| 摘要 |
An interferometer comprises a delay element and a phase shift element. The delay element delays an optical DQPSK signal by one-symbol time. The phase shift element shifts the optical DQPSK signal by &pgr;/8. A pair of photodiodes converts each of a pair of optical signals output from the interferometer into an electric signal. A photodetector circuit converts differential current obtained by a pair of the photodiodes into voltage and outputs as a detection signal. A first decision circuit outputs one-bit information based on the voltage of the detection signal. A second decision circuit outputs one-bit information based on a squared value of the voltage of the detection signal.
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