Patent ID: 7417796

Claim:
A spectral filter, comprising: a plurality of birefringent elements coupled along a signal propagation path for successively advancing a polarization state of a discrimination wavelength from a predetermined polarization alignment at an input up to a predetermined polarization alignment at a selection polarizer, each of the birefringent elements having a fast axis and a slow axis at a right angle to one another, wherein each of the birefringent elements comprises an electrically tunable liquid crystal abutted with a fixed birefringent retarder, the liquid crystal having fast and slow axes aligned parallel to respective fast and slow axes of the fixed birefringent retarder, thereby defining a total thickness and birefringence, and the tunable liquid crystal retarders of each of said birefringent elements are coupled to a control voltage to be controlled in unison for equal retardation, so as to discriminate for at least one discrimination wavelength; wherein a first of the birefringent elements along the signal propagation path is arranged with one of the respective fast and slow axes thereof at a rotational angle less than 45° to said polarization alignment at the input, whereby a light component oriented at the polarization alignment is divided unequally into said fast and slow axes; wherein at least one subsequent one of the birefringent elements is rotationally displaced relative to the first of the birefringent elements; wherein the plurality of birefringent elements occupies a series of rotation angles up to the selection polarizer; wherein the selection polarizer passes a polarization component parallel to the predetermined polarization alignment of the selection polarizer and blocks a polarization component orthogonal thereto; and, wherein the discrimination wavelength is tunably selectable over a range extending to infrared wavelengths.