Patent ID: 7391978
Filing Date: 2008-06-24
Classification: H04B,H04J

Abstract:
1. An optical signal transmitter for applying frequency modulation to amplitude-modulated electric signals that have undergone frequency division multiplexing to optically transmit the electric signals, the optical signal transmitter comprising: a distribution circuit for distributing the electric signals into a plurality of signal parts and outputting the signal parts; a plurality of frequency modulation means for applying frequency modulation to each output of the distribution circuit and emitting each output, the plurality of frequency modulation means being substantially equal to each other in frequency deviation and in intermediate frequency and being substantially identical in phase of each output; a multiplexing means for multiplexing outputs of the plurality of frequency modulation means and outputting multiplexed outputs; and a transmitting circuit for outputting optical signals subjected to intensity modulation by the output of the multiplexing means to an optical transmission path, wherein the distribution circuit distributes the electrical signals into two signals (N=2) and outputs these signals, the frequency modulation means includes: a first optical frequency modulation portion for outputting a first frequency-modulated optical signal applied frequency modulation to one of the two electric signals input from the distribution circuit as a modulated input; a first optical frequency local oscillation portion for outputting a first optical local oscillation signal having an optical frequency apart from an optical center frequency of the first frequency-modulated optical signal output from the first optical frequency modulation portion by a frequency substantially equal to an intermediate frequency; a first optical multiplexer for multiplexing the first frequency-modulated optical signal and the first optical local oscillation signal so as to become identical in polarization direction and outputting a first multiplexed optical signal; a second optical frequency modulation portion for outputting a second frequency-modulated optical signal applied frequency modulation to the other one of the two electric signals input from the distribution circuit as a modulated input; a second optical multiplexer for multiplexing the second frequency-modulated optical signal and the second optical local oscillation signal together so as to become identical in polarization direction and outputting a second multiplexed optical signal, the multiplexing means is a third optical multiplexer for multiplexing the first multiplexed optical signal output from the first optical multiplexer and the second multiplexed optical signal output from the second optical multiplexer so that a polarization direction of the first multiplexed optical signal becomes perpendicular to a polarization direction of the second multiplexed optical signal and outputting a third multiplexed optical signal, the optical signal transmitter further comprising: an optical detector for applying heterodyne detection to the third multiplexed optical signal output from the third optical multiplexer and outputting an electric signal having a frequency equal to a difference between an optical frequency of the first frequency-modulated optical signal and an optical frequency of the first optical local oscillation signal and an electric signal having a frequency equal to a difference between an optical frequency of the second frequency-modulated optical signal and an optical frequency of the second optical local oscillation signal; and a transmitting circuit for outputting an optical signal subjected to intensity modulation by an output of the optical detector to an optical transmission path; wherein the first optical frequency modulation portion and the second optical frequency modulation portion are set to be substantially equal to each other in frequency deviation, wherein a phase of an electric signal obtained by subjecting the first multiplexed optical signal to heterodyne detection in the optical detector is set to be substantially identical to a phase of an electric signal obtained by subjecting the second multiplexed optical signal to heterodyne detection in the optical detector.