Selective intensity modulation of channels in a multiplexed optical communication system

• US 6,529,307 B1
• Filed: 06/04/2001
• Issued: 03/04/2003
• Est. Priority Date: 06/05/2000
• Status: Expired due to Term

• Current Assignee: II-VI Delaware, Inc. (Coherent Corp.)
• Original Assignee: Avanex Corporation (Oclaro (North America), Inc.)
• Inventors: Peng, Song, Li, Ming
• Primary Examiner(s): Mai, Huy
• Assistant Examiner(s): TRA, TUYEN Q
• Application Number:
  US09/874,474
• Time in Patent Office: 638 Days
• Field of Search: 349/196, 349/193, 359/115, 359/117, 359/122, 359/128, 359/129, 359/130, 359/131, 359/256, 359/245, 359/315, 359/246, 359/316, 359/250, 359/310, 359/722, 359/589, 385/16, 385/18, 385/33, 385/35, 385/36, 385/37
• US Class Current: 359/256
• CPC Class Codes: H04B 10/2569 due to polarisation mode dispersion [PMD]
• CPC Class Codes: H04B 10/532 Polarisation modulation

Abstract

A method and apparatus for achieving dynamic intensity modulation of the channels in a wavelength-division multiplexed optical communication system is presented. Wavelengths are spatially separated into a plurality of channels, the polarization states of which are individually modulated. The channels can be combined or filtered by polarization states to achieve the desired intensity in the output signal. An exemplary embodiment includes at least a polarization modulator, a birefringent wedge, a lens, and a dispersive element (e.g., diffraction grating) arranged in various order. Each segment of the polarization modulator can be made to rotate the polarization direction of an incident channel by a specified angle. A half-wave plate may be inserted between the second dispersive element and the second birefringent wedge to eliminate polarization-dependent loss. Optionally, a parallel birefringent plate may be inserted after the second birefringent wedge to reduce polarization mode dispersion.

Claims

1. A wavelength-selective intensity modulator comprising:

a birefringent element having optic axis aligned to spatially separate a beam of light according to the direction of polarization;

a dispersive element optically coupled to said birefringent element; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator comprises a plurality of segments, wherein a first polarized beam comprising a first linear polarization and a second polarized beam comprising a second linear polarization pass through separate segments.

2. The wavelength-selective intensity modulator of claim 1, wherein said segments are tunable within a predefined range, said segments aligned along an axis perpendicular to the direction in which said beam of light travels through said polarization modulator.

3. The wavelength-selective intensity modulator of claim 1, wherein said segments are placed in one of a plurality of settings, each setting being associated with changing an incident channel of light by a predetermined number of degrees.

4. The wavelength-selective intensity modulator of claim 3, further comprising:

a second dispersive element optically coupled to said polarization modulator;

a second birefringent element optically coupled to said second dispersive element; and

a second lens optically coupled to said second dispersive element and said polarization modulator.

5. The wavelength-selective intensity modulator of claim 4, wherein said first birefringent element and said second birefringent element are connected to two opposing sides of said polarization modulator.

6. The wavelength-selective intensity modulator of claim 4, further comprising a half-wave plate coupled to said second dispersive element and said second birefringent element.

7. The wavelength-selective intensity modulator of claim 4, wherein the distance between said second lens and said polarization modulator is substantially equal to the focal length of said second lens in the medium between said lens and said polarization modulator.

8. The wavelength-selective intensity modulator of claim 4 wherein at least one of said first and second birefringent elements comprises one of calcite crystals, yttrium vanadate, and yttrium orthovanadate.

9. The wavelength-selective intensity modulator of claim 1, further comprising a first lens optically coupled to said dispersive element and said polarization modulator.

10. The wavelength-selective intensity modulator of claim 9, further comprising:

a reflective surface on said polarization modulator; and

a half-wave waveplate optically coupled to said dispersive element and said first lens and positioned to intersect beams traveling from said reflective surface to said dispersive element.

11. The wavelength-selective intensity modulator of claim 9, wherein said first lens comprises a cylindrical lens.

12. The wavelength-selective intensity modulator of claim 1, wherein said dispersive element is wavelength-dispersive.

13. The wavelength-selective intensity modulator of claim 1, wherein said dispersive element is a diffraction grating.

14. A wavelength-selective intensity modulator comprising:

a birefringent element having optic axis aligned to spatially separate a beam of light according to the direction of polarization;

a dispersive element optically coupled to said birefringent element;

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator comprises a plurality of segments tunable within a predefined range, said segments aligned along an axis perpendicular to the direction in which said beam of light travels through said polarization modulator; and

an angled reflector coupled to said birefringent element, said angled reflector comprising two or more reflective surfaces positioned at an angle with respect to one another.

15. A wavelength-selective intensity modulator comprising:

a birefringent element having optic axis aligned to spatially separate a beam of light according to the direction of polarization;

a dispersive element optically coupled to said birefringent element; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator comprises a plurality of segments tunable within a predefined range, said segments aligned along an axis perpendicular to the direction in which said beam of light travels through said polarization modulator, wherein segments are aligned along an axis parallel to the direction in which said beam of light travels from said dispersive element, further comprising a first prism and a second prism coupled to two opposing sides of said polarization modulator, said first prism reflecting a channel toward said second prism and said second prism reflecting said As channel toward said dispersive element.

