Chirped-pulse amplifier using photonic-crystal-rod (PCR) waveguides and associated method Number:7,436,585 from the United States Patent and Trademark Office (PTO) owispatent A method and apparatus use a photonic-crystal fiber having a very large core while maintaining a single transverse mode. In some fiber lasers and amplifiers having large cores problems exist related to energy being generated at multiple-modes (i.e., polygamy), and of mode hopping (i.e., promiscuity) due to limited control of energy levels and fluctuations. The problems of multiple-modes and mode hopping result from the use of large-diameter waveguides, and are addressed by the invention. This is especially true in lasers using large amounts of energy (i.e., lasers in the one-megawatt or more range). By using multiple small waveguides in parallel, large amounts of energy can be passed through a laser, but with better control such that the aforementioned problems can be reduced. An additional advantage is that the polarization of the light can be maintained better than by using a single fiber core.<H associated, waveguides, Chirped, pulse, a, 7436585.html, Patent, pto, 7436585

Title: Chirped-pulse amplifier using photonic-crystal-rod (PCR) waveguides and associated method

Abstract: A method and apparatus use a photonic-crystal fiber having a very large core while maintaining a single transverse mode. In some fiber lasers and amplifiers having large cores problems exist related to energy being generated at multiple-modes (i.e., polygamy), and of mode hopping (i.e., promiscuity) due to limited control of energy levels and fluctuations. The problems of multiple-modes and mode hopping result from the use of large-diameter waveguides, and are addressed by the invention. This is especially true in lasers using large amounts of energy (i.e., lasers in the one-megawatt or more range). By using multiple small waveguides in parallel, large amounts of energy can be passed through a laser, but with better control such that the aforementioned problems can be reduced. An additional advantage is that the polarization of the light can be maintained better than by using a single fiber core.

Patent Number: 7,436,585 Issued on 10/14/2008 to Di Teodoro, et al.

Inventors: Di Teodoro; Fabio (Everett, WA), Brooks; Christopher D. (Kenmore, WA)
Assignee: Aculight Corporation (Bothell, WA)

Appl. No.: 11/420,755

Filed: May 28, 2006

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
11420729	May., 2006
60703822	Jul., 2005
60746166	May., 2006
60797931	May., 2006

Current U.S. Class: 359/341.1 ; 359/341.5; 385/126

Current International Class: H01S 3/00 (20060101); G02B 6/04 (20060101); G02B 6/02 (20060101)

Field of Search: 359/341.1,341.5 385/126

References Cited [Referenced By]

U.S. Patent Documents


3728117	April 1973	Heidenhain et al.
4313648	February 1982	Yano et al.
4367040	January 1983	Goto
4424435	January 1984	Barnes, Jr.
4862257	August 1989	Ulich
4895790	January 1990	Swanson et al.
5319668	June 1994	Luecke
5379310	January 1995	Papen et al.
5440416	August 1995	Cohen et al.
5526155	June 1996	Knox et al.
5608826	March 1997	Boord et al.
5802236	September 1998	DiGiovanni et al.
5818630	October 1998	Fermann et al.
5847863	December 1998	Galvanauskas et al.
5907436	May 1999	Perry et al.
5974060	October 1999	Byren et al.
6014249	January 2000	Fermann et al.
6023361	February 2000	Ford
6028879	February 2000	Ershov
6031952	February 2000	Lee
6097863	August 2000	Chowdhury
6192062	February 2001	Sanchez-Rubio et al.
6208679	March 2001	Sanchez-Rubio et al.
6212310	April 2001	Waarts et al.
6226077	May 2001	Dunne
6275623	August 2001	Brophy et al.
6330523	December 2001	Kacyra
6339662	January 2002	Koteles et al.
6381008	April 2002	Branagh et al.
6381388	April 2002	Epworth et al.
6496301	December 2002	Koplow et al.
6625364	September 2003	Johnson et al.
6665471	December 2003	Farmer et al.
6717655	April 2004	Cheng et al.
6754006	June 2004	Barton et al.
6765724	July 2004	Kramer
6798960	September 2004	Hamada
6819871	November 2004	Baldwin et al.
6822796	November 2004	Takada et al.
6829421	December 2004	Forbes et al.
6845204	January 2005	Broeng et al.
6865344	March 2005	Johnson et al.
6882431	April 2005	Teich et al.
6901197	May 2005	Hasegawa et al.
6914916	July 2005	Pezeshki et al.
6917631	July 2005	Richardson et al.
6937795	August 2005	Squires et al.
6950692	September 2005	Gelikonov et al.
6952510	October 2005	Karlsen et al.
6958859	October 2005	Hoose et al.
6960027	November 2005	Krah et al.
6963354	November 2005	Scheps
6996343	February 2006	Neilson
7043127	May 2006	Hasegawa
7072553	July 2006	Johnson et al.
7106932	September 2006	Birks et al.
7113327	September 2006	Gu et al.
7116469	October 2006	Bragheri et al.
7142757	November 2006	Ward
7190705	March 2007	Fermann et al.
7199924	April 2007	Brown
7242835	July 2007	Busse et al.
7280730	October 2007	Dong et al.
7340140	March 2008	Xu et al.
2002/0122644	September 2002	Birks
2002/0181856	December 2002	Sappey et al.
2003/0068150	April 2003	Ariel et al.
2003/0165313	September 2003	Broeng et al.
2003/0189758	October 2003	Baer et al.
2004/0033043	February 2004	Monro et al.
2004/0095968	May 2004	Avizonis et al.
2004/0114849	June 2004	Shah et al.
2004/0114852	June 2004	Brown
2005/0041702	February 2005	Fermann et al.
2005/0105866	May 2005	Grudinin et al.
2005/0157998	July 2005	Dong et al.
2005/0163426	July 2005	Fermann et al.
2005/0169590	August 2005	Alkeskjold
2005/0173817	August 2005	Fauver et al.
2005/0226286	October 2005	Liu et al.
2005/0259933	November 2005	Temelkuran et al.
2005/0259934	November 2005	Temelkuran et al.
2006/0067632	March 2006	Broeng et al.
2006/0120418	June 2006	Harter et al.
2006/0198246	September 2006	Frederick et al.
2006/0204195	September 2006	Kurosawa et al.
2006/0204795	September 2006	Kurosawa et al.
2006/0227816	October 2006	Liu
2006/0233554	October 2006	Ramachandran et al.
2006/0263024	November 2006	Dong et al.
2007/0127123	June 2007	Brown et al.

