Method and apparatus for optical add/drop multiplexing in optical networks
View Patent ↗An optical add/drop multiplexing (OADM) apparatus and method are disclosed for detecting pilot tones, removing ghost tones, and re-inserting pilot tones on WDM signals in an optical network. The OADM apparatus comprises a smart processing and control unit (PCU) for detecting pilot tones used for channel identifications, including identifying and removing ghost tones associated with respective optical channels, and re-inserting the processed pilot tones into the respective optical channels. The OADM apparatus and method may incorporate digital PCU or analog PCU for processing pilot tones and removing ghost tones.
1. An optical add/drop multiplexing (OADM) apparatus comprising:
a demultiplexer for demultiplexing a WDM input signal into individual optical channels (wavelengths), each channel being marked with a pilot tone for channel identification and monitoring;
a smart processing and control unit (PCU) for processing the pilot tones including identifying and removing ghost tones associated with the respective pilot tones, and re-inserting the processed pilot tones into the respective optical channels; and
a multiplexer for receiving the optical channels with the re-inserted pilot tones and multiplexing said optical channels into a WDM output signal.
2. An OADM apparatus as described in claim 1 , further comprising:
a plurality of electronically controlled variable optical attenuators (eVOAs), each being inserted into an optical signal path of one of said individual optical channels;
a plurality of optical tap couplers following the eVOAs for detecting the optical channel power at the outputs of said eVOAs; and
a plurality of optical to electrical (O/E) converters at the inputs to and outputs of said eVOAs for converting the optical signals to electrical signals.
3. An OADM apparatus as described in claim 1 , wherein the smart PCU comprises means for measuring frequency spectra at the inputs to and outputs of the eVOAs; and means for comparing and processing said measured frequency spectra.
4. An OADM apparatus as described in claim 3 , wherein the means for comparing and processing said measured frequency spectra comprises means for processing pilot tones and removing the ghost tones associated with the respective pilot tones.
5. An OADM apparatus as described in claim 4 , wherein the means for processing pilot tones comprises means for detecting pilot tones; filtering out electronically the pilot tones; registering the pilot tones and associated optical channels; and re-inserting the processed pilot tones into the respective optical channels.
6. An OADM apparatus as described in claim 4 , wherein the means for removing the ghost tones associated with the respective pilot tones comprises means for detecting ghost tones on each of the optical channels; comparing the ghost tones are with a pre-determined signal-to-noise ratio (SNR) threshold; generating the ghost tones cancellation tones to cancel ghost tone on each of the optical channels; and modulating the outputs of the eVOAs by applying the ghost tones cancellation tones.
7. An OADM apparatus as described in claim 3 , wherein the smart PCU is a digital PCU.
8. An OADM apparatus as described in claim 7 , wherein the means for measuring the frequency domain spectra at the inputs to and outputs of the eVOAs are tone monitors.
9. An OADM apparatus as described in claim 7 , wherein the means for comparing and processing said measured frequency spectra is a comparator unit.
10. An OADM apparatus as described in claim 9 , wherein the comparator unit comprises:
a comparator for comparing the measured frequency spectra at the inputs to and outputs of the eVOAs;
a tone generator for generating cancellation tones to cancel the ghost tones on each of the optical channels; and
a tone modulator for removing the ghost tones at the output of the eVOAs by applying the cancellation tones to the respective optical channels.
11. An OADM apparatus as described in claim 10 , wherein the tone generator comprises means for filtering and registering the pilot tones from respective optical channels; and the tone modulator comprises means for re-inserting the filtered pilot tones into respective optical channels; the comparator unit further comprising a controller for using the optical channels with the re-inserted pilot tones for controlling attenuations of said eVOAs.
12. An OADM apparatus as described in claim 10 , wherein the tone generator further comprises means for detecting ghost tones on each of the optical channels and comparing the ghost tones with a pre-determined signal-to-noise ratio (SNR) threshold; and the tone modulator comprises means for modulating the outputs of the eVOAs by applying the ghost tones cancellation tones.
13. An OADM apparatus as described in claim 7 , wherein the digital PCU further comprises A/D converters for converting analog electrical signals to digital signals; and D/A converters for converting digital signals to the analog electrical signals.
14. An OADM apparatus as described in claim 3 , wherein the smart PCU is an analog PCU.
15. An OADM apparatus as described in claim 14 , wherein the means for measuring the frequency domain spectra at the inputs to and outputs of the eVOAs are monitor circuitries.
16. An OADM apparatus as described in claim 14 , wherein the means for comparing and processing said measured frequency spectra is a comparator unit.
17. An OADM apparatus as described in claim 16 , wherein the comparator unit comprises:
a comparator for comparing the measured frequency spectra at the inputs to and outputs of the eVOAs;
a tone generation circuitry for generating cancellation tones to cancel the ghost tones on each of the optical channels; and
a tone modulation circuitry for removing the ghost tones at the output of the eVOAs by applying the cancellation tones to the respective optical channels.
18. An OADM apparatus as described in claim 17 , wherein the tone generation circuitry comprises means for filtering and registering the pilot tones from respective optical channels; and the tone modulation circuitry comprises means for re-inserting the filtered pilot tones into respective optical channels; the comparator unit further comprising a control circuit for using the optical channels with the re-inserted pilot tones for controlling attenuations of said eVOAs.
19. An OADM apparatus as described in claim 17 , wherein the tone generation circuitry further comprises means for detecting ghost tones on each of the optical channels and comparing the ghost tones with a pre-determined signal-to-noise ratio (SNR) threshold; and the tone modulation circuitry comprises means for modulating the outputs of the eVOAs by applying the ghost tones cancellation tones.
20. An OADM apparatus as described in claim 1 , wherein the demultiplexer and multiplexer are arrayed-waveguide gratings (AWGs).
21. A method for detecting pilot tones, removing ghost tones, and re-inserting pilot tones on WDM signals in an optical network, the method comprising the steps of:
demultiplexing a WDM input signal into individual optical channels (wavelengths), each channel being marked with a pilot tone;
processing the pilot tones including identifying and removing ghost tones associated with the respective pilot tones, and re-inserting the processed pilot tones into the respective optical channels; and
multiplexing the optical channels received with the re-inserted pilot tones into a WDM output signal.
22. A method as described in claim 21 , wherein the step of processing comprises measuring frequency spectra at the inputs to and outputs of the eVOAs; and comparing and processing said measured frequency spectra.
23. A method as described in claim 22 , wherein the step of processing pilot tones comprises detecting pilot tones; filtering out electronically the pilot tones, registering the pilot tones and associated optical channels; and re-inserting the processed pilot tones into the respective optical channels.
24. A method as described in claim 22 , wherein the step of removing the ghost tones at the outputs of the eVOAs comprises:
detecting ghost tones on each of the optical channels;
determining if the ghost tones are greater than a pre-determined signal-to-noise ratio (SNR) threshold;
generating the ghost tones cancellation tones to cancel ghost tone on each of the channels; and
modulating the output of the eVOAs by applying the ghost tones cancellation tones whenever the ghost tones are greater than the pre-determined SNR threshold.
25. A method as described in claim 21 , wherein the step of demultiplexing comprises filtering amplified spontaneous emission (ASE) on the individual optical channels.