Patent ID: 8306431

Claim:
An optical transmission system comprising: an optical transmitter that includes: a first and a second light source, a first phase modulator that modulates light from the first light source, a second phase modulator that modulates light from the second light source, and a polarized beam combiner that combines the light output from the first phase modulator and the light output from the second phase modulator into an optical signal and outputs the optical signal; and an optical receiver that includes: a local oscillator, a polarization beam splitter that splits, according to polarization, the optical signal transmitted from the optical transmitter, and a first and a second digital coherent receiver respectively corresponding to the first phase modulator and the second phase modulator, and including a frontend that mixes light from the oscillator and the optical signal split according to polarization by the polarization beam splitter to output an electrical signal of a real part and an imaginary part, an analog-digital converter that converts the electrical signal of the real part and the imaginary part output from the frontend to a digital signal, and a digital signal processor that performs phase estimation with respect to the digital signal and extracts a signal, wherein the first and the second digital coherent receivers respectively calculate a difference in frequency between the optical signal from the local oscillator and the light split according to polarization, the optical receiver, based on the differences in frequency calculated respectively by the first and the second digital coherent receivers, controls the frequency of the light from the local oscillator.