Generation of a wavelength-tunable laser oscillation in a wave-guiding gain medium based on passive mode lock

• US 6,373,867 B1
• Filed: 06/21/2000
• Issued: 04/16/2002
• Est. Priority Date: 07/11/1997
• Status: Expired due to Term

• Current Assignee: Calmar Optcom, Inc.
• Original Assignee: Calmar Optcom, Inc.
• Inventors: Lin, Hong, Lin, Katherine Y.
• Primary Examiner(s): Davie, James W.
• Assistant Examiner(s): Inzirillo, Gioacchino
• Application Number:
  US09/575,675
• Time in Patent Office: 664 Days
• Field of Search: 372/6, 372/18, 372/75, 372/98
• US Class Current: 372/18
• CPC Class Codes: H01S 3/067 Fibre lasers
• CPC Class Codes: H01S 3/0675 Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
• 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 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1394 by using an active reference, e.g. second laser, klystron or other standard frequency source

Abstract

A passive mode-locked linear-resonator fiber laser using polarization-maintaining fibers and a saturable absorber to produce ultra short pulses and a long-term reliable operation with reduced maintenance. Such a fiber laser can be configured to produce tunable pulse repetition rate and tunable laser wavelength.

Claims

1. A laser, comprising:

a pump light source which includes a semiconductor light source to produce a pump beam at a pump wavelength;

a wave-guiding gain medium to absorb light at said pump wavelength to emit light at a laser wavelength within a spectral gain profile and is different from said pump wavelength;

an optical coupler to couple said pump beam from said pump light source into said wave-guiding gain medium;

first and second reflective elements disposed to form a linear optical resonator that encloses said wave-guiding gain medium and supports a plurality of longitudinal modes;

a saturable absorber disposed in said optical resonator and formed of a material to exhibit an intensity-dependent absorption to effectuate a mode-locking mechanism that locks said longitudinal modes in phase to produce optical pulses at said laser wavelength, said material in said saturable absorber selected to exhibit a slow saturation process with a low saturation intensity to initiate said optical pulses and a fast saturation process with a high saturation intensity to shorten a temporal width of said pulses; and

a tuning element located in said optical resonator to change at least one property of said optical pulses.

2. The laser as in claim 1, wherein said wave-guiding gain medium includes a fiber segment that is doped with rare-earth ions.

3. The laser as in claim 1, wherein said wave-guiding gain medium includes a segment of an optical waveguide.

4. The laser as in claim 1, wherein said waveguiding-medium and said optical resonator are configured to maintain a polarization of said optical pulses at said laser wavelength along a fixed direction.

5. The laser as in claim 1, wherein said tuning element includes a resonator-length tuning element which is operable to change an optical path length within said optical resonator so as to alter a repetition rate of said optical pulses.

6. The laser as in claim 1, wherein said tuning element includes a wavelength-tuning element which is operable to change said laser wavelength within said spectral gain profile of said wave-guiding gain medium.

7. The laser as in claim 1, wherein said saturable absorber includes a semiconductor material having a bandgap equal to or less than a photon energy corresponding to said laser wavelength.

8. The laser as in claim 1, further comprising:

a first fiber segment coupled in an optical path between said first reflective element and said wave-guiding gain medium; and

a second fiber segment coupled in an optical path between said second reflective element and said wave-guiding gain medium,

wherein an intensity of said pump beam is less than a first threshold level above which a nonlinear optical effect occurs in said first and said second fiber segments, and is above a second threshold level above which an optical absorption in said saturable absorber is saturated.

9. The laser as in claim 1, wherein said optical coupler is positioned in an optical path of said optical resonator between said wave-guiding medium and said saturable absorber.

10. The laser as in claim 1, wherein said optical coupler and said saturable absorber are positioned in an optical path between said wave-guiding gain medium and said second reflective element, and wherein said first reflective element is operable to reflect light only at said laser wavelength.

