Patent ID: 8885989

Claim:
An optical hybrid circuit comprising: a multimode interference coupler including a pair of input channels provided at positions symmetrical with each other with respect to a center position in a widthwise direction thereof, a pair of first output channels adjacent with each other to output a pair of first optical signals having an in-phase relationship with each other, and a pair of second output channels adjacent with each other to output a pair of second optical signals having an in-phase relationship with each other, the multimode interference coupler being adapted to convert multilevel modulation signal light into the pair of first optical signals having an in-phase relationship with each other and the pair of second optical signals having an in-phase relationship with each other; a first optical coupler coupled to one of the first output channels and the second output channels, having two channels on the input side thereof and two channels on the output side thereof, and having a branching ratio of 85:15 or 15:85, the first optical coupler being adapted to convert the first optical signals into a pair of third optical signals having a 45- or 135-degree phase relationship with each other; a second optical coupler coupled to the other of the first output channels and the second output channels, having two channels on the input side thereof and two channels on the output side thereof, and having a same branching ratio as the branching ratio of the first optical coupler, the second optical coupler being adapted to convert the second optical signals into a pair of fourth optical signals having a 135- or 45-degree phase relationship with each other; and at least one phase controlling region provided on at least one of one of the pair of first output channels and one of the pair of second output channels.