Other References

Di Teodoro, Fabio , et al., "1.1 MW peak-power, 7 W average-power, high-spectral-brightness, diffraction-limited pulses from a photonic crystal fiber amplifier", Optics Letters vol. 30, No. 20, (Oct. 15, 2005), 2694-2696. cited by other .
Di Teodoro, Fabio , et al., "Diffraction-limited, 300-kW peak-power pulses from a coiled multimode fiber amplifier", Optics Letters vol. 27, No. 7, (Apr. 1, 2002), 518-520. cited by other .
Di Teodoro, Fabio , et al., "Harmonic generation of an Yb-doped photonic-crystal fiber amplifier to obtain 1ns pulses of 410, 160, and 190kW peak-power at 531, 354, and 265nm wavelength", Advanced Solid-State Photonics 29 Technical Digest, Paper ME3, (2006). cited by other .
Galvanauskas, A. , et al., "Fiber-laser-based femtosecond parametric generator in bulk periodically poled LiNbO3", Optics Letters vol. 22, No. 2, (Jan. 15, 1997), 105-107. cited by other .
Kristiansen, Rene E., et al., "Microstructured fibers and their applications", Proceedings of the 4th Reunion Espanola of Optoelectronics (OPTOEL), CI-5, (2005), 37-49. cited by other .
Roser, F. , et al., "131 W 220 fs fiber laser system", Optics Letters vol. 30, No. 20, (Oct. 15, 2005), 2754-2756. cited by other .
Blazephonics (Company), "Hollow Core Photonic Bandgap Fiber HC-580-01 Product Description", "http://www.crystal-fibre.com/datasheets/HC-580-01.pdf", Feb. 10, 2006. cited by other .
Blazephonics (Company), "High NA Multimode Fiber MM-37-01 Product Description", "http://www.crystal-fibre.com/datasheets/MM-37-01.pdf", 2005. cited by other .
Crystal Fibre (Company), "High-Power Fiber Laser and Amplifier Subassembly Modules Product Description", "http://www.crystal-fibre.com/products/subassemblies.shtm#description", 2005 (copyright). cited by other .
Crystal Fibre (Company), "Multimode Ultra High NA Photonic Crystal Fiber MM-HNA-110 Product Description", "http://www.crystal-fibre.com/datasheets/MM-HNA-110.pdf", Apr. 2005. cited by other .
Crystal Fibre (Company), "Multimode Ultra High NA Photonic Crystal Fiber MM-HNA-200 Product Description", "http://www.crystal-fibre.com/datasheets/MM-HNA-200.pdf", Apr. 2005. cited by other .
Crystal Fibre (Company), "Multimode Ultra High NA Photonic Crystal Fiber MM-HNA-35 Product Description", "http://www.crystal-fibre.com/datasheets/MM-HNA-35.pdf", Apr. 2005. cited by other .
Crystal Fibre (Company), "Multimode Ultra High NA Photonic Crystal Fiber MM-HNA-5 Product Description", "http://www.crystal-fibre.com/datasheets/MM-HNA-5.pdf", Apr. 2005. cited by other .
Galvanauskas, Almantas, "Mode-scalable fiber-based chirped pulse amplification systems", "IEEE Journal on Selected Topics in Quantum Electronics", Jul. 2001, pp. 504-517, vol. 7, No. 4. cited by other .
Krause, J. T., et al., "Arc Fusion Splices with Near Pristine Strengths and Improved Optical Loss", "22nd European Conference on Optical Communication--EEOC'96, Oslo, Norway", 1996, pp. 237-240, vol. 2. cited by other .
Sorensen, T., et al., "Metal-assisted coupling to hollow-core photonic crystal fibres", "Electronics Letters", Jun. 9, 2005, vol. 41, No. 12. cited by other .
Crystal Fibre (Company), Towards 100 kW fiber laser systems Scaling up power in fiber lasers for beam combining, http://www.crystal-fibre.com/support/White.sub.--Paper.sub.---.sub.--Towa- rds.sub.--100kW.sub.--fiber.sub.--laser.sub.--systems.sub.---.sub.--Scalin- g.sub.--up.sub.--power.sub.--in.sub.--fiber.sub.--lasers.sub.--for.sub.--b- eam.sub.--combining.pdf, Feb. 28, 2006. cited by other .
