Costas Loop Homodyne Detection for 20-Gb/s QPSK Signal on the Optical Frequency Synchronous Network

This paper demonstrates a decision-directed, stable and high-sensitivity Costas loop homodyne receiver for 20-Gb/s QPSK signals based on our concept of the “optical frequency synchronous network.” The receiver sensitivity is shown to approach the theoretical shot-noise limit (as degraded by spontaneous emission) and the symbol-error rate is stably measured to be around 10^-3 even though the optical signal was chirped by fiber chromatic dispersion and was delayed by 50% of the symbol period. This high-performance phase-locked loop can be realized by utilizing optical frequency stabilized (synchronized) several kilohertz spectral line-width external cavity laser diodes (E-LD´s), compensating the frequency modulation (FM) response of the E-LDs, and consisting of only differential mode logical circuits. The compensation of the E-LD´s FM response by the loop filter improves the receiver sensitivity by 3 dB.