IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 22, NOVEMBER 15, 2014 2295 High Energy and High Peak Power Nanosecond Pulses Generated by Fiber Ampliﬁer Haitao Zhang, Xinglai Shen, Dan Chen, Chao Zheng, Ping Yan, and Mali Gong Abstract— Generation of nanosecond pulses with 55-mJ energy and 4.8-MW peak power are demonstrated in a multistage ﬁber ampliﬁer system, which is seeded with 10-nm broadband pulses from a superluminescence diode. The pulses are at low repetition of 10 Hz and with width of 10 ns. High energy and high peak power for nanosecond pulses are obtained simultaneously from ﬁber ampliﬁer. In particular, the peak power is at least two times the previous peak power for nanosecond pulses. Index Terms— Yb-doped ﬁber ampliﬁer, high energy, high peak power, broadband seed. I. I NTRODUCTION H IGH energy and high peak power nanosecond pulses have numerous practically important applications in many ﬁelds, such as materials processing [1], remote sensing, and inertial conﬁned fusion (ICF) [2]. Fiber lasers and ampli- ﬁers are emerging as an important and practical technology to produce these pulses due to its compactness, robustness and high efﬁciency. However, there are many difﬁculties in scaling up pulse energies and peak powers generated by ﬁber lasers and ampliﬁers which mainly originate in the limited size of the ﬁber core [3]. Therefore utilizing very large mode ﬁeld area (VLMA) ﬁber with mode ﬁeld diameters beyond 50 μm is one of the promising solutions toward higher energies and peak powers for its ability of storing a large amount of energy and mitigating intensity related nonlinearity such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) [4]. Q-switch ﬁber laser and master oscillator power ampliﬁer (MOPA) are two main schemes using VLMA ﬁbers to deliver nanosecond high-energy and high-peak- power pulses. By using Q-switched ﬁber laser format, pulses with 22 mJ energy, 20 ns width and near diffraction limited beam quality have been reported [5] with rod photonic crystal ﬁber whose core diameter is 135 μm. Using MOPA format, S. Maryashin and his coworkers reported 300 ns 10 mJ energy Yb-doped laser from all-ﬁber format at 1–50 kHz variable repetition frequencies with 65 μm core diameter multimode ﬁber [6]. Using ﬁber with 200 μm core diameter in the ﬁnal Manuscript received April 21, 2014; revised July 28, 2014; accepted August 20, 2014. Date of publication August 28, 2014; date of current version October 24, 2014. This work was supported in part by the National High Technology Research and Development Program of China, in part by the National Natural Science Foundation of China under Grant 61475081, and in part by the State Key Laboratory of Tribology, Tsinghua University, Beijing, China, under Grant SKLT11B05. The authors are with the State Key Laboratory of Tribology, Center for Photonics and Electronics, Department of Precision Instruments, Tsinghua University, Beijing 100084, China (e-mail: zhanghaitao@mail. tsinghua.edu.cn; gongml@mail.tsinghua.edu.cn). Color versions of one or more of the ﬁgures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/LPT.2014.2352864 ampliﬁer stage, pulses with >82 mJ of pulse energy, 25 Hz repetition rate, and 500 ns pulse width are reported by using a cascade of four-stage Ytterbium-doped ﬁber ampliﬁers [7]. The maximum peak power for the nanosecond pulses in the above work is 2.4 MW with 4-ns pulses which is mainly restricted by the SBS threshold which is proportional to the bandwidth of pulses. To scaling up peak power further, we present a ﬁber super- radiation pulsed ampliﬁcation (SPA) technology which utilizes wideband super-radiation pulsed light with non longitudinal mode and chaotic mode phase, such as the superluminescent diode light, super ﬂuorescence and spontaneous emission light, as the seed source of the cascaded ﬁber ampliﬁers. Obvi- ously, wide bandwidth helps to increase the SBS threshold in the VLMA ﬁber further. Moreover, the SPA features of non longitudinal mode and chaotic phase help to reduce modal interference, and make the intensity in the ﬁber more uniform. The uniformity of the intensity distribution results in the suppressed small-size nonlinearity in the ﬁber, which makes pulses with higher peak power delivered by ﬁber more achievable. In addition, super-radiation light has a lot of unique application, such as being beneﬁcial for beam smoothing in ﬁber ampliﬁer network [8], [9] and speckleless laser projection display. Using SPA technology, our group have reported nanosecond pulses with peak power of 12 kW delivered by 15 μm core diameter single-mode ﬁber [10]; nanosecond pulses with near diffraction limit beam quality, peak power of 600 kW and average power of 60 W delivered by 30 μm core diameter ﬁber [11]; nanosecond pulses with energy up to 30 mJ and peak power up to 3 MW delivered by all-ﬁber cascade ampliﬁer [12]. In this letter, we report, for the ﬁrst time to our knowledge, nanosecond (∼10 ns) pulses with high energy (55 mJ) and high peak power (4.8 MW) simultaneously are generated in a multistage ﬁber ampliﬁer system seeded by a superluminescent light source which provides as broad as 10 nm bandwidth. Multistage scheme is used to provide enough injection pulsed energy for saturation ampliﬁcation of the ﬁnal ﬁber ampliﬁer stage. The ﬁber with 200 μm core is used as the ﬁnal stage gain ﬁber which can provide large storage energy to extract and increase the threshold for intensity based nonlinearity such as SBS and SRS. Consequently we obtained high energy and high peak power simultaneously in nanosecond pulses from ﬁber ampliﬁer. The peak power is at least two times the previous peak power in nanosecond pulses. II. EXPERIMENTAL SETUP In [12], our group reported a cascade six-stage ﬁber ampli- ﬁer system where 200 μm core-diameter ﬁber is used in the last ampliﬁer stage. Pulses with energy up to 30 mJ are obtained from the ﬁber ampliﬁer system with 10 ns pulse 1041-1135 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.