Stable Costas Loop Homodyne Detection for 20-Gbit/s QPSK Signal Fiber Transmission

An experiment confirms stable homodyne detection of a fiber transmitted 20-Gbit/s QPSK signal by a decision-directed Costas loop circuit. Our low phase noise PLL is shown to compensate external cavity laser diode FM response and achieve high sensitivity homodyne detection of the QPSK signal.