Polarization insensitive coherent lightwave detector

Claim 1

A coherent lightwave demodulator which is capable of recovering a message signal from a received lightwave signal (E R (t)), said demodulator comprising coupling means responsive to both said transmitted lightwave signal and a locally generated lightwave oscillating signal of a known polarization state (E L (t)). said coupling means generating as separate outputs a first signal (Ei(t)) and a second signal (E z (t)) comprising portions of both said transmitted message signal and said locally generated lightwave oscillating signal; CHARACTERIZED IN THAT the demodulator is capable of recovering said message signal without determining the polarization state ø of the received lightwave signal, said demodulator further comprising a first polarization-selective beam splitter (12) aligned with said known polarization state of said local oscillating signal for receiving as an input the first signal generated by said coupling means, said first beam splitter for evenly dividing the power of said locally generated lightwave oscillating signal and providing as separate outputs a first horizontally polarized lightwave signal (E 1H (t) and a first vertically polarized lightwave signal (E 1 v (t)); a second polarization-selective beam splitter (14) aligned with said known polarization state of said local oscillating signal for receiving as an input the second signal generated by said coupling means, said second beam splitter for evenly dividing the power of said locally generated lightwave oscillating signal and providing as separate outputs a second horizontally polarized lightwave signal (E 2H (t)) and a second vertically polarized lightwave signal (E 2 v(t)); a first balanced optical receiver responsive (16) to said first and said second vertically polarized lightwave signals for providing as an output a first photocurrent (i v (t)) related to the difference between said first and said second vertically polarized lightwave signals; a second balanced optical receiver (24) responsive to said first and second second horizontally polarized lightwave signals for providing as an output a second photocurrent (i h (t)) related to the difference between said first and second second vertically polarized lightwave signals; first squaring means (32) responsive to said first photocurrent output from said first balanced receiver for generating a squared representation (D v (t)) of said first photocurrent; second squaring means (34)) responsive to said second photocurrent output from said second balanced receiver for generating a squared representation (D h (t)) of said second photocurrent; and summing means (36) responsive to both said first and said second squaring means for adding together said squared representations of said first and said second photocurrents to provide as an output the recovered message signal (D(t)).