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Dostovalov A, Kokhanovskiy A, Golikov E, Revyakin A, Munkueva Z, Kharenko D, Babin S. Fiber Bragg grating inscription assisted by a spatial light modulator. OPTICS LETTERS 2024; 49:1077-1080. [PMID: 38359257 DOI: 10.1364/ol.514896] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/04/2023] [Accepted: 01/30/2024] [Indexed: 02/17/2024]
Abstract
In this Letter, we proposed a new technique for point-by-point fiber Bragg grating (FBG) writing in a static fiber by using a spatial light modulator to control the position of the focal point inside the fiber core. Various types of short-length FBGs (uniform, phase-shifted, and apodized) were demonstrated by this inscription technique. Moreover, the capability to tailor the transverse dimension of a grating pitch from a single point to more complex shapes, such as a wide plane covering a whole fiber core or a transverse ring, was shown.
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Agrawal GP. Spatial beam narrowing in multimode graded-index fiber amplifiers: an analytic approach. OPTICS LETTERS 2023; 48:259-262. [PMID: 36638432 DOI: 10.1364/ol.479060] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/23/2022] [Accepted: 12/01/2022] [Indexed: 06/17/2023]
Abstract
Doped and optically pumped graded-index (GRIN) fibers can be used to amplify an optical beam such that its spatial quality is improved at the output end of the fiber compared with that of the unamplified beam. We develop a simple model of the amplification process in such GRIN fiber amplifiers and show that the resulting equations can be solved analytically with suitable approximations. The solution shows that the width of the amplifying beam oscillates but also becomes narrower because of the radial dependence of the optical gain. The main advantage of our simplified approach is that it provides an analytic expression for the damping distance of beam-width oscillations that shows clearly the role played by various physical parameters.
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3
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Kharenko DS, Gervaziev MD, Kuznetsov AG, Podivilov EV, Wabnitz S, Babin SA. Mode-resolved analysis of pump and Stokes beams in LD-pumped GRIN fiber Raman lasers. OPTICS LETTERS 2022; 47:1222-1225. [PMID: 35230332 DOI: 10.1364/ol.449119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/19/2021] [Accepted: 01/25/2022] [Indexed: 06/14/2023]
Abstract
All-fiber Raman lasers have demonstrated their potential for efficient conversion of highly multimode pump beams into high-quality Stokes beams. However, the modal content of these beams has not yet been investigated. In this work, based on a mode decomposition technique, we are able to reveal the details of intermodal interactions in the different operation regimes of continuous wave multimode graded-index fiber Raman lasers. We observed that, above the laser threshold, the residual pump beam is strongly depleted in its transverse modes with principal quantum number below 10. However, the generated Stokes signal beam mainly consists of the fundamental mode, but higher-order modes are also present, albeit with exponentially decreasing population.
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High-Peak Power Frequency Modulation Pulse Generation in Cascaded Fiber Configurations with Inscribed Fiber Bragg Grating Arrays. PHOTONICS 2021. [DOI: 10.3390/photonics8110471] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
We explored the dynamics of frequency-modulated (FM) pulses in a cascaded fiber configuration comprising one active and one passive optical fiber with multiple fiber Bragg gratings (FBGs) of different periods inscribed over the fiber configuration length. We present a theoretical formalism to describe the mechanisms of the FM pulse amplification and pulse compression in such fiber cascades resulting in peak powers up to ~0.7 MW. In combination with the decreasing dispersion fibers, the considered cascade configuration enables pico- and sub-picosecond pulse trains with a sub-terahertz repetition rate and sub-kW peak power generated directly from the continuous optical signal.
