Optical-wireless hybrid transmission system and optical-wireless hybrid transmission method

An optical-wireless hybrid transmission system in which a central office has an optical transmitter and an optical receiver, said optical transmitter transmits an optical carrier signal to a base station via an optical transmission line, said base station receives an RF signal (frequency:

• fRF) that is modulated according to transmit-data, optically modulates said optical carrier signal according to the received RF signal, and transmits a resulting modulated optical signal to said central office via an optical transmission line, and said optical receiver receives and demodulates the modulated optical signal and reproduces said transmit-data, the system characterized in:
• that said optical transmitter comprises:
• a first single-mode optical source which outputs a first single-mode optical signal (center frequency:
• fC1);
• a second single-mode optical source which outputs a second single-mode optical signal (center frequency:
• fC2);
• a third single-mode optical source which outputs a third single-mode optical signal (center frequency:
• fC3);
• and
• a polarization-coupling part which orthogonal-polarization-couples said second single-mode optical signal with said third single-mode optical signal while adjusting polarization directions and optical powers of two waves so as to make the polarization directions orthogonal and the optical powers identical, and outputs a resulting polarization-coupled optical signal, in which:
• the center frequencies fC1, fC2, and fC3 of said first, second, and third single-mode optical signals are controlled so as to satisfy
• |fC1−fC2|=fRF±fIF1
• |fC1−fC3|=fRF±fIF2
• where fRF is a frequency of said RF signal and fIF1 and fIF2 are prescribed intermediate frequencies;
• and
• said polarization-coupled optical signal is transmitted to said base station as said optical carrier signal and said first single-mode optical signal is output to said optical receiver;
• and
• that said optical receiver comprises:
• an optical coupler which couples the modulated optical signal transmitted from said base station with said first single-mode optical signal output from said optical transmitter;
• a photodetector which photodetects a coupled optical signal that is output from said optical coupler, and thereby outputs an electrical signal having the intermediate frequencies fIF1 and fIF2;
• an electrical demodulator which demodulates the electrical signal having the intermediate frequencies fIF1 and fIF2 and output from said photodetector;
• and
• a low-pass filter which filters an output signal of said electrical demodulator and thereby outputs said transmit-data.

An optical-wireless hybrid transmission method in which a central office has an optical transmitter and an optical receiver, said optical transmitter transmits an optical carrier signal to a base station via an optical transmission line, said base station receives an RF signal (frequency:

• fRF) that is modulated according to transmit-data, optically modulates said optical carrier signal according to the received RF signal, and transmits a resulting modulated optical signal to said central office via an optical transmission line, and said optical receiver receives and demodulates the modulated optical signal and reproduces said transmit-data, characterized by comprising the steps of:
• transmitting, by said optical transmitter, a first single-mode optical signal (center frequency:
• fC1) to said base station as said optical carrier signal;
• outputting, by said optical transmitter, a polarization-coupled optical signal from said optical transmitter to said optical receiver, the polarization coupled optical signal being obtained by orthogonal-polarization-coupling a second single-mode optical signal (center frequency:
• fC2) with a third single-mode optical signal (center frequency:
• fC3) so as to give two waves orthogonal polarization directions and a same optical power;
• controlling, by said optical transmitter, the center frequencies fC1, fC2, and fC3 of said first, second, and third single-mode optical signals so that they satisfy
• |fC1−fC2|=fRF±fIF1
• |fC1−fC3|=fRF±fIF2
• where fRF is a frequency of said RF signal and fIF1 and fIF2 are prescribed intermediate frequencies;
• coupling, by said optical receiver, the modulated optical signal transmitted from said base station with said polarization-coupled optical signal output from said optical transmitter;
• demodulating, by said optical receiver, an electrical signal having the intermediate frequencies fIF1 and fIF2 that is obtained by photodetecting a resulting coupled optical signal;
• and
• generating, by said optical receiver, said transmit-data by filtering a resulting output signal.

An optical-wireless hybrid transmission method in which a central office has an optical transmitter and an optical receiver, said optical transmitter transmits an optical carrier signal to a base station via an optical transmission line, said base station receives an RF signal (frequency:

• fRF) that is modulated according to transmit-data, optically modulates said optical carrier signal according to the received RF signal, and transmits a resulting modulated optical signal to said central office via an optical transmission line, and said optical receiver receives and demodulates the modulated optical signal and reproduces said transmit-data, characterized by comprising the steps of:
• outputting, by said optical transmitter, a first single-mode optical signal (center frequency:
• fC1) to said optical receiver;
• generating, by said optical transmitter, a polarization-coupled optical signal by orthogonal-polarization-coupling a second single-mode optical signal (center frequency:
• fC2) with a third single-mode optical signal (center frequency:
• fC3) so as to give two waves orthogonal polarization directions and a same optical power;
• transmitting, by said optical transmitter, the generated polarization-coupled optical signal to said base station as said optical carrier signal;
• and
• controlling, by said optical transmitter, the center frequencies fC1, fC2, and fC3 of said first, second, and third single-mode optical signals so that they satisfy
• |fC1−fC2|=fRF±fIF1
• |fC1−fC3|=fRF±fIF2
• where fRF is a frequency of said RF signal and fIF1 and fIF2 are prescribed intermediate frequencies;
• coupling, by said optical receiver, the modulated optical signal transmitted from said base station with said optical signal output from said optical transmitter;
• demodulating, by said optical receiver, an electrical signal having the intermediate frequencies fIF1 and fIF2 that is obtained by photodetecting a resulting coupled optical signal;
• and
• generating, by said optical receiver, said transmit-data by filtering a resulting output signal.