Patent ID: 7778553

Claim:
An optical transmitter comprising: a light source which generates continuous light, a first optical modulator to which is applied a drive signal corresponding to a data signal, for performing multivalued phase modulation with respect to the continuous light output from said light source, and a second optical modulator to which is applied a drive signal corresponding to a clock signal having a frequency corresponding to a bit rate of said data signal, for pulsing the optical signal having been subjected to the multivalued phase modulation in said first optical modulator, wherein said optical transmitter further comprises: an output monitor section which acquires an electrical spectrum by photoelectrically converting a part of the optical signal output from said second optical modulator, and extracts a low frequency component excluding a direct current component from said electrical spectrum by a low pass filter, to measure a power of said low frequency component; and a controller which controls a relative phase of respective drive signals applied to said first and second optical modulators, so that the power measured in said output monitor section becomes a minimum.