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Abstract
We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas, for the second order, both a linear cavity consisting of two FBGs and a half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored. LDs with different wavelengths (915 and 940 nm) are used for pumping enabling Raman lasing at different wavelengths of the first (950, 954 and 976 nm), second (976, 996 and 1019 nm) and third (1065 nm) Stokes orders. Output power and efficiency, spectral line shapes and widths, beam quality and shapes are compared for different configurations. It is shown that the RDFB cavity provides higher slope efficiency of the second Stokes generation (up to 70% as that for the first Stokes wave) with output power up to ~30 W, limited by the third Stokes generation. The best beam quality parameter of the second Stokes beam is close to the diffraction limit (M2~1.3) in both linear and half-open cavities, whereas the line is narrower (<0.2 nm) and more stable in the case of the linear cavity with two FBGs. However, an optimization of the FBG reflection spectrum used in the half-open cavity allows this linewidth value to be approached. The measured beam profiles show the dip formation in the output pump beam profile, whereas the first and second Stokes beams are Gaussian-shaped and almost unchanged with increasing power. A qualitative explanation of such behavior in connection with the power evolution for the transmitted pump and generated first, second and third Stokes beams is given. The potential for wavelength tuning of the cascaded Raman lasers based on LD-pumped multimode fibers is discussed.
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Fan C, Xiao H, Yao T, Xu J, Chen Y, Leng J, Zhou P. Kilowatt level Raman amplifier based on 100 µm core diameter multimode GRIN fiber with M 2 = 1.6. OPTICS LETTERS 2021; 46:3432-3435. [PMID: 34264231 DOI: 10.1364/ol.431273] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/12/2021] [Accepted: 06/07/2021] [Indexed: 06/13/2023]
Abstract
In this Letter, we demonstrate a high-power Raman fiber amplifier with excellent beam quality based on graded-index fiber. The Yb-doped fiber laser (YDFL) and bandwidth-tunable amplified spontaneous emission (ASE) source are employed as the pump source to compare the laser performance separately. When the ASE with a bandwidth of 8 nm is employed, a maximum power of 943 W at 1130 nm is achieved, which is twice that pumped by YDFL. The beam quality factor M2 at maximum output power is 1.6, with a brightness enhancement (BE) factor of 27. To the best of our knowledge, this is the best beam quality and BE factor based on pure Raman gain with output power of over 100 W.
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Kuznetsov AG, Nemov IN, Wolf AA, Kablukov SI, Babin SA. Multimode LD-pumped all-fiber Raman laser with excellent quality of 2 nd-order Stokes output beam at 1019 nm. OPTICS EXPRESS 2021; 29:17573-17580. [PMID: 34154297 DOI: 10.1364/oe.425639] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/23/2021] [Accepted: 05/03/2021] [Indexed: 06/13/2023]
Abstract
A multimode all-fiber Raman laser enabling cascaded generation of high-quality 1019-nm output beam at direct pumping by highly-multimode (M2>30) 940-nm laser diodes has been demonstrated. The laser is made of a 100/140 graded-index fiber with special in-fiber Bragg gratings which secure sequential generation of the 1st (976 nm) and 2nd (1019 nm) Stokes orders. Comparing different 1019-nm cavity structures shows that the half-open cavity with one FBG and distributed feedback via random Rayleigh backscattering provides excellent quality (M2∼1.3) with higher slope efficiency of pump-to-2nd Stokes conversion than in the conventional 2-FBG cavity. The maximum achieved slope efficiency amounts to about 40% at output powers of up to 12 W limited by the 3rd Stokes generation.
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Chen Y, Fan C, Yao T, Xiao H, Leng J, Zhou P, Nemov IN, Kuznetsov AG, Babin SA. Brightness enhancement in random Raman fiber laser based on a graded-index fiber with high-power multimode pumping. OPTICS LETTERS 2021; 46:1185-1188. [PMID: 33649688 DOI: 10.1364/ol.416740] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2020] [Accepted: 02/02/2021] [Indexed: 06/12/2023]
Abstract
A brightness-enhanced random Raman fiber laser (RRFL) with maximum power of 306 W at 1120 nm is demonstrated. A half-open cavity is built based on a graded-index (GRIN) passive fiber and single high-reflective fiber Bragg grating written in it directly. Due to the beam cleanup effect in the GRIN fiber enhanced in the half-open RRFL cavity, the output beam quality factor M2 is improved from 9.15 (pump) to 1.76-2.35 (Stokes) depending on power, while the pump-Stokes brightness enhancement (BE) factor increases proportionally to output power reaching 6.1 at maximum. To the best of our knowledge, this is the highest power GRIN RRFL with BE.
