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1 uJ sub 300fs pulse generation from a compact thulium doped chirped pulse amplifier seeded by Raman shifted erbium doped fiber laser_FANGZHOU2016

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300 fs pulses from a thulium chirp pulsed amplifier
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  1 μ J, sub-300 fs pulse generation from a compact thulium-doped chirped pulse amplifier seeded by Raman shifted erbium-doped fiber laser F ANGZHOU  T AN , H ONGXING  S HI , R UOYU  S UN , P ENG  W ANG ,  AND P U  W ANG *    Institute of Laser Engineering, Beijing University of Technology, Beijing Engineering Research Center of Laser Applied Technology, Beijing 100124, China * wangpuemail@bjut.edu.cn Abstract:  We present a compact thulium-doped chirped pulse amplifier producing 241 fs  pulses with 1 μ J energy. The system is seeded with the Raman shifted soliton generated by the combination of an erbium-doped femtosecond laser and a nonlinear fiber. The Tm-doped large mode area fiber yields output power of 71 W, corresponding to pulse energy of 2.04 μ J, with a slope efficiency of 52.2%. The amplified pulses have been compressed to a duration time of 241 fs, using a folded Treacy grating setup. The pulse energy is measured to be 1.02 μ J, corresponding to a peak power of ~3 MW. To the best of our knowledge, this is the highest average power and pulse energy generated from an all-fiber, Raman shifted soliton seeded thulium-doped chirped pulse amplifier system. © 2016 Optical Society of America OCIS codes:  (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (140.3280) Laser amplifiers; (140.3538) Lasers, pulsed; (320.5520) Pulse compression; (320.7090) Ultrafast lasers. References and Links 1. C. W. Rudy, M. J. F. Digonnet, and R. L. Byer, “Advances in 2- μ m Tm-doped mode-locked fiber lasers,” Opt. Fiber Technol. 20 (6), 642–649 (2014). 2. P. Kadwani, J. Chia, F. Altal, R. A. Sims, C. Willis, L. Shah, D. Killinger, and M. C. Richardson, “Atmospheric absorption spectroscopy using Tm: fiber sources around two microns,” Proc. SPIE 7924 , 79240L (2011). 3. K. D. Polder and S. Bruce, “Treatment of melasma using a novel 1,927-nm fractional thulium fiber laser: a pilot study,” Dermatol. Surg. 38 (2), 199–206 (2012). 4. I. Mingareev, F. Weirauch, A. Olowinsky, L. Shah, P. Kadwani, and M. Richardson, “Welding of polymers using a 2 μ m thulium fiber laser,” Opt. Laser Technol. 44 (7), 2095–2099 (2012). 5. F. Adler and S. A. Diddams, “High-power, hybrid Er:fiber/Tm:fiber frequency comb source in the 2 μ m wavelength region,” Opt. Lett. 37 (9), 1400–1402 (2012). 6. C. W. Rudy, A. Marandi, K. L. Vodopyanov, and R. L. Byer, “Octave-spanning supercontinuum generation in in situ tapered As 2 S 3  fiber pumped by a thulium-doped fiber laser,” Opt. Lett. 38 (15), 2865–2868 (2013). 7. K. Liu, J. Liu, H. Shi, F. Tan, and P. Wang, “High power mid-infrared supercontinuum generation in a single-mode ZBLAN fiber with up to 21.8 W average output power,” Opt. Express 22 (20), 24384–24391 (2014). 8. N. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, “Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser,” Opt. Express 20 (7), 7046–7053 (2012). 9. M. Gebhardt, C. Gaida, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann, “Sub-200 fs, nJ-level stretched- pulse thulium-doped fiber oscillator at 23MHz repetition rate,” in  Advanced Solid State Lasers , OSA Technical Digest (online) (Optical Society of America, 2014), paper AM5A.43. 10. M. Gebhardt, C. Gaida, F. Stutzki, S. Hädrich, C. Jauregui, J. Limpert, and A. Tünnermann, “Impact of atmospheric molecular absorption on the temporal and spatial evolution of ultra-short optical pulses,” Opt. Express 23 (11), 13776–13787 (2015). 11. R. Gumenyuk, I. Vartiainen, H. Tuovinen, and O. G. Okhotnikov, “Dissipative dispersion-managed soliton 2 μ m thulium/holmium fiber laser,” Opt. Lett. 36 (5), 609–611 (2011). 12. A. Wienke, F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Ultrafast, stretched-pulse thulium-doped fiber laser with a fiber-based dispersion management,” Opt. Lett. 37 (13), 2466–2468 (2012). 