Methods and systems for periodic optical filtering to identify tone modulated optical signals

1. A method for identifying tone modulated optical signals, the method comprising:

receiving an optical signal transmitted over an optical transport network, wherein the optical signal is transmitted with N number of optical channels that are respectively modulated at tone signal frequencies v n that are smaller than a baud rate for the optical signal, wherein n is selected from values of 1 to N;

scanning the optical signal with a periodic comb filter over a free optical spectral range W fs of the periodic comb filter to generate a filtered signal, wherein the free optical spectral range W fs corresponds to K number of optical channels in the optical signal;

demodulating the filtered signal to generate a power spectrum within the free spectral range for a first frequency selected from the frequencies v n that corresponds to a first optical channel selected from the N optical channels; and

based on the power spectrum for the first frequency, identifying a first center optical frequency of the first optical channel,

wherein the optical signal is a dense wavelength division multiplexed signal (DWDM), and wherein a center optical frequency f n for an optical channel n is given by:

f n =m*W fs +W c1 +ΔW n

where:

m=floor ((n−1)/K);

W c1 is an initial scanning frequency of a first periodic passband of the periodic comb filter; and

ΔW n is an optical frequency shift with respect to W fs at which a central alignment feature in the power spectrum is detected.

2. The method of claim 1 , wherein the first optical channel is modulated using frequency modulation (FM), and wherein the central alignment feature is a relative minima in the power spectrum.

3. The method of claim 1 , wherein the first optical channel is modulated using amplitude modulation (AM), and wherein the central alignment feature is a relative maxima in the power spectrum.

4. The method of claim 1 , wherein demodulating the filtered signal includes:

based on the frequencies v n , respectively, for the N number of optical channels, demodulating the filtered signal to generate N number of power spectra respectively corresponding to the frequencies v n .

5. The method of claim 1 , wherein demodulating the filtered signal includes:

when a second frequency selected from the frequencies v n is not detected from the filtered signal, determining that a second optical channel that corresponds to the second frequency is not included in the optical signal received.

6. The method of claim 1 , wherein the optical signal is a superchannel that is transmitted with N number of subcarriers.

7. The method of claim 1 , wherein demodulating the filtered signal includes:

using a digital signal processor to detect at least some of the frequencies v n .

8. An optical transport network enabled for identifying optical channels modulated with tone signals, the optical transport network comprising:

an optical channel monitor enabled to:

receive an optical signal transmitted over an optical transport network, wherein the optical signal is transmitted with N number of optical channels that are respectively modulated at tone signal frequencies v n that are smaller than a baud rate for the optical signal, wherein n is selected from values of 1 to N;

scan the optical signal with a periodic comb filter over a free optical spectral range W fs of the periodic comb filter to generate a filtered signal, wherein the free optical spectral range W fs corresponds to K number of optical channels in the optical signal;

demodulate the filtered signal to generate a power spectrum within the free spectral range for a first frequency selected from the frequencies v n that corresponds to a first optical channel selected from the N optical channels; and

based on the power spectrum for the first frequency, identify a first center optical frequency of the first optical channel,

wherein the optical signal is a dense wavelength division multiplexed signal (DWDM), and wherein a center optical frequency f n for an optical channel n is given by:

f n =m*W fs +W c1 +ΔW n

where:

m=floor ((n−1)/K);

W c1 is an initial scanning frequency of a first periodic passband of the periodic comb filter; and

ΔW n is an optical frequency shift with respect to W fs at which a central alignment feature in the power spectrum is detected.

9. The optical transport network of claim 8 , wherein the first optical channel is modulated using frequency modulation (FM), and wherein the central alignment feature is a relative minima in the power spectrum.

10. The optical transport network of claim 8 , wherein the first optical channel is modulated using amplitude modulation (AM), and wherein the central alignment feature is a relative maxima in the power spectrum.

11. The optical transport network of claim 8 , wherein the optical channel monitor demodulates the filtered signal, based on the frequencies v n , respectively, for the N number of optical channels, to generate N number of power spectra respectively corresponding to the frequencies v n .

12. The optical transport network of claim 8 , wherein the optical channel monitor determines, when a second frequency selected from the frequencies v n is not detected from the filtered signal, that a second optical channel that corresponds to the second frequency is not included in the optical signal received.

13. The optical transport network of claim 8 , wherein the optical signal is a superchannel that is transmitted with N number of subcarriers.

14. The optical transport network of claim 8 , wherein the optical channel monitor demodulating the filtered signal further comprises using a digital signal processor to detect at least some of the frequencies v n .