Sample timing control in an optical receiver
View Patent ↗A method of compensating phase error between parallel digital signals of a received optical signal, in a coherent optical receiver. A phase differential between the digital signals is determined. A digital skew is imposed between the digital signals, an amount of skew imposed being an integer number of sample periods calculated to most nearly compensate the phase differential. The skewed digital signals are then digitally processed, in the frequency domain, to compensate a residual portion of the phase differential.
1. In an optical receiver, a method of compensating sample phase error between parallel multi-bit-digital signals of a received optical signal, the method comprising steps of:
identifying a first digital signal having a greatest amount of propagation delay;
determining a respective sample phase differential between the first digital signal and a second digital signal;
imposing a digital skew between the digital signals, an amount of skew imposed being an integer number of sample periods calculated to most nearly compensate the sample phase differential; and
digitally processing the skewed digital signals, in the frequency domain, to compensate a residual portion of the sample phase differential.
2. A method as claimed in claim 1 , wherein the step of imposing a digital skew comprises a step of delaying the second digital signal by the calculated number of sample periods, relative the first digital signal.
3. A method as claimed in claim 1 , wherein the step of digitally processing the skewed digital signals comprises steps of:
computing an array {R A X } representative of a spectrum of the received optical signal, based on at least one of the digital signals;
applying a selected frequency domain function to the array {R A X } to derive a modified array {V A X };
deriving a time-domain output vector based on the modified array {V A X }; and
wherein the frequency domain function imposes a phase delay that compensates the residual portion of the phase differential.
4. A method as claimed in claim 3 , wherein the first and second digital signals correspond to respective In-phase and Quadrature components of a received polarization of the optical signal, and wherein the array {R A X } representative of a spectrum of the received optical signal is computed based on both the first and second digital signals taken together.
5. A method as claimed in claim 3 , wherein the first and second digital signals are derived from respective different received polarizations of the optical signal, and wherein the array {R A X } representative of a spectrum of the received optical signal is computed based on a selected one of the first and second digital signals, and a third digital signal derived from the same received polarization as the selected digital signal.
6. A method as claimed in claim 3 , wherein the step of applying the frequency domain function to the array {R A X } comprises a step of multiplying the array {R A X } by a coefficient vector implementing the frequency domain function.
7. A method as claimed in claim 3 , wherein the step of applying the frequency domain function to the array {R A X } comprises steps of:
multiplying the array {R A X } and a transpose array { R X A } by a respective coefficient vector implementing the frequency domain function; and
adding the multiplication results.