High-speed dispersion compensation control apparatus
An optical signal, which is a low-speed signal superimposed on a high-speed phase modulated optical signal by intensity modulation, is used. In an optical receiver apparatus 40 , a received signal is split, and one of the split signals is O/E converted and low frequency component alone is extracted via a filter 46 . A clock is extracted from low-frequency component by CDR, and is compared with a preset frequency. Using the frequency difference obtained, dispersion compensation is performed with low accuracy. Next, the amount of phase control of the delay interferometer 21 is adjusted so that the amplitude of the electrical signal is maximized. An error rate is measured, and fine adjustment is performed to improve the error rate.
1. A dispersion compensation control apparatus for receiving an optical signal with a predetermined frequency, which is phase-modulated light applied with intensity modulation slower in the modulation speed than the phase modulation with a predetermined frequency, comprising:
a variable dispersion compensation unit, which can vary an amount of dispersion compensation, to perform dispersion compensation of the optical signal;
a clock recovery unit to extract an intensity modulation component from the received optical signal and to recover a clock from the extracted intensity modulation component;
a comparator to compare a frequency of the recovered clock with the predetermined frequency;
a demodulator to demodulate the phase modulated optical signal;
an error rate calculation unit to calculate an error rate of a demodulation result; and
a dispersion compensation amount controller, based on the comparison result, to control an amount of dispersion compensation of the variable dispersion compensation unit so that the difference in the frequency of the clock becomes small, and, after completion of reducing the difference in the frequency of the clock, to control an amount of dispersion compensation of the variable dispersion compensation unit so that the error rate becomes small.
2. The dispersion compensation control apparatus according to claim 1 , wherein
the demodulator unit comprises:
an optical interferometer to demodulate the phase modulation;
an O/E converter unit to convert an output of the optical interferometer into an electrical signal; and
a phase control unit to control a phase shift that the optical interferometer provides to the optical signal so that amplitude of the O/E converted signal is maximized.
3. The dispersion compensation control apparatus according to claim 1 wherein
an adjustment of the amount of dispersion compensation for reducing the difference from the frequency of the clock is rougher than an adjustment of the amount of dispersion compensation for reducing the error rate.
4. The dispersion compensation control apparatus according to claim 1 , wherein
the phase modulation is differential 2n-ary phase modulation where n is a natural number.
5. The dispersion compensation control apparatus according to claim 4 , wherein the optical interferometer is a delay interferometer comprising:
a splitter to split an input optical signal into two;
a delay unit to delay one of the split optical signals for 1 bit;
a phase shifter to provide a phase shift to another one of the split optical signals; and
an interference unit to interfere the output of the delay unit and the output of the phase shifter.
6. The dispersion compensation control apparatus according to claim 1 , wherein
the comparator comprises a phase locked loop to perform comparison.
7. The dispersion compensation control apparatus according to claim 2 , wherein
the dispersion compensation performed so as to reduce the difference in the clock frequencies is performed first, phase control is performed next, and the dispersion compensation performed so as to reduce the error rate is performed last.
8. The dispersion compensation control apparatus according to claim 2 , wherein
the dispersion compensation performed so as to reduce the difference in the clock frequencies and the phase control are performed simultaneously, and the dispersion compensation performed so as to reduce the error rate is performed last.
9. The dispersion compensation control apparatus according to claim 1 , wherein
the dispersion compensation control apparatus operates during the operation of a communication system.
10. The dispersion compensation control apparatus according to claim 1 , wherein
the intensity-modulated signal is a sine-wave signal.
11. The dispersion compensation control apparatus according to claim 1 , wherein
a control is performed so that difference between the recovered clock frequency and the predetermined intensity-modulated frequency becomes 0.