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Bobkov K, Levchenko A, Kashaykina T, Aleshkina S, Bubnov M, Lipatov D, Laptev A, Guryanov A, Leventoux Y, Granger G, Couderc V, Février S, Likhachev M. Scaling of average power in sub-MW peak power Yb-doped tapered fiber picosecond pulse amplifiers. OPTICS EXPRESS 2021; 29:1722-1735. [PMID: 33726380 DOI: 10.1364/oe.413528] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/29/2020] [Accepted: 12/20/2020] [Indexed: 06/12/2023]
Abstract
Prospects for average power scaling of sub-MW output peak power picosecond fiber lasers by utilization of a Yb-doped tapered fiber at the final amplification stage were studied. In this paper, it was shown experimentally that a tapered fiber allows the achievement of an average power level of 150 W (limited by the available pump power) with a peak power of 0.74 MW for 22 ps pulses with no signs of transverse mode instability. Measurements of the mode content using the S2 technique showed a negligible level of high order modes (less than 0.3%) in the output radiation even for the maximum output power level. Our reliability tests predict no thermal issues during long-term operation (105 hours) of the developed tapered fiber laser up to kilowatt output average power levels.
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Kuznetsov AG, Evmenova EA, Dontsova EI, Kablukov SI, Babin SA. Frequency doubling of multimode diode-pumped GRIN-fiber Raman lasers. OPTICS EXPRESS 2019; 27:34760-34768. [PMID: 31878659 DOI: 10.1364/oe.27.034760] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/05/2019] [Accepted: 10/16/2019] [Indexed: 06/10/2023]
Abstract
Frequency doubling of multimode diode-pumped GRIN-fiber Raman laser with improved beam quality (M2=1.9-2.6 depending on configuration) in a simple single-pass scheme with 5-mm PPLN crystal is studied. After scheme optimization and elimination of back reflection and crystal heating effects, an efficient conversion into blue spectral range with output power of about 0.4 W@488 nm and 0.64 W@477 nm has been demonstrated.
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Evmenova EA, Kuznetsov AG, Nemov IN, Wolf AA, Dostovalov AV, Kablukov SI, Babin SA. 2nd-order random lasing in a multimode diode-pumped graded-index fiber. Sci Rep 2018; 8:17495. [PMID: 30504776 PMCID: PMC6269461 DOI: 10.1038/s41598-018-35767-9] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2018] [Accepted: 11/09/2018] [Indexed: 11/09/2022] Open
Abstract
Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950-954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.
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Affiliation(s)
| | - Alexey G Kuznetsov
- Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia
| | - Ilya N Nemov
- Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia
| | - Alexey A Wolf
- Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia
- Novosibirsk State University, Novosibirsk, 630090, Russia
| | - Alexandr V Dostovalov
- Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia
- Novosibirsk State University, Novosibirsk, 630090, Russia
| | - Sergey I Kablukov
- Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia
- Novosibirsk State University, Novosibirsk, 630090, Russia
| | - Sergey A Babin
- Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia.
- Novosibirsk State University, Novosibirsk, 630090, Russia.
