Patent ID: 8582977
Filing Date: 2013-11-12
Classification: H04B,H04J

Abstract:
1. An optical transmission system for transmitting a high-frequency signal having a frequency equal to or higher than 20 GHz through an optical fiber over a long distance, comprising: a two-lightwave generator for generating, from laser light that has been input, an optical signal having a first wavelength and an optical signal having a second wavelength, which differ from each other by a frequency of a first microwave signal being the high-frequency signal, by using the first microwave signal; a first optical coupler for distributing two optical signals generated by the two-lightwave generator; a polarization beam splitter for guiding one pair of the two optical signals distributed by the first optical coupler to the optical fiber; a second optical coupler for distributing the two optical signals transmitted through the optical fiber; a first photodetector for detecting a second microwave signal from one pair of the two optical signals distributed by the second optical coupler; an optical modulator for frequency-shifting another pair of the two optical signals distributed by the second optical coupler by a frequency of a third microwave signal; a Faraday reflector for reflecting the two optical signals frequency-shifted by the optical modulator by applying Faraday rotation of 90 degrees thereto; a third optical coupler for mixing the two optical signals that have been reflected by the Faraday reflector, frequency-shifted again by the optical modulator, transmitted by the second optical coupler and the optical fiber, and guided by the polarization beam splitter, with another pair of the two optical signals distributed by the first optical coupler; an optical demultiplexer for wavelength-dividing four optical signals mixed by the third optical coupler into the optical signal having the first wavelength and the optical signal having the second wavelength; a second photodetector for detecting a beat signal of the optical signal having the first wavelength that has been wavelength-divided by the optical demultiplexer; a third photodetector for detecting a beat signal of the optical signal having the second wavelength that has been wavelength-divided by the optical demultiplexer; and a phase difference detector for detecting a phase difference between the beat signal of the optical signal having the first wavelength and the beat signal of the optical signal having the second wavelength, which are detected by the second photodetector and the third photodetector, respectively, wherein the phase difference detector comprises: a first analog-digital converter for analog/digital-converting the beat signal of the optical signal having the first wavelength detected by the second photodetector; a second analog-digital converter for analog/digital-converting the beat signal of the optical signal having the second wavelength detected by the third photodetector; a Direct Digital synthesizer for generating a reference signal having a frequency obtained by shifting a frequency twice as high as the frequency of the third microwave signal by a predetermined frequency and a reference clock signal having the same frequency as the frequency of the reference signal; a first digital mixer for digitally frequency-converting an output signal from the first analog-digital converter by multiplying the output signal by the reference signal; a second digital mixer for digitally frequency-converting an output signal from the second analog-digital converter by multiplying the output signal by the reference signal; a first digital filter for removing an unnecessary wave from the output signal from the first digital mixer; a second digital filter for removing an unnecessary wave from the output signal from the second digital mixer; and a phase difference detection circuit for performing a correlation integration processing on a first output signal and a second output signal from the first digital filter and the second digital filter, respectively, by using the reference clock signal and an orthogonal wave detection method, calculating respective phases of the first output signal and the second output signal, and calculating and outputting a phase difference between the first output signal and the second output signal, and wherein the phase difference output from the phase difference detection circuit and the second microwave signal output from the first photodetector are output to the outside, so that a signal processing including an integration processing is performed by using the second microwave signal, and the processing time of the integration processing is three seconds or shorter, and an off-line correction is performed, by using the phase difference, on the signal obtained by the signal processing.