Control of LO signal frequency offset between optical transmitters and receivers

Patent metadata

Field	Value
Publication number	US-9735888-B2
Application number	US-201514921222-A
Country	US
Kind code	B2
Filing date	Oct 23, 2015
Priority date	Oct 23, 2015
Publication date	Aug 15, 2017
Grant date	Aug 15, 2017

Abstract

In an optical receiver, an optical local oscillator (LO) frequency is generated. A modulated optical frequency is received at the optical receiver. An LO-signal frequency offset between the received modulated optical frequency and the optical LO frequency is determined. A determination is made as to whether the LO-signal frequency offset is in one of multiple predefined non-overlapping target windows that cover respective non-zero LO-signal frequency offsets. If it is determined that the LO-signal frequency offset is not in one of the target windows, the optical LO frequency is tuned to drive the LO-signal frequency offset toward one of the target windows to ensure the LO-signal frequency offset is non-zero.

First claim

What is claimed is:

1. A method comprising:
in an optical receiver:
generating an optical local oscillator (LO) frequency;
receiving a modulated optical frequency;
determining a LO-signal frequency offset between the received modulated optical frequency and the optical LO frequency;
determining whether the LO-signal frequency offset is in one of multiple predefined non-overlapping target windows that cover respective non-zero LO-signal frequency offsets; and
if the LO-signal frequency offset is determined not to be in one of the target windows, tuning the optical LO frequency to drive the LO-signal frequency offset toward one of the target windows to ensure the LO-signal frequency offset is non-zero.

2. The method of claim 1 , further comprising, in the optical receiver:
if the LO-signal frequency offset is determined to be in one of the target windows, maintaining the optical LO frequency so that the LO-signal frequency offset remains in the one of the target windows.

3. The method of claim 1 , wherein the determining the LO-signal frequency offset includes:
detecting, using the optical LO frequency, at or near baseband in-phase (I) and quadrature (Q) signals representative of a digital modulation constellation conveyed by the modulated optical frequency and that rotates at a rate equal to the LO-signal frequency offset; and
determining the LO-signal frequency offset based on the detected I and Q signals.

4. The method of claim 3 , wherein the detecting includes:
frequency down-converting the modulated optical frequency toward baseband using the optical LO frequency; and
detecting the I and Q signals from results of the frequency down-converting.

5. The method of claim 3 , wherein the digital modulation constellation includes a dual-polarization Binary Phase Shift Keying (BPSK) constellation.

6. The method of claim 1 , wherein the target windows include a positive target window centered at a positive target LO-signal frequency offset from baseband and a negative target window centered at a negative target LO-signal frequency offset from baseband.

7. The method of claim 6 , wherein the target windows further include a dead-band window centered at a dead-band target LO-signal frequency offset between baseband and the positive target window.

8. The method of claim 1 , further comprising, in the optical receiver, determining which of the target windows is nearest the LO-signal frequency offset, wherein the tuning includes tuning the optical LO frequency to drive the LO-signal frequency offset toward the target window determined to be nearest the LO-signal frequency offset.

9. The method of claim 8 , wherein the tuning includes tuning the optical LO frequency to drive the LO-signal frequency offset toward the determined nearest target window such that the LO-signal frequency offset resulting from the tuning does not exceed the target LO-signal frequency offset from baseband at which the determined nearest target window is centered.

10. An apparatus comprising:
a laser configured to generate a tunable optical local oscillator (LO) frequency:
an optical receiver configured to receive a modulated optical frequency and configured to:
determine a LO-signal frequency offset between the received modulated optical frequency and the optical LO frequency;
determine whether the LO-signal frequency offset is in one of multiple predefined non-overlapping target windows that cover respective non-zero LO-signal frequency offsets; and
if the LO-signal frequency offset is determined not to be in one of the target windows, tune the optical LO frequency to drive the LO-signal frequency offset toward one of the target windows to ensure the LO-signal frequency offset is non-zero.

11. The apparatus of claim 10 , wherein the optical receiver is further configured to:
if the LO-signal frequency offset is determined to be in one of the target windows, maintain the optical LO frequency so that the LO-signal frequency offset remains in the one of the target windows.

12. The apparatus of claim 10 , wherein the optical receiver includes:
an optical/electrical (O/E) converter configured to detect, using the optical LO frequency, at or near baseband in-phase (I) and quadrature (Q) signals representative of a digital modulation constellation conveyed by the modulated optical frequency and that rotates at a rate equal to the LO-signal frequency offset; and
a digital processor to determine the LO-signal frequency offset based on the detected I and Q signals.

13. The apparatus of claim 12 , wherein the 0 /E converter includes:
an optical mixer configured to frequency down-convert the modulated optical frequency toward baseband using the optical LO frequency; and
photo-detectors configured to detect the I and Q signals from results output from the optical mixer.

14. The apparatus of claim 10 , wherein the target windows include a positive target window centered at a positive target LO-signal frequency offset from baseband and a negative target window centered at a negative target LO-signal frequency offset from baseband.

15. The apparatus of claim 14 , wherein the target windows further include a dead-band window centered at a dead-band target LO-signal frequency offset between baseband and the positive target window.

16. The apparatus of claim 10 , wherein the optical receiver is further configured to determine which of the target windows is nearest the LO-signal frequency offset, wherein the optical receiver is configured to tune by tuning the optical LO frequency to drive the LO-signal frequency offset toward the target window determined to be nearest the LO offset.

17. The apparatus of claim 16 , wherein the optical receiver is further configured to tune by tuning the optical LO frequency to drive the LO-signal frequency offset toward the determined nearest target window such that the LO-signal frequency offset resulting from the tuning does not exceed the target LO-signal frequency offset from baseband at which the determined nearest target window is centered.

18. A method comprising:
in an optical receiver:
generating an optical local oscillator (LO) frequency;
receiving a modulated optical frequency;
determining a LO-signal frequency offset between the received modulated optical frequency and the optical LO frequency; and
controlling the optical LO frequency to ensure the LO-signal frequency offset is non-zero, the controlling including:
if the LO-signal frequency offset is in one of multiple predefined non-overlapping target windows covering respective non-zero LO-signal frequency offsets, maintaining the optical LO frequency so that the LO-signal frequency offset remains in one of the target windows; and
if the LO-signal frequency offset is not in one of the target windows, tuning the optical LO frequency to drive the LO-signal frequency offset toward one of the target windows.

19. The method of claim 18 , further comprising, in the optical receiver:
determining whether the LO-signal frequency offset is in one of the target windows; and
performing the controlling responsive to results of the determining.

20. The method of claim 18 , wherein the target windows include a positive target window centered at a positive target LO-signal frequency offset from baseband and a negative target window centered at a negative target LO-signal frequency offset from baseband.

21. The method of claim 20 , wherein the target windows further include a dead-band window centered at a dead-band target L

Assignees

• Cisco Tech Inc

Inventors

• Fludger Christopher
• Duthel Thomas
• Leykauf Joerg
• Michaelsen Karsten

Classifications

• H04L27/00

  Modulated-carrier systems · CPC title

• H04B10/5561

  Digital phase modulation · CPC title

• H04B10/6164 Primary

  Estimation or correction of the frequency offset between the received optical signal and the optical local oscillator · CPC title

• H04B10/532

  Polarisation modulation · CPC title

• H04L7/0075

  with photonic or optical means · CPC title