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Bai Z, Williams RJ, Kitzler O, Sarang S, Spence DJ, Mildren RP. 302 W quasi-continuous cascaded diamond Raman laser at 1.5 microns with large brightness enhancement. OPTICS EXPRESS 2018; 26:19797-19803. [PMID: 30119299 DOI: 10.1364/oe.26.019797] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/18/2018] [Accepted: 05/25/2018] [Indexed: 06/08/2023]
Abstract
We report a second Stokes diamond Raman laser at 1.49 μm capable of high power and large-scale-factor brightness enhancement. Using a quasi-continuous 1.06 μm pump of power 823 W (0.85% duty cycle) and M2 up to 6.4, a maximum output power of 302 W was obtained with an M2 = 1.1 providing an overall brightness enhancement factor of 6.0. The pulse length of ~210 μs was selected to ensure operation was representative of steady-state continuous lasing conditions in the diamond bulk. Accompanying theoretical calculations indicate that even more strongly aberrated pumps may be used to efficiently generate high beam quality output and with higher brightness enhancement factors. This diamond-based beam conversion technique addresses needs for high brightness and efficient eye-safe sources using low-brightness 1 μm pumps and reveals a widely-applicable route to practical high brightness lasers of increased wavelength range.
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Kharenko DS, Efremov VD, Evmenova EA, Babin SA. Generation of Raman dissipative solitons near 1.3 microns in a phosphosilicate-fiber cavity. OPTICS EXPRESS 2018; 26:15084-15089. [PMID: 30114760 DOI: 10.1364/oe.26.015084] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/16/2018] [Accepted: 05/19/2018] [Indexed: 06/08/2023]
Abstract
An external-cavity generation of powerful ultrashort pulses in an all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse near 1.3 microns is observed. Optimization of Stokes output spectrum depending on pump pulse duration (chirp), energy and output coupling ratio of the cavity is performed. As result, the output energy of highly-chirped pulses compressible to 570 fs reaches 1.6 nJ.
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14
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Glick Y, Shamir Y, Wolf AA, Dostovalov AV, Babin SA, Pearl S. Highly efficient all-fiber continuous-wave Raman graded-index fiber laser pumped by a fiber laser. OPTICS LETTERS 2018; 43:1027-1030. [PMID: 29489772 DOI: 10.1364/ol.43.001027] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/29/2017] [Accepted: 01/24/2018] [Indexed: 06/08/2023]
Abstract
We report for the first time, to the best of our knowledge, an all-fiber Raman graded-index (GRIN) fiber laser pumped by a fiber laser. This configuration points to potential future power and brightness increases. Continuous-wave power of 135 W with an M2 value of 2.5 was obtained at a wavelength of 1081 nm with an optical-to-optical efficiency of 68%. A commercial GRIN core fiber acts as the Raman fiber in a power oscillator configuration that includes fiber Bragg gratings (FBGs) written onto the GRIN fiber. The efficiency and brightness demonstrated here are, to the best of our knowledge, the highest reported in any Raman GRIN fiber laser. A brightness enhancement of the pump beam by a factor of 5.6 is attained due to the transverse profiles of Raman gain and FBG reflection in the GRIN fiber.
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Bai Z, Williams RJ, Jasbeer H, Sarang S, Kitzler O, Mckay A, Mildren RP. Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion. OPTICS LETTERS 2018; 43:563-566. [PMID: 29400841 DOI: 10.1364/ol.43.000563] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/30/2017] [Accepted: 12/21/2017] [Indexed: 06/07/2023]
Abstract
High average power lasers with high beam quality are critical for emerging applications in industry and research for defense, materials processing, and space applications. However, overcoming thermal effects in the gain medium remains the key challenge for increasing laser brightness at high powers. Here we report a means for increasing the beam brightness of high-power continuous-wave (CW) beams based on external cavity Raman lasers using diamond, a material with thermal properties far superior to any other laser material. With pump beam quality in the range M2=2.3-7.3, efficient pump-limited conversion to an M2=1.1 Stokes beam is achieved in all cases, with increases in brightness from the pump by factors as high as 12.7. The influence of pump beam quality on laser threshold and slope efficiency is analyzed. This Letter foreshadows an alternative approach for scaling the brightness of CW lasers using high-power, moderate beam quality pumps up to M2=20 or more, such as thin-disk and slab lasers and fiber lasers operating in a mode instability regime.
