# US 8,543,012 B2 · Compensating for end-to-end group delay ripples

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- Number: US 8,543,012 B2
- Document ID: US8543012B2
- Status: Granted
- Kind: B2
- Application number: 12/780,283
- Filed: 2010-05-14
- Published: 2011-11-17
- Granted: 2013-09-24
- Earliest claimed: 2010-05-14
- Assignees: Verizon Patent and Licensing Inc.
- Applicants: Tiejun J. Xia; Glenn A. Wellbrock
- Inventors: Tiejun J. Xia; Glenn A. Wellbrock
- Web page: https://patentgrep.com/patent/US8543012B2

## CPC

- H04B 10/2513: due to chromatic dispersion
- H04B: TRANSMISSION

## Abstract

A device may include a group delay monitor and a signal receiver. The group delay monitor may be configured to obtain group delay data corresponding to group delay of an optical signal and provide the group delay data to a signal receiver. The signal receiver configured to obtain a time-domain digital signal corresponding to the optical signal, convert the time-domain digital signal into a frequency-domain signal, apply a digital filter constructed based on the group delay data to the frequency-domain signal to obtain an output signal, and transmit the output signal.

## Claims

### Claim 1

A method comprising:
receiving, at different signal receivers of a device, an optical signal included in network traffic carried via different channels and destined for a client device;
performing, by the device, in-service measurement of first values of original group delay ripples exhibited over select channels of the different channels of the optical signal without impacting the network traffic;
determining, by the device, second values of compensating group delay ripples associated with each of the select channels, based on the in-service measurement of the first values;
generating, based on the second values, the compensating group delay ripples associated with each of the select channels;
injecting, by the different signal receivers of the device, the compensating group delay ripples into a digital signal, corresponding to the optical signal, to generate an output signal in which the original group delay ripples and the compensating group delay ripples sum to zero; and
transmitting, from the device, the output signal from the device to the client device.

### Claim 2 (depends on 1)

The method of claim 1, wherein the determining second values of compensating group delay ripples includes constructing a digital filter based on the second values of the in-service measurement of the first values, and wherein the injecting the compensating group delay ripples includes applying the constructed digital filter to the digital signal.

### Claim 3 (depends on 1)

The method of claim 1, wherein the compensating group delay ripples, when plotted as a function of frequency, are a mirror image of a plot of the original group delay ripples.

### Claim 4 (depends on 1)

The method of claim 1, wherein the original group delay ripples includes at least one of:
group delay caused by the optical signal propagating through an optical fiber;
group delay caused by optical filters on an optical path through which the optical signal has traveled; or
group delay due to chromatic dispersion.

### Claim 5 (depends on 1)

The method of claim 1, wherein the device includes one of:
a muxponder; or
a transponder.

### Claim 6 (depends on 1)

The method of claim 1, wherein the injecting the compensating group delay ripples includes applying a fast Fourier transform to a time-domain digital signal corresponding to the optical signal.

### Claim 7 (depends on 1)

The method of claim 1, wherein the in-service measurement includes:
monitoring a timer; and
measuring, by the device, the first values of the original group delay ripples when the device determines that the timer indicates a threshold amount of time has been reached for performing the in-service measurement.

### Claim 8 (depends on 1)

The method of claim 1, wherein the in-service measurement includes:
determining, based on information received via a network interface of the device, whether an optical path for the optical signal has changed; and
measuring, by the device, the first values of the original group delay ripples when the device determines that the optical path has changed.

### Claim 9

A device comprising:
a group delay monitor configured to:
obtain group delay data corresponding to group delay of an optical signal, included in network traffic destined for a client device, without impacting the network traffic;
provide the group delay data to a signal receiver; and
the signal receiver configured to:
determine, based on the group delay data, that the group delay of the optical signal corresponds to accumulated group delay ripples;
obtain a time-domain digital signal corresponding to the optical signal;
convert the time-domain digital signal, within a time window, into a frequency-domain signal;
apply a digital filter, constructed based on the group delay data to compensate for the accumulated group delay ripples, to the frequency-domain signal to obtain an output signal in which the accumulated group delay ripples have been removed; and
transmit the output signal to the client device.

### Claim 10 (depends on 9)

The device of claim 9, wherein the device comprises:
a muxponder or transponder.

### Claim 11 (depends on 9)

The device of claim 9, further comprising one of:
an optical switch to select the optical signal among optical signals, in the network traffic, at different wavelengths; or
an arrayed waveguide grating to combine optical signals received from a reconfigurable optical multiplexer (ROADM) into a single optical signal.

### Claim 12 (depends on 11)

The device of claim 11, wherein the group delay monitor is further configured to:
obtain group delay of a second optical signal when the optical switch selects the second optical signal among the optical signals.

### Claim 13 (depends on 11)

The device of claim 11, wherein the group delay monitor is further configured to:
obtain an update of the group delay of the optical signal when an optical path of the optical signal changes.

### Claim 14 (depends on 11)

The device of claim 11, wherein the optical signals include optical signals from a reconfigurable optical multiplexer (ROADM).

### Claim 15 (depends on 9)

The device of claim 9, wherein the group delay monitor includes:
a narrow band tunable filter to filter the optical signal;
an optical detector to extract a data signal from the filtered optical signal;
a clock recovery unit for extracting a clock signal from one of:
the optical signal or
an analog signal corresponding to the optical signal;
a phase detector that determines, at different frequencies, a phase differences between the clock signal and the data signal; and
a data collector to obtain the group delay data based on the phase differences.

### Claim 16 (depends on 15)

The device of claim 15, further comprising:
an amplifier to amplify the optical signal and to provide the amplified signal to the group delay monitor.

### Claim 17 (depends on 9)

The device of claim 9, wherein the signal receiver includes the group delay monitor.

### Claim 18 (depends on 9)

The device of claim 9, wherein the signal receiver includes:
a digital signal processor to convert the time-domain digital signal into the frequency domain signal; or
a software component to convert the time-domain digital signal into the frequency domain signal.

### Claim 19

A device comprising:
a plurality of signal receivers, each of the signal receivers configured to:
obtain, without impacting network traffic destined for a client device, group delay data corresponding to group delay of optical signals, included in the network traffic, input to the signal receivers;
determine, based on the group delay data, that the group delay corresponds to group delay ripples exhibited in the optical signal;
configure a digital filter, based on the group delay data, to compensate for the group delay ripples;
obtain a time-domain digital signal from the optical signal;
convert the time-domain digital signal into a frequency domain signal;
apply the digital filter to the frequency domain signal to obtain an output signal in which the group delay ripples have been removed; and
transmit the output signal to the client device.

### Claim 20 (depends on 19)

The device of claim 19, further comprising an optical switch to select an optical signal corresponding to one of the signal receivers.