Multi-mode fiber amplifier

• US 20030202547A1
• Filed: 04/25/2003
• Published: 10/30/2003
• Est. Priority Date: 11/25/1998
• Status: Active Grant

• Current Assignee: IMRA America Incorporated (Aisin Corp.)
• Original Assignee: IMRA America Incorporated (Aisin Corp.)
• Inventors: Harter, Donald J., Fermann, Martin E.
• Granted Patent: US 8,761,211 B2
• Time in Patent Office: Days
• US Class Current: 372/6
• CPC Class Codes: G02B 6/26 Optical coupling means G02B6/36, G02B6/42 take precedence
• CPC Class Codes: H01S 3/0092 Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity nonlinear frequency conversion per se G02F1/35
• CPC Class Codes: H01S 3/067 Fibre lasers
• CPC Class Codes: H01S 3/06704 Housings; Packages
• CPC Class Codes: H01S 3/06708 Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering optical fibres as passive waveguides G02B6/02
• CPC Class Codes: H01S 3/06712 Polarising fibre; Polariser
• CPC Class Codes: H01S 3/06716 Fibre compositions per se C03C13/04or doping with active elements lasing materials in general H01S3/14
• CPC Class Codes: H01S 3/06725 Fibre characterized by a specific dispersion, e.g. for pulse shaping in soliton lasers or for dispersion compensating [DCF]
• CPC Class Codes: H01S 3/06729 Peculiar transverse fibre profile
• CPC Class Codes: H01S 3/06733 Fibre having more than one cladding
• CPC Class Codes: H01S 3/06737 Fibre having multiple non-coaxial cores, e.g. multiple active cores or separate cores for pump and gain
• CPC Class Codes: H01S 3/06745 Tapering of the fibre, core or active region
• CPC Class Codes: H01S 3/06783 Amplifying coupler
• CPC Class Codes: H01S 3/08045 Single-mode emission
• CPC Class Codes: H01S 3/08054 Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation quarter-wave plates in a Q-switch laser H01S3/1124, H01S3/115
• CPC Class Codes: H01S 3/094007 Cladding pumping, i.e. pump light propagating in a clad surrounding the active core
• CPC Class Codes: H01S 3/094019 Side pumped fibre, whereby pump light is coupled laterally into the fibre via an optical component like a prism, or a grating, or via V-groove coupling
• CPC Class Codes: H01S 3/094042 of a fibre laser
• CPC Class Codes: H01S 3/094069 Multi-mode pumping
• CPC Class Codes: H01S 3/1106 Mode locking
• CPC Class Codes: H01S 3/1109 : Active mode locking
• CPC Class Codes: H01S 3/1115 : using intracavity saturable absorbers
• CPC Class Codes: H01S 3/1118 : Semiconductor saturable absorbers, e.g. semiconductor saturable absorber mirrors [SESAMs]; Solid-state saturable absorbers, e.g. carbon nanotube [CNT] based
• CPC Class Codes: H01S 3/1608 : erbium
• CPC Class Codes: H01S 3/1618 : ytterbium

Abstract

A laser utilizes a cavity design which allows the stable generation of high peak power pulses from mode-locked multi-mode fiber lasers, greatly extending the peak power limits of conventional mode-locked single-mode fiber lasers. Mode-locking may be induced by insertion of a saturable absorber into the cavity and by inserting one or more mode-filters to ensure the oscillation of the fundamental mode in the multi-mode fiber. The probability of damage of the absorber may be minimized by the insertion of an additional semiconductor optical power limiter into the cavity.

To amplify and compress optical pulses in a multi-mode (MM) optical fiber, a single-mode is launched into the MM fiber by matching the modal profile of the fundamental mode of the MM fiber with a diffraction-limited optical mode at the launch end, The fundamental mode is preserved in the MM fiber by minimizing mode-coupling by using relatively short lengths of step-index MM fibers with a few hundred modes and by minimizing fiber perturbations. Doping is confined to the center of the fiber core to preferentially amplify the fundamental mode, to reduce amplified spontaneous emission and to allow gain-guiding of the fundamental mode. Gain-guiding allows for the design of systems with length-dependent and power-dependent diameters of the fundamental mode. To allow pumping with high-power laser diodes, a double-clad amplifier structure is employed. For applications in nonlinear pulse-compression, self phase modulation and dispersion in the optical fibers can be exploited. High-power optical pulses may be linearly compressed using bulk optics dispersive delay lines or by chirped fiber Bragg gratings written directly into the SM or MM optical fiber. High-power cw lasers operating in a single near-diffraction-limited mode may be constructed from MM fibers by incorporating effective mode-filters into the laser cavity. Regenerative fiber amplifiers may be constructed from MM fibers by careful control of the recirculating mode. Higher-power Q-switched fiber lasers may be constructed by exploiting the large energy stored in MM fiber amplifiers.