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Guenard R, Krupa K, Dupiol R, Fabert M, Bendahmane A, Kermene V, Desfarges-Berthelemot A, Auguste JL, Tonello A, Barthélémy A, Millot G, Wabnitz S, Couderc V. Nonlinear beam self-cleaning in a coupled cavity composite laser based on multimode fiber. OPTICS EXPRESS 2017; 25:22219-22227. [PMID: 29041536 DOI: 10.1364/oe.25.022219] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/27/2017] [Accepted: 07/26/2017] [Indexed: 05/24/2023]
Abstract
We study a coupled cavity laser configuration where a passively Q-switched Nd:YAG microchip laser is combined with an extended cavity, including a doped multimode fiber. For appropriate coupling levels with the extended cavity, we observed that beam self-cleaning was induced in the multimode fiber thanks to nonlinear modal coupling, leading to a quasi-single mode laser output. In the regime of beam self-cleaning, laser pulse duration was reduced from 525 to 225 ps. We also observed a Q-switched mode-locked operation, where spatial self-cleaning was accompanied by far-detuned nonlinear frequency conversion in the active multimode fiber.
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Leconte B, Gilles H, Robin T, Cadier B, Laroche M. Transverse mode selection in a Nd-doped fiber amplifier at 910 nm. OPTICS EXPRESS 2017; 25:18314-18319. [PMID: 28789318 DOI: 10.1364/oe.25.018314] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/27/2017] [Accepted: 07/08/2017] [Indexed: 06/07/2023]
Abstract
Selective amplifications of LP01 fundamental mode and higher order modes LP11 and LP01 are demonstrated in a double-pass Nd-doped LMA fiber amplifier operating at 910 nm. A multimode core fiber Bragg grating is employed to select a single guided mode by simply adjusting the wavelength of the seed signal. Although the M2 parameter of the output beam from the amplifier was ~2.5 in a single-pass configuration, a double-pass configuration with LP01 mode selection reduces the value of the M2 parameter to 1.06 in spite of the multimode nature of the core (V~5). In addition, it is shown that this amplifier configuration permits to lower both the power saturation and the parasitic emission at 1060 nm, which consequently increase the pump-to-signal conversion efficiency at 910 nm.
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Zlobina EA, Kablukov SI, Wolf AA, Nemov IN, Dostovalov AV, Tyrtyshnyy VA, Myasnikov DV, Babin SA. Generating high-quality beam in a multimode LD-pumped all-fiber Raman laser. OPTICS EXPRESS 2017; 25:12581-12587. [PMID: 28786613 DOI: 10.1364/oe.25.012581] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/2017] [Accepted: 05/06/2017] [Indexed: 06/07/2023]
Abstract
We report on the first demonstration of an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode fiber-coupled laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the graded-index fiber core, results in high-efficiency conversion of a multimode (M2~26) pump at 915 nm into a high-quality (M2~2.6) output beam at 954 nm. About 50 W output power has been obtained with slope efficiency of 67%. The proposed development and integration of key multimode fiber technologies opens the door to new type of LD-pumped high-power high-beam-quality fiber lasers that may operate at almost any wavelength defined by available LDs.
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Guenard R, Krupa K, Dupiol R, Fabert M, Bendahmane A, Kermene V, Desfarges-Berthelemot A, Auguste JL, Tonello A, Barthélémy A, Millot G, Wabnitz S, Couderc V. Kerr self-cleaning of pulsed beam in an ytterbium doped multimode fiber. OPTICS EXPRESS 2017; 25:4783-4792. [PMID: 28380747 DOI: 10.1364/oe.25.004783] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
We experimentally demonstrate that Kerr spatial self-cleaning of a pulsed beam can be obtained in an amplifying multimode optical fiber. An input peak power of 500 W only was sufficient to produce a quasi-single-mode emission from the double-clad ytterbium doped multimode fiber (YMMF) with non-parabolic refractive index profile. We compare the self-cleaning behavior observed in the same fiber with loss and with gain. Laser gain introduces new opportunities to achieve spatial self-cleaning of light in multimode fibers at a relatively low power threshold.
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