Patent ID: 7526210

Claim:
A method for demodulating a DQPSK optical signal comprising: splitting, the DQPSK signal into two optical signals; further splitting, each of the two optical signals into two sub-signals through the effect of a single, free-space beam splitter; directing, the two sub-signals split from one of the two optical signals along two, unequal length optical paths, wherein the path length difference between the unequal length optical paths is maintained to be substantially equal to the distance traversed by the optical signal in one bit period; shifting the phase of one or more of sub-signals by a predetermined fixed amount prior to interference through the effect of a single phase shifter interposed in at least one of the optical paths; generating, constructive and destructive interference signals from the two sub-signals through the effect of the free-space beam splitter; detecting, a signal resulting from the constructive interference of the two sub-signals after said sub-signals traverse the unequal length optical paths; detecting, a signal resulting from the destructive interference of the two sub-signals after said sub-signals traverse the unequal length optical paths; and determining, a tributary of the DQPSK signal from the detected constructive interference signal and the detected destructive interference signal; wherein the shifted phase difference between two-sub signals that originate from two different optical signals but share the same path is substantially equal to π/2.