherein said discrimination wavelength is equal across said angular span.

An undersea communication method, comprising:

• providing a source of a light signal having at least one wavelength related aspect, and a receiver responsive to the wavelength aspect, and signaling from the source to the receiver in a situation wherein at least one of the source and the receiver has at least one of an indefinite location, an indefinite orientation and a light scattering medium in water between the source and the receiver, such that the light signal is incident on the receiver over a predetermined external field of view;
• controlling in the receiver an optical wavelength filter with a plurality of tunable birefringent retarders in at least one stage with at least one polarizer mounted in a housing to define a light propagation path along an optical axis, wherein the retarders comprise stacked plates normal to the axis, rotationally oriented relative to one another and relative to the polarizer, each of the plates including at least one liquid crystal element tunable in unison with other liquid crystals of others of said plates, so as to discriminate for a discrimination wavelength, the receiver comprising at least one lens operable to refract light incident on an input aperture so as to propagate through the retarders and the polarizer to a sensor;
• employing the lens to reduce the external field of view to an angular span substantially encompassing the retarders and the polarizer, wherein the angular span includes a range of propagation angles from parallel to the optical axis up to at least ±7.5° oblique to the axis;
• and,
• sensing the same said discrimination wavelength across said angular span.