Claims

1. A laser for generating ultra-short optical pulses, comprising:

a cavity which repeatedly passes light energy along a cavity axis;

a length of multi-mode optical fiber doped with a gain medium and positioned along said cavity axis;

a pump for exciting said gain medium;

a mode locking mechanism positioned on said cavity axis; and

an optical guide positioned on said cavity axis which confines the light amplified by said multi-mode optical fiber to preferentially the fundamental mode of said multi-mode optical fiber.

2. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said mode locking mechanism comprises a passive mode locking element.

3. A laser for generating ultra-short optical pulses as defined in claim 2 wherein said passive mode locking element comprises a saturable absorber.

4. A laser for generating ultra-short optical pulses as defined in claim 3 wherein said saturable absorber comprises InGaAsP.

5. A laser for generating ultra-short optical pulses as defined in claim 3 additionally comprising a power limiter for protecting said saturable absorber.

6. A laser for generating ultra-short optical pulses as defined in claim 5 wherein said power limiter comprises a two photon absorber.

7. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said optical guide comprises a single-mode mode-filter fiber on said cavity axis.

8. A laser for generating ultra-short optical pulses as defined in claim 7 wherein said single-mode mode-filter fiber is fusion spliced onto one end of said multi-mode optical fiber.

9. A laser for generating ultra-short optical pulses as defined in claim 8 wherein said multi-mode fiber is tapered at said fusion splice.

10. A laser for generating ultra-short optical pulses as defined in claim 8 wherein said single-mode mode-filter fiber is tapered at said fusion splice.

11. A laser for generating ultra-short optical pulses as defined in claim 8 wherein both said single-mode mode-filter fiber and said multi-mode fiber are tapered at said fusion splice.

12. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said pump is coupled to said multi-mode fiber along said cavity axis.

13. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said pump is coupled to the side of said multi-mode fiber.

14. A laser for generating ultra-short optical pulses as defined in claim 13 additionally comprising an optical coupler for coupling said pump to said multi-mode fiber.

15. A laser for generating ultra-short optical pulses as defined in claim 13 additionally comprising a v-groove on said multi-mode optical fiber for coupling said pump to said multi-mode fiber.

16. A laser for generating ultra-short optical pulses as defined in claim 1 additionally comprising a polarization beam splitter for outputting said ultra-short optical pulses from said laser.

17. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said cavity comprises a pair of reflectors at its opposite ends.

18. A laser for generating ultra-short optical pulses as defined in claim 17 wherein one of said pair of reflectors is partially reflecting and provides the output for said cavity.

19. A laser for generating ultra-short optical pulses as defined in claim 17 wherein said mode locking mechanism comprises a saturable absorber, and wherein one of said reflectors is formed on a surface of said saturable absorber.

20. A laser for generating ultra-short optical pulses as defined in claim 19 wherein said mode locking mechanism additionally comprises a power limiter for protecting said saturable absorber, and wherein said saturable absorber is formed on a surface of said power limiter opposite said one of said reflectors.

21. A laser for generating ultra-short optical pulses as defined in claim 20 wherein said power limiter comprises a two-photon absorber.

22. A laser for generating ultra-short optical pulses as defined in claim 1 additionally comprising a linear phase drift compensator on said cavity axis.

23. A laser for generating ultra-short optical pulses as defined in claim 22 wherein said linear phase drift compensator comprises a Faraday rotator.

24. A laser for generating ultra-short optical pulses as defined in claim 23 wherein said linear phase drift compensator comprises a pair of Faraday rotators.

25. A laser for generating ultra-short optical pulses as defined in claim 22 additionally comprising a linear polarization transformer on said cavity axis.

26. A laser for generating ultra-short optical pulses as defined in claim 25 wherein said linear polarization transformer comprises a wave plate.

27. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said mode locking mechanism comprises an active mode locking element.

28. A laser for generating ultra-short optical pulses as defined in claim 27 wherein said active mode locking element comprises an optical amplitude modulator.

