Multiple-FSR DPSK demodulator

Abstract

A switchable free-spectral-range mode selector is used to change the free spectral range of a free-space delay-line interferometer. The mode selector consists of a rotatable device with at least one transparent plate selected to produce the desired change in the free spectral range of the delay-line interferometer. The device may be rotated in and out of the free-space optical path of on of the interferometer arms. If used as a DPSK demodulator, the device enables operation at multiple predetermined free spectral ranges. In the preferred embodiment, the demodulator includes a 50/50 beam-splitter cube combined with two cavities. The mode selector consists of a plurality of different transparent slabs attached to a rotatable shaft so that any one of the slabs or none may be inserted in the appropriate optical path to obtain the desired FSR mode of operation.

Dependencies are inferred from the claim text.