11. The laser as in claim 1, further comprising:

a wavelength-tuning mechanism in said optical resonator configured to (1) select radiation at said laser wavelength to travel in said optical resonator between said first and said second reflective elements and to suppress radiation at other wavelengths in said optical resonator and (2) to tune said laser wavelength from one wavelength to another within said spectral gain profile; and

a resonator-length tuning mechanism in said optical resonator configured to change an optical path length between said first and said second reflective elements of said optical resonator so as to alter a temporal repetition rate of said optical pulses.

12. The laser as in claim 1, wherein an intensity of said pump beam is selected and said optical resonator is configured so that said optical pulses are soliton optical pulses.

13. The laser as in claim 4, wherein said wave-guiding gain medium includes a doped polarization-maintaining fiber segment that produces optical gain at said laser wavelength within said spectral gain profile when pumped at said pump wavelength, and further comprising:

a plurality of polarization-maintaining fiber segments coupled between said first and second reflective elements to form a polarization-maintaining optical conduit with said wave-guiding medium in said optical resonator.

14. The laser as in claim 5, further comprising at least one fiber segment within said optical resonator, where said resonator-length tuning element includes a fiber stretcher engaged to said fiber segment.

15. The laser as in claim 5, wherein said resonator-length tuning element includes a positioning element engaged to change a position of at least one of said first and said second reflective elements.

16. The laser as in claim 5, wherein said optical resonator has an optical path length so that a temporal separation between two adjacent pulses is substantially equal to a round trip time for a pulse to travel within said optical resonator.

17. The laser as in claim 5, further comprising:

a photo detector positioned to receive a portion of said optical pulses from said optical resonator; and

an electronic control, coupled to receive a detector signal from said photo detector and operable to determine said pulse repetition rate from said detector signal and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate,

wherein said electronic control is operable to control said resonator-length tuning element to adjust and maintain said pulse repetition rate at said reference clock rate.

18. The laser as in claim 6, wherein said wavelength-tuning element includes a tunable optical bandpass filter.

19. The laser as in claim 6 wherein said wavelength-tuning element includes a tunable grating.

20. The laser as in claim 7, wherein said semiconductor material has a linear absorption at said laser wavelength from about 25% to about 75%.

21. The laser as in claim 11, wherein said wavelength-tuning mechanism includes a tunable optical bandpass filter.

22.The laser as in claim 11, wherein said wavelength-tuning mechanism includes a tunable optical grating.

22. The laser as in claim 11, wherein said wavelength-tuning mechanism includes a tunable optical grating.

23. The laser as in claim 11, wherein said resonator-length tuning mechanism includes a positioning device that changes a relative position between said first and said second reflective elements.

24. The laser as in claim 11, wherein said resonator-length tuning mechanism includes a device coupled to said wave-guiding medium and operable to change a length of said wave-guiding medium.

25. The laser as in claim 11, further comprising:

a photo detector positioned to receive a portion of said optical pulses from said optical resonator; and

an electronic control, coupled to receive a detector signal from said photo detector and operable to determine said pulse repetition rate from said detector signal and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate,

wherein said electronic control is operable to control said resonator-length tuning element to adjust and maintain said pulse repetition rate at said reference clock rate.

26. The laser as in claim 12, further comprising a wavelength-tuning mechanism in said optical resonator configured to (1) select radiation at said laser wavelength to travel in said optical resonator between said first and said second reflective elements and to suppress radiation at other wavelengths in said optical resonator and (2) to tune said laser wavelength from one wavelength to another within said spectral gain profile.

27. The laser as in claim 12, further comprising a resonator-length tuning mechanism in said optical resonator configured to change an optical path length between said first and said second reflective elements of said optical resonator so as to alter a temporal repetition rate of said optical pulses.

28. The laser as in claim 13, wherein said doped polarization-maintaining fiber segment is doped with rare-earth ions.

29. The laser as in claim 19, wherein said tunable grating includes a tunable fiber grating operable to select radiation at said laser wavelength to reflect and to adjust said laser wavelength in response to a control signal.