Liem, A., et al., 100-W single-frequency master-oscillator fiber power amplifier, Optics Letters, Sep. 1, 2003, pp. 1537-1539, vol. 28, No. 17. cited by other .
Champert, P. A., etal , "3.5 W frequency-doubled fiber-based laser source at 772 nm", "Applied Physics Letters", Apr. 23, 2001, pp. 2420-2421, vol. 78, No. 17. cited by other .
Limpert, J., etal , "High power Q-switched Yb-doped photonic crystal fiber laser producing sub-10 ns pulses", "Appl. Phys. B 81", 2005, pp. 19-21. cited by other .
Tunnermann, A., etal , "The renaissance and bright future of fibre lasers", "Journal of Physics B: Atomic, Molecular and Optical Physics", 2005, pp. S681-S693, vol. 38. cited by other .
Augst, S. J.., etal , "Wavelength beam combining of ytterbium fiber lasers", "Opt. Lett.", 2003, pp. 331-333, vol. 28. cited by other .
Cooper, L. J.., etal , "High-power Yb-doped multicore ribbon fiber laser", Nov. 1, 2005, pp. 2906-2908, vol. 30, No. 21. cited by other .
Fan, T. Y., "Laser Beam Combining for High-Power, High Radiance Sources", "IEEE Journal of Selected Topics in Quantum Electronics", 2005, pp. 567-577, vol. 11. cited by other .
Hehl, Karl, etal, "High-efficiency dielectric reflection gratings: design, fabrication, and analysis", "Appl. Opt.", 1999, pp. 6257-6271, vol. 38. cited by other .
Liu, F., etal , "Cost-effective wavelength selectable light source using DFB fibre laser array", "Electronics Letters", Mar. 30, 2000, pp. 620-621, vol. 36, No. 7. cited by other .
Liu, A., etal , "Spectral beam combining of high power fiber lasers", "Proceedings of SPIE", 2004, pp. 81-88, vol. 5335. cited by other .
Perry, M. D.., etal , "High-efficiency multilayer dielectric diffraction gratings", "Opt. Lett.", 1995, pp. 940-942, vol. 20. cited by other .
"Sensor Systems for Biological Agent Attacks: Protecting Buildings and Military Bases", Committee on Materials and Manufacturing Processes for Advanced Sensors, National Research Council National Academy Press, Washington D.C. web address: http://darwin.nap.edu/books/030909576X/html, (2005), 73. cited by other .
Brooks, Christopher D., et al., "1-mJ energy, 1-MW peak-power, 10-W average power, spectrally narrow, diffraction-limited pulses from a photonic-crystal fiber amplifier", Optics Express vol. 13, No. 22, (Oct. 31, 2005), 8999-9002. cited by other .
Davitt, Kristina, et al., "290 and 340 nm UV LED arrays for fluorescence detection from single airborne particles", Optics Express, vol. 13, No. 23, 9548-9555. cited by other .
Dunne, Mike, "Laser-driven particle accelerators", Science, vol. 312, (Apr. 21, 2006), 374-376. cited by other .
Henderson, Angus, et al., "Low threshold, singly-resonant CW OPO pumped by an all-fiber pump source", Optics Express vol. 14, No. 2, (Jan. 23, 2006), 767-772. cited by other .
Limpert, J., et al., "High-power rod-type photonic crystal fiber laser", Optics Express vol. 13, No. 4, "Source date" estimated from other Limpert article uploaded, (Feb. 21, 2005), 1055-1058. cited by other .
Limpert, J., et al., "Low-nonlinearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier", Optics Express vol. 12, No. 7, (Apr. 5, 2004), 1313-1319. cited by other .
Moutzouris, Konstantinos, et al., "Highly efficient second, third and fourth harmonic generation from a two-branch femtosecond erbium fiber source", Optics Express vol. 14, No. 5, (Mar. 6, 2006), 1905-1912. cited by other .
Schreiber, T., et al., "Stress-induced single-polarization single-transverse mode photonic crystal fiber with low nonlinearity", Optics Express vol. 13, No. 19, (Sep. 19, 2005), 7621-7630. cited by other.