13. F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Monotonically chirped pulse evolution in an ultrashort pulse thulium-doped fiber laser,” Opt. Lett. 37 (6), 1014–1016 (2012). 14. J. P. Gordon, “Theory of the soliton self-frequency shift,” Opt. Lett. 11 (10), 662–664 (1986). Vol. 24, No. 20 | 3 Oct 2016 | OPTICS EXPRESS 22461 #272983 http://dx.doi.org/10.1364/OE.24.022461 Journal © 2016Received 3 Aug 2016; revised 12 Sep 2016; accepted 12 Sep 2016; published 19 Sep 2016  15. G. Imeshev and M. Fermann, “230-kW peak power femtosecond pulses from a high power tunable source based on amplification in Tm-doped fiber,” Opt. Express 13 (19), 7424–7431 (2005). 16. R. Herda and A. Zach, “All-fiber generation of few-cycle pulses at 1950 nm by triple-stage compression of a Thulium-doped laser system,” in  Proceedings of IEEE Conference on Photonics Conference  (IEEE, 2013), pp. 621–622. 17. C. Gaida, M. Gebhardt, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann, “Thulium-doped fiber chirped- pulse amplification system with 2 GW of peak power,” Opt. Lett. 41 (17), 4130–4133 (2016). 18. F. Tauser, F. Adler, and A. Leitenstorfer, “Widely tunable sub-30-fs pulses from a compact erbium-doped fiber source,” Opt. Lett. 29 (5), 516–518 (2004). 19. A. Sell, G. Krauss, R. Scheu, R. Huber, and A. Leitenstorfer, “8-fs pulses from a compact Er:fiber system: quantitative modeling and experimental implementation,” Opt. Express 17 (2), 1070–1077 (2009). 20. S. Kumkar, G. Krauss, M. Wunram, D. Fehrenbacher, U. Demirbas, D. Brida, and A. Leitenstorfer, “Femtosecond coherent seeding of a broadband Tm:fiber amplifier by an Er:fiber system,” Opt. Lett. 37 (4), 554–556 (2012). 21. D. Brida, G. Krauss, A. Sell, and A. Leitenstorfer, “Ultrabroadband Er:fiber lasers,” Laser Photonics Rev. 8 (3), 409–428 (2014). 22. R. A. Sims, P. Kadwani, A. S. Shah, M. Richardson, and M. Richardson, “1 μ J, sub-500 fs chirped pulse amplification in a Tm-doped fiber system,” Opt. Lett. 38 (2), 121–123 (2013). 23. M. E. Fermann, V. I. Kruglov, B. C. Thomsen, J. M. Dudley, and J. D. Harvey, “Self-similar propagation and amplification of parabolic pulses in optical fibers,” Phys. Rev. Lett. 84 (26), 6010–6013 (2000). 24. Y. Kim, Y. J. Kim, S. Kim, and S. W. Kim, “Er-doped fiber comb with enhanced f  ceo  S/N ratio using Tm:Ho-doped fiber,” Opt. Express 17 (21), 18606–18611 (2009). 1. Introduction Pulsed 2 μ m fiber laser sources have attracted intense interests in recent years [1] for its wide applications in remote sensing [2], medicine [3], plastic engineering [4], mid-IR frequency comb [5], nonlinear mid-IR supercontinuum generation [6,7], optical parametric generation [8] and so on. To produce picosecond or femtosecond pulses at 2 μ m wavelength regime, mode-locking and nonlinear frequency conversion are two major approaches. Thulium fiber  based mode-locked oscillators are usually operating at the wavelengths below 2 μ m, which may be affected by water absorption around 1.9 μ m [9,10]. Because the material dispersion values of silica glass around 2 μ m is positive and relatively high, to generate pulses with  pulse durations from picoseconds to hundreds of femtoseconds and repetition rates of MHz, normally dispersion-managed mode-locked thulium-doped fiber lasers are employed [11– 13]. Using erbium-doped femtosecond fiber lasers to generate Raman shifted solitons have  been proved to be a powerful and effective way to obtain ultrafast pulses at 2 μ m regime, in comparison with the alternative 2 μ m mode-locked oscillators. This is because the well-developed 1.5 μ m fiber-based components are mature, stable and costless. The center wavelength of the generated Raman solitons beyond 1.5 μ m can be shifted by controlling the  pumping power level [5,14–16,18–21]. This can be achieved through either the soliton self-frequency shift (SSFS) [14,15] or spectral broadening in highly nonlinear fibers [5,16,18– 21]. So far, the shortest pulse width of 27 fs has been achieved using this approach, with three additional compression stages at 