| 摘要 |
PROBLEM TO BE SOLVED: To synchronize an optical phase of local oscillation light for multi-level phase modulation signal light of 16-level or more, by using a COSTAS circuit for QPSK.SOLUTION: Between an optical receiver for inputting multi-level phase modulation signal light of 16-level or more, and local oscillation light to an optical 90 degree hybrid circuit, and converting the signal light of I component and Q component, outputted from the optical 90 degree hybrid circuit, respectively, into signals I, Q, and a COSTAS circuit for QPSK, a QPSK component extraction circuit for extracting a signal, corresponding to the code of QPSK having a phase of ±&pgr;/4 or ±3&pgr;/4, from the component extraction circuit signals I, Q and inputting to the COSTAS circuit for QPSK is provided. The COSTAS circuit for QPSK is configured to perform phase synchronization control of local oscillation light by using a signal corresponding to the code of the QPSK, out of the multi-level phase modulation signal light. |
