Method and apparatus for transmitter IQ skew and insertion loss detection for coherent optical systems

A coherent optical transmitter comprising:

• a processor configured to receive a digital data signal carrying digitized user data, the digital data signal including an in-phase (I) component and a quadrature (Q) component, and convert the digital data signal into an analog I signal and an analog Q signal;
• an optical transmitter frontend comprising:
• a first dither signal generator configured to generate a first dither signal,
• a second dither signal generator configured to generate a second dither signal,
• a first combiner configured to combine the first dither signal with the analog I signal to generate an I combined signal,
• a second combiner configured to combine the second dither signal with the analog Q signal to generate a Q combined signal, and
• an optical modulator configured to generate an optical signal corresponding to the I and Q combined signals for transmission over an optical fiber;
• and
• a dither tone processor configured to calculate, based on the optical signal, at least one of an IQ skew and an insertion loss associated with the coherent optical transmitter and provide the at least one of the IQ skew and the insertion loss to the processor to correct for an impairment associated with the at least one of IQ skew and the insertion loss,
• wherein the dither tone processor is further configured to:
• measure a first optical power of the optical signal as a function of frequency, phase and difference in time delay between an I path and a Q path in the coherent optical transmitter, wherein the optical signal is generated based on the first and second dither signals;
• measure a second optical power of a second optical signal as a function of frequency, phase and difference in time delay between the I path and the Q path in the coherent optical transmitter, wherein the second optical signal is generated based on third and fourth dither signals;
• calculate an amplitude difference between the first optical power and the second optical power;
• and
• calculate the IQ skew based on a primary frequency and phases of the primary frequency in the I and Q paths that result in the amplitude difference between the first and second optical powers being zero.

A method performed by a coherent optical transmitter, the method comprising:

• receiving a digital data signal carrying digitized user data, the digital data signal including an in-phase (I) component and a quadrature (Q) component;
• generating a first dither signal;
• generating a second dither signal;
• combining the first dither signal with the I component of the digital data signal to generate an I combined signal;
• combining the second dither signal with the Q component of the digital data signal to generate a Q combined signal;
• converting the I combined signal into an I analog waveform;
• converting the Q combined signal into a Q analog waveform;
• generating an optical signal corresponding to the I and Q analog waveforms for transmission over an optical fiber;
• and
• calculating, based on the optical signal, at least one of an IQ skew and an insertion loss associated with the coherent optical transmitter to correct for an impairment associated with the at least one of IQ skew and the insertion loss,
• measuring a first optical power of the optical signal as a function of frequency, phase and difference in time delay between an I path and a Q path in the coherent optical transmitter, wherein the optical signal is generated based on the first and second dither signals;
• measuring a second optical power of a second optical signal as a function of frequency, phase and difference in time delay between the I path and the Q path in the coherent optical transmitter, wherein the second optical signal is generated based on third and fourth dither signals;
• calculating an amplitude difference between the first optical power and the second optical power;
• and
• calculating the IQ skew based on a primary frequency and phases of the primary frequency in the I and Q paths that result in the amplitude difference between the first and second optical powers being zero.