Tunable filter

• US 20030214700A1
• Filed: 11/29/2002
• Published: 11/20/2003
• Est. Priority Date: 11/30/2001
• Status: Active Grant

• Current Assignee: Optitune PLC
• Original Assignee: Optitune PLC
• Inventors: Al-Chalabi, Salah A., Damirji, Abdul H., Sidorin, Yakov
• Granted Patent: US 7,027,469 B2
• Time in Patent Office: Days
• US Class Current: 359/334
• CPC Class Codes: H01S 3/08059 Constructional details of the reflector, e.g. shape mirrors in general G02B5/08; mountings for mirrors G02B7/18
• CPC Class Codes: H01S 5/005 Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping H01S5/026, H01S5/18388 take precedence
• CPC Class Codes: H01S 5/141 using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence
• CPC Class Codes: H01S 5/146 using a fiber as external cavity
• CPC Class Codes: H01S 5/405 Two-dimensional arrays
• CPC Class Codes: H01S 5/4062 with an external cavity or using internal filters, e.g. Talbot filters

Abstract

In a tunable optical source, a diffraction device 115 is used to provide wavelength selective feedback to a laser diode 100. The diffraction device 115 is at least partially fabricated in a material whose refractive index is controllable so as to control the diffraction performance of the device. This allows the diffraction device 115 to be used in tuning the optical source, potentially without using moving parts. For instance, the refractive index can be controlled using a temperature change across material of the device 115. The diffraction device 115 can be mounted in free space in an external cavity with respect to the laser diode 100. Novel forms of diffraction device 115 are described. Tunable optical sources of this type can be used for instance in communications, particularly wavelength division multiplexing.

Claims

1. A tunable optical source which comprises:

i) a gain section for use in generating an optical output;

ii) a feedback section to provide optical feedback to the gain section, the feedback section comprising a diffraction device arranged to provide wavelength selection in said optical feedback by diffraction,

wherein the tunable optical source further comprises control means for controlling the refractive index of material of the diffraction device to modify said wavelength selection.

2. A tunable optical source according to claim 1 wherein the feedback section comprises an external cavity.

3. A tunable optical source according to claim 2 wherein the external cavity comprises free space and the diffraction device is mounted in said free space.

4. A tunable optical source according to either one of claims 2 or 3 wherein the diffraction device provides an end facet of the external cavity and plays a part in defining its length.

5. A tunable optical source according to any one of claims 2, 3 or 4 further comprising collimating means for collimating optical radiation travelling in the external cavity.

6. A tunable optical source according to any one of the preceding claims wherein optical radiation incident on the diffraction device, in use, is collimated and the diffraction device extends primarily in a direction which is transverse to the direction of the incident optical radiation.

7. A tunable optical source according to claim 6 wherein the diffraction device extends wholly across the path of the collimated incident optical radiation.

8. A tunable optical source according to any one of the preceding claims wherein the diffraction device comprises a grating to provide diffraction of incident optical radiation.

9. A tunable optical source according to any one of the preceding claims wherein the control means comprises means to apply a voltage across material of the diffraction device.

10. A tunable optical source according to any one of the preceding claims wherein the control means comprises means to apply a temperature change to material of the diffraction device.

11. A tunable optical source according to any one of the preceding claims wherein the diffraction device comprises a grating to provide diffraction of incident optical radiation and the diffraction device comprises more than one layer of material, the material whose refractive index is controlled by the control means being material of a layer adjacent to or containing the grating and through which optical radiation is incident on the grating in use.

12. A tunable optical source according to any one of the preceding claims wherein material of the diffraction device whose refractive index is controlled by the control means is selected from the group comprising glasses, photonic crystals, electro-optic polymers, thermo-optic polymers, liquid crystals, organic crystals, semiconductor materials selected from group IV of the periodic table, compound semiconductor materials selected from one or more of the III-V, II-VI and IV-VI groups of the periodic table, and photorefractive materials.

13. A tunable optical source according to any one of the preceding claims tunable to provide an optical radiation output tunable over a wavelength range of at least 10 nm.

14. A tunable optical source according to any one of the preceding claims tunable to provide an optical radiation output tunable over a wavelength range of at least 30 nm.

15. A tunable optical source according to any one of the preceding claims having an optical radiation output tunable by temperature change to show a shift in wavelength of at least 0.3 nm per °

C.

