PMD measurements for an in-service fiber span

1. A method of testing for Polarization Mode Dispersion (PMD) in-service on a fiber span between a first terminal in a network and a second terminal in the network, the method comprising:

in a control system, selecting a plurality of available data channels from data channels of a data bandwidth within the fiber span;

in a light system, transmitting test signals based on the plurality of available data channels;

in a filter system, filtering the test signals to prevent interference with the unavailable data channels;

in a multiplexer located in the first terminal, receiving data signals from a plurality of transmitters in the first terminal, receiving the test signals from the filter system, transmitting the data signals in the data channels of the data bandwidth within the fiber span, wherein the fiber span has a bandwidth that comprises a first subsection and a second subsection and wherein the data bandwidth comprises the first subsection of the bandwidth, and transmitting the test signals over the plurality of available data channels in the data bandwidth concurrently with transmitting the data signals, wherein the data signals occupy a subset of the data channels of the data bandwidth and the test signals occupy the plurality of available data channels in the data bandwidth;

in a de-multiplexer located in the second terminal, receiving the data signals and the test signals; and

in a PMD measurement system coupled to the de-multiplexer, measuring PMD of the fiber span based on the test signals.

2. The method of claim 1 wherein

selecting the plurality of available data channels in the control system further comprises:

(a) identifying the plurality of available data channels in the data bandwidth;

(b) selecting a one of the plurality of available data channels;

(c) generating control signals to control the light system to transmit the test signals over the one of the plurality of available data channels;

(d) transmitting the control signals to the light system; and

(e) selecting a next one of the plurality of available data channels and repeating steps (c) through (e).

3. The method of claim 1 wherein

selecting the plurality of available data channels in the control system further comprises:

(a) identifying the plurality of available data channels in the data bandwidth;

(b) selecting one of the plurality of available data channels;

(c) determining a plurality of sub-channels in the one of the plurality of available data channels;

(d) generating control signals to control the light system to transmit the test signals over the sub-channels of the one of the plurality of available data channels;

(e) transmitting the control signals to the light system; and

(f) selecting a next one of the plurality of available data channel and repeating steps (c) through (f).

4. The method of claim 1 wherein the fiber span bandwidth comprises an L-band, an S-band, and a C-band and wherein the data bandwidth comprises the C-band.

5. A test system for testing for Polarization Mode Dispersion (PMD) in-service on a fiber span between a first terminal in a network and a second terminal in the network, the test system comprising:

a control system configured to select a plurality of available data channels from data channels of a data bandwidth within the fiber span;

a light system configured to transmit test signals based on the plurality of available data channels;

a filter system configured to filter the test signals to prevent interference with the unavailable data channels;

a multiplexer located in the first terminal and configured to receive data signals from a plurality of transmitters, receive test signals from the filter system, transmit the data signals in the data channels of the data bandwidth within the fiber span, wherein the fiber span has a bandwidth that comprises a first subsection and a second subsection and wherein the data bandwidth comprises the first subsection of the bandwidth, and transmit the test signals over the plurality of available data channels in the data bandwidth concurrently with the data signals, wherein the data signals occupy a subset of the data channels of the data bandwidth and the test signals occupy the plurality of available data channels in the data bandwidth;

a de-multiplexer located in the second terminal and configured to receive the data signals and the test signals; and

a PMD measurement system configured to measure PMD of the fiber span based on the test signals.

6. The test system of claim 5 wherein to select the plurality of available data channels the control system is further configured to:

(a) identify the plurality of available data channels in the data bandwidth;

(b) select one of the plurality of available data channels;

(c) generate control signals to control the light system to transmit the test signals over the one of the plurality of available data channels;

(d) transmit the control signals to the light system; and

(e) select a next one of the plurality of available data channels and repeat steps (c) through (e).

7. The test system of claim 5 wherein to select the plurality of available data channels the control system is further configured to:

(a) identify the plurality of available data channels in the data bandwidth;

(b) select a one of the plurality of available data channels;

(c) determine a plurality of sub-channels in the plurality of available data channels;

(d) generate control signals to control the light system to transmit the test signals over the sub-channels of the plurality of available data channels;

(e) transmit the control signals to the light system; and

(f) select a next one of the plurality of available data channels and repeat steps (c) through (f).

8. The test system of claim 5 wherein the fiber span bandwidth comprises an L-band, an S-band, and a C-band and wherein the data bandwidth comprises the C-band.