Versatile endless optical polarization controller/tracker/demultiplexer

Following an initial discussion of control error signal generation, we present new developments and applications of automatic endless optical polarization control based on a commercial electrooptic LiNbO(3) polarization transformer: (i) Fast tracking and subsequent demultiplexing of DPSK/DQPSK/QAM polarization channels was hitherto limited to a fairly fixed optical input power. With APD photoreceivers used for residual interference detection, we demonstrate here an optical level tolerance of at least 7 dB, compared to only 3 dB for PIN photoreceivers. DPSK channel polarizations are tracked at up to 40 krad/s and higher speed on the Poincaré sphere. (ii) High-order optical modulation schemes require increased accuracy of the polarization controller in the demultiplexer. This is possible at the expense of a reduced tracking speed. We achieve a mean polarization extinction ratio of >40 dB or <0.02 rad error while tracking arbitrary endless polarization changes of up to 1000 rad/s. (iii) While electronic polarization tracking in coherent receivers is currently limited to a symbol rate of about 28 GBaud we show optical polarization tracking of a signal with 1 THz bandwidth.