Optical multilevel signal pre-equalization circuit, optical multilevel signal pre-equalization transmitter, and polarization-multiplexed pre-equalization transmitter

An optical multilevel pre-equalization transmitter comprising:

• a middle code conversion circuit that converts an input one of a plurality of first middle codes based on symmetry of a constellation of multilevel signals of a multilevel code into one of a plurality of second middle codes from which symmetry is eliminated, the input one of the first middle codes corresponding to one of the multilevel signals which is to be transmitted;
• a look-up table in which a plurality of waveform response components are stored in association with the plurality of second middle codes;
• a symmetry correction operation circuit which corrects the waveform response components from the look-up table which are associated with the converted one of the second middle codes based on the input one of the first middle codes, and outputs corrected waveform response components;
• one or more operation circuits to which the corrected waveform response components are input from the symmetry correction operation circuit, and which output a pre-equalized output waveform corresponding to the one of the multilevel signals to be transmitted;
• and
• an optical field modulator that outputs the pre-equalized waveform outputted from the optical multilevel signal pre-equalization circuit after conversion into a complex optical field,
• wherein the first middle codes and the second middle codes are bit strings.

A polarization multiplexed optical pre-equalization transmitter comprising:

• a first optical multilevel pre-equalization transmitter and a second optical multilevel pre-equalization transmitter, each including:
• a middle code conversion circuit that converts an input one of a plurality of first middle codes based on symmetry of a constellation of multilevel signals of a multilevel code into one of a plurality of second middle codes from which symmetry is eliminated, the input one of the first middle codes corresponding to one of the multilevel signals which is to be transmitted,
• a look-up table in which a plurality of waveform response components are stored in association with the plurality of second middle codes,
• a symmetry correction operation circuit which corrects the waveform response components from the look-up table which are associated with the converted one of the second middle codes based on the input one of the first middle codes, and outputs corrected waveform response components,
• one or more operation circuits to which the corrected waveform response components are input from the symmetry correction operation circuit, and which output a pre-equalized output waveform corresponding to the one of the multilevel signals to be transmitted, and
• a polarization multiplexer that outputs optical signals outputted from the first and second optical multilevel pre-equalization transmitters after polarization multiplexing or polarization encoding,
• wherein the first middle codes and the second middle codes are bit strings.