Patent ID: 7630022

Claim:
A multi-stage birefringent optical filter that produces an overall light output signal from an input light, said filter, comprising: an arrangement of optical elements, configured to form an initial multi-stage filter having at least two filter stages, including a plurality of polarizers, each having a polarization axis, with one of said polarizers serving as an input polarizer and another one of said polarizers serving as an output polarizer, having said polarization axis of the output polarizer in a given orientation, and at least one retarder element positioned between the input polarizer and the output polarizer such that the arrangement of optical elements cooperates to define an optical path along which at least said input light travels from an optical input to an initial optical output to exhibit an initial spectral filter response which produces an initial optical output that exhibits a desired transmission feature and an undesired transmission peak; an additional polarizer following said output polarizer, said additional polarizer having an additional polarization axis that is oriented at least approximately orthogonal to said given orientation; and an adjustable retarder, as a sole optical element, disposed directly between said output polarizer and said additional polarizer to cooperate with said output polarizer and said additional polarizer, as an add-on stage, to receive said initial optical output from said optical path and to produce therefrom an overall spectral filter response that at least generally eliminates said undesired transmission peak from said overall light output signal, while passing the desired transmission feature.