Monolithic signal coupler for high-aspect ratio solid-state gain media

• US 7,860,360 B2
• Filed: 01/23/2009
• Issued: 12/28/2010
• Est. Priority Date: 01/23/2009
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Shkunov, Vladimir V., Rockwell, David Alan
• Primary Examiner(s): Pak; Sung H
• Application Number:
  US12/358,901
• Publication Number:
  US 20100189392A1
• Time in Patent Office: 704 Days
• Field of Search: None
• US Class Current: 385/50
• CPC Class Codes: G02B 6/1245 Geodesic lenses
• CPC Class Codes: G02B 6/32 having lens focusing means positioned between opposed fibre ends with lens being an integral part of the single fibre end G02B6/262
• CPC Class Codes: H01S 3/042 for solid state lasers H01S3/0401 takes precedence
• CPC Class Codes: H01S 3/06704 Housings; Packages
• CPC Class Codes: H01S 3/06729 Peculiar transverse fibre profile
• CPC Class Codes: H01S 3/06745 Tapering of the fibre, core or active region
• CPC Class Codes: H01S 3/06758 Tandem amplifiers

Abstract

A coupler and method of coupling a signal beam between from a circular-core fiber to a rectangular-core cross section fiber includes providing an optical coupler having an optical core with a high-aspect ratio cross section at one end and a circular cross section at an opposite end to receive a signal beam having a circular cross section. The signal beam is propagated from the circular fiber to the rectangular-core cross section fiber in a narrow, fast-axis direction while maintaining the size and divergence of the signal beam as it propagates. The signal beam is expanded in size while producing a collimated beam from the circular cross section end to the rectangular-core cross section end in a wide, slow-axis direction.

Claims

1. A method of coupling a signal beam from a circular-core fiber to a rectangular-core cross section fiber having a narrow, fast-axis direction and a wide, slow-axis direction, the method comprising:

providing an optical coupler having an optical core with a high-aspect ratio cross section to receive a signal beam having a circular cross section;

propagating the signal beam through the optical core in a narrow, fast-axis direction while maintaining a size and a divergence of the signal beam; and

expanding the signal beam size while producing a collimated beam in a wide, slow-axis direction.

2. The method according to claim 1, further comprising exciting a lowest-order spatial mode of the signal beam in the wide, slow-axis direction.

3. The method according to claim 1, wherein said expanding the signal beam size comprises expanding the size by a factor greater than about 10 in the rectangular-core cross section fiber.

4. The method according to claim 1, wherein the maintaining is accomplished by a passive structure that is configured to guide the signal beam in the narrow, fast-axis direction.

5. The method according to claim 1, wherein in the slow-axis direction, the signal beam is allowed to undergo free diffraction while in the presence of an effective refractive index having a parabolic profile in the slow axis direction in the core of the coupler.

6. The method according to claim 5, wherein the parabolic index profile includes a gradient-index lens in the slow-axis direction.

7. The method according to claim 1, comprising:

attaching the coupler directly at either end to the high-aspect ratio core fiber and to the circular core fiber.

8. The method according to claim 7, wherein the coupler is directly attached by bonding, splicing, and/or fusing the coupler to the high aspect ratio core fiber and to the circular core fiber.

9. The method according to claim 1, wherein the coupler enables a monolithic architecture between the high aspect ratio fiber and the circular core fiber.

10. The method according to claim 1, wherein the optical core is an active core.

11. The method according to claim 1, wherein the optical core is a passive core.

37. The method of claim 1, wherein the expanding the signal beam size is controlled by a lensing action of the optical coupler.

38. The method of claim 37, wherein the lensing action includes varying a thickness of the optical core in the narrow, fast-axis direction.

39. The method of claim 37, wherein the lensing action includes applying a transverse refractive-index in the wide, slow-axis direction of the optical core.

40. The method of with claim 37, wherein the lensing action includes applying a spatially varying strain to the optical core.

12. A coupler configured to couple a single-transverse-mode signal beam emerging from a round core fiber into a lowest order planar mode of a rectangular core of a semi-guiding high-aspect ratio core fiber, the coupler comprising:

an optical coupler including an optical core having a high-aspect ratio cross section to receive a signal beam having a circular cross section, wherein

the optical coupler is arranged to propagate the signal beam through the optical core in a narrow, fast-axis direction while maintaining a size and a divergence of the signal beam,

wherein the optical coupler is arranged to allow the signal beam to expand in size and also to be collimated in a wide, slow-axis direction of the high aspect ratio cross section.

