Multimode planar waveguide spectral filter

• US 20040208466A1
• Filed: 05/11/2004
• Published: 10/21/2004
• Est. Priority Date: 03/16/2000
• Status: Active Grant

• Current Assignee: Steyphi Services DE LLC (Intellectual Ventures LLC)
• Original Assignee: LightSmyth Technologies, Inc. (II-VI Incorporated)
• Inventors: Greiner, Christoph M., Iazikov, Dmitri, Mossberg, Thomas W.
• Granted Patent: US 6,987,911 B2
• Time in Patent Office: Days
• US Class Current: 385/129
• CPC Class Codes: G02B 2006/12164 Multiplexing; Demultiplexing
• CPC Class Codes: G02B 5/203 having holographic or diffractive elements diffraction gratings per se G02B5/18; holographic elements per se G02B5/32; generating the spectrum using diffraction elements G01J3/18
• CPC Class Codes: G02B 5/32 Holograms used as optical elements
• CPC Class Codes: G02B 6/124 Geodesic lenses or integrated gratings
• CPC Class Codes: G02B 6/29325 of the slab or planar or plate like form, i.e. confinement in a single transverse dimension only integrated circuit planar waveguide arrangements G02B6/12007; specially adapted for lighting G02B6/0011
• CPC Class Codes: G03H 1/0005 Adaptation of holography to specific applications holographic optical element G02B5/32; holographic scanner G02B26/106; recognition using holographic mask G06V10/88; holographic memories G11B7/0065, G11C13/042
• CPC Class Codes: G03H 1/0248 Volume holograms

Abstract

A spectral filter comprises a planar optical waveguide having at least one set of diffractive elements. The waveguide confines in one transverse dimension an optical signal propagating in two other dimensions therein. The waveguide supports multiple transverse modes. Each diffractive element set routes, between input and output ports, a diffracted portion of the optical signal propagating in the planar waveguide and diffracted by the diffractive elements. The diffracted portion of the optical signal reaches the output port as a superposition of multiple transverse modes. A multimode optical source may launch the optical signal into the planar waveguide, through the corresponding input optical port, as a superposition of multiple transverse modes. A multimode output waveguide may receive, through the output port, the diffracted portion of the optical signal. Multiple diffractive element sets may route corresponding diffracted portions of optical signal between one or more corresponding input and output ports.

Claims

1. An optical apparatus, comprising a planar optical waveguide having at least one set of diffractive elements, the planar optical waveguide substantially confining in one transverse spatial dimension an optical signal propagating in two other spatial dimensions therein, wherein:

the planar waveguide supports multiple optical transverse modes in the confined transverse dimension;

each diffractive element set routes, between a corresponding input optical port and a corresponding output optical port, a corresponding diffracted portion of the optical signal propagating in the planar waveguide that is diffracted by the diffractive element set;

the optical signal is successively incident on the diffractive elements; and

the corresponding diffracted portion of the optical signal reaches the corresponding output optical port as a superposition of multiple optical transverse modes supported by the planar optical waveguide.

2. The apparatus of claim 1, further comprising at least one corresponding multimode optical source positioned and aligned so as to launch the optical signal into the planar waveguide through the corresponding input optical port as a superposition of multiple optical transverse modes supported by the planar optical waveguide.

3. The apparatus of claim 2, wherein the multimode optical source comprises a multimode channel optical waveguide.

4. The apparatus of claim 2, wherein the multimode optical source comprises a multimode optical fiber.

5. The apparatus of claim 2, wherein the multimode optical source comprises a light-emitting diode.

6. The apparatus of claim 2, wherein the multimode optical source comprises a laser diode.

7. The apparatus of claim 1, further comprising at least one corresponding multimode output optical waveguide positioned and aligned so as to receive the corresponding diffracted portion of the optical signal through the corresponding output optical port.

8. The apparatus of claim 7, wherein the multimode output optical waveguide comprises a multimode channel optical waveguide.

9. The apparatus of claim 7, wherein the multimode output optical waveguide comprises a multimode optical fiber.

10. The apparatus of claim 1, further comprising at least one corresponding photodetector positioned and aligned at the corresponding output optical port so as to receive the corresponding diffracted portion of the optical signal through the corresponding output optical port.

11. The apparatus of claim 1, wherein the diffractive elements are curvilinear diffractive elements.

12. The apparatus of claim 11, wherein the diffractive elements are arranged as focusing elements, and the corresponding input optical port and the corresponding output optical port are located at corresponding conjugate image points defined by the focusing elements.

