To provide a technique accurately reproducing data from an optical signal modulated using a differential M phase shift modulation system.
A demodulator 1 has a heater 21 for applying phase shift to an optical signal obtained by branching an optical signal PS. A phase shift amount in the heater 212 is controlled by a control unit 50. The control unit 50 includes: a signal generator 52 for generating a low-frequency signal LFS1; an envelope detector 51 for detecting an envelope of an electric signal ES1; a manipulated variable determination section 53 for determining a control voltage VT1 for the heater 212 on the basis of a detection signal DS1 output from the envelope detector 51 and the low-frequency signal LFS1; and an adder 54 for outputting, as a control signal CS1, a signal obtained by adding the low-frequency signal LFS1 to the control voltage VT1. The heater 212 varies the phase shift amount in response to the control signal CS1.
GOTODA MITSUNOBU
SHIMAKURA YASUHISA
YOSHIARA KIICHI
Takahiro Arita
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