Method and apparatus for dynamic equalization in wavelength division multiplexing

• US 6,782,205 B2
• Filed: 01/15/2002
• Issued: 08/24/2004
• Est. Priority Date: 06/25/2001
• Status: Active Grant

• Current Assignee: Silicon Light Machines Corporation (SCREEN Holdings Co., Ltd.)
• Original Assignee: Silicon Light Machines Corporation (SCREEN Holdings Co., Ltd.)
• Inventors: Carlisle, Clinton B., Trisnadi, Jahja I.
• Primary Examiner(s): Pascal, Leslie
• Assistant Examiner(s): Leung, Christina Y
• Application Number:
  US10/051,972
• Publication Number:
  US 20020196492A1
• Time in Patent Office: 952 Days
• Field of Search: 359/291, 359/124, 359/125, 359/126, 359/127, 359/128, 359/130, 359/131, 398/94, 398/158, 398/79, 398/81, 398/197
• US Class Current: 398/94
• CPC Class Codes: G02B 6/2713 cascade of polarisation selective or adjusting operations
• CPC Class Codes: G02B 6/272 comprising polarisation means for beam splitting and combining
• CPC Class Codes: G02B 6/2793 Controlling polarisation dependent loss, e.g. polarisation insensitivity, reducing the change in polarisation degree of the output light even if the input polarisation state fluctuates
• CPC Class Codes: G02B 6/2931 Diffractive element operating in reflection
• CPC Class Codes: G02B 6/29311 Diffractive element operating in transmission
• CPC Class Codes: G02B 6/29314 by moving or modifying the diffractive element, e.g. deforming
• CPC Class Codes: G02B 6/29391 Power equalisation of different channels, e.g. power flattening
• CPC Class Codes: H04B 10/25073 using spectral equalisation, e.g. spectral filtering
• CPC Class Codes: H04B 2210/258 treating each wavelength or wavelength band separately
• CPC Class Codes: H04J 14/0221 Power control, e.g. to keep the total optical power constant

Abstract

An adjustable diffractive light modulator is used to equalize the power level of wavelength multiplexed signals in wavelength division multiplexing applications. An optical channel transmits a multi-wavelength input signal through a wavelength demultiplexer. The input signal comprises a plurality of component signals defined according to a plurality of wavelengths. The de-multiplexer spatially separates the multi-wavelength input signal into its respective plurality of component signals, which are transmitted through a plurality of optical channels onto a plurality of controllable diffractive light modulators. The diffractive light modulators are advantageously comprised of grating light valves. Component signals are reflected off their respective diffractive light modulators and collected in a plurality of optical channels for re-transmission into a wavelength multiplexer. The reflected signals are combined through a multiplexing process and transmitted as an output beam. Control of the diffractive light modulator is achieved by sensors measuring the power level of signals passing through the plurality of optical channels. The measured power levels of light are converted to digital values which are input into an electronic controller for processing. Control output signals from the controller are used to independently control individual pixels within the diffractive light modulator to equalize the reflected light signals.

Claims

1. An apparatus for selectively adjusting power levels of component signals of a wavelength division multiplexed signal including a first wavelength signal and a second wavelength signal, the apparatus comprising a diffractive light modulator with a first pixel configured to receive the first wavelength signal and a second pixel configured to receive the second wavelength signal, wherein each pixel is controllable to selectively direct a received signal into a first mode and a second mode, and further wherein each first mode is collected such that a power level of the first wavelength signal is selectively adjusted relative to a power level of the second wavelength signal.

2. The apparatus according to claim 1 wherein the diffractive light modulator comprises a grating light valve.

3. An apparatus for selectively adjusting power levels of component signals of a wavelength division multiplexed signal including a first wavelength signal and a second wavelength signal, the apparatus comprising:

a. a demultiplexer configured to de-multiplex the wavelength division multiplexed signal into the component signals, including the first wavelength signal and the second wavelength signal;

b. a diffractive light modulator with a first pixel configured to receive the first wavelength signal and a second pixel configured to receive the second wavelength signal from the multiplexer, wherein each pixel is controllable to selectively direct a received signal into a first mode and a second mode, and further wherein each first mode is collected such that a power level of the first wavelength signal is selectively adjusted relative to a power level of the second wavelength signal thereby forming first and second collected wavelength signals; and

c. a multiplexer configured to multiplex the first and second collected wavelength signals into an output signal.

4. The apparatus of claim 3 wherein the first mode comprises a reflection mode and further wherein the second mode comprises a diffraction mode.

5. The apparatus of claim 3 further comprising:

a. a light sensor for each pixel for determining a power level proportional to the first mode from the pixel; and

b. a controller electrically coupled to each light sensor and the diffractive light modulator such that in operation the light sensors detect power levels proportional to the first and second collected wavelength signals and further such that in operation the controller dynamically modulates the first and the second pixels of the diffractive light modulator to maintain a desired level of power in the first mode for each pixel relative to each other.

6. The apparatus of claim 3 wherein the multiplexer and the demultiplexer comprise separate components.

7. The apparatus of claim 3 wherein the multiplexer and the demultiplexer comprise a bi-directional multiplexer/demultiplexer.

8. The apparatus of claim 7 further comprising a circulator such that in operation the wavelength division multiplexed signal couples to the bi-directional multiplexer/demultiplexer through the circulator from an optical input.

9. The apparatus of claim 8 wherein the output signal couples from the bi-directional multiplexer/demultiplexer through the circulator to an optical output.

10. The apparatus of claim 3 further comprising first and second lenses configured such that, in operation, the first lens directs the first wavelength signal onto the first pixel, and the second lense directs the second wavelength signal onto the second pixel.