13. The apparatus of claim 12, wherein the focusing elements define a non-unity conjugate ratio.

14. The apparatus of claim 11, wherein the diffractive elements are arranged so as to yield a substantially collimated diffracted portion of the optical signal.

15. The apparatus of claim 1, wherein:

the planar optical waveguide comprises a core surrounded by first and second claddings, a core index n_core being larger than respective cladding indices n₁ and n₂;

each localized portion of each diffractive element set diffracts a corresponding portion of the output signal that falls within a set of mode-to-mode diffracted spectral peaks, wherein center wavelengths of the mode-to-mode diffracted spectral peaks fall within a corresponding diffractive element set local bandpass Δ

λ

_loc at a corresponding diffractive element set nominal local wavelength λ

_loc, where λ

_loc is twice a local optical path difference, within the planar optical waveguide between adjacent diffractive elements, divided by a selected diffracted order; and

a diffractive element set local fractional bandpass ratio Δ

λ

_loc/λ

_loc is substantially equal to (n_core−

n_clad)/n_core, where n_clad is the larger of n₁ and n₂.

16. The apparatus of claim 15, wherein λ

_loc is substantially spatially uniform along at least one of the diffractive element sets.

17. The apparatus of claim 16, wherein the diffractive element set with substantially spatially uniform λ

_loc is arranged so that the mode-to-mode spectral peaks are sufficiently spectrally broad so as to overlap, resulting in substantially continuous diffraction of the optical signal across the bandpass Δ

λ

_loc.

18. The apparatus of claim 15, wherein λ

_loc spatially varies along at least one of the diffractive element sets.

19. The apparatus of claim 18, wherein λ

_loc is spatially chirped along at least one of the diffractive element sets.

20. The apparatus of claim 18, wherein λ

_loc is spatially segmented along at least one of the diffractive element sets.

21. The apparatus of claim 18, wherein spectral shifting of individual mode-to-mode diffracted spectral peaks due to the spatial variation of λ

_loc results in substantially continuous diffraction of the optical signal across a bandpass Δ

λ

_set, where Δ

λ

_set is about equal to Δ

λ

_loc plus the range of variation of λ

_loc along the diffractive element set.

22. The apparatus of claim 1, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements are located at an interface between the core and cladding.

23. The apparatus of claim 1, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements are located entirely within the core.

24. The apparatus of claim 1, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements span the entire thickness of the core.

25. The apparatus of claim 1, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements are located in the cladding.

26. The apparatus of claim 1, wherein:

the optical element has multiple sets of diffractive elements;

each diffractive element set routes, between a corresponding input optical port and a corresponding output optical port, a corresponding portion of the optical signal that is diffracted by the diffractive element set; and

each corresponding portion of the optical signal reaches the corresponding output optical port as a superposition of multiple optical transverse modes supported by the planar optical waveguide.

27. The apparatus of claim 26, wherein at least two of the corresponding diffracted portions of the optical signal fall within distinct spectral passbands.

28. The apparatus of claim 26, wherein at least two of the multiple diffractive element sets are overlaid.

29. The apparatus of claim 26, wherein at least two of the multiple diffractive element sets are positioned sequentially.

30. The apparatus of claim 26, wherein at least two of the multiple diffractive element sets are interleaved.

31. The apparatus of claim 26, further comprising multiple optical input ports, wherein at least two diffracted portions of the optical signal are routed from different ones of the multiple optical input ports.

32. The apparatus of claim 26, further comprising multiple optical output ports, wherein at least two diffracted portions of the optical signal are routed to different ones of the multiple optical output ports.

33. A method, comprising:

launching an optical signal through an input optical port into a planar waveguide, the planar optical waveguide substantially confining in one transverse dimension the optical signal propagating in two other dimensions therein; and

receiving from the planar optical waveguide through an output optical port at least one diffracted portion of the optical signal diffracted by a corresponding one of at least one diffractive element set of the planar waveguide,

wherein;

the planar waveguide supports multiple optical transverse modes in the confined transverse dimension;

each diffractive element set routes, between a corresponding input optical port and a corresponding output optical port, the corresponding diffracted portion of the optical signal propagating in the planar waveguide that is diffracted by the diffractive element set; and

the corresponding diffracted portion of the optical signal received through the output optical port reaches the output optical port as a superposition of multiple transverse optical modes supported by the planar optical waveguide.

34. The method of claim 33, wherein at least one corresponding multimode optical source is positioned and aligned so as to launch the optical signal into the planar waveguide through the corresponding input optical port as a superposition of multiple optical transverse modes supported by the planar optical waveguide.

