Mode locking methods and apparatus

• US 20060187537A1
• Filed: 01/20/2006
• Published: 08/24/2006
• Est. Priority Date: 01/20/2005
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Taira, Kenji, Huber, Robert, Fujimoto, James
• Granted Patent: US 7,414,779 B2
• Time in Patent Office: Days
• US Class Current: 359/337.220
• CPC Class Codes: G01B 9/02004 using frequency scans
• CPC Class Codes: G01B 9/02069 Synchronization of light source or manipulator and detector
• CPC Class Codes: G01B 9/02091 Tomographic interferometers, e.g. based on optical coherence
• CPC Class Codes: H01S 2301/08 Generation of pulses with special temporal shape or frequency spectrum
• CPC Class Codes: H01S 3/063 Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength waveguide gas lasers H01S3/0315
• CPC Class Codes: H01S 3/067 Fibre lasers
• 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 3/06791 Fibre ring lasers fibre laser gyrometers G01C19/72
• CPC Class Codes: H01S 3/08013 Resonator comprising a fibre, e.g. for modifying dispersion or repetition rate the active medium being a fibre H01S3/067
• CPC Class Codes: H01S 3/08027 by a filter, e.g. a Fabry-Perot filter is used for wavelength setting
• CPC Class Codes: H01S 3/082 defining a plurality of resonators, e.g. for mode selection or suppression
• CPC Class Codes: H01S 3/083 Ring lasers fibre ring lasers H01S3/06791
• CPC Class Codes: H01S 3/106 by controlling devices placed within the cavity H01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 3/1062 using a controlled passive interferometer, e.g. a Fabry-Perot etalon
• CPC Class Codes: H01S 3/1068 using an acousto-optical device
• CPC Class Codes: H01S 3/107 using electro-optic devices, e.g. exhibiting Pockels or Kerr effect H01S3/1061, H01S3/1063, H01S3/1065 take precedence
• CPC Class Codes: H01S 3/1109 Active mode locking
• CPC Class Codes: H01S 3/1121 Harmonically mode locking lasers, e.g. modulation frequency equals multiple integers or a fraction of the resonator roundtrip time
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase
• CPC Class Codes: H01S 3/2383 : Parallel arrangements
• CPC Class Codes: H01S 5/065 : Mode locking; Mode suppression; Mode selection ; Self pulsating
• 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

Abstract

In one aspect the invention relates to a frequency varying wave generator. The generator includes a gain element adapted to amplify a wave having a wavelength; a time varying tunable wavelength selective filter element in communication with the gain element, the tunable filter element adapted to selectively filter waves during a period T; and a feedback element in communication with the tunable filter element and the gain element, wherein the tunable wavelength selective filter element, the gain element and the feedback element define a circuit such that the roundtrip time for the wave to propagate through the circuit is substantially equal to a non-zero integer multiple of the period T.

Claims

1. A frequency varying wave generator comprising:

a gain element adapted to amplify a wave having a wavelength;

a time varying tunable wavelength selective filter element in communication with the gain element, the tunable filter element adapted to selectively filter waves during a period T; and

a feedback element in communication with the tunable filter element and the gain element,

wherein the tunable wavelength selective filter element, the gain element and the feedback element define a circuit such that the roundtrip time for the wave to propagate through the circuit is substantially equal to a non-zero integer multiple of the period T.

2. The wave generator of claim 1 wherein the waves are electromagnetic waves and have wavelengths in the range of about 600 nm to about 2000 nm.

3. The wave generator of claim 1 wherein the waves generated are electromagnetic waves.

4. The wave generator of claim 3 wherein the waves are electromagnetic waves and have wavelengths in the range of about 1200 nm to about 1600 nm.

5. The wave generator of claim 1 wherein the tunable filter element is a tunable band-pass filter.

6. The wave generator of claim 1 wherein the time varying tunable wave length selective filter element and the gain element are a single apparatus.

7. The wave generator of claim 1 further comprising an optical compressor in optical communication with the circuit.

8. The wave generator of claim 1 wherein the filter element is a passive filter.

9. The wave generator of claim 1 wherein the filter element is self-tuning.

10. The wave generator of claim 1 wherein the wavelength is optical and the tunable filter element is selected from the group consisting of:

an acoustic-optical filter, an electro-optical filter, a Bragg filter, a graded interferometric filter, a prismatic filter, and a grating filter.

11. The wave generator of claim 1 wherein the tunable filter element is a Fabry Perot filter.

12. The wave generator of claim 1 wherein the tunable filter element is a Mach Zehnder filter.

13. The wave generator of claim 1 wherein the feedback element further comprises a means for performing dispersion management.

14. The wave generator of claim 13 wherein the means for performing dispersion management is selected from the group consisting of:

an optical fiber, a free space element, a telescope element, and a multi-pass cavity element.

15. The wave generator of claim 1 wherein the feedback element is selected from the group consisting of:

an optical fiber, a free space element, a waveguide, a telescope element, and a multi-pass cavity element.

16. The wave generator of claim 1 further comprising an energy extraction element in optical communication with the circuit.

17. The wave generator of claim 1 wherein the gain element is selected from the group consisting of:

a semiconductor optical amplifier, a fiber amplifier, a dye amplifier, an excimer amplifier, a solid state amplifier, a Raman amplifier, a gas amplifier, a Brillouin amplifier, and a parametric amplifier.

18. The wave generator of claim 1 wherein the feedback element comprises at least two mirrors.

19. The wave generator of claim 1, wherein the energy is extracted at two or more different points in the circuit to generate temporally shifted output waves.

