Reconfigurable crystal-based demodulator for both differential-phase-shift keying and differential-quadrature-phase-shift keying formats: principle, design and performance

A reconfigurable demodulator for both differential-phase-shift keying (DPSK) and differential-quadrature-phase-shift keying (DQPSK) formats is theoretically and experimentally presented. In this crystal-based demodulator, a rotatable quarter-wave plate is implemented to switch the operation of the device between DPSK and DQPSK modes. To mitigate polarisation-dependent-frequency shift (PDFS), the authors control uniformity of optical path length differences for the orthogonal polarisation states in the delay crystal, a liquid crystal phase-tuning retarder is used to improve the tuning time. The design and fabrication consideration of this device are also discussed. The packaged device has a low PDFS of within ±0.2±GHz over C-band, fast tuning time of 20.5±ms, error-free demodulation of optical transponder unit-3 DPSK and DQPSK signals can be achieved.