Primary Examiner: Bolda; Eric
Attorney, Agent or Firm: Lemaire; Charles A. Lemaire Patent Law Firm, P.L.L.C.

Government Interests

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract FA9451-04-D-0412/0001 awarded by the U.S. Air Force. The Government has certain rights in the invention.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/420,729 entitled "FIBER- OR ROD-BASED OPTICAL SOURCE FEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OF HIGH-POWER PULSED RADIATION AND METHOD" filed on May 26, 2006, which claims benefit of U.S. Provisional Patent Application 60/703,822 filed on Jul. 29, 2005, titled "FIBER-BASED OPTICAL SOURCE FEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC CRYSTAL FIBER FOR GENERATION OF HIGH POWER PULSED RADIATION," and U.S. Provisional Patent Application 60/746,166 filed on May 1, 2006, titled "FIBER- OR ROD-BASED OPTICAL SOURCE FEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OF HIGH-POWER PULSED RADIATION AND METHOD," and U.S. Provisional Patent Application 60/797,931 filed on May 5, 2006, titled "FIBER- OR ROD-BASED OPTICAL SOURCE FEATURING A LARGE-CORE, RARE-EARTH-DOPED PHOTONIC-CRYSTAL DEVICE FOR GENERATION OF HIGH-POWER PULSED RADIATION AND METHOD," which are each hereby incorporated by reference in their entirety. This application is also related to U.S. patent application Ser. No. 11/420,730 entitled "MULTI-SEGMENT PHOTONIC-CRYSTAL-ROD WAVEGUIDES FOR AMPLIFICATION OF HIGH-POWER PULSED OPTICAL RADIATION AND ASSOCIATED METHOD" filed on May 26, 2006, which is incorporated herein by reference in its entirety.

Claims

What is claimed is:

1. An apparatus comprising: a first photonic-crystal rod (PCR), having rare-earth-doped core with a diameter of at least 50 microns and an external diameter of at least 1 mm such that the rod is therefore thick enough to substantially hold its shape when released, wherein the rare-earth-doped photonic-crystal rod (REDPCR) receives, as signal input, spectrally broad chirped optical pulses having a FWHM spectral linewidth of at least 10 nm and a duration of ins of less, and amplifies the pulses to obtain an output pulse energy of at least 0.5 mJ, peak power of at least 1 megawatt (MW), and beam-quality M.sup.2 value of less than 1.5.

2. The apparatus of claim 1, wherein the spectrally broad optical input pulses are obtained by temporally stretching and spectrally chirping optical pulses having duration of 100 ps or less emitted from an external optical source.

3. The apparatus of claim 1, wherein the rare-earth-doped photonic-crystal rod includes internal stress rods that induce birefringence in the core thereby ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB.

4. The apparatus of claim 1, wherein the rare-earth-doped photonic-crystal rod includes internal elements that induce birefringence in the core thereby ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB.

5. The apparatus of claim 1, wherein the peak power is at least 4 MW.

6. The apparatus of claim 1, further comprising: at least one solid-body fiber; at least one seed laser subsystem configured to emit chirped pulses; and at least one photonic-crystal fiber, wherein the first photonic-crystal rod, the solid-body fiber, the laser subsystem, and the photonic-crystal fiber are parts of a series of optical components separated by free-space optical subassemblies that provide pump light into the series of optical components.

7. The apparatus of claim 1, further comprising: at least one solid-body fiber; at least one seed laser subsystem configured to emit chirped pulses; and at least one photonic-crystal fiber, wherein the first photonic-crystal rod, the solid-body fiber, the laser subsystem, and the photonic-crystal fiber are parts of a series of optical components separated by free-space optical subassemblies that provide pump light into the series of optical components, and wherein the rare-earth-doped photonic-crystal rod includes internal elements that induce birefringence in the core thereby ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB.

8. A method comprising: providing a first photonic-crystal rod (PCR) having rare-earth-doped core with a diameter of at least 50 microns and an external diameter of at least 1 mm such that the rod is therefore thick enough to substantially hold its shape when released; optically coupling as signal input into the rare-earth-doped photonic-crystal rod (REDPCR) spectrally broad chirped optical pulses having a FWHM spectral linewidth of at least 10 nm and a duration of 1 ns of less; and amplifying the pulses in the REDPCR to obtain an output pulse energy of at least 0.5 mJ, peak power of at least 1 MW, and beam-quality M.sup.2 value of less than 1.5.

9. The method of claim 8, further comprising: obtaining optical pulses having duration of 100 ps or less; and temporally stretching and spectrally chirping the optical pulses to produce the spectrally broad optical input pulses.