20. The wave generator of claim 1, wherein the energy is extracted at two or more different point in the circuit to generate temporally shifted output waves which are combined in an external device.

21. An apparatus adapted for generating waves having temporally varying frequencies, the apparatus comprising:

a gain element adapted for amplifying waves having different wavelengths;

a time varying tunable wavelength selective filter element;

a feedback element in communication with the gain element and the tunable filter element

the tunable filter element, gain element and feedback element defining a circuit such that the roundtrip time of the wave substantially equals a non-zero integer multiple of a periodic time T during which the tunable filter element is tuned.

22. The apparatus of claim 21, wherein the energy is extracted at two or more different points in the circuit to generate temporally shifted output waves.

23. The apparatus of claim 21, wherein the energy is extracted at two or more different points in the circuit to generate temporally shifted output waves which are combined in an external device.

24. The apparatus of claim 21 wherein the time varying tunable wavelength selective filter element has an associated substantially periodic varying wavelength dependent loss or gain.

25. The apparatus of claim 21 further comprising at least one energy extraction mechanism adapted for transmitting energy from the circuit.

26. The apparatus of claim 21 wherein the waves are electromagnetic waves.

27. The apparatus of claim 21 wherein the feedback element contains an element for performing dispersion management.

28. The apparatus of claim 21 wherein the tunable wavelength selective filter element is a tunable bandpass filter and is tuned continuously over time to generate a frequency sweep.

29. The apparatus of claim 21 wherein the circuit defined by the filter element, gain element and feedback element is selected from the group consisting of:

a laser having a linear laser cavity, a laser having a ring cavity, and a laser having a sigma-ring cavity.

30. The apparatus of claim 21 wherein the circuit defined by the filter element, gain element and feedback element is a ring laser cavity.

31. The apparatus of claim 21 wherein the circuit defined by the filter element, gain element and feedback element is a sigma-ring laser cavity.

32. The apparatus of claim 21 wherein the tunable wavelength selective filter element is selected from the group consisting of:

a Fabry Perot filter, a tunable etalon filter, an optical grating filter, a spectral filter, an acousto optic spectral filter, an electro-optic spectral filter, a Mach Zehnder or Michelson based filter and a thin film interferometric filter.

33. The apparatus of claim 21 wherein tunable wavelength selective filter element and the gain element are one physical entity such that the gain medium is adapted to provide a tunable wavelength dependent gain factor.

34. The apparatus of claim 21 wherein the feedback element is an optical delay line selected from the group consisting of:

an optical fiber, a free space beam, a multi-pass cavity, a waveguide, and a telescope assembly.

35. The apparatus of claim 21 wherein the gain element is selected from the group consisting of:

a rare earth doped fiber amplifier, a dye amplifier, an excimer amplifier, a solid state amplifier, a semiconductor optical amplifier, a Raman amplifier, a gas amplifier, a Brillouin amplifier, and a parametric amplifier.

36. A method of frequency sweeping a wave generator comprising a gain element, a tunable filter having a tuning time period T in communication with the gain element and a feedback element in communication with the tunable filter and the gain element, the tunable filter, gain element and feedback element defining a circuit, the method comprising the steps of:

generating waves; and

periodically tuning a tunable filter over a period T, such that the waves travel from the filter element through the circuit and back to the filter element during a time which is a non-zero integer multiple of time T.

37. The method of claim 36 further comprising the step of performing heterodyne measurements using waves from different filter tuning periods.

38. The method of claim 36 further comprising the step of superimposing electric fields of two different waves separated in time, wherein the separation time is longer than one roundtrip time.

39. The method of claim 36 wherein the waves are substantially short pulses generated using a compressor.

40. A laser for generating light having time varying wavelengths, the laser comprising:

a gain element having a spectral bandwidth;

a time varying, tunable optical bandpass filter having an optical pass band that is varied during a substantially periodic time T; and

a feedback element in communication with the gain element and the filter, forming a laser resonator, wherein the roundtrip time of the optical waves in the resonator is substantially equal to a non-zero integer multiple of the time T.

41. The laser of claim 40 wherein the laser resonator is an optical fiber based laser resonator.

42. The laser of claim 40 wherein the filter is a tunable Fabry Perot filter.

43. The laser of claim 40 wherein the feedback element is a single mode optical fiber.

44. The laser of claim 40 wherein the gain element is a semiconductor optical amplifier.

45. The claim 40 wherein the resonator comprises an optical isolator.

46. The laser of claim 41 further comprising a fiber coupler, the fiber coupler adapted to extra energy from the resonator.

47. The laser of claim 40 wherein the feedback element contains a means for performing dispersion management.

48. The laser of claim 40 further comprising an external optical compressor in optical communication with the laser.

49. The laser of claim 48 wherein the compressor is adapted to substantially compensate for a relative time delay associated with waves emitted from the laser by regulating the transit time through the compressor for waves having different wavelengths.

50. The laser of claim 48 wherein the compressor is a dispersion compensating optical fiber adapted to generate temporally short optical pulses from the laser.

51. The laser of claim 48 further comprising a pulse analysis system and a feedback control system in communication with the filter.

52. The laser of claim 48 wherein the compressor is a grating type compressor.

53. The laser of claim 40 further comprising an interferometer, the interferometer adapted to superimpose electric fields of two different waves generated by the laser, wherein the waves are separated by a time period longer than one roundtrip time.