Pre-distortion techniques for optical modulators to improve constellation point separation

A method comprising:

• receiving as input, data to be transmitted from a transmitter to a receiver over an optical fiber in a coherent optical communications system;
• converting the data to sequences of one of at least four levels according to a multilevel quadrature amplitude modulation (QAM) scheme, the levels including two outer levels and two or more inner levels, wherein converting the data to sequences of four or more levels comprises generating sequences of in-phase (I) modulation levels and sequences of quadrature-phase (Q) modulation levels;
• adjusting the Q modulation levels to compensate for a non-ideal phase response of a first optical modulator based on the I modulation levels;
• adjusting the I modulation levels to compensate for a non-ideal phase response of a second optical modulator based on the Q modulation levels;
• and
• driving the first optical modulator with adjusted I modulation levels and driving the second optical modulator with adjusted Q modulation levels.

An apparatus comprising:

• a modulation mapper circuit configured to convert transmit data to sequences of one of four or more levels according to a multilevel quadrature amplitude modulation (QAM) scheme, the four or more levels including two outer levels and two or more inner levels;
• a distortion circuit configured to distort only the two outer levels to reduce amplitudes of the two outer levels;
• a modulator driver circuit configured to generate a modulator drive waveform from the output of the distortion circuit and the modulation mapper circuit;
• and
• an optical modulator configured to receive as input the modulator drive waveform and to modulate a light beam for transmission from a transmitter to a receiver over an optical fiber in a coherent optical communications system, so that, as a result of the distorting only the two outer levels, eye openings of a received optical field at the receiver are equalized.

An apparatus comprising:

• a modulation mapper circuit configured to convert transmit data to sequences of one of four or more levels according to a multilevel quadrature amplitude modulation (QAM) scheme, the four or more levels including two outer levels and two or more inner levels, wherein the modulator mapper circuit is configured to generate sequences of in-phase (I) modulation levels and sequences of quadrature-phase (Q) modulation levels;
• a first optical modulator and a second optical modulator;
• a compensation circuit configured to adjust the Q modulation levels to compensate for a non-ideal phase response of the first optical modulator based on the I modulation levels to produce adjusted Q modulation levels and to adjust the I modulation levels to compensate for a non-ideal phase response of the second optical modulator based on the Q modulation levels to produce adjusted I modulation levels;
• and
• wherein the first optical modulator is driven on the basis of the adjusted I modulation levels and the second optical modulator is driven on the basis of the adjusted Q modulation levels.

One or more tangible computer readable storage media encoded with instructions that, when executed by a processor, cause the processor to:

• receive as input, data to be transmitted over an optical fiber from a transmitter to a receiver in a coherent optical communications system;
• convert the data to sequences of one of at least four levels according to a multilevel quadrature amplitude modulation (QAM) scheme, the levels including two outer levels and two or more inner levels;
• distort only the two outer levels to reduce amplitudes of the two outer levels;
• and
• supply the at least four levels, after distorting the two outer levels, to drive an optical modulator so that, as a result of the distorting of the at least two outer levels, eye openings of a received optical field at the receiver are equalized.

A tangible computer readable storage media encoded with instructions that, when executed by a processor, cause the processor to:

• obtain as input, data to be transmitted over an optical fiber from a transmitter to a receiver in a coherent optical communications system;
• convert the data to sequences of one of at least four levels according to a multilevel quadrature amplitude modulation (QAM) scheme, the levels including two outer levels and two or more inner levels, the sequences including sequences of in-phase (I) modulation levels and sequences of quadrature-phase (Q) modulation levels;
• adjust the Q modulation levels to compensate for a non-ideal phase response of a first optical modulator based on the I modulation levels;
• adjust the I modulation levels to compensate for a non-ideal phase response of a second optical modulator based on the Q modulation levels;
• and
• drive the first optical modulator on the basis of the adjusted I modulation levels and drive the second optical modulator on the basis of the adjusted Q modulation levels.