10. The method of claim 8, further comprising: ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB by including, in the rare-earth-doped photonic-crystal rod, internal stress rods that induce birefringence in the core of the rare-earth-doped photonic-crystal rod.

11. The method of claim 8, further comprising: ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB by including, in the rare-earth-doped photonic-crystal rod, internal elements that induce birefringence in the core of the rare-earth-doped photonic-crystal rod.

12. The method of claim 8, wherein the amplifying generates peak power of at least 4 MW.

13. The method of claim 8, further comprising: serially connecting at least one solid-body fiber, at least one seed laser subsystem configured to emit chirped pulses, and at least one photonic-crystal fiber, as parts of a series of optical components separated by free-space optical subassemblies; and injecting pump light into the series of optical components through the free-space optical subassemblies.

14. The method of claim 8, further comprising: serially connecting at least one solid-body fiber, at least one seed laser subsystem configured to emit chirped pulses, and at least one photonic-crystal fiber, as parts of a series of optical components separated by free-space optical subassemblies; injecting pump light into the series of optical components through the free-space optical subassemblies; and ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB by including, in the rare-earth-doped photonic-crystal rod, internal elements that induce birefringence in the core of the rare-earth-doped photonic-crystal rod.

15. An apparatus comprising: photonic-crystal-rod means for amplifying, the means for amplifying having rare-earth-doped means for waveguiding with a diameter of at least 50 microns and means for substantially holding the shape of the means for amplifying when released; means for optically coupling as signal input into the rare-earth-doped photonic-crystal rod (REDPCR) spectrally broad chirped optical pulses having a FWHM spectral linewidth of at least 10 nm and a duration of ins of less; and means for pumping the means for amplifying in order to obtain an output pulse energy of at least 0.5 mJ, peak power of at least 1 MW, and beam-quality M.sup.2 value of less than 1.5.

16. The apparatus of claim 15, further comprising: means for obtaining optical pulses having duration of 100 ps or less; and means for temporally stretching and spectrally chirping the optical pulses to produce the spectrally broad optical input pulses.

17. The apparatus of claim 15, further comprising: means for ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB including internal means for stressing the means for waveguiding.

18. The apparatus of claim 15, further comprising: means for ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB including means for inducing birefringence in the means for waveguiding.

19. The apparatus of claim 15, wherein the means for amplifying generates peak power of at least 4 MW.

20. The apparatus of claim 15, further comprising: means for serially connecting at least one solid-body fiber, at least one seed laser subsystem configured to emit chirped pulses, and at least one photonic-crystal fiber, as parts of a series of optical components separated by free-space optical subassemblies; and means for injecting pump light into the series of optical components through the free-space optical subassemblies.

21. The apparatus of claim 15, further comprising: means for serially connecting at least one solid-body fiber, at least one seed laser subsystem configured to emit chirped pulses, and at least one photonic-crystal fiber, as parts of a series of optical components separated by free-space optical subassemblies; means for injecting pump light into the series of optical components through the free-space optical subassemblies; and means for ensuring that the optical beam emitted by the photonic-crystal rod is linearly polarized and the degree of polarization is at least 15 dB including means for inducing birefringence in the means for waveguiding.

22. The apparatus of claim 6, wherein at least one of the free-space optical subassemblies that provide pump light into the series of optical components includes an integrated pump block having a fiber signal input port, a fiber pump input port and a fiber signal output port, and is configured to propagate pump light from the fiber pump input port to the fiber signal input port across a free-space gap.

Description

FIELD OF THE INVENTION

The invention relates generally to high-power optical amplifiers and lasers and more particularly to methods and apparatus applicable for photonic-crystal optical fibers and similar structures suitable for very high peak-power and average-power optical output, near-diffraction-limited beam quality, multi-kHz pulse-repetition rate, highly controlled spectral properties including narrow line width and high signal-to-noise ratios.

BACKGROUND OF THE INVENTION

Rare-earth (RE) doped, pulsed fiber lasers and amplifiers constitute efficient and compact optical sources that can emit a diffraction-limited Gaussian beam of highly controlled spectral quality. The output power generated by these sources is limited, however, by parasitic nonlinear optical effects, amplified spontaneous emission, and damage to optical components due to high optical power.

Nonlinear effects include stimulated Brillouin and Raman scattering (SBS and SRS), self- and cross-phase modulation (SPM and XPM), and four-wave mixing (FWM). The common origin of these effects is the high optical intensity in the fiber core and long path for the nonlinear interaction between the in-fiber optical beam and fiber material (e.g., silica). These effects hamper in particular the generation of high-peak-power pulses by causing unwanted spectral broadening, distortion of the pulse temporal profile, and sudden power instabilities that result in optical damages.

The build-up of amplified spontaneous emission (ASE) is due to the high optical gain available in the fiber core in the time interval between pulses. ASE constitutes an unwanted continuous-wave (CW) noise, which degrades the pulse/background contrast and, most importantly, limits the attainable pulse energy by using up gain.