16. A wavelength-selective intensity modulator comprising:

a birefringent element having optic axis aligned to spatially separate a beam of light according to the direction of polarization;

a dispersive element optically coupled to said birefringent element, wherein said dispersive element is an Echelle grating; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator comprises a plurality of segments tunable within a predefined range, said segments aligned along an axis perpendicular to the direction in which said beam of light travels through said polarization modulator, wherein segments are aligned along an axis parallel to the direction in which said beam of light travels from said dispersive element, further comprising a first prism and a second prism coupled to two opposing sides of said polarization modulator, said first prism reflecting a channel toward said second prism and said second prism reflecting said channel toward said dispersive element.

17. A method of modulating the intensity of a multiplexed optical signal, said method comprising:

spatially separating said optical signal into a first light bin comprising a first polarization and a second light beam comprising a second polarization;

spatially separating the first light beam into a plurality of first channel light beams and the second light beam into a plurality of second channel light beams according to wavelength;

selectively changing the direction polarization of at least some of said plurality of first and second channel light beams, wherein the changing is performed pair-wise upon one of the first channel light beams and one of the second channel light beams;

using a birefringent element to create an output signal and an attenuation beam, said attenuation beam being spatially separate from said output signal; and

coupling only said output signal into a signal transfer medium.

18. The method of claim 17, further comprising inserting a parallel birefringent plate between said birefringent element and said signal transfer medium to compensate for polarization mode dispersion, the optic axis of said parallel birefringent plate being perpendicular to the optic axis of said birefringent element.

19. The method of claim 17, further comprising reflecting said light beams off of a reflective surface after using said birefringent element to spatially separate said channels.

20. The method of claim 17, further comprising passing said channels through an aperture to achieve the desired degree of attenuation.

21. A method of modulating the intensity of a multiplexed optical signal, said method comprising:

spatially separating said optical signal into a plurality of light beams according to the direction of polarization;

selectively changing the direction of polarization of at least some of said plurality of light beams;

using a birefringent element to create an output signal and an attenuation beam, said attenuation beam being spatially separate from said output signal;

coupling only said output signal into a signal transfer medium;

reflecting said light beams off of a reflective surface after using said birefringent element to spatially separate channels, wherein said reflective surface comprises two or more angled reflective surfaces that reflect different incident channels in different directions.

22. A wavelength-selective intensity modulator comprising:

a birefringent element having optic axis aligned to spatially separate a beam of light according to the direction of polarization;

a dispersive element optically coupled to said birefringent element; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator comprises a plurality of segments, each of said segments being placed in one of a plurality of settings, each setting being associated with changing an incident channel of light by a predetermined number of degrees, wherein segments are aligned along an axis parallel to the direction in which said beam of light travels from said dispersive element, further comprising a first prism and a second prism coupled to two opposing sides of said polarization modulator, said first prism reflecting said channel toward said second prism and said second prism reflecting said channel toward said dispersive element.

23. A wavelength-selective intensity modulator comprising:

a birefringent element having optic axis aligned to spatially separate a beam of light according to the direction of polarization;

a dispersive element optically coupled to said birefringent element, wherein said dispersive element is an Echelle grating; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator comprises a plurality of segments, each of said segments being placed in one of a plurality of settings, each setting being associated with changing an incident channel of light by a predetermined number of degrees, wherein segments are aligned along an axis parallel to the direction in which said beam of light travels from said dispersive element, further comprising a first prism and a second prism coupled to two opposing sides of said polarization modulator, said first prism reflecting said channel toward said second prism and said second prism reflecting said channel toward said dispersive element.

24. A wavelength-selective intensity modulator comprising:

a birefringent element having an optic axis aligned to spatially separate a light beam according to a direction of polarization;

a dispersive element optically coupled to said birefringent element; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator is operable for transforming at least one portion of said separated light beam into an elliptically polarized light beam, wherein said polarization modulator comprises a plurality of segments tunable within a predefined range, said segments aligned along an axis perpendicular to the direction in which said beam of light travels through said polarization modulator.

25. A wavelength-selective intensity modulator comprising:

a birefringent element having an optic axis aligned to spatially separate a light beam according to a direction of polarization;

a dispersive element optically coupled to said birefringent element; and

a polarization modulator optically coupled to said dispersive element, wherein said polarization modulator is operable for transforming at least one portion of said separated light beam into an elliptically polarized light beam, wherein said polarization modulator comprises a plurality of segments, each of said segments being placed in one of a plurality of settings, each setting being associated with transforming a polarized light beam into a different state of elliptical polarization.

26. A method of modulating the intensity of a multiplexed optical signal, said method comprising:

spatially separating said optical signal into a plurality of channel light beams according to wavelength;

spatially separating each channel light beam into a first polarized channel light beam comprising a first polarization and a second polarized channel light beams comprising a second polarization;

selectively changing the direction of polarization of at least some of said plurality of first polarized channel light beams and second polarized channel light beams, wherein the changing is performed pair-wise upon a first polarized channel light beam and a second polarized channel light beam;

using a birefringent element to create an output signal and an attenuation beam, said attenuation beam being spatially separate from said output signal; and