Method for controlling polarizations of signal lights in an optical

Abstract

Frequency division multiplex signal lights are propagated through an optical fiber, and polarizations of the lights are collectively controlled by a polarization controller. The polarization-controlled signal lights are received separately in receiving systems including optical heterodyne or homodyne receivers. In the collective polarization control, a step is selected from steps of controlling relative polarization states of local oscillation lights to coincide to each other, receiving a control signal from one of the receiving systems which receives a signal light of a frequency to be allocated in the center of a frequency band of the signal lights or the vicinity thereof, and dividing the frequency division multiplex signal lights into plural groups, thereby controlling polarizations collectively in each of the plural groups. One of the polarizations of the signal lights may be orthogonal to the others, and one of the polarizations of the local oscillation lights may be orthogonal to the others correspondingly.