Signal identification in optical communications networks
Signals in an optical communications network, such as optical channels in an optical WDM network for example, are each identified by at least two low frequency dither tones with which the signal is modulated. The dither tones alternate with a predetermined periodicity to produce a cyclically repeated sequence of dither tones. A network parameter, such as a channel identifier for example, is obtained by the detection of the particular combination of dither tones in the sequence. To detect a number of network parameters a signal is modulated with a number of cyclically repeated sequences of dither tones each uniquely identifying a respective network parameter. In some implementations each dither tone in a cyclically repeated sequence of dither tones is repeated with substantially the same phase and coherent averaging is performed over a number of periods to detect dither tones on low power signals.
1. A method for detecting one or more low frequency dither tones modulated onto an optical signal sequentially with a cyclically repeated periodicity, each low frequency dither tone being repeated in the cyclically repeated periodicity with substantially the same phase, the method being performed at a detection arrangement having a processor, the method comprising:
(a) performing a plurality of frequency analysis operations on the optical signal to produce respective frequency analysis results; and
(b) performing coherent averaging of the frequency analysis results over the plurality of the frequency analysis operations, including accumulating the frequency analysis results in accordance with respective phase relationships for successive frequency analysis operation periods, to detect the low frequency dither tones.
2. The method as described in claim 1 , wherein the step (a) comprises performing discrete transform operations.
3. The method as described in claim 2 , wherein the performing of the discrete transform operations comprises performing Fast Fourier Transform (FFT) operations.
4. The method as described in claim 2 , wherein the performing FFT operations comprises accumulating FFT amplitudes in accordance with their respective phase relationships for successive FFT periods.
5. The method as described in claim 2 , wherein the discrete transform comprises one or more of the following transforms: Discrete Cosine Transform (CDCT), Discrete Sinus Transform (DST), Inverse Discrete Cosine Transform (IDCT), Inverse Discrete Sinus Transform (IDST), Fast Walsh Transform (FWT), and Fast Hadamard Transform (FHT).
6. The method as described in claim 1 , wherein the steps (a) and (b) of detecting are synchronized in time relative to modulating the dither tones onto the optical signal.
7. A method for detecting one or more low frequency dither tones modulated onto an optical signal with a cyclically repeated sequence having a predetermined periodicity, the method performed at a detection arrangement having a processor, the method comprising:
(a) performing a plurality of frequency analysis operations on the optical signal to produce respective frequency analysis results; and
(b) performing coherent averaging of the frequency analysis results over the plurality of the frequency analysis operations to detect the low frequency dither tones;
wherein the detecting and modulating of dither tones are synchronized by using a global clock in an optical network so that time intervals of modulating and detecting respective dither tones have same duration and start at an instant of switching between dither tone frequencies.
8. The method as described in claim 7 , wherein the instant of switching is when an amplitude of the dither tone frequency to be switched from is close to zero.
9. The method as described in claim 7 , wherein the step (a) comprises performing discrete transform operations.
10. The method as described in claim 9 , wherein the performing of the discrete transform operations comprises performing Fast Fourier Transform (FFT) operations.
11. The method as described in claim 10 , wherein the performing FFT operations comprises accumulating FFT amplitudes in accordance with their respective phase relationships for successive FFT periods.
12. The method as described in claim 9 , wherein the discrete transform comprises one or more of the following: Discrete Cosine Transform (CDCT), Discrete Sinus Transform (DST), Inverse Discrete Cosine Transform (IDCT), Inverse Discrete Sinus Transform (IDST), Fast Walsh Transform (FWT), and Fast Hadamard Transform (FHT).
13. An optical detection arrangement, comprising:
a detector for detecting one or more low frequency dither tones modulated onto an optical signal sequentially with a cyclically repeated periodicity, each low frequency dither tone being repeated in the cyclically repeated periodicity with substantially the same phase; and
a processor for:
(a) performing a plurality of frequency analysis operations on the optical signal to produce respective frequency analysis results; and
(b) performing coherent averaging of the frequency analysis results over the plurality of the frequency analysis operations, including accumulating the frequency analysis results in accordance with respective phase relationships for successive frequency analysis operation periods, to detect the low frequency dither tones.
14. The optical detection arrangement as described in claim 13 , wherein the processor comprises a processor for performing discrete transform operations.
15. The optical detection arrangement as described in claim 14 , wherein the discrete transform comprises Fast Fourier Transform (FFT).
16. The optical detection arrangement as described in claim 14 , wherein the discrete transform comprises one or more of the following: Discrete Cosine Transform (CDCT), Discrete Sinus Transform (DST), Inverse Discrete Cosine Transform (IDCT), Inverse Discrete Sinus Transform (IDST), Fast Walsh Transform (FWT), and Fast Hadamard Transform (FHT).
17. The optical detection arrangement as described in claim 13 , further comprising means for synchronizing the detecting relative to modulating the dither tones onto the optical signal.
18. An optical detection arrangement, comprising:
a detector for detecting one or more low frequency dither tones modulated onto an optical signal with a cyclically repeated sequence having a predetermined periodicity; and
a processor for:
(a) performing a plurality of frequency analysis operations on the optical signal to produce respective frequency analysis results; and
(b) performing coherent averaging of the frequency analysis results over the plurality of the frequency analysis operations to detect the low frequency dither tones;
a global clock for synchronizing the detecting and modulating of dither tones in an optical network so that time intervals of modulating and detecting respective dither tones have same duration and start at an instant of switching between dither tone frequencies.
19. The optical detection arrangement as described in claim 18 , wherein the processor comprises a processor for performing discrete transform operations.
20. The optical detection arrangement as described in claim 19 , wherein the discrete transform comprises Fast Fourier Transform (FFT).
21. The optical detection arrangement as described in claim 18 , further comprising means for synchronizing the detecting relative to modulating the dither tones onto the optical signal.