Spectral power monitors with active alignment compensation

• US 6,504,976 B1
• Filed: 11/14/2001
• Issued: 01/07/2003
• Est. Priority Date: 09/20/2001
• Status: Expired due to Term

• Current Assignee: Capella Photonics, Inc. (Nokia Corporation)
• Original Assignee: Capella Photonics, Inc. (Nokia Corporation)
• Inventors: Polynkin, Pavel G., Wilde, Jeffrey P., Timmons, Michael J.
• Primary Examiner(s): Healy, Brian
• Assistant Examiner(s): Wood, Kevin S
• Application Number:
  US09/992,778
• Time in Patent Office: 419 Days
• Field of Search: 385/15, 385/24, 385/37, 385/52, 359/115, 359/124, 359/130
• US Class Current: 385/37
• CPC Class Codes: G01J 2003/2866 Markers; Calibrating of scan
• CPC Class Codes: G01J 3/0224 using polarising or depolarising elements
• CPC Class Codes: G01J 3/2803 using photoelectric array detector
• CPC Class Codes: G01J 3/447 Polarisation spectrometry
• CPC Class Codes: G02B 6/2713 cascade of polarisation selective or adjusting operations
• 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 6/4213 the intermediate optical elements being polarisation selective optical elements G02B6/27 takes precedence
• CPC Class Codes: G02B 6/4215 the intermediate optical elements being wavelength selective optical elements, e.g. variable wavelength optical modules or wavelength lockers G02B6/4246 takes precedence
• CPC Class Codes: G02B 6/4225 by a direct measurement of the degree of coupling, e.g. the amount of light power coupled to the fibre or the opto-electronic element
• CPC Class Codes: G02B 6/4227 Active alignment methods, e.g. procedures and algorithms

Abstract

This invention provides a spectral power monitoring apparatus that uses a diffraction grating to separate a multi-wavelength optical signal along with a reference signal by wavelength into multiple spectral channels and a reference spectral component having a predetermined relative alignment, impinging onto an array of optical power sensors. The optical power sensor array may be configured such that the power levels of the spectral channels impinging onto the optical power sensor array can be related to the electrical signals thus produced by a predetermined conversion matrix. The spectral power monitoring apparatus of the present invention further includes an alignment compensation unit, which monitors the real-time impinging position of the reference spectral component and ensures that the corresponding conversion matrix is used for converting the measured electrical signals to the optical power levels of interest. The spectral power monitoring apparatus of the present invention is well suited for optical networking applications.

Claims

1. An optical apparatus comprising:

a) an input port, providing a multi-wavelength optical signal and a reference signal;

b) a wavelength-disperser that separates said multi-wavelength optical signal and said reference signal by wavelength into multiple spectral channels and a reference spectral component having a predetermined relative arrangement;

c) an array of optical power sensors, including a reference-position-sensing element for receiving said reference spectral component and a plurality of channel-sensing elements for receiving said spectral channels; and

d) an alignment compensation unit, including an alignment-adjusting element for adjusting an alignment of said spectral channels along with said reference spectral component and a processing element;

wherein said processing element monitors an impinging position of said reference spectral component on said reference-position-sensing element and provides control of said alignment-adjusting element accordingly, so as to maintain said reference spectral component at a predetermined location on said reference-position-sensing element and thereby ensure a particular alignment between said spectral channels and said channel-sensing elements.

2. The optical apparatus of claim 1 wherein said alignment-adjusting element comprises an actuation device, coupled to said array of optical power sensors, for causing said array of optical power sensors to move.

3. The optical apparatus of claim 1 further comprising a beam-focuser for focusing said spectral channels and said reference spectral component into corresponding focused spots, impinging onto said array of optical power sensors, wherein said alignment-adjusting element comprises an actuation device coupled to said beam-focuser, for causing said beam-focuser to move.

4. The optical apparatus of claim 1 wherein said array of optical power sensors comprises a photodiode array.

5. The optical apparatus of claim 4 wherein each channel-sensing element receives a separate one of said spectral channels.

6. The optical apparatus of claim 4 wherein said photodiode array possesses a continuous overall photo-response function.

7. The optical apparatus of claim 6 wherein said reference-position-sensing element comprises two adjacent channel-sensing elements in said photodiode array.

8. The optical apparatus of claim 1 further comprising a signal processor, for deriving power levels of said spectral channels impinging onto said channel-sensing elements from output signals produced by said channel-sensing elements.

9. The optical apparatus of claim 8 wherein said signal processor contains a predetermined conversion matrix that relates said output signals to said power levels.

10. The optical apparatus of claim 1 wherein said wavelength-disperser comprises an element selected from the group consisting of ruled diffraction gratings, holographic diffraction gratings, echelle gratings, curved diffraction gratings, transmission gratings, and dispersing prisms.

11. The optical apparatus of claim 1 wherein said input port comprises a fiber collimator, coupled to an input optical fiber.

12. The optical apparatus of claim 11 further comprising an optical combiner, for coupling a reference light source to said input optical fiber, wherein said input optical fiber transmits said multi-wavelength optical signal and said reference light source provides said reference signal.