13. The coupler according to claim 12, wherein the coupler is made as a miniature planar waveguiding channel.

14. The coupler according to claim 13, wherein the coupler is arranged with a core thickness at one end that corresponds to a diameter of an input fiber core and a thickness of the semi-guiding high-aspect ratio core fiber at the opposite end.

15. The coupler according to claim 14, wherein the core thickness is varied along the slow-axis direction to produce a collimated beam along the slow-axis direction.

16. The coupler according to claim 14, wherein the core thickness is arranged to be thickest along a longitudinal axis of the coupler and with the thickness gradually increasing along the slow-axis direction toward either edge of the core.

17. The coupler according to claim 14, wherein the planar waveguiding channel is arranged to provide a focusing effect in a plane of the channel due to a profile of the planar channel.

18. The coupler according to claim 17, wherein the focusing effect is controlled by arranging a transverse profile for a refractive index of the optical core that provides a higher refractive index near a center of the beam and a gradual reduction at an edge of the beam.

19. The coupler according to claim 12, wherein a length of the coupler is equal to a quarter-pitch distance.

20. The coupler in accordance with claim 12, wherein the coupler is configured to be directly attached at either end to the high-aspect ratio core fiber and to the circular core fiber.

21. The coupler in accordance with claim 20, wherein the coupler is directly attached by bonding, splicing, and/or fusing the coupler to the high aspect ratio core fiber and to the circular core fiber.

22. The coupler in accordance with claim 12, wherein the coupler enables a monolithic architecture between the high aspect ratio fiber and the circular core fiber.

23. The coupler in accordance with claim 12, wherein the optical core is an active core.

24. The coupler in accordance with claim 12, wherein the optical core is a passive core.

41. The coupler of with claim 12, wherein the signal beam is expanded in size by a lensing action of the optical coupler.

42. The coupler of claim 41, wherein the lensing action includes varying a thickness of the optical core in the narrow, fast-axis direction.

43. The coupler of with claim 41, wherein the lensing action includes applying a transverse refractive-index in the wide, slow-axis direction of, the optical core.

44. The coupler of with claim 41, wherein the lensing action includes applying a spatially varying strain to the optical core.

25. A method of coupling a signal beam from a rectangular-core fiber having a narrow, fast-axis direction and a wide, slow-axis direction to a circular-core cross section fiber, the method comprising:

providing an optical coupler having an optical core having a high-aspect ratio cross section;

propagating the signal beam through the optical core in a narrow, fast-axis direction while maintaining a size and a divergence of the signal beam; and

changing the signal beam size while producing a collimated beam in a wide, slow-axis direction.

26. The method in accordance with claim 25, comprising:

attaching the coupler directly at either end to the high-aspect ratio core fiber and to the circular core fiber.

27. The method in accordance with claim 26, wherein the coupler is directly attached by bonding, splicing, and/or fusing the coupler to the high aspect ratio core fiber and to the circular core fiber.

28. The method in accordance with claim 25, wherein the coupler enables a monolithic architecture between the high aspect ratio fiber and the circular core fiber.

29. The method in accordance with claim 25, wherein the optical core is an active core.

30. The method in accordance with claim 25, wherein the optical core is a passive core.

31. A coupler configured to couple a single-transverse-mode signal beam emerging from a high-aspect ratio rectangular core fiber to a circular core fiber, the coupler comprising:

an optical coupler including an optical core having a high aspect ratio cross section, wherein

the optical coupler is arranged to propagate the signal beam through the optical core in a narrow, fast-axis direction while maintaining a size and a divergence of the signal beam,

wherein the optical coupler is arranged to collimate and narrow in size the signal beam in an initially wide, slow-axis direction.

32. The coupler in accordance with claim 31, wherein the coupler is configured to be directly attached at either end to the high-aspect ratio core fiber and to the circular core fiber.

33. The coupler in accordance with claim 32, wherein the coupler is directly attached by bonding, splicing, and/or fusing the coupler to the high aspect ratio core fiber and to the circular core fiber.

34. The coupler in accordance with claim 31, wherein the coupler enables a monolithic architecture between the high aspect ratio fiber and the circular core fiber.

35. The coupler in accordance with claim 31, wherein the optical core is an active core.

36. The coupler in accordance with claim 31, wherein the optical core is a passive core.