Finally, optical damages can occur in the fiber because of material breakdown in the presence of high optical intensities. The fiber facets are especially vulnerable because exposed to potential contaminants and subject to defects that can initiate damage.

There is a need for fiber lasers and optical amplifiers configured to emit pulses of considerably higher energy and peak power than currently available. These sources must be designed so as to circumvent the limitations described above.

SUMMARY OF THE INVENTION

In some embodiments, the present invention provides one or more optical-pulse amplifiers based on photonic-crystal-fiber technology, which simultaneously provide one or more of the following: pulse peak power in excess of 1 megawatt (MW), near-diffraction-limited beam quality (M.sup.2<1.5), multi-kHz pulse-repetition rate, and highly controlled spectral properties that include, in some embodiments, pulse linewidth of 50 GHz or less and signal-to-noise ratio of 30 dB or more. M.sup.2 is a widely used dimensionless beam-propagation-quality parameter and the definition adopted hereafter is the same provided in the current ISO Standard for beam quality characterization (ISO 11146). For a pure Gaussian beam, M.sup.2=1. In this document, signal-to-noise ratio is defined as the intensity ratio between the pulse's spectral peak and that of background radiation at wavelengths other than those of the pulse.

In some embodiments, the present invention provides pulsed fiber lasers and amplifiers for applications that require pulses that are a few nanoseconds long at multi-kHz pulse-repetition rates (PRR), exhibiting one or more of the following characteristics: high peak power (useful for applications such as, e.g., wavelength conversion, materials processing, and ranging), high pulse energy (useful for applications such as, e.g., illumination and imaging), and narrow spectral linewidth (useful for applications such as, e.g., remote sensing and wavelength conversion). The present invention provides fiber-based sources that generate higher pulse energies and peak powers than are conventionally available, while also achieving compactness, efficiency, and high beam quality.

In some embodiments, the present invention provides high-power pulsed fiber lasers and amplifiers based on photonic-crystal-fiber technology, which produce high peak power (>500 kilowatt (kW)) linearly polarized output beams of near-diffraction-limited beam quality and narrow spectral linewidth that can be effectively used for generation of high-peak-power visible and ultraviolet radiation by means of frequency conversion in nonlinear crystals. In some embodiments, the outputs from several linearly polarized, spectrally narrow, high-peak-power fiber lasers and/or amplifiers based on the same technology and arranged in a suitable pattern can be combined spectrally by using an external dispersive optical element to produce a beam of near-diffraction-limited beam quality and peak power/pulse energy approximately equal to the sum of the peak powers/pulse energies from each individual fiber laser/amplifier. The benefit of this beam-combination scheme is that it produces a peak power in a single beam that is much higher than the damage threshold for an individual fiber.

Optical fibers are waveguides, in which a certain number of transverse modes of radiation can exist and propagate with low loss. Different transverse modes correspond to different transverse profiles of optical intensity. In step-index fibers, the fundamental mode profile is very similar to a Gaussian. Fibers that support only this mode (usually referred to as "single-mode fibers") inherently produce the best beam quality. For a given refractive index step between core and cladding, the number of transverse modes supported by a fiber is proportional to the core diameter. Therefore, large-core fibers tend to be multimode, which degrades the beam quality.

Moreover, in multimode fibers, thermal and mechanical perturbations can effect uncontrolled changes in the mode pattern and beam pointing. In large-core RE-doped fibers, different modes exhibit different spatial overlap with the dopant distribution, and hence different modes experience different gain, therefore sudden mode pattern changes result in power instabilities. These instabilities can also result in sudden intensity spikes that damage the fiber facet or body, especially in fiber lasers/amplifiers emitting high peak power (e.g., in the range of 1 MW or higher). The present invention addresses these problems, among others.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a block diagram of a master-oscillator/power-amplifier (MOPA) system 100 having high-peak-power optical amplifiers including one or more gain stages, pump blocks and rare-earth-doped photonic-crystal-fiber (PCF) power amplifiers.

FIG. 1B is a schematic diagram of a system 101 having high-peak-power optical amplifiers including one or more gain stages, pump blocks and rare-earth-doped photonic-crystal-fiber (PCF) power amplifiers.

FIG. 1C is a block diagram of a pump block 118.

FIG. 1D is a schematic diagram of a pump block 118.

FIG. 1E is a schematic diagram of a compact system 102 having high-peak-power rare-earth-doped photonic-crystal-fiber (PCF)/photonic-crystal-rod (PCR) optical power amplifiers.

FIG. 1F is a schematic diagram of a compact system 103 having high-peak-power rare-earth-doped PCF/PCR optical power amplifiers.

FIG. 1G is a schematic diagram of a pump block 119.

FIG. 1H is a schematic diagram of a compact system 104 having high-peak-power rare-earth-doped PCF/PCR optical power amplifiers.

