Optical 16-QAM signal homodyne detection by extracting +/-π/4 and +/-3π/4-phase symbols

This paper demonstrates, for the first time, stable 16-Gbit/s 16-QAM signal homodyne detection. By extracting +/-π/4 and +/-3π/4-phase data symbols from 16-QAM optical signal, our decision-driven Costas loop for QPSK code achieved the local oscillator light phase-locking to the signal carrier wave modulated at 16-QAM. Homodyne phase-lock is confirmed in an experiment by observing eye opening of 4-level amplitude modulation signals in I- and Q-arm.