Method and system for synchronizing optical clocks

• US 20030058010A1
• Filed: 09/25/2001
• Published: 03/27/2003
• Est. Priority Date: 09/25/2001
• Status: Active Grant

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Jacobowitz, Lawrence, DeCusatis, Casimer M.
• Granted Patent: US 7,197,249 B2
• Time in Patent Office: Days
• US Class Current: 327/141
• CPC Class Codes: G06F 1/105 in which the distribution is at least partially optical
• CPC Class Codes: G06F 1/12 Synchronisation of different clock signals provided by a plurality of clock generators
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines
• CPC Class Codes: H04L 7/0075 with photonic or optical means

Abstract

A method and system for synchronizing a local optical clock with a master optical clock, wherein the local clock generates a local clock signal at a local clock wavelength, and said master clock generates a master clock signal at a master clock wavelength. The method comprises the steps of dithering the wavelengths of the local and master clock signals, and generating a control signal representing the difference between the dithered wavelengths of the local and master clock signals. That control signal is then used to synchronize the local clock with the master clock. Preferably, a wavelength-locked loop is used as a means of phase or wavelength control for the optically distributed clock signals. The wavelength-locked loop provides a technique to control the wavelength of a local optical clock source in a manner that will allow the local clock to follow variations in the wavelength of another optical signal.

Claims

1. A method of synchronizing a local optical clock with a master optical clock, said local clock generating a local clock signal at a local clock wavelength, and said master clock generating a master clock signal at a master clock wavelength, the method comprising:

dithering the wavelengths of the local and master clock signals;

generating a control signal representing the difference between the dithered wavelengths of the local and master clock signals; and

using said control signal to synchronize the local clock with the master clock.

2. A method according to claim 1, wherein the generating step includes the steps of:

conducting at least a portion of the local clock signal through a wavelength selective device implementing a peaked passband function including a center wavelength; and

generating an output signal representing the difference between the wavelength of the local clock signal and the center wavelength of said peaked passband function.

3. A method according to claim 2, wherein the conducting step includes the step of conducting at least a portion of the local clock signal through a filter that implements said passband function.

4. A method according to claim 1, wherein the dithering step includes the step of dithering the local and master clock signals with approximately the same dither rate.

5. A method according to claim 1, wherein the dithering step includes the step of dithering the local and master clock signals with substantially the same dither rate.

6. A method according to claim 1, wherein:

the dithering step includes the step of generating a local dither signal, and using the local dither signal to dither the local clock signal; and

the generating step includes the steps of

i) conducting at least a portion of the local clock signal through a wavelength selective device implementing a peaked passband function to generate a feedback signal, and

ii) mixing the feedback signal with the local dither signal.

7. A method according to claim 1, wherein the local clock frequency is above 100 Mhz.

8. A distributed clock tree, comprising:

a master optical clock for generating a master optical clock signal having a master clock wavelength;

a master dither source for dithering the master clock signal;

a plurality of distributed, local devices; and

means for transmitting the master clock signal to each of the local devices; and

wherein each of the local devices includes

i) a local optical clock for generating a local optical clock signal having a local clock

wavelength,

ii) a local dither source,

iii) an input for receiving the master optical clock signal, and

iv) a control circuit for generating a control signal representing the difference between the dithered wavelengths of the local and master clock signals, and for using said control signal to synchronize the local clock with the master clock.

9. A distributed clock tree according to claim 8, wherein the control circuit includes:

a wavelength selective device implementing a peaked passband function including a center wavelength;

means for conducting at least a portion of the local clock signal through the wavelength selective device; and

a feedback circuit for generating an output signal representing the difference between the wavelength of the local clock signal and the center wavelength of said peaked passband function, and for processing said output circuit to generate the control signal.

10. A distributed clock tree according to claim 8, wherein the master dither source dithers the master clock signal dithers at a given rate, and the local dither source dithers the local clock signal at a rate approximately the same as the given rate.

11. A distributed clock tree according to claim 8, wherein:

the local dither source generates a local dither signal, and uses the local dither signal to dither the local clock signal; and

the control circuit includes

i) a wavelength selective device implementing a peaked passband function to generate a feedback signal,

ii) means for conducting at least a portion of the local clock signal through the wavelength selective device, and

iii) a mixer for mixing the feedback signal with the local dither signal.

12. A system for synchronizing a local optical clock with a master optical clock, said local clock generating a local clock signal at a local clock wavelength, and said master clock generating a master clock signal at a master clock dithered wavelength, the system comprising:

a local dither source for dithering the wavelengths of the local clock signal; and

a control circuit for generating a control signal representing the difference between the dithered wavelengths of the local and master clock signals, and for using said control signal to synchronize the local clock with the master clock.

13. A system according to claim 12, wherein the control circuit includes:

a wavelength selective device implementing a peaked passband function including a center wavelength;

means for conducting at least a portion of the local clock signal through the wavelength selective device; and

a detector for generating an output signal representing the difference between the wavelength of the local clock signal and the center wavelength of said peaked passband function.

14. A system according to claim 12, wherein wavelength selective device comprises a filter that implements said passband function.

15. A system according to claim 12, wherein the wavelength of the master clock signal dithers at a given rate, and the local dither source dithers the local clock signal at a rate approximately the same as the given rate.

16. A system according to claim 12, wherein the wavelength of the master clock signal dithers at a given rate, and the local dither source dithers the local clock signal at a rate substantially the same as the given rate.

17. A system according to claim 12, wherein:

the dither source generates a local dither signal, and uses the local dither signal to dither the local clock signal; and

the control circuit includes

iv) a wavelength selective device implementing a peaked passband function to generate a feedback signal,

v) means for conducting at least a portion of the local clock signal through the wavelength selective device, and

vi) a mixer for mixing the feedback signal with the local dither signal.

18. A system according to claim 12, wherein the frequency of the local clock signal is above 100 Mhz.