30. The laser as in claim 25, wherein said wave-guiding medium is a doped fiber and said device is a fiber stretcher.

31. The laser as in claim 28, further comprising:

a photo detector positioned to receive a portion of said optical pulses from said optical resonator; and

an electronic control, coupled to receive a detector signal from said photo detector and operable to determine said pulse repetition rate from said detector signal and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate,

wherein said electronic control is operable to control said resonator-length tuning element to adjust and maintain said pulse repetition rate at said reference clock rate.

32. The laser as in claim 30, further comprising a fiber grating controller coupled to said tunable fiber grating and operable to produce said control signal to adjust said tunable fiber grating.

38. The fiber laser as in claim 1, further comprising a tuning element to change an optical path length within said optical resonator so as to alter a repetition rate of said optical pulses.

43. The fiber laser as in claim 38, wherein said second reflective element has a high reflectivity at said laser wavelength and low reflectivities at other wavelengths.

51. The fiber laser as in claim 43, wherein said semiconductor absorber includes a multiple quantum well structure and said second reflective element includes a semiconductor Bragg reflector.

52. The fiber laser as in claim 43, wherein said second polarization-maintaining fiber segment has an end facet that is directly engaged to said semiconductor absorber.

53. The fiber laser as in claim 43, wherein said second polarization-maintaining fiber segment has an end facet that interfaces with said semiconductor absorber, and further comprising a lens between said end facet and said semiconductor absorber.

54. The fiber laser as in claim 43, further comprising an optical attenuator between said end facet and said semiconductor absorber.

58. The fiber laser as in claim 54, further comprising a polarizer between said end facet and said semiconductor absorber.

60. The fiber laser as in claim 58, wherein said resonator-length tuning mechanism includes a positioning device that changes a relative position between said first and said second reflective elements.

61. The fiber laser as in claim 58, wherein said resonator-length tuning mechanism includes a device coupled to said fiber gain medium and operable to change a length of said fiber gain medium.

62. The fiber laser as in claim 58, further comprising:

a photo detector positioned to receive a portion of said optical pulses from said optical resonator; and

an electronic control, coupled to receive a detector signal from said photo detector and operable to determine said pulse repetition rate from said detector signal and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate,

wherein said electronic control is operable to control said resonator-length tuning element to adjust and maintain said pulse repetition rate at said reference clock rate.

63. The fiber laser as in claim 60, wherein said end facet of said first polarization-maintaining fiber segment is coated with an anti-reflection coating.

64. The fiber laser as in claim 60, wherein said end facet is substantially perpendicular to an optic axis of said first polarization-maintaining fiber segment.

65. The fiber laser as in claim 60, wherein said end facet forms an acute angle with respect to an optic axis of said first polarization-maintaining fiber segment.

66. The fiber laser as in claim 61, wherein said positioning device is engaged to change a position of said optical grating.

67. The fiber laser as in claim 66, wherein said optical axis of said first polarization-maintaining fiber segment forms an angle with respect to an optical axis of said lens.

33. The laser as in claim 33, wherein said fiber controller includes a device that is operable to change a length of said tunable fiber grating to tune said laser wavelength.

34. A fiber laser, comprising:

a pump light source which includes a semiconductor light source to produce a pump beam at a pump wavelength;

first and second reflective elements disposed relative to each other to form an optical resonator having a plurality of longitudinal modes;

a fiber gain medium having optical transitions operable to absorb photons at said pump wavelength and to emit photons at a laser wavelength within a spectral gain profile that is different from said pump wavelength, said fiber gain medium configured to maintain light polarization in a specified direction perpendicular to an optic axis of said fiber gain medium;

a semiconductor absorber disposed in said optical resonator and configured to exhibit a saturable intensity-dependent absorption and to have a bandgap equal to or less than a photon energy corresponding to said laser wavelength, said semiconductor absorber operable to lock a plurality of oscillating longitudinal modes in phase to produce optical pulses at said laser wavelength and configured to exhibit a slow saturation process with a low saturation intensity to initiate said optical pulses and a fast saturation process with a high saturation intensity to shorten a temporal width of said pulses;

