Patent ID: 8170422

Claim:
A frequency shift keying (FSK) communication system comprising: an optical FSK modulator; an FSK demodulator comprising: a means for branching an optical signal according to wavelengths thereof; a means for adjusting a delay time of two lights branched by the branching means; a means for detecting one optical signal branched by the branching means; a means for detecting a remaining optical signal branched by the branching means; and a means for calculating a difference between an output signal of the first photodetector and an output signal of the second photodetector, wherein the optical signal in the means for branching an optical signal is optical FSK signal and the means for branching an optical signal is a means for branching into an upper side band (USB) signal and a lower side band (LSB) signal of an optical FSK signal, and wherein the means for adjusting a delay time is capable of changing the adjusted delay time; a dispersive medium connecting the FSK modulator and the FSK demodulator; wherein the optical FSK modulator comprises: a first sub Mach-Zehnder waveguide (MZ A ); a second sub Mach-Zehnder waveguide (MZ B ); a main Mach-Zehnder waveguide (MZ C ) comprising the MZ A , the MZ B , an input portion for a light and an output portion for a modulated light; a first RF electrode (RF A electrode) inputting a radio frequency (RF) signal to two arms composing the MZ A ; a second RF electrode (RF B electrode) inputting an RF signal to two arms composing the MZ B ; and a third RF electrode (RF C electrode) comprising a traveling-wave-type electrode controlling a frequency of the light outputted from the output portion by controlling a frequency of the RF signal inputted; and modulating the frequency of the light outputted from the output portion by controlling a frequency of a modulating signal inputted to the RF C electrode that is the traveling-wave-type electrode corresponding to the RF signal.