Patent ID: 8184355

Claim:
A light modulation device, comprising: a light modulator including a phase modulator which is provided for each of a plurality of input lights and which performs phase modulation of the input light, and a phase shifter which is provided at the upstream or downstream of at least one of the phase modulators and which shifts a phase of the input light or a phase of the phase-modulated light; and a light interference section to cause a plurality of output lights outputted from the light modulator to interfere with each other, wherein the light interference section causes the output lights to interfere with each other to generate a multilevel phase modulated signal and a monitoring interference light having a phase state different from that of the multilevel phase modulated signal, wherein the first to the m-th phase modulators (m is an integer equal to or more than 2) are provided, wherein the light interference section includes: an n-th output interference section to cause output lights from an n-th phase modulator (n is an integer of 1 to m) to interfere with each other and to output an n-th modulated light and an n-th non-phase modulated light; a multilevel phase modulated signal generating section to multiplex the first to the n-th modulated lights to generate the multilevel phase modulated signal; and a monitoring interference light generating section to cause the first to the n-th non-phase modulated lights to interfere with each other to generate the monitoring interference light having a phase state different from that of the multilevel phase modulated signal, and wherein the n-th output interference section (n is an integer of 1 to m) includes an n-th 2×2 optical coupler having two input ports and two output ports, in which two emitting waveguides of the n-th phase modulator and the two input ports are respectively connected to each other, and the n-th modulated light is outputted from the one output port as well as the n-th non-phase modulated light is outputted from the other output port.