Optical communication system and optical receiver

1. An optical communication system for transmitting an optical signal via an optical transmission line, comprising:

an optical signal generator configured to generate an optical signal that transmits a first data signal and a second data signal, in a transmitter;

an optical front-end configured to generate a digital signal that represents the optical signal, in a receiver;

a detector configured to detect a state of the optical signal by using the digital signal, and output state information that represents the state of the optical signal, in the receiver;

a state controller configured to control the digital signal on the basis of the state information in order to recover the first data signal, in the receiver; and

a data recovery configured to recover the second data signal on the basis of the state information, in the receiver, wherein

the optical signal generator modulates a carrier frequency of the optical signal in accordance with the second data signal, and

the data recovery recovers the second data signal by determining an amount of a fluctuation in the carrier frequency.

2. The optical communication system according to claim 1 , wherein

the optical front-end comprises a local oscillator for generating locally oscillated light intended to detect the optical signal,

the detector detects, as the state information, a frequency offset between the carrier frequency of the optical signal and a frequency of the locally oscillated light,

the state controller compensates for the frequency offset, and

the data recovery recovers the second data signal on the basis of a fluctuation in the frequency offset.

3. The optical communication system according to claim 2 , wherein

the transmitter comprises an encoder for performing differential encoding on the second data signal,

the optical signal generator controls the carrier frequency of the optical signal to be a first frequency or a second frequency in accordance with a value of each symbol of the encoded second data signal,

the receiver comprises a decoder for performing differential decoding on the state information that represents the frequency offset, and

the data recovery decides each symbol of the second data signal by comparing an output of the decoder with a threshold value.

4. The optical communication system according to claim 1 , wherein

the optical signal generator comprises a vector modulator for modulating continuous wave light by using a mixed signal of the first and the second data signals.

5. The optical communication system according to claim 1 , wherein

a symbol rate of the second data signal is lower than a symbol rate of the first data signal, and

the second data signal is transmitted by the optical signal by being superimposed on the first data signal.

6. An optical communication system for transmitting an optical signal via an optical transmission line, comprising:

an optical signal generator configured to generate an optical signal that transmits a first data signal and a second data signal, in a transmitter;

an optical front-end configured to generate a digital signal that represents the optical signal, in a receiver;

a detector configured to detect a state of the optical signal by using the digital signal, and output state information that represents the state of the optical signal, in the receiver;

a state controller configured to control the digital signal on the basis of the state information in order to recover the first data signal, in the receiver; and

a data recovery configured to recover the second data signal on the basis of the state information, in the receiver

wherein

the optical signal generator modulates a carrier phase of the optical signal in accordance with the second data signal, and

the data recovery recovers the second data signal on the basis of a fluctuation in the carrier phase.

7. The optical communication system according to claim 6 , wherein

the optical front-end comprises a local oscillator for generating locally oscillated light intended to detect the optical signal,

the detector detects, as the state information, a phase error between the carrier phase of the optical signal and a phase of the locally oscillated light,

the state controller compensates for the phase error, and

the data recovery recovers the second data signal on the basis of a fluctuation in the phase error.

8. An optical communication system for transmitting an optical signal via an optical transmission line, comprising:

an optical signal generator configured to generate an optical signal that transmits a first data signal and a second data signal, in a transmitter;

an optical front-end configured to generate a digital signal that represents the optical signal, in a receiver;

a detector configured to detect a state of the optical signal by using the digital signal, and output state information that represents the state of the optical signal, in the receiver;

a state controller configured to control the digital signal on the basis of the state information in order to recover the first data signal, in the receiver; and

a data recovery configured to recover the second data signal on the basis of the state information, in the receiver

wherein

the optical signal generator controls a polarization of the optical signal in accordance with the second data signal, and

the data recovery recovers the second data signal by determining an amount of a fluctuation in polarization state of the optical signal.

9. The optical communication system according to claim 8 , wherein

the receiver is a polarization diversity receiver that extracts first and second polarization components from the optical signal, and recovers the first data signal by using the first and the second polarization components,

the state controller is a polarization tracking circuit for splitting the optical signal into orthogonal polarized waves by using a digital filter,

the state information is a filter coefficient provided to the digital filter, and

the data recovery recovers the second data signal on the basis of a fluctuation in the filter coefficient.

10. An optical receiver for receiving an optical signal that transmits a first data signal and a second data signal, comprising:

an optical front-end configured to generate a digital signal that represents the optical signal;

a detector configured to detect a state of the optical signal by using the digital signal, and output state information that represents the state of the optical signal;

a state controller configured to control the digital signal on the basis of the state information in order to recover the first data signal; and

a data recovery configured to recover the second data signal on the basis of the state information, wherein

a carrier frequency of the optical signal is modulated in accordance with the second data signal, and

the data recovery recovers the second data signal by determining an amount of a fluctuation in the carrier frequency.

11. A digital signal processing circuit used in an optical receiver for receiving an optical signal that transmits a first data signal and a second data signal, comprising:

a detector configured to detect a state of the optical signal by using a digital signal that represents the optical signal, and output state information that represents the state of the optical signal;

a state controller configured to control the digital signal on the basis of the state information in order to recover the first data signal; and

a data recovery configured to recover the second data signal on the basis of the state information, wherein

a carrier frequency of the optical signal is modulated in accordance with the second data signal, and

the data recovery recovers the second data signal by determining an amount of a fluctuation in the carrier frequency.