29. A laser for generating ultra-short optical pulses as defined in claim 27 wherein said active mode locking element comprises an optical frequency modulator.

30. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said ultra-short optical pulses preferentially in the fundamental mode of said multi-mode optical fiber have a pulse width below 500 psec.

31. A laser for generating ultra-short optical pulses as defined in claim 1 additionally comprising an environmental stabilizer on said cavity axis to assure that said cavity remains environmentally stable.

32. A laser for generating ultra-short optical pulses as defined in claim 31 wherein said environmental stabilizer comprises a Faraday rotator.

33. A laser for generating ultra-short optical pulses as defined in claim 32 wherein said environmental stabilizer comprises a pair of Faraday rotators.

34. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said optical guide comprises an optical fiber doped with an amplifying medium to provide gain guiding.

35. A laser for generating ultra-short optical pulses as defined in claim 34 wherein said amplifying medium is concentrated centrally within a fraction of the core diameter of said optical fiber.

36. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said optical guide comprises a single-mode optical fiber on said cavity axis.

37. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said optical guide comprises a mode-filter on said cavity axis.

38. A laser for generating ultra-short optical pulses as defined in claim 37 wherein said mode filter excites the fundamental mode of said multi-mode fiber.

39. A laser for generating ultra-short optical pulses as defined in claim 38 wherein said mode filter excites the fundamental mode of said multi-mode fiber with an efficiency of at least 90%.

40. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said cavity additionally comprises a positive dispersion element.

41. A laser for generating ultra-short optical pulses as defined in claim 40 wherein said positive dispersion element comprises a length of single-mode positive dispersion fiber positioned along said cavity axis.

42. A laser for generating ultra-short optical pulses as defined in claim 41 additionally comprising an output coupler for limiting the light energy at said single-mode positive dispersion fiber to less than 10% of the peak power in said cavity.

43. A laser for generating ultra-short optical pulses as defined in claim 42 additionally comprising a frequency converter for compressing pulses generated by said cavity.

44. A laser for generating ultra-short optical pulses as defined in claim 43 wherein said frequency converter comprises a frequency doubler.

45. A laser for generating ultra-short optical pulses as defined in claim 44 wherein said frequency doubler comprises chirped periodically poled LiNbO3.

46. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said multi-mode fiber includes a core, and wherein said gain medium in said multi-mode optical fiber is concentrated centrally within the core of said multi-mode fiber.

47. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said multi-mode optical fiber is polarization-maintaining.

48. A laser for generating ultra-short optical pulses as defined in claim 47 wherein said polarization-maintaining multi-mode fiber has an elliptical core.

49. A laser for generating ultra-short optical pulses as defined in claim 47 wherein said polarization maintaining multi-mode fiber comprises stress-producing regions.

50. A laser for generating ultra-short optical pulses as defined in claim 1 wherein said cavity additionally comprises a fiber grating written onto said multi-mode fiber, said grating primarily reflecting the fundamental mode of said multi-mode fiber.

51. A method of generating ultra-short pulses, comprising:

providing a length of optical fiber doped with a gain medium;

repeatedly passing signal light through said length of optical fiber to produce said ultra-short pulses; and

providing sufficient stored energy within said gain medium to amplify said pulses to a peak power above 1 KW.

52. A method of generating ultra-short pulses as defined in claim 51 additionally comprising:

environmentally stabilizing said optical fiber.

53. A method of generating ultra-short pulses as defined in claim 51 additionally comprising mode-locking said optical fiber.

54. A method of generating ultra-short pulses as defined in claim 51 wherein said providing step comprises providing a multi-mode fiber doped with a gain medium.

55. A method of generating ultra-short optical pulses, comprising:

circulating light energy within a cavity;

amplifying said light energy within said cavity in a multi-mode fiber; and

confining said light energy within said cavity substantially to the fundamental mode of said multi-mode fiber.

56. A method of generating ultra-short optical pulses as defined in claim 55 additionally comprising mode locking said light energy.

57. A method of generating ultra-short optical pulses as defined in claim 55 wherein said confining comprises mode filtering said light energy.

58. A mode-locked laser for generating high power ultra-short optical pulses, comprising:

a multi-mode optical fiber doped with gain material for amplifying optical energy;

means for pumping said optical fiber; and

means for confining the optical energy amplified by said multi-mode optical fiber to substantially the fundamental mode of said multi-mode optical fiber.