Patent ID: 7603007

Claim:
A quadrature phase shift keying modulator comprising: a first modulator which performs phase-shift keying modulation of input light with a first driving signal that is based on first data; a second modulator which performs phase-shift keying modulation of input light with a second driving signal that is a driving signal based on second data and is different from the first driving signal in eye crossing percentage; a phase shifting unit which performs phase shift processing in such a manner that a second optical signal, which is phase-shift keying modulated with a first optical signal being phase-shift keying modulated by said first modulator, and a second optical signal, which is phase-shift keying modulated by said second modulator, form a phase difference of π/2 therebetween; and a combining unit which combines the first optical signal and the second optical signal, which are subjected to the phase shift processing performed by said phase shifting unit, and hereby outputs quadrature phase-shift keying modulated light, said quadrature phase shift keying modulator being constructed in such a manner that an average light output power of the quadrature phase-shift keying modulation light output from said combining unit changes in accordance with a phase difference between a component of the first optical signal and a component of the second optical signal after being combined by said combining unit by means of applying driving signals different in eye crossing percentage by said first modulator and said second modulator, respectively, said quadrature phase shift keying modulator further comprising: a power monitor which monitors an average light output power of the quadrature phase-shift keying modulation light output from said combining unit; and a phase shift controlling unit which performs feedback control of the phase shift amount in said phase shifting unit based on the average light output power monitored by said power monitor.