DIFFERENTIAL FOUR-PHASE SHIFT MODULATOR AND METHOD FOR CONTROLLING PHASE SHIFT AMOUNT THEREOF

摘要

A differential four-phase shift modulator wherein a low-cost photoelectric conversion element, the response rate of which is sufficiently lower than the bit rate, is used to achieve a feedback control of a phase shift amount such that two multiplexed differential phase shift modulated optical components, which exhibit differential four-phase shift modulated light, have appropriate phase difference therebetween. For this purpose, the inventive apparatus is composed such that drive signals, which have different duty ratios, are applied to a first modulator (2-1) and a second modulator (2-2), whereby the average optical output power of a differential four-phase shift modulated light outputted by a multiplexing part (16b) can vary in accordance with the phase difference between the first and second optical signal components as multiplexed by the multiplexing part (16b). The inventive apparatus comprises a power monitor (18) that monitors the average optical output power of the differential four-phase shift modulated light; and a phase shift control part (19) that performs, based on the average optical output power monitored by the power monitor (18), a feedback control of the phase shift amount in a phase shifting part (14).