Patent ID: 8095018
Filing Date: 2012-01-10
Classification: G02F,H04B,H04L

Abstract:
1. A quaternary phase modulator, comprising: an optical branching filter that splits light from a first light source into two parts, and outputs the resulting lights; first and second phase modulators that modulate the phases of the two lights outputted from the branching filter, respectively, and output the resulting lights; a phase adjuster that shifts the phase of the light outputted from the first phase modulator, and outputs the resulting light; an optical multiplexer that multiplexes the outputted light of the phase adjuster and the outputted light of the second phase modulator, and outputs the resulting light; a second light source that outputs a backward light propagated in an opposite direction from that of the light outputted from the first light source; an optical coupler that introduces the backward light outputted from the second light source into an output end of the multiplexer; a first controller that performs feedback control of a bias voltage of a modulating electrical signal applied to the first and second phase modulators so that a light intensity monitor result of the backward light at an input end of the optical branching filter is a minimum value, or does not exceed a first predetermined value; and a second controller that performs feedback control of the voltage applied to the phase adjuster, so that the result of monitoring the outputted light of the optical multiplexer by a photodiode having a frequency response characteristic not exceeding the communication bit rate is a minimum value, or does not exceed a second predetermined value, wherein the quaternary phase modulator performs feedback control in the first controller after performing feedback control in the second controller.