16. A tunable optical source according to any one of the preceding claims, comprising an array of two or more tunable gain sections.

17. A tunable optical source according to claim 16 wherein, in use, a common diffraction device provides wavelength selection in the feedback to more than one gain section.

18. A tunable optical source according to claim 16 wherein, in use, at least two diffraction devices provide wavelength selection in the feedback to two respective gain sections.

19. A tunable optical source according to any one of the preceding claims wherein the feedback section comprises optical path control means for controlling the length of the optical path of radiation in the feedback section.

20. A tunable optical source according to claim 19 wherein the optical path control means comprises a material whose refractive index is modifiable and the optical source comprises means to modify said refractive index so as to control the length of the optical path of radiation in the feedback section.

21. A wavelength division multiplexing system comprising an optical source according to any one of the preceding claims.

22. A method of selecting the output wavelength or wavelength range of an optical source, the source comprising a gain section and a wavelength selective feedback section, the feedback section comprising a diffraction device arranged to provide at least part of the wavelength selectivity, which method comprises modifying the refractive index of material of the diffraction device so as to modify one or more diffraction characteristics of the diffraction device.

23. A method according to claim 22 which further comprises modifying the refractive index of material of the diffraction device so as to modify reflectivity of the diffraction device.

24. A tunable optical source which comprises:

i) a gain section for use in generating an optical output;

ii) a feedback section to provide optical feedback to the gain section,

wherein the feedback section comprises a diffraction device having a diffractive structure arranged to provide wavelength selection in said optical feedback by diffraction, the diffraction device being arranged in relation to the gain section such that, in use, radiation from the gain section is incident on the diffractive structure through material of the diffraction device.

25. A tunable optical source according to claim 24 wherein the diffractive structure is formed in or adjacent to a surface of a layer of material and the diffraction device is arranged in relation to the gain section such that, in use, radiation from the gain section passes through the layer of material, or substantially through it, to the diffractive structure.

26. A tunable optical source according to either one of claims 24 or 25 wherein the diffraction device is mounted in free space and such that the diffractive structure extends primarily or wholly in a direction which is transverse to the direction of radiation incident on the diffractive structure in use.

27. A diffraction device for providing wavelength selection in optical radiation, the device comprising:

i) a body of material having a controllable refractive index;

ii) an entry point to the body of material for an incident beam of optical radiation;

iii) a diffractive structure; and

iv) control means for controlling said refractive index,

wherein the diffractive structure is arranged to extend at least generally transverse to the optical path of the optical radiation in use of the diffraction device, but non-parallel to said entry point,

whereby changes in the controllable refractive index provide said wavelength selection.

28. A diffraction device according to claim 27 wherein the incident beam is incident in a direction normal to the plane of said entry point.

29. A diffraction device according to either one of claims 27 or 28 wherein the entry point is provided at a surface of the body of material.

30. A diffraction device according to any one of claims 27 to 29 wherein the entry point is provided at an interface between the body of material and a further material of the diffraction device.

31. A diffraction device according to any one of claims 27 to 30 wherein the body of material is at least partly defined by the extent of a controllable electric field in the diffraction device in use.

32. A diffraction device according to any one of claims 27 to 31 wherein the body of material is provided as a prism, having a triangular or trapezoidal cross section, the prism having a first surface providing the entry point and a second surface carrying the diffractive structure.

33. A diffraction device according to claim 32 wherein said control means comprises one or more control elements to control the refractive index of the material of the prism, said one or more control elements being disposed on one or more facets of the prism which are not in said optical path.

34. A diffraction device according to claim 33, comprising two or more control elements, the two or more control elements being disposed on respective side facets of the prism which are parallel to the direction of the optical path.

35. A diffraction device according to any one of claims 27 to 34 wherein the diffractive structure is mounted in a Littrow configuration with respect to the incident beam.

36. A diffraction device according to any one of claims 27 to 35, wherein the grating is covered by further material, the refractive index of the further material being selected or controlled to determine the reflectivity or transmissivity of the grating in use.

37. A diffraction device according to claim 36 wherein the refractive index of the further material is controllable separately from that of the body of material.

38. A diffraction device according to any one of claims 27 to 30, further comprising a collimating device to collimate radiation received at the entry point.

39. A diffraction device according to claim 38 wherein the collimating device is provided as a lens constructed from material supported by the first surface of the prism.

40. A diffraction device according to claim 39 wherein the lens is a diffractive lens.