Pulsed laser sources

• US 9,653,868 B2
• Filed: 07/12/2016
• Issued: 05/16/2017
• Est. Priority Date: 07/25/2003
• Status: Expired due to Fees

• Current Assignee: IMRA America Incorporated (Aisin Corp.)
• Original Assignee: IMRA America Incorporated (Aisin Corp.)
• Inventors: Hartl, Ingmar, Fermann, Martin E., Imeshev, Gennady
• Primary Examiner(s): Sanghavi, Hemang
• Application Number:
  US15/207,959
• Publication Number:
  US 20170063015A1
• Time in Patent Office: 308 Days
• Field of Search: None
• CPC Class Codes: G02F 1/3517 using an interferometer
• CPC Class Codes: G02F 1/3528 for producing a supercontinuum
• CPC Class Codes: H01S 3/0014 Monitoring arrangements not otherwise provided for photometry G01J1/00, e.g. G01J1/4257; radiation pyrometry G01J5/00; measuring coherence of light G01J9/00; measuring wavelength of light G01J9/00, e.g. G01J9/0246; measuring optical pulses G01J11/00; calorimetrically measuring power of laser beams G01K17/003
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108
• CPC Class Codes: H01S 3/0078 Frequency filtering
• 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 3/0675 Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/094042 of a fibre laser
• CPC Class Codes: H01S 3/10 Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
• CPC Class Codes: H01S 3/105 by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity lengthH01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 3/1053 Control by pressure or deformation
• CPC Class Codes: H01S 3/106 by controlling devices placed within the cavity H01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 3/1067 using pressure or deformation
• CPC Class Codes: H01S 3/109 Frequency multiplication, e.g. harmonic generation
• CPC Class Codes: H01S 3/1106 Mode locking
• 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 3/1616 thulium

Abstract

Modelocked fiber laser resonators may be coupled with optical amplifiers. An isolator optionally may separate the resonator from the amplifier. A reflective optical element on one end of the resonator having a relatively low reflectivity may be employed to couple light from the resonator to the amplifier. Enhanced pulse-width control may be provided with concatenated sections of both polarization-maintaining and non-polarization-maintaining fibers. Apodized fiber Bragg gratings and integrated fiber polarizers may also be included in the laser cavity to assist in linearly polarizing the output of the cavity. Very short pulses with a large optical bandwidth may be obtained by matching the dispersion value of the grating to the inverse of the dispersion of the intra-cavity fiber. Frequency comb sources may be constructed from such modelocked fiber oscillators. Low dispersion and an in-line interferometer that provides feedback may assist in controlling the frequency components output from the comb source.

Claims

1. A frequency comb source comprising:

a mode-locked fiber oscillator comprising optical fiber disposed in an optical cavity;

a non-linear optical element disposed to receive pulsed output from said mode-locked fiber oscillator and to generate a supercontinuum comprising a first train of optical pulses having a first set of frequency components;

an interferometer disposed to receive said first train of optical pulses, said interferometer comprising a frequency converter comprising a nonlinear waveguide and configured to generate a second train of optical pulses having a second set of frequency components that are shifted in frequency with respect to the first set of frequency components, said second train of optical pulses temporally overlapping said first set of optical pulses, said interferometer configured to interfere at least a portion of said first train and said second train of optical pulses to generate an optical interference signal comprising beat frequencies; and

an optical detector configured to detect said optical interference signal and output said beat frequencies.

2. The frequency comb source according to claim 1, wherein said interferometer comprises an f to 2f interferometric arrangement.

3. The frequency comb source according to claim 1, wherein said non-linear optical element comprises a highly non-linear fiber.

4. The frequency comb source according to claim 1, wherein said nonlinear waveguide comprises PPLN.

5. The frequency comb source according to claim 1, wherein said nonlinear waveguide comprises a periodically poled grating section.

6. The frequency comb source according to claim 1, further comprising an optical filter disposed upstream from said optical detector, said filter substantially transmitting portions of the supercontinuum that contribute to signals representative of said beat frequencies.

7. The frequency comb source according to claim 1, wherein said pulsed output from said mode-locked fiber oscillator comprises frequency components having a repetition frequency, f_rep, and a carrier envelope offset frequency, f_ceo, said frequency comb source further comprising:

a feedback system having an input for receiving a beat frequency related to f_ceo,

said feedback system comprising a phase lock loop to compare said beat frequency related to f_ceo with a reference frequency,

said feedback system connected to said mode-locked fiber oscillator to control said carrier envelope offset frequency, f_ceo, based on said beat frequencies.

8. The frequency comb source according to claim 1, said frequency comb source further comprising optical comb lines characterized by a repetition frequency, f_rep, and a carrier envelope offset frequency, f_ceo, said frequency comb source further comprising:

a first feedback system configured to lock at least one of the optical comb lines to an optical frequency reference.

9. The frequency comb source according to claim 8, further comprising a second feedback system configured to control f_ceo.

10. The frequency comb source according to claim 9, wherein said second feedback system configured to control f_ceo is configured to adjust a pump power for the modelocked fiber oscillator.

