Transmitter self-calibration of time delay and frequency response for high baud-rate QAM transceivers

An apparatus comprising:

• an optical modulator having an in-phase path and a quadrature phase path;
• a photodetector coupled to an output of the optical modulator;
• a measurement unit coupled to the photodetector and including a first mixer and a second mixer;
• wherein during a calibration procedure, the optical modulator is configured to:
• set phase bias so that the in-phase path and the quadrature path are in phase;
• apply a first stimulus signal at a first frequency to one of the in-phase path or the quadrature path, and a second stimulus signal at a second frequency to another of the in-phase path or the quadrature path of the optical modulator, the second frequency being equal to a sum of the first frequency and a relatively small frequency difference;
• detect, with the photodetector, the output of optical modulator at a plurality of frequency steps over which the first frequency is swept during the calibration procedure;
• mix, with the first mixer, an output of the photodetector with a first reference signal at a reference frequency that is equal to the relatively small frequency difference, to produce a first mixing output;
• mix, with the second mixer, the output of the photodetector with a second reference signal at the reference frequency, to produce a second mixing output, wherein the first reference signal and the second reference signal are orthogonal in phase to each other;
• and
• at each frequency step over the frequency range, with the measurement unit, derive a first measurement value from the first mixing output and a second measurement value from the second mixing output.

A non-transitory processor readable medium storing instructions that, when executed by a processor, cause the processor to perform operations as part of a calibration procedure of an optical transmitter, the operations including:

• causing phase bias of an optical modulator of the optical transmitter to be set so that an in-phase path and a quadrature path of the optical modulator are in phase;
• supplying a first stimulus signal at a first frequency to one of the in-phase path or the quadrature path of the optical modulator, and a second stimulus signal at a second frequency to another of the in-phase path or the quadrature path of the optical modulator, the second frequency being equal to a sum of the first frequency and a relatively small frequency difference;
• causing the first frequency to be swept over a frequency range at a plurality of frequency steps;
• obtaining from the optical modulator a first measurement value and a second measurement value for each of the plurality of frequency steps, the first measurement value and second measurement value being derived by the optical modulator from an optical output of the optical modulator at the plurality of frequency steps over the frequency range;
• and
• computing at least one of a frequency response and a time delay between the in-phase path and the quadrature path, based on the first measurement value and the second measurement value obtained at each frequency step over the frequency range.