11. The apparatus of claim 3 further comprising first and second optical fibers for respectively directing the first and second wavelength signals onto the first and second pixels.

12. The apparatus of claim 3 wherein the diffractive light modulator comprises a grating light valve.

13. The apparatus of claim 3 wherein the demultiplexer comprises a dispersion device.

14. The apparatus of claim 13 wherein the dispersion device comprises a prism.

15. The apparatus of claim 13 wherein the dispersion device comprises a grating.

16. The apparatus of claim 15 wherein the grating comprises an array waveguide grating.

17. A method of adjusting a power level of each of a plurality of component wavelength signals of a wavelength division multiplexed signal comprising the steps of:

a. de-multiplexing an input signal into component wavelength signals including first and second wavelength signals;

b. directing the first wavelength signal onto a first pixel and the second wavelength signal onto a second pixel;

c. modulating the first pixel to selectively direct the first wavelength signal into a first mode and a second mode;

d. modulating the second pixel to selectively direct the second wavelength signal into the first mode and the second mode; and

e. collecting the first mode of the first wavelength signal and the first mode of the second wavelength signal such that a power level of the first wavelength signal is adjusted relative to a power level of the second wavelength signal.

18. The method of claim 17 wherein the first mode comprises a reflection mode and further wherein the second mode comprises a diffraction mode.

19. An apparatus for adjusting a power level of each of a plurality of component wavelength signals of a wavelength division multiplexed signal comprising:

a. means for de-multiplexing an input signal into component wavelength signals including first and second wavelength signals;

b. means for directing the first wavelength signal onto a first pixel and the second wavelength signal onto a second pixel;

c. means for modulating the first pixel to selectively direct the first wavelength signal into a first mode and a second mode;

d. means for modulating the second pixel to selectively direct the second wavelength signal into the first mode and the second mode; and

e. means for collecting the first mode of the first wavelength signal and the first mode of the second wavelength signal such that a power level of the first wavelength signal is adjusted relative to a power level of the second wavelength signal.

20. The apparatus according to claim 19 wherein the first mode comprises a reflection mode and further wherein the second mode comprises a diffraction mode.

21. A channel equalizer for a wavelength division multiplexing system comprising:

a. a demultiplexer configured to separate the wavelength division multiplexed input signal into a plurality of input wavelength signals, including a first input wavelength signal;

b. a diffractive light modulator comprising a plurality of pixels configured to modulate the plurality of input wavelength signals into a plurality of modulated signals, including a first pixel configured to controllably modulate the first input wavelength signals into a first modulated signal, such that a modulation of a pixel controls a percent of a modulated signal forming a reflected component and a percent of a modulated signal forming a diffracted component, wherein a percent of the first modulated signal modulated into a first diffracted signal is within the range of zero percent to one hundred percent of the first modulated signal;

c. a multiplexer configured to combine the reflected components of the plurality of modulated signals into a wavelength division multiplexed output signal.

22. The channel equalizer of claim 21 wherein the diffractive light modulator comprises a grating light valve.

23. The channel equalizer of claim 22 further comprising a light sensor configured to measure a reflected portion of the first modulated signal.

24. The channel equalizer of claim 22 further comprising a light sensor configured to measure a portion of the first diffracted signal.

25. The channel equalizer of claim 22 further comprising a controller coupled to the light sensor, wherein the controller is configured to control the first pixel according to a power level of light measured by the light sensor.

26. The channel equalizer according to claim 22 further comprising a circulator coupled to both the wavelength division multiplexed input signal and a wavelength division multiplexed output signal.

27. The channel equalizer according to claim 22 wherein the grating light valve is a step grating light valve.

28. The channel equalizer according to claim 22 wherein the grating light valve is a center well grating light valve.

29. The channel equalizer according to claim 22 wherein the grating light valve is a fine-grating grating light valve.

30. The channel equalizer of claim 22 wherein power levels of the plurality of input wavelength signals are equalized to a reference power level.

31. The channel equalizer of claim 22 wherein power levels of the plurality of input wavelength signals are equalized to a reference power-level function.

32. A method of selectively equalizing respective power levels among a plurality of component wavelength signals of a wavelength division multiplexed signal, the method comprising the steps:

a. directing the plurality of component incoming wavelength signals onto a respective plurality of pixels of a diffractive light modulator, wherein a first component incoming wavelength signal is directed onto a first pixel;

b. controlling the respective pixels by a controller to control an amount of modulation imposed on a component incoming wavelength signal;

c. producing a plurality of modulated signals, wherein each of the modulated signals comprises a reflected component, and wherein at least one modulated signal comprises a diffracted component; and

d. equalizing respective power levels among the reflected components of the modulated signals.

33. The method according to claim 32 wherein the diffractive light modulator is a grating light valve.

34. The method according to claim 32 further comprising the steps of:

a. collecting a portion of the plurality of modulated signals though a respective plurality of light sensors; and

b. measuring a power level of light collected within each of the plurality of light sensors.

35. The method according to claim 34 wherein light collected within a light sensor comprises a reflected portion of a modulated signal.

36. The method according to claim 34 wherein light collected within a light sensor comprises a diffracted portion of a modulated signal.

37. The method according to claim 35 further comprising the steps:

a. converting the power levels of the light collected in each of the plurality of light sensors to a respective digital value, and

b. using a digital value to control the first pixel.

38. The method according to claim 33 further comprising the steps:

a. collecting a plurality of collected portions of reflected signals into a wavelength multiplexer; and

b. combining the plurality of collected portions of the reflected signals reaction signals into a multiplexed output signal.