35. The method of claim 34, wherein the multimode optical source comprises a multimode channel optical waveguide.

36. The method of claim 34, wherein the multimode optical source comprises a multimode optical fiber.

37. The method of claim 34, wherein the multimode optical source comprises a light-emitting diode.

38. The method of claim 34, wherein the multimode optical source comprises a laser diode.

39. The method of claim 33, wherein at least one corresponding multimode output optical waveguide is positioned and aligned so as to receive the corresponding diffracted portion of the optical signal through the corresponding output optical port.

40. The method of claim 39, wherein the multimode output optical waveguide comprises a multimode channel optical waveguide.

41. The method of claim 39, wherein the multimode output optical waveguide comprises a multimode optical fiber.

42. The method of claim 33, wherein at least one corresponding photodetector is positioned and aligned at the corresponding output optical port so as to receive the corresponding diffracted portion of the optical signal through the corresponding output optical port.

43. The method of claim 33, wherein the diffractive elements are curvilinear diffractive elements.

44. The method of claim 43, wherein the diffractive elements are arranged as focusing elements, and the corresponding input optical port and the corresponding output optical port are located at corresponding conjugate image points defined by the focusing elements.

45. The method of claim 44, wherein the focusing elements define a non-unity conjugate ratio.

46. The method of claim 43, wherein the diffractive elements are arranged so as to yield a substantially collimated diffracted portion of the optical signal.

47. The method of claim 33, wherein:

the planar optical waveguide comprises a core surrounded by first and second claddings, a core index n_core being larger than respective cladding indices n₁ and n₂;

each localized portion of each diffractive element set diffracts a corresponding portion of the output signal that falls within a set of mode-to-mode diffracted spectral peaks, wherein center wavelengths of the mode-to-mode diffracted spectral peaks fall within a corresponding diffractive element set local bandpass Δ

λ

_loc at a corresponding diffractive element set nominal local wavelength λ

_loc, where λ

_loc is twice a local optical path difference, within the planar optical waveguide between adjacent diffractive elements, divided by a selected diffracted order; and

a diffractive element set local fractional bandpass ratio Δ

λ

_loc/λ

_loc is substantially equal to (n_core−

n_clad)/n_core, where n_clad is the larger of n₁ and n₂.

48. The method of claim 47, wherein λ

_loc is substantially spatially uniform along at least one of the diffractive element sets.

50. The method of claim 47, wherein λ

_loc spatially varies along at least one of the diffractive element sets.

51. The method of claim 50, wherein λ

_loc is spatially chirped along at least one of the diffractive element sets.

52. The method of claim 50, wherein λ

_loc is spatially segmented along at least one of the diffractive element sets.

53. The method of claim 50, wherein spectral shifting of individual mode-to-mode diffracted spectral peaks due to the spatial variation of λ

_loc results in substantially continuous diffraction of the optical signal across a bandpass Δ

λ

_set, where Δ

λ

_set is about equal to Δ

λ

_loc plus the range of variation of λ

_loc along the diffractive element set.

54. The method of claim 33, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements are located at an interface between the core and cladding.

55. The method of claim 33, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements are located entirely within the core.

56. The method of claim 33, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements span the entire thickness of the core.

57. The method of claim 33, wherein the planar optical waveguide comprises a core surrounded by lower-index cladding, and the diffractive elements are located in the cladding.

58. The method of claim 33, wherein:

the optical element has multiple sets of diffractive elements;

each diffractive element set routes, between a corresponding input optical port and a corresponding output optical port, a corresponding portion of the optical signal that is diffracted by the diffractive element set; and

each corresponding portion of the optical signal reaches the corresponding output optical port as a superposition of multiple optical transverse modes supported by the planar optical waveguide.

59. The method of claim 58, wherein at least two of the corresponding diffracted portions of the optical signal fall within distinct spectral passbands.

60. The method of claim 58, wherein at least two of the multiple diffractive element sets are overlaid.

61. The method of claim 58, wherein at least two of the multiple diffractive element sets are positioned sequentially.

62. The method of claim 58, wherein at least two of the multiple diffractive element sets are interleaved.

63. The method of claim 58, wherein the planar waveguide includes multiple optical input ports, wherein at least two diffracted portions of the optical signal are routed from different ones of the multiple optical input ports.

64. The method of claim 58, wherein the planar waveguide includes multiple optical output ports, wherein at least two diffracted portions of the optical signal are routed to different ones of the multiple optical output ports.

49. The method of claim 49, wherein the diffractive element set with substantially spatially uniform λ

_loc is arranged so that the mode-to-mode spectral peaks are sufficiently spectrally broad so as to overlap, resulting in substantially continuous diffraction of the optical signal across the bandpass Δ

λ

_loc.