Polarization-multiplexed optical transmission system, polarization-Multiplexed optical transmitter, and polarization-multiplexed optical receiver

A polarization-multiplexed optical transmitter comprising:

• two optical modulators and a polarization beam combiner, wherein
• the polarization-multiplexed optical transmitter modulates input light input to the two optical modulators using mutually different multilevel information signals being one of one-dimensional and two-dimensional to generate an optical multilevel signal;
• the polarization-multiplexed optical transmitter allows the polarization beam combiner to multiplex the optical multilevel signal into mutually orthogonal polarizations for transmission,
• the polarization-multiplexed optical transmitter supplies a specified frequency difference to a center frequency of an optical multilevel signal for the mutually orthogonal polarizations,
• center frequencies of each of the mutually orthogonal polarizations:
• i) are first matched, and ii) are subsequently shifted by different amounts with respect to the matching center frequencies, and
• the specified frequency difference is approximately 20 % or less of an entire bandwidth occupied by a transmitted optical signal.

A polarization-multiplexed optical receiver in a polarization-multiplexed optical transmission system that also includes a polarization-multiplexed optical transmitter, the receiver comprising:

• an adaptive polarization splitting circuit for adaptively separating two orthogonal polarization components from a polarization-multiplexed signal;
• and
• two sets of demodulation parts for demodulating two separated polarization components, wherein
• the polarization-multiplexed optical receiver detects a frequency difference component in two received polarization components and identifies or separates a multiplexed original orthogonal polarization component based on a detected frequency component, and
• the polarization-multiplexed optical transmitter:
• i)includes two optical modulators and a polarization beam combiner,
• ii) modulates input light input to the two optical modulators using mutually different multilevel information signals being one of one-dimensional and two-dimensional to generate an optical multilevel signal,
• iii) allows the polarization beam combiner to multiplex the optical multilevel signal into mutually orthogonal polarizations for transmission,
• iv) supplies a specified frequency difference to a center frequency of an optical multilevel signal for the mutually orthogonal polarizations,
• center frequencies of each of the mutually orthogonal polarization components:
• i) are first matched, and ii) are subsequently shifted by different amounts with respect to the matching center frequencies, and
• the specified frequency difference is approximately 20% or less of an entire bandwidth occupied by a transmitted optical signal.