an optical coupler, disposed relative to said fiber gain medium to couple said pump beam into said fiber gain medium; and

a first polarization-maintaining fiber segment disposed to transmit light between said first reflective element and said fiber gain medium; and

a second polarization-maintaining fiber segment disposed to transmit light between said fiber gain medium and said second reflective element, wherein each of said first and said second fiber segments has a principal polarization axis aligned with said specified direction defined by said fiber gain medium.

35. The fiber laser as in claim 35, wherein said optical coupler is positioned in an optical path between said fiber gain medium and said semiconductor absorber to direct said pump beam towards said fiber gain medium.

36. The fiber laser as in claim 35, wherein said optical coupler includes a dichroic beam splitter.

37. The fiber laser as in claim 35, wherein said first reflective element includes a reflective grating to selectively reflect light of said laser wavelength.

39. The fiber laser as in claim 35, wherein said first reflective element includes a diffractive optical grating operable to receive light from said first polarization-maintaining fiber segment and direct only light that is at said laser wavelength back to said first polarization-maintaining fiber segment.

40. The fiber laser as in claim 35, wherein said first reflective element includes a reflective coating directly formed on an end facet of said first polarization-maintaining fiber segment and configured to reflect only light at a wavelength within said spectral gain profile of said fiber gain medium.

41. The fiber laser as in claim 35, wherein said first polarization-maintaining fiber segment has an end facet coupled to said first reflective element, and wherein said first reflective element includes a reflector a reflective coating formed on an end facet of said first polarization-maintaining fiber segment and configured to reflect only light at a wavelength within said spectral gain profile of said fiber gain medium.

42. The fiber laser as in claim 35, wherein said semiconductor absorber is engaged to said second reflective element.

44. The fiber laser as in claim 39, wherein said tuning element includes a fiber stretcher within said optical resonator.

45. The fiber laser as in claim 39, wherein said tuning element includes a positioning element engaged to and operable to change a position of at least one of said first and said second reflective elements.

46. The fiber laser as in claim 39, wherein said optical resonator has an optical path length so that a temporal separation between two adjacent pulses is substantially equal to a round trip time for a pulse to travel within said optical resonator.

47. The fiber laser as in claim 40, wherein said optical grating is operable to change an orientation relative to a direction of said light from said first polarization-maintaining fiber segment so as to change said laser wavelength from one wavelength to another within said spectral gain profile of said fiber gain medium.

48. The fiber laser as in claim 42, further comprising a mechanical coupler having an opening to receive said first polarization-maintaining fiber segment and configured to engage said end facet of said polarization-maintaining fiber segment to said reflector.

49. The fiber laser as in claim 42, further comprising a lens between said reflector and said end facet of said first polarization-maintaining fiber segment.

50. The fiber laser as in claim 42, further comprising a tunable optical bandpass filter between said reflector and said end facet of said first polarization-maintaining fiber segment, said optical bandpass filter operable to transmit light of said laser wavelength and to change said laser wavelength within said spectral gain profile of said fiber grain medium when an orientation of said optical bandpass filter is changed.

55. The fiber laser as in claim 48, further comprising an optical collimator between said optical grating and said first polarization-maintaining fiber segment.

56. The fiber laser as in claim 48, further comprising a lens between said optical grating and said first polarization-maintaining fiber segment.

57. The fiber laser as in claim 48, further comprising a resonator-length tuning mechanism in said optical resonator configured to change an optical path length between said first and said second reflective elements of said optical resonator so as to alter a temporal repetition rate of said optical pulses.

59. The fiber laser as in claim 57, wherein said lens is spaced from said optical grating by a distance greater than a focal length of said lens to image an end facet of said first polarization-maintaining fiber segment to said optical grating.