11. The frequency comb source according to claim 8, further comprising an electro-mechanical transducer configured to controllably alter the cavity length of said optical cavity in response to a signal in the first feedback system.

12. The frequency comb source according to claim 1, wherein said pulsed output from said mode-locked fiber oscillator comprises frequency components having a repetition frequency, f_rep, and a carrier envelope offset frequency, f_ceo,

said optical cavity comprising an addressable intra-cavity element,

said frequency comb source further comprising a feedback system connected to said mode-locked fiber oscillator and said addressable intra-cavity element to control said carrier envelope offset frequency, f_ceo,

said addressable intra-cavity element configured to perturb said mode-locked fiber oscillator in response to feedback from said feedback system.

13. The frequency comb source according to claim 12, wherein said addressable intra-cavity element comprises a heating element, an electro-mechanical transducer, or a piezo-electric transducer.

14. The frequency comb source according to claim 1, wherein:

said frequency comb source is substantially fiber based,

said non-linear optical element comprises a highly non-linear fiber, and

said frequency comb source comprises coupling fibers that are spliced to the highly non-linear fiber so as to couple said supercontinuum from the highly non-linear fiber to the nonlinear waveguide.

15. The frequency comb source according to claim 1, wherein said frequency comb source is substantially fiber based, and said frequency comb source comprises a coupling fiber attached to the non-linear waveguide for output coupling the output of said non-linear waveguide.

16. The frequency comb source according to claim 1, wherein said optical fiber disposed in said cavity has a total length L, and said mode-locked fiber oscillator has a total dispersion in the range from −

10,000 femtosec²/m×

L to +2,500 femtosec²/m×

L.

17. The frequency comb source according to claim 1, wherein said mode-locked fiber oscillator has a total absolute dispersion less than about 5,000 femtosec².

18. The frequency comb source according to claim 1, wherein said nonlinear optical element is fiber based and has a dispersion in the range from −

5,000 femtosec²/m to +2,000 femtosec²/m.

19. The frequency comb source according to claim 1, wherein said optical cavity of said mode-locked fiber oscillator is configured with sufficiently low cavity dispersion such that a carrier envelope offset frequency beat signal (f_ceo beat signal) having a frequency bandwidth smaller than approximately 200 kHz is obtainable from said frequency comb source.

20. The frequency comb source according to claim 1, further comprising a fiber amplifier disposed between said mode-locked fiber oscillator and said non-linear optical element.

21. The frequency comb source according to claim 18, further comprising an isolator disposed between said mode-locked fiber oscillator and said fiber amplifier.

22. The frequency comb source according to claim 1, wherein said interferometer comprises a group delay compensator configured to compensate for a difference in optical path between said at least a portion of said first train and second train of optical pulses interfering in said interferometer, said group delay compensator configured to reduce said optical path difference.

23. A frequency comb source comprising:

a mode-locked fiber oscillator comprising a least two optical fibers with different dispersion characteristics disposed in an optical cavity and arranged such that said mode-locked cavity dispersion is in the range from −

10,000 femtosec²/m×

L to +2,500 femtosec²/m×

L where L is the total intra-cavity fiber length, said mode-locked fiber oscillator configured to output pulses having a carrier envelope offset frequency f_ceo;

a non-linear optical element disposed downstream from said mode-locked fiber oscillator configured to receive pulses originating from said mode-locked fiber oscillator to generate a supercontinuum; and

an interferometer configured to generate a beat signal representative of the f_ceo of said mode-locked fiber oscillator.

24. The frequency comb source according to claim 23, further comprising a polarization-maintaining fiber amplifier.

25. The frequency comb source according to claim 24, said fiber amplifier configured to be operated in a nonlinear regime such that said pulses are subject to substantial levels of self-phase modulation, producing spectral broadening in the output from the fiber amplifier.

26. The frequency comb source according to claim 25, further comprising a compressor downstream of said fiber amplifier, said compressor comprising step-index fiber.

27. The frequency comb source according to claim 26, wherein said compressor is configured to generate pulses with a width of less than approximately 200 fs, to preserve coherence in supercontinuum generation.

28. The frequency comb source according to claim 26, wherein said compressor is configured to generate pulses with a width of less than approximately 100 fs, to preserve coherence in supercontinuum generation.

29. The frequency comb source according to claim 26, wherein said compressor is configured to generate pulses with a width of less than approximately 70 fs, to preserve coherence in supercontinuum generation.

30. The frequency comb source according to claim 23, wherein said mode-locked fiber oscillator comprises a fiber pig-tailed cavity component.

31. The frequency comb source according to claim 23 wherein at least one of said mode-locked fiber oscillator or said nonlinear optical element comprises polarization maintaining fiber.

32. The frequency comb source according to claim 23, wherein said beat signal representative of said f_ceo of said mode-locked fiber oscillator has a spectral width of less than approximately 200 kHz.

33. The frequency comb source according to claim 23, wherein said mode-locked fiber oscillator has a total absolute dispersion less than about 5,000 femtosec².

34. The frequency comb source according to claim 23, wherein said nonlinear optical element is fiber based and has a dispersion in the range from −

5,000 femtosec²/m to +2,000 femtosec²/m.