Short-wavelength infrared (SWIR) multi-conjugate liquid crystal tunable filter

摘要

A SWIR hyperspectral imaging filter has serial stages along an optical signal path with angularly distributed birefringent retarders and polarizers. The retarders can include active retarders such as tunable liquid crystal birefringent elements, passive retarders such as fixed retarders, and/or combinations thereof. Distinctly different periodic transmission spectra are provided by different filter stages, each having multiple retarders, in particular with some stages having broad bandpass peaks at wide spectral spacing and other stages have very narrow closely spaced peaks. The respective spectra include at least one tunably selectable band at which the transmission spectra of the filter stages coincide, whereby the salutary narrow bandpass and wide spectral spacing ranges of different stages apply together, resulting in a high finesse wavelength filter suitable for spectral imaging. The filter may be configured to provide faster switching speed and increased angle of acceptance and may operate in the rage of approximately 850-1700 nm.

主权项

1. A spectral imaging filter, comprising:
at least two spectral filter stages coupled along an optical signal path, wherein each of the at least filter stages has a periodic transmission characteristic with bandpass peaks separated by free spectral bandpass gaps; wherein each of the filter stages comprises a set of birefringent retarders and at least one polarizer, and the birefringent retarders in the set of at least one of said stages is characterized by retardation that is different from a retardation of the birefringent retarders in at least one other of the stages, such that said one of the at least two filter stages has a greater free spectral range than the other of the two filter stages, between the bandpass peaks of said one of the filter stages, and said other of the two filter stages has narrower bandpass peaks than said one of the two filter stages; wherein the bandpass peaks of said at least two filter stages overlap in an operative state of the filter, whereby a transmission characteristic of the spectral imaging filter as a whole is characterized by said greater free spectral range and said narrower bandpass peaks; and wherein said set of birefringent retarders of each filter stage are rotationally distributed leading to an output polarizer, whereby the output polarizer for leading stages functions as an input polarizer for following stages, wherein a number of said stages, and a number and respective thickness of retarders within the stages, are chosen to provide a free spectral range (FSR) from about 850-1700 nm.