Monitoring polarization-mode dispersion and signal-to-noise ratio in optical signals based on polarization analysis

1. A method for monitoring a polarization mode dispersion (PMD) effect in an optical signal, comprising:

splitting an input optical signal with an input optical signal bandwidth into a first optical signal with a first optical signal bandwidth less than the input optical signal bandwidth and a second optical signal with a second optical signal bandwidth different from the first optical signal bandwidth, wherein the first optical signal and the second optical signal are centered at a common optical wavelength which is at a center of signal spectral bands of the first optical signal with the first optical signal bandwidth and the second optical signal with the second optical signal bandwidth;

measuring a degree of polarization of the first optical signal and a degree of polarization of the second optical signal, respectively; and

processing the measured degree of polarization of the first optical signal and the measured degree of polarization of the second optical signal to determine a depolarization factor of the input optical signal to reflect the PMD effect in the input optical signal and to determine an optical to signal noise ratio of the input optical signal.

2. The method as in claim 1 , comprising:

using an optical coupler to split the input optical signal into the first optical signal in a first optical path and the second optical signal in a second, separate optical path; and

using a first detector in the first optical path to measure the degree of polarization of the first optical signal and a second detector in the second optical path to measure the degree of polarization of the second optical signal, respectively.

3. The method as in claim 1 , comprising:

using a first optical switch to switch the input optical signal as the first optical signal in a first optical path at one time and to switch the input optical signal as the second optical signal in a second, separate optical path at a different time;

using a second optical switch coupled to the first and the second optical paths to switch the first optical signal in the first optical path into a common detector to measure the degree of polarization of the first optical signal when the first optical switch switches the input optical signal as the first optical signal into the first optical path; and

using the second optical switch to switch the second optical signal in the second optical path into the common detector to measure the degree of polarization of the second optical signal when the first optical switch switches the input optical signal as the second optical signal into the second optical path.

4. The method as in claim 1 , comprising:

using an optical filter to filter one of the first and second optical signals to make the second optical bandwidth different from the first optical bandwidth.

5. The method as in claim 1 , comprising:

using a first optical filter to filter the first optical signal and a second optical filter to filter the second optical signal to make the first optical bandwidth of the first optical signal, after being filtered by the first optical filter, to be different from the second optical bandwidth of the second optical signal after being filtered by the second optical filter.

6. The method as in claim 1 , comprising:

using an input optical filter to filter the input optical signal before the splitting to produce a filtered input optical signal having spectral components in a transmission band of the input optical filter while rejecting other spectral components in the input optical signal; and

downstream from the input optical filter, splitting the input optical signal into the first and second optical signals.

7. The method as in claim 6 , wherein:

the input optical filter is a bandpass filter centered at a wavelength division multiplexed (WDM) wavelength grid under a WDM standard by the International Telecommunications Union (ITU) to select one WDM channel in the input optical signal while rejecting other WDM channels.

8. The method as in claim 7 , wherein:

the input optical filter is a tunable bandpass filter, and

the method comprises tuning the input optical filter to select one WDM channel to measure the depolarization factor and the optical to signal noise ratio of the selected WDM channel at a time and subsequently select another WDM channel to measure the depolarization factor and the optical to signal noise ratio of the subsequently selected another WDM channel.

9. The method as in claim 8 , comprising:

using a Fabry-Perot filter in an optical path of at least one of the first and second optical signals to perform optical filtering, wherein the Fabry-Perot filter has transmission resonance peaks that coincide with ITU WDM frequencies.

10. A device for monitoring a polarization mode dispersion (PMD) effect in an optical signal, comprising:

a first optical path that transmits light;

a second optical path that is separate from the first optical path and transmits light;

a first optical switch coupled to a first end of the first optical path and a first end of the second optical path to switch an input optical signal received by the first optical switch into either the first optical path as a first optical signal or the second optical path as a second optical signal, wherein the first optical signal and the second optical signal are centered at a common optical wavelength;

a first optical filter in the first optical path to filter the first optical signal to have a first optical signal bandwidth that is less than an input optical signal bandwidth of the input optical signal received by the first optical switch and is different from a second optical signal bandwidth of the second optical signal;

a second optical switch coupled to a second end of the first optical path and a second end of the second optical path to receive the first and second optical signals and to select either the first optical signal in the first optical path or the second optical signal in the second optical path as an optical output of the second optical switch;

an optical detector coupled to receive the optical output of the second optical switch to measure a degree of polarization of the received optical output; and

a processing unit in communication with the optical detector to receive output of the optical detector, the processing unit processing the measured degree of polarization of the first optical signal and the measured degree of polarization of the second optical signal to determine a depolarization factor of the input optical signal to reflect the PMD effect in the input optical signal and to determine an optical to signal noise ratio of the input optical signal.

11. The device as in claim 10 , comprising:

a second optical filter in the second optical path to filter the second optical signal to have the second optical bandwidth that is different from the first optical bandwidth of the first optical signal after being filtered by the first optical filter.

12. The device as in claim 10 , comprising:

an input optical filter placed in the optical path of the input optical signal upstream to the first optical switch to filter the input optical signal to allow transmission of spectral components in a transmission band of the input optical filter while rejecting other spectral components in the input optical signal.

13. The device as in claim 12 , wherein:

the input optical filter is a tunable bandpass filter to select one wavelength division multiplexed (WDM) optical channel in the input optical signal while rejecting other WDM optical channels; and

the first optical filter is a Fabry-Perot filter that has transmission resonance peaks that coincide with WDM frequencies under a WDM standard by the International Telecommunications Union (ITU).

14. The device as in claim 11 , comprising:

a second optical filter in the second optical path to filter the second optical signal to have the second optical bandwidth that is different from the first optical bandwidth of the first optical signal after being filtered by the first optical filter.