Optical DQPSK video system with heterodyne detection

An optical heterodyne subcarrier-multiplexed (SCM) quadrature-phase-shift keying (QPSK) system is described that employs differential encoding to transmit multiple video channels. With this differential encoding scheme, only a simple delay-and-multiply demodulator is necessary, considerably reducing the signal processing complexity. The demodulators reproduce the data in a parallel fashion so no baseband demultiplexing or four-level decision circuit is required. This approach also avoids the complications of implementing a carrier recovery circuit (CRC) and a phase-locked loop associated with coherent microwave demodulation schemes. In addition to bandwidth efficiency and simplicity, differential QPSK also provides very good receiver sensitivity. With heterodyne detection, a receiver sensitivity of -41 dBm is achieved.<>