FIG. 1I is a schematic diagram of a pump block 121.

FIG. 2 is a schematic diagram of a high-peak-power rare-earth-doped photonic-crystal-rod optical power amplifier 200.

FIG. 3A is an end-view schematic diagram of a high-peak-power rare-earth-doped photonic-crystal rod 300 having a beam-expanding endcap.

FIG. 3B is a side-view schematic diagram of PCR 300 having a beam-expanding endcap.

FIG. 3C is a cross-section schematic diagram of PCR 300 having a beam-expanding endcap.

FIG. 3D is an end-view schematic diagram of a high-peak-power rare-earth-doped photonic-crystal rod 301 partially fabricated into having a beam-expanding endcap.

FIG. 3E is a side-view schematic diagram of PCR 301.

FIG. 3F is a cross-section schematic diagram of PCR 301.

FIG. 3G is an end-view schematic diagram of a high-peak-power rare-earth-doped photonic-crystal rod 302 partially fabricated into having a beam-expanding endcap after tapering.

FIG. 3H is a side-view schematic diagram of PCR 302.

FIG. 3I is a cross-section schematic diagram of PCR 302.

FIG. 3J is a microphotograph of PCR 304 after tapering the end and before removing the epoxy cap and collapsing the air cladding to form the beam-expanding endcap.

FIG. 4A is a side-view schematic diagram of a high-peak-power rare-earth-doped photonic-crystal rod (PCR) 401 partially fabricated into having a beam-expanding endcap.

FIG. 4B is a side-view schematic diagram of PCR 402 after partially collapsing the holey region of the endcap.

FIG. 4C is a diagram of PCR 403 after further collapsing the holey region of the endcap.

FIG. 4D shows PCR 404 after yet more collapsing the holey region of the endcap.

FIG. 4E shows PCR 404 after angle polishing the end of the endcap to form the rod facet.

FIG. 4F is a side-view schematic diagram of PCR 405 after injecting an index matching compound into the air cladding at the rod end and polishing the rod end to form a beam-expanding endcap and facet.

FIG. 5A is a side-view schematic diagram of a high-peak-power rare-earth-doped photonic-crystal fiber (PCF) 500 after faceting and collapsing the holey region of the endcap.

FIG. 5B is a microphotograph of PCF 500.

FIG. 6 is a microphotograph of a photonic-crystal rod (PCR) 600 after faceting and collapsing the holey region of the endcap.

FIG. 7A is a side-view block diagram of a system 700 for forming a beam-expanding endcap onto high-peak-power rare-earth-doped photonic-crystal rod 310 at a time before collapsing the holey region of the endcap.

FIG. 7B is a side-view block diagram of a system 700 at a time after collapsing the end portion of the holey region of the endcap.

FIG. 7C is a side-view block diagram of a system 700 at a time after moving the endcap further into the heating region.

FIG. 7D is a side-view block diagram of a system 700 at a time after further collapsing the end portion of the holey region of the endcap.

FIG. 7E is a side-view block diagram of photonic-crystal rod 310 at a time after collapsing the holey region of the endcap and angle-polishing the end.

FIG. 7F is an end-view block diagram of a system 700.

FIG. 7G is an end-view block diagram of a system 701 for forming a beam-expanding endcap onto ribbon-like high-peak-power rare-earth-doped photonic-crystal rod.

FIG. 8A is a cross-section-view schematic diagram of a ribbon-PCR system 800 having a ribbon-like high-peak-power rare-earth-doped photonic-crystal rod (PCR ribbon).

FIG. 8B is a perspective-view schematic diagram of a ribbon-like high-peak-power rare-earth-doped photonic-crystal-rod spectral-beam combiner output-stage system 808.

FIG. 8C is a plan-view schematic diagram of a master-oscillator/power-amplifier (MOPA) high-peak-power rare-earth-doped photonic-crystal-ribbon laser system 870 using a ribbon-like high-peak-power rare-earth-doped photonic-crystal rod spectral-beam combiner output-stage system 808.

FIG. 8D is a cross-section-view schematic diagram of a preform 861 configured to compensate for lateral shrinkage in later forming of a ribbon-like high-peak-power rare-earth-doped photonic-crystal rod inner-cladding/core portion 862.

FIG. 8E is a cross-section-view schematic diagram of a ribbon-like high-peak-power rare-earth-doped photonic-crystal-rod inner-cladding/core portion 862.

FIG. 8F is a cross-section-view schematic diagram of a polarizing high-peak-power rare-earth-doped photonic-crystal rod 880.

FIG. 8G is a cross-section-view schematic diagram of a single-polarization high-peak-power rare-earth-doped PCF or PCR ribbon 881.

FIG. 8H is a cross-section-view schematic diagram of ribbon PCR 887 having stress elements 886 to induce birefringence in cores 868.

FIGS. 8-I is a cross-section-view schematic diagram of a central portion of a high-peak-power rare-earth-doped photonic-crystal rod 890.

