Bias Error Correction in an Optical Coherent Transponder

A system, comprising:

• circuitry to:
• receive optical signals transmitted in an optical modulation format by an optical coherent transponder under test;
• an optical power meter comprising circuitry to:
• perform a first plurality of measurements of optical power of positive harmonics of an optical signal transmitted by the optical coherent transponder, each measurement to be performed while a respective different amount of skew is introduced into the optical signal by a de-skewing filter of the optical coherent transponder;
• perform a second plurality of measurements of optical power of negative harmonics of the optical signal, each measurement to be performed while a respective different amount of skew is introduced into the optical signal by a de-skewing filter of the optical coherent transponder;
• a bias error corrector comprising circuitry to:
• detect, dependent on the first plurality of measurements and the second plurality of measurements, a bias error in a modulator in the optical coherent transponder that when correctly biased produces a constant phase offset of π/2 radians between in-phase and quadrature components of the optical signals transmitted by the optical coherent transponder, including circuitry to determine that a curve to represent the first plurality of measurements and a curve to represent the second plurality of measurements are not orthogonal.

A method for correcting a bias error in an optical coherent transponder, comprising:

• performing a first plurality of measurements of optical power of positive harmonics of an optical signal transmitted in an optical modulation format by the optical coherent transponder, each measurement being performed while a respective different amount of skew is introduced into the optical signal by a de-skewing filter of the optical coherent transponder;
• performing a second plurality of measurements of optical power of negative harmonics of the optical signal, each measurement being performed while a respective different amount of skew is introduced into the optical signal by a de-skewing filter of the optical coherent transponder;
• detecting, dependent on the first plurality of measurements and the second plurality of measurements, a bias error in a modulator in the optical coherent transponder that when correctly biased produces a constant phase offset of π/2 radians between in-phase and quadrature components of the optical signals transmitted by the optical coherent transponder;
• determining a bias voltage value for the modulator that corrects the bias error;
• and
• changing the bias voltage of the modulator to the determined bias voltage value.