Investigation of parasitic stimulated emission in a nanosecond diode-seeded high gain fiber pre-amplifier Chun-Lin Chang a* , Yen-Yin Li a , Yi-Ping Lai a , and Sheng-Lung Huang a,b a Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan b Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan Presenting author e-mail address: *louischang@cc.ee.ntu.edu.tw Abstract: Parasitic stimulated emission produces a front spike in a pulsed fiber MOPA system at a gain threshold of ~11.5 dB. Mitigation of the parasitic stimulated emission can enable high gain for diode-seeded fiber pre-amplifier. 1. Introduction Over the last decade, the output powers produced by the near-diffraction-limited fiber lasers have shown an average increment of 2 dB per year which is much steeper than that of bulk solid-state lasers [1]. Moreover, fiber lasers also possess many characteristics, such as robust single transverse mode operation, ultrahigh energy efficiency and to fully fiber-based cavity as a compact system. These features differentiate them in terms of versatility, performance and practicality for the commercial interest and scientific applicability. One of the most exciting possibilities of nanosecond pulsed fiber laser system is to allow the adaptive pulse shaping [2,3] which can be achieved by rapid modulation techniques such as fast electro-optic modulators or electro- absorption modulators [4], or just simply directly-modulated seed diode [2,3]. This enables a fully-customized pulse shapes for a specific end application such as material processing [5], frequency conversion [6], and laser plasma interaction [7]. Nevertheless, the peak power of state-of-the-art seed diode is still watt-level, resulting in only a few nanojoules to tens of nanojoules in nanosecond-scale duration [8]. For a several millijoule-level high power fiber laser system using the directly-modulated seed diode as master oscillator [9], the utilization of fiber master oscillator power amplifier (FMOPA) scheme is therefore very important for the availability of high gain. To reach the saturated input into the power amplifier stage for efficient energy extraction, a well-designed high gain fiber pre-amplifier stage is necessary. How to obtain a distortion-free signal with high enough gain before entering the power amplifier series [10] is a key point to achieve a high efficient nanosecond multi-millijoule pulse energy with arbitrary shapes. In this paper, we present the investigation of parasitic stimulated emission generating a spike at the front foot in diode-seeded high gain ytterbium-doped fiber pre-amplifier in pulsed operation. Firstly, the direct evidence indicates it is caused by a few additional transient spectral components of seed diode to grow up during the propagation of the fiber amplifier, instead of from continuous amplified spontaneous emission generated by optically-pumped amplifier. Secondly, the dynamic gain dependence of the spike is confirmed at various pump powers and repetition rates. Finally, the influence of passive in-line bandpass filter between seed and amplifiers is discussed. 2. Experimental results A two-stage core-pumped amplifier system is used here for study. The 2-meter-long single-mode ytterbium- doped gain fiber (Nufern PM-YSF-HI) is pumped by 976-nm single-mode pump diode for both stages for hundreds milliwatt level. Two in-line isolators of 55 dB and 35 dB (AFR PMI and HPMI series) are placed at the seed (Lumics LU1064 M010) output and between amplifiers, respectively. The in-line 1064-nm bandpass filter used here has bandwidth of 10nm. The angle-cleaved end facet is used before exiting the fiber end to block back reflections. Fig. 1. (a) The normalized output spectrums of 1064-nm, 20-ns and 20-nJ FBG-stabilized pulsed diode seed laser after passing isolator, seed in- line bandpass filter or both at 20 kHz. The inset is the corresponding seed pulse shapes. (b) The temporal pulse shapes and (c) the corresponding normalized spectrums at the output of the first fiber pulsed amplifier with 20-dB gain pumped by 292 mW backwardly. ACOFT Presentation ● IQEC/CLEO Pacific Rim 2011 ● 28 August - 1 September 2011 ● Sydney, Australia 978-0-9775657-7-1  2011 AOS 1151