FIG. 9A is a perspective-view schematic diagram of a high-peak-power rare-earth-doped PCF or PCR ribbon MOPA laser system 900.

FIG. 9B is a plan-view schematic diagram of MOPA laser system 900.

FIG. 9C is an elevation-view schematic diagram of MOPA laser system 900.

FIG. 10 is a plan-view schematic diagram of MOPA laser system 1000 having a segmented final gain section having fiber splices.

FIG. 11A is a perspective-view schematic diagram of a high-peak-power rare-earth-doped laser-welded PCF or PCR MOPA laser system 1100.

FIG. 11B is an elevation-view schematic diagram of MOPA laser system 1100.

FIG. 11C is an end-view schematic diagram of MOPA laser system 1100.

FIG. 11D is a perspective-view schematic diagram of another high-peak-power rare-earth-doped laser-welded PCF or PCR MOPA laser system 1101.

FIG. 11E is an elevation-view schematic diagram of MOPA laser system 1101.

FIG. 11F is an end-view schematic diagram of MOPA laser system 1101.

FIG. 11G is a perspective-view schematic diagram of yet another high-peak-power rare-earth-doped laser-welded PCF or PCR MOPA laser system 1102.

FIG. 11H is an elevation-view schematic diagram of MOPA laser system 1102.

FIG. 11-I is an end-view schematic diagram of MOPA laser system 1102.

FIG. 12A is a schematic diagram of a high-peak-power rare-earth-doped PCF or PCR MOPA laser system 1200 having an improved delivery fiber 1230.

FIG. 12B is a cross-section-view schematic diagram of an output end 1210 of improved delivery fiber 1230.

FIG. 12C is a cross-section-view schematic diagram of an input end 1220 of improved delivery fiber 1230.

FIG. 12D is a cross-section-view schematic diagram of an alternative input end 1222 of improved delivery fiber 1230.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics for the purpose of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiments of the invention is set forth without any loss of generality to, and without imposing limitations upon the claimed invention.

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

The leading digit(s) of reference numbers appearing in the Figures generally corresponds to the Figure number in which that component is first introduced, such that the same reference number is used throughout to refer to an identical component that appears in multiple figures. Signals and connections may be referred to by the same reference number or label, and the actual meaning will be clear from its use in the context of the description.

As used herein, an optical-waveguide device is defined as any device that provides a constrained guided optical path in a solid, for example, an optical fiber having one or more waveguide cores or an optical slab or monolithic substrate having a width and length each larger than the thickness, and having one or more waveguides formed therein (e.g., laterally spaced waveguides formed by diffusion of a index-modifying material through a mask to form surface or near-surface waveguides). An optical fiber is defined as any device having one or more cores or internal waveguides and a length much longer than a transverse width, for example a glass fiber drawn from a melt or preform or extruded from an extruder. A thin optical fiber is defined as a fiber that is thin enough to be readily bent to some non-infinite radius (e.g., a conventional optical fiber). A rod-like fiber (also referred to hereafter as "rod waveguide" or simply "rod") is defined as a fiber that is thick enough to readily hold its shape when released (e.g., a glass rod having a diameter of 1 millimeter (mm) or more). An optical ribbon is defined as a fiber having two or more signal cores laterally spaced across a width of the fiber. An optical ribbon rod is defined as a fiber having two or more signal cores laterally spaced across a width of the fiber and that is thick enough to readily hold its shape when released.

Major factors limiting the pulse peak power and energy in pulsed fiber-based sources are parasitic nonlinear optical effects (NLEs) and inter-pulse amplified spontaneous emission (ASE).

A great deal of prior art and published literature has attempted to address the issue of parasitic NLEs in fiber pulse amplifiers. These parasitic nonlinear optical effects arise, at sufficiently high pulse peak power, from the nonlinear interaction between the optical pulses confined in the fiber core and the silica-based material of the fiber. NLEs include stimulated Brillouin scattering and stimulated Raman scattering (SBS and SRS), self- and cross-phase modulation (SPM and XPM), and four-wave mixing (FWM, also referred to as "cross-talk"). Different NLEs have the following in common: a) the pulse optical power at which they set on (referred to as "threshold power" and coinciding with the maximum pulse power achievable) is proportional to the fiber core area and inversely proportional to the fiber length. In other words, long fibers of small core favor NLEs; and b) they cause: i. unwanted spectral broadening of the optical pulses and/or wavelength conversion, and ii. optical feedback, power instabilities and ensuing potential damages to the optical components.

Because of the above-described dependence of the NLE threshold power on fiber length and core area, a widely adopted method for avoiding NLEs has been to resort to fibers featuring as large a core as possible. These fibers are best known in the art as large-mode-area (LMA) fibers and exhibit core areas more than an order of magnitude larger than those of conventional single-mode fibers used for telecommunications.

A major problem with this approach is that for cores that are la