Patent ID: 7697846

Claim:
A method for changing a frequency in a radio optical fusion communication system including a base station and a remote antenna station, the base station being adapted to generate a modulated radio signal, to electro-optically convert the generated signal into an optical signal while the modulation mode is kept, and to transmit the converted signal to the remote antenna station over an optical fiber path, the remote antenna station being adapted to opto-electrically convert the received optical signal to extract the modulated radio signal and transmitting the signal through an antenna by radio, the base station including a first light source and a second light source for generating optical signals of different frequencies, an intermediate-frequency signal generating means for generating a modulating signal at an intermediate frequency band, a modulator for modulating the optical signal from the first light source into an unsuppressed-carrier single-sideband (SSB) or double-sideband (DSB) modulated optical signal using the intermediate-frequency signal, and an optical mixer for mixing the modulated optical signal with the optical signal from the second light source to obtain an optical transmission signal, the method comprising: controlling the frequency of at least one of the optical signals from the first and second light sources so that the difference in frequency between the optical signals is a desired frequency of the modulated radio signal, and the frequency channel of the modulated radio signal extracted by the remote antenna station is switched, shifting the frequency of the optical signal from at least one of the first and second light sources through an optical frequency shifter provided downstream of the light source, wherein the optical frequency shifter has optical waveguides including a main Mach-Zehnder integrated with two sub Mach-Zehnders, and driving the optical frequency shifter in accordance with a predetermined frequency oscillation signal for determination of the amount of frequency shift, and the frequency is shifted as much as the frequency of the oscillation signal by changing a voltage applied to the optical frequency shifter such that the optical waveguides have predetermined phase differences therebetween.