System and method for controlling a difference in optical phase and an optical signal transmitter

Abstract

An optical phase difference control system is provided for controlling a phase difference of a single optical time-division multiplexed signal obtained by multiplexing a plurality of modulated optical signals encoded. The control system includes an interferometer and a low-frequency extractor. The interferometer is used for receiving part of the optical time-division multiplexed signal to split it into first and second signals, giving, between the first and second signals, a phase difference equivalent to one bit of the optical time-division multiplexed signal, and thereafter multiplexing the first and second signals. The low-frequency extractor is used for adding together signals, output from the interferometer, which have the similar intensity every two successive bits, and extracting a low-frequency waveform signal as a signal for controlling the phase difference of the single optical time-division multiplexed signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram showing a preferred embodiment of an optical signal transmitter according to the present invention;

FIG. 2 shows the relative phase of interference light-wave in a conventional optical phase difference control method;

FIG. 3 plots intensity changes of interference light-wave by the conventional phase difference control method;

FIG. 4 shows the phase differences and the eye pattern of an interference signal in a first interferometer; and

FIG. 5 plots 40-GHz components calculated and contained in the interference signal.

Claims

1. An optical phase difference control system for controlling a phase difference of a single optical time-division multiplexed signal obtained by multiplexing a plurality of modulated optical signals encoded, comprising: an interferometer for receiving part of the optical time-division multiplexed signal to split it into first and second signals, and giving, between the first and second signals, a phase difference equivalent to one bit of the optical time-division multiplexed signal, and thereafter multiplexing the first and second signals; anda low-frequency extractor for adding together signals, output from said interferometer, which have similar intensity every two successive bits, and extracting a low-frequency waveform signal as a signal for controlling the phase difference of the single optical time-division multiplexed signal.

2. An optical phase difference control method of controlling a phase difference of a single optical time-division multiplexed signal obtained by multiplexing a plurality of modulated optical signals encoded, comprising: an interference step of receiving part of the optical time-division multiplexed signal to split it into first and second signals, and multiplexing the first and second signals after giving, between the first and second signals, a phase difference equivalent to one bit of the optical time-division multiplexed signal; anda low-frequency extracting step of adding together signals, output in said interference step, which have similar intensity every two successive bits, and extracting a low-frequency waveform signal as a signal for controlling the phase difference of the single optical time-division multiplexed signal.

3. An optical signal transmitter comprising: a plurality of modulators for receiving a plurality of optical short-pulse trains and encoding the plurality of optical short-pulse trains to output a plurality of modulated optical signals;an interferometer for receiving part of a single optical time-division multiplexed signal, obtained by multiplexing the plurality of modulated optical signals encoded by said modulators, to split it into first and second signals, and giving, between the first and second signals, a phase difference equivalent to one bit of the optical time-division multiplexed signal, and thereafter multiplexing the first and second signals;a low-frequency extractor for adding together signals, output from said interferometer, which have similar intensity every two successive bits, and extracting a low-frequency waveform signal as a signal for controlling the phase difference of the single optical time-division multiplexed signal; anda controller connected to said modulators and said interferometer for adjusting said modulators so that the signal extracted by said low-frequency extractor has a maximum value.

4. An optical signal transmitter comprising: a plurality of modulators for receiving a plurality of optical short-pulse trains and encoding the plurality of optical short-pulse trains to output a plurality of modulated optical signals;a first interferometer for receiving part of a single optical time-division multiplexed transmission signal, obtained by multiplexing the plurality of modulated optical signals encoded by said modulators, to split it into first and second signals, and giving, between the first and second signals, a phase difference equivalent to one bit of the optical time-division multiplexed signal, and thereafter multiplexing the first and second signals;a low-frequency extractor for adding together signals, output from the first interferometer, which have similar intensity every two successive bits, and extracting a low-frequency waveform signal as a signal for controlling the phase difference of the single optical time-division multiplexed signal;a second interferometer for receiving one of two optical time-division multiplexed transmission signals being under multiplexing, obtained by multiplexing the plurality of modulated optical signals, to split it into third and fourth signals, and giving, between the third and fourth signals, a phase difference equivalent to one bit of the one optical time-division multiplexed signal, and thereafter multiplexing the third and fourth signals;a third interferometer for receiving another of the two optical time-division multiplexed transmission signals to split it into fifth and sixth signals, and giving, between the fifth and sixth signals, a phase difference equivalent to one bit of the other optical time-division multiplexed signal, and thereafter multiplexing the fifth and sixth signals; anda controller connected to said modulators and said first, second and third interferometers for receiving the optical time-division multiplexed signals from said second and third interferometers, and adjusting said modulators so that an optical phase difference in the two optical time-division multiplexed signals is zero, and for adjusting said modulators so that the signal extracted by the low-frequency extractor has a maximum value.

5. The transmitter in accordance with claim 4, wherein said second and third interferometers are disposed between said modulators and a point at which the optical time-division multiplexed transmission signal is obtained.

6. The transmitter in accordance with claim 3, wherein each of said first, second and third interferometers has an optoelectronic circuit for converting optical intensity of the optical time-division multiplexed signal into corresponding electrical intensity.