2 μ m [16]. However, there are still some drawbacks for this kind of sources. For example, extra noises can be introduced from the nonlinear frequency generation processes [5], and the configuration of the laser source can be complex compared with thulium doped mode-locking oscillators. Moreover, the Raman-shifted source usually delivers pulse energy of pJ- to nJ-level, which is an obvious barrier to extend this sort of sources for some applications requiring higher pulse energy. For further power scaling, nonlinear amplification and chirped pulse amplification (CPA) are the two common ways to generate 2 μ m high-energy pulses with sub-ps pulse duration. To date, the highest reported pulse energy is 31 nJ using the nonlinear amplification approach, seeded by the Raman shifted soliton from an ultrafast erbium-doped fiber laser [15]. In that work, the shifted Raman soliton pulses were positively pre-chirped by a segment of normal dispersion fiber (NDF), before being amplified in a thulium-doped large mode area (LMA) fiber. Pulses with a width of 108 fs were obtained during the nonlinear amplification Vol. 24, No. 20 | 3 Oct 2016 | OPTICS EXPRESS 22462  with center wavelength of 1980 nm. However, there is a trade-off between the fiber nonlinearity and the dispersion during the nonlinear amplification. This limits the achievable  pulse energy since excessively high nonlinearity will cause pulse breakup. On the other hand, pulse energy of 470 μ J with peak power up to 2 GW has been realized in thulium-doped chirped pulse amplification system seeded by 2 μ m thulium-doped fiber oscillator. The generation of the pulses with such high energy is benefited from the usage of a large-pitch thulium-doped fiber, as well as complex free-space stretchers and high efficiency grating compressors [17]. Insteadly, R. A. Sims et al reported chirped pulse amplification seeded by Raman soliton  pulses from a 1.9 μ m mode-locked thulium doped fiber laser [22]. The shifted Raman solitons were positively stretched by a chirped Bragg grating (CBG). The pulse repetition rate is then reduced down to 100 kHz by an electro-optic modulator. After the amplification of a thulium-doped LMA fiber and the compression of grating pairs, pulses with energy of 1 μ J and pulse width below 500 fs were finally obtained. The CBG in the experiment was also used as spectral filter to remove the unwanted spectral portion lying in the thulium gain  bandwidth. However, the system is not an all-fiber device, due to the insertion of the free-space CBG stretcher. This makes the system complex and inconvenient. In this work, we demonstrate a compact all-fiber thulium-doped chirped pulse amplification system seeded by a Raman-shifted erbium-doped fiber laser. The average output power of the amplified pulses was measured to be 71 W with a slope efficiency of 52.2%. The repetition rate has been measured to be 34.8 MHz. The pulses are then compressed to a duration time of 241 fs with 35.4 W average output power corresponding to a peak power of ~3 MW. 2. Experimental setup Fig. 1. Experimental setup of thulium-doped CPA system. ISO: isolator; TDF: thulium-doped fiber; LMA-TDF: large mode area thulium-doped fiber. The schematic setup of Tm-doped fiber CPA system is shown in Fig. 1. It consists of an all fiber Raman soliton generator, a fiber stretcher, two stages of fiber amplifiers and a free-space grating compressor. The Raman soliton generator is based on an amplified mode-locked erbium-doped fiber (EDF) laser and a short piece of highly nonlinear fiber (HNLF). The mode-locked EDF laser consists of a wavelength division multiplexer (WDM), a 20% output coupler, a fiber optical circulator coupled with a commercial available SESAM (semiconductor saturable absorber mirror) mode locker and a piece of EDF with the dispersion of − 12 ps/nm/km at 1550 nm. The total cavity length is about 5.75 m, which corresponds to 34.8 MHz fundamental repetition rate. A fiber polarization controller (PC) is  placed before the amplification stage to optimize the compression pulse quality. The amplifier consists of ~4 m