13. The optical apparatus of claim 1 further comprising a beam-focuser, for focusing said spectral channels and said reference spectral component into corresponding focused spots.

14. The optical apparatus of claim 1 further comprising a polarization-separating element and a polarization-rotating element, in optical communication with said input port and said wavelength-disperser, wherein said polarization-separating element decomposes said multi-wavelength optical signal along with said reference signal into first and second polarization components, and said first polarization-rotating element rotates a polarization of said second polarization component by 90-degrees.

15. The optical apparatus of claim 14 wherein said polarization-separating element comprises an element selected from the group consisting of polarizing beam splitters and birefringent beam displacers.

16. The optical apparatus of claim 14 wherein said polarization-rotating element comprises an element selected from the group consisting of half-wave plates, liquid crystal rotators, and Faraday rotators.

17. The optical apparatus of claim 14 further comprising an auxiliary polarization-rotating element, in optical communication between said wavelength-disperser and said array of optical power sensors, such that dispersed optical beams originating from said first polarization component each undergo a 90-degree rotation in polarization.

18. The optical apparatus of claim 17 wherein said auxiliary polarization-rotating element comprises an element selected from the group consisting of half-wave plates, Faraday rotators, and liquid crystal rotators.

19. An optical apparatus, comprising:

a) an input port, providing a multi-wavelength optical signal and a reference signal;

b) a wavelength-disperser that separates said multi-wavelength optical signal and said reference signal by wavelength into multiple spectral channels and a reference spectral component having a predetermined relative arrangement;

c) an array of optical power sensors, including a reference-position-sensing element for receiving said reference spectral component and a plurality of channel-sensing elements for receiving said spectral channels; and

d) an alignment compensation unit, in communication with said array of optical power sensors;

wherein said alignment compensation unit monitors an impinging position of said reference spectral component on said reference-position-sensing element, and wherein said alignment compensation unit further includes a predetermined calibration table containing a plurality of conversion matrices each corresponding to a particular impinging position of said reference spectral component, whereby for each impinging position of said reference spectral component detected, said alignment compensation unit looks up a corresponding conversion matrix from said calibration table that relates output signals from said channel-sensing elements to power levels of said spectral channels impinging onto said channel-sensing elements.

20. The optical apparatus of claim 19 wherein said array of optical power sensors comprises a photodiode array.

21. The optical apparatus of claim 20 wherein said photodiode array possesses a continuous overall photo-response function.

22. The optical apparatus of claim 21 wherein said reference-position-sensing element comprises at least two adjacent channel-sensing elements in said photodiode array.

23. The optical apparatus of claim 19 wherein said wavelength-disperser comprises an element selected from the group consisting of ruled diffraction gratings, holographic gratings, echelle gratings, curved diffraction gratings, transmission gratings and dispersing prisms.

24. The optical apparatus of claim 19 wherein said input port comprises a fiber collimator coupled to an input optical fiber.

25. The optical apparatus of claim 24 further comprising an optical combiner, for coupling a reference light source to said input optical fiber, wherein said input optical fiber transmits said multi-wavelength optical signal and said reference light source provides said reference signal.

26. The optical apparatus of claim 19 further comprising a beam-focuser that focuses said spectral channels and said reference spectral component into corresponding focused spots.

27. The optical apparatus of claim 19 further comprising a polarization-separating element and a polarization-rotating element, in optical communication with said input port and said wavelength-disperser, wherein said polarization-separating element decomposes said multi-wavelength optical signal along with said reference signal into first and second polarization components, and said polarization-rotating element rotates a polarization of said second polarization component by 90-degrees.

28. The optical apparatus of claim 27 wherein said polarization-separating element comprises an element selected from the group consisting of polarizing beam splitters and birefringent beam displacers.

29. The optical apparatus of claim 27 said polarization-rotating element comprises an element selected from the group consisting of half-wave plates, liquid crystal rotators, and Faraday rotators.

30. The optical apparatus of claim 27 further comprising an auxiliary polarization-rotating element, in optical communication between said wavelength-disperser and said array of optical power sensors, such that dispersed optical beams originating from said first polarization component each undergo a 90-degree rotation in polarization.

31. The optical apparatus of claim 30 wherein said auxiliary polarization-rotating element comprises an element selected from the group consisting of half-wave plates, Faraday rotators, and liquid crystal rotators.

32. A method of using software-based alignment compensation in spectral power monitoring, comprising:

a) providing a multi-wavelength optical signal with a reference signal;

b) spatially separating said multi-wavelength optical signal and said reference signal by wavelength into multiple spectral channels and a reference spectral component having a predetermined relative arrangement;

c) impinging said reference spectral component and said spectral channels onto an array of optical power sensors; and

d) determining an impinging position of said reference spectral component, and looking up a corresponding conversion matrix from a predetermined calibration table that relates output signals from said array of optical power sensors to power levels of said spectral channels impinging onto said array of optical power sensors, thereby providing a power spectrum of said multi-wavelength optical signal.

33. The method of claim 32 further comprising the step of performing a calibration, so as to obtain said predetermined calibration table.

34. The method of claim 32 wherein said array of optical power sensors possesses a continuous overall photo-response function.