Method and apparatus for robust clock recovery in coherent optical systems
An optical channel between a coherent optical transmitter and a coherent optical receiver may include one or more components that act as a bandpass filter with a passband that is narrower than the signal bandwidth. Such a narrow filter may significantly attenuate the signal content close to the band edge of the data signal. As a result, timing error detection may work less effectively, and therefore clock recovery may be less effective or fail. Methods and systems are disclosed in which a single optical carrier is used to transmit a data signal that has multiple bands, and timing error detection is performed at the receiver using one or more inner bands of the multiple bands. The timing error detection may therefore be made more robust to the effects of the narrow filtering.
1. A method performed at an optical receiver comprising:
converting a received optical signal on a single optical carrier into an electrical signal to obtain a received multi-band signal, the received multi-band signal having a plurality of frequency bands comprising k≥1 inner frequency bands interposed between a first outer frequency band and a second outer frequency band;
separating the received multi-band signal into a plurality of signals, the plurality of signals including a first signal corresponding to the first outer frequency band, k signals each corresponding to a respective one of the k inner frequency bands, and a second signal corresponding to the second outer frequency band; and
computing a timing error value for use in clock recovery by using at least one of the k signals without using the first signal or the second signal.
2. The method of claim 1 , further comprising:
performing chromatic dispersion compensation on a particular signal of the k signals to obtain a dispersion compensated signal; and
computing the timing error value using the dispersion compensated signal.
3. The method of claim 2 , further comprising:
computing the timing error value using a Godard method.
4. The method of claim 3 , further comprising:
computing a fine delay value based on taps of a filter in the receiver; and
computing the timing error value using the fine delay value.
5. The method of claim 4 , wherein the filter is for performing polarization mode dispersion compensation.
6. The method of claim 4 , wherein the computing the timing error value comprises:
computing an initial value using the dispersion compensated signal; and
adjusting the initial value by the fine delay value in order to obtain the timing error value.
7. The method of claim 4 , wherein the dispersion compensated signal is a first dispersion compensated signal, wherein the filter is a first filter, wherein the fine delay value is a first fine delay value, and wherein the method further comprises:
performing chromatic dispersion compensation on another signal of the k signals to obtain a second dispersion compensated signal, wherein the another signal of the k signals is different from the particular signal;
filtering the second dispersion compensated signal with a second filter to perform polarization mode dispersion compensation;
computing a second fine delay value based on taps of the second filter; and
further using the second fine delay value to compute the timing error value.
8. The method of claim 7 , comprising:
averaging the first fine delay value and the second fine delay value to obtain an average fine delay value; and
computing the timing error value using the average fine delay value.
9. The method of claim 1 , further comprising:
correcting a timing offset based on the timing error value.
10. The method of claim 1 , wherein the received multi-band signal corresponds to a transmitted multi-band signal having each frequency band pulse-shaped, wherein a pulse-shaping filter used to pulse-shape an outer frequency band signal has a roll-off factor smaller than a roll-off factor of another pulse-shaping filter used to pulse shape an inner frequency band.
11. An optical receiver comprising:
an opto-electronic front end to convert a received optical signal on a single optical carrier into an electrical signal to obtain a received multi-band signal, the received multi-band signal having a plurality of frequency bands comprising k≥1 inner frequency bands interposed between a first outer frequency band and a second outer frequency band;
a band slicer to separate the received multi-band signal into a plurality of signals, the plurality of signals including a first signal corresponding to the first outer frequency band, k signals each corresponding to a respective one of the k inner frequency bands, and a second signal corresponding to the second outer frequency band; and
a timing error detection computation unit to compute a timing error value for use in clock recovery by using at least one of the k signals without using the first signal or the second signal.
12. The optical receiver of claim 11 , further comprising:
a chromatic dispersion compensator to perform chromatic dispersion compensation on a particular signal of the k signals to obtain a dispersion compensated signal;
wherein the timing error detection computation unit is to compute the timing error value using the dispersion compensated signal.
13. The optical receiver of claim 12 , wherein the timing error detection computation unit is to compute the timing error value using a Godard method.
14. The optical receiver of claim 12 , further comprising:
a fine delay computation unit to compute a fine delay value based on taps of a filter in the receiver;
wherein the timing error detection computation unit is to compute the timing error value using the fine delay value.
15. The optical receiver of claim 14 , wherein the filter is for performing polarization mode dispersion compensation.
16. The optical receiver of claim 14 , wherein the timing error detection computation unit is to compute the timing error value by:
computing an initial value using the dispersion compensated signal; and
adjusting the initial value by the fine delay value in order to obtain the timing error value.
17. The optical receiver of claim 14 , wherein the chromatic dispersion compensator is a first chromatic dispersion compensator, wherein the dispersion compensated signal is a first dispersion compensated signal, wherein the filter is a first filter, wherein the fine delay computation unit is a first fine delay computation unit, wherein the fine delay value is a first fine delay value, and wherein the optical receiver further comprises:
a second chromatic dispersion compensator to perform chromatic dispersion compensation on another signal of the k signals to obtain a second dispersion compensated signal, wherein the another signal of the k signals is different from the particular signal;
a second filter to filter the second dispersion compensated signal to perform polarization mode dispersion compensation; and
a second fine delay computation unit to compute a second fine delay value based on taps of the second filter;
wherein the timing error detection computation unit is to compute the timing error value by also using the second fine delay value.
18. The optical receiver of claim 17 , wherein the timing error detection computation unit is to:
average the first fine delay value and the second fine delay value to obtain an average fine delay value; and
compute the timing error value using the average fine delay value.
19. The optical receiver of claim 11 , further comprising:
a retimer to correct a timing offset based on the timing error value.
20. The optical receiver of claim 11 , wherein the received multi-band signal corresponds to a transmitted multi-band signal having each frequency band pulse-shaped, wherein a pulse-shaping filter used to pulse-shape an outer frequency band signal has a roll-off factor smaller than a roll-off factor of another pulse-shaping filter used to pulse shape an inner frequency band.