EDF with the same parameters as the oscillator gain fiber. After amplification, the pulses are compressed with 110 cm single mode fiber (SMF-28). A short segment of HNLF was spliced directly to the SMF-28 fiber to generate the supercontinuum. Vol. 24, No. 20 | 3 Oct 2016 | OPTICS EXPRESS 22463  The HNLF has a mode field diameter of 2.23 μ m, a dispersion of 2.187 ps/nm/km at 1550 nm, and a nonlinearity γ  of ~9 W − 1 km − 1  respectively. An additional 16 cm-long Tm:Ho-doped fiber (TH512, Coractive Inc.) is used to further enhance the signal intensity at 2010 nm. Then the enhanced Raman soliton pulses are temporally stretched using ~110 m ultrahigh numerical aperture fiber (UHNA-4, Nufern Inc.) with a dispersion of ~-48.5  ps/nm/km at 1.9 μ m and amplified in two-stage thulium-doped fiber amplifier. The pre-amplifier consists of a 3.5 m-long double clad single mode thulium-doped fiber (TDF). The TDF is with a core diameter of 10 μ m, an outer diameter of 130 μ m (Nufern Inc.), and an NA of 0.46 for the second cladding with cladding absorption of ~3 dB/m at 793 nm. A commercial pump combiner is used to deliver pump light to the gain fiber from the multimode pump diode, which has a center wavelength of 793 nm and output power of 12 W with 0.22 NA, 105/125 μ m fiber pigtail. In the main amplifier stage, 1.9 m LMA thulium doped fiber (Nufern Inc.) is used to boost the signal power to tens of watts. The active fiber has a core diameter of 25 μ m, an NA of 0.09, an inner cladding diameter of 250 μ m and a cladding NA of 0.46 with cladding absorption of ~9.5 dB/ m at 793 nm. The active fiber is water-cooled down to 12 °C to dissipate the heat accumulation as well as promoting efficient two-for-one cross-relaxation during high power operation. The pump source consists of 5 diodes operating at 793 nm with 105 μ m (NA = 0.22) pigtail fibers. The total output power of these pump diodes is ~148 W. A (6 + 1) × 1 high power pump combiner (ITF Technologies Inc.) is used to deliver pump light to the gain fiber with a coupling efficiency of ~95%. The output end of the LMA thulium-doped fiber was angle cleaved to frustrate parasitic lasing. After amplification a dichroic mirror (DM) is used to filter the unabsorbed pump light and the pulses are compressed with a folded Treacy grating compressor consisting of two 560 grooves per mm, fused silica transmission gratings. A fused silica wedge is placed after the compression output to measure the optical and radio-frequency (RF) spectra as well as pulse characteristics. 3. Experimental results The oscillator is a passively SESAM mode-locked erbium-doped fiber laser operating at stretched pulse mode-locking regime. The net dispersion of the cavity is estimated to be − 0.048 ps 2 . The oscillator delivers an average output power of 1.5 mW with a pulse width of 1.1 ps and a repetition rate of 34.8 MHz. The output spectrum is shown in Fig. 2 (as the black line) with a center wavelength of 1559 nm and a spectrum bandwidth of 12 nm, indicating that the pulses are negatively chirped. The output pulses are amplified to about 68 mW by the erbium-doped fiber amplifier (EDFA). The combination of the distributed gain, the normal dispersion of the EDFA as well as self-phase modulation (SPM) effect leads to self-similar amplification with appropriate input pulse energy, in which the pulses have been evolved to  parabolic shape with linear chirp [23]. After amplification, the width of the spectrum expands to over 80 nm with nearly parabolic shape (see the red line in Fig. 2) and the amplified pulse width is measured to be 1.15 ps. In the experiment, the SMF-28 with anomalous dispersions is used to compress the chirped pulses. By carefully tailoring the SMF-28 fiber length with cutback method, the pulse compression is maximized when the SMF-28 length is 110 cm. The pulse duration after the compression fiber is measured to be ~70 fs and the bandwidth of the compressed pulse spectrum (as the blue line shown in Fig. 2) is ~84 nm. Vol. 24, No. 20 | 3 Oct 2016 | OPTICS EXPRESS 22464
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