Development of a Femtosecond High-Power Ytterbium-doped Fiber Laser System and Study on Spiky Spectral Modulation in Hig
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Development of a Femtosecond High-Power Ytterbium-doped Fiber Laser System and Study on Spiky Spectral Modulation in Highly Nonlinear Chirped-Pulse Fiber Amplifiers Suwon Kim∗ Quantum Optic Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea and Accelerator and Nuclear Fusion Physical Engineering, Korea University of Science and Technology, Daejeon 34113, Korea
Yong-Ho Cha and Do-Young Jeong Quantum Optic Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea (Received 12 February 2020; revised 27 April 2020; accepted 27 April 2020) We have developed and analyzed a femtosecond high-power fiber laser system based on chirpedpulse amplification. The system consists of a homemade femtosecond ytterbium-doped fiber oscillator, a fiber pulse stretcher, a pulse picker, three ytterbium-doped fiber amplifiers and a grating-based pulse compressor. A single-mode photonic crystal fiber with a large mode-field diameter of 30 μm is used in the final amplifier, and the special homemade mode-field adapter based on a graded refractive index fiber has been developed for the photonic crystal fiber. Pedestal-free pulses of 15 μJ are generated after pulse compression at a repetition rate of 500 kHz, and the pulse width is 300 fs. When small-core-size fibers are used at the final amplifier, the generation of spiky modulations in amplified spectra is observed. We analyze the physical origin of the spiky spectral modulation and investigate the effect of the spectral modulation on the pulse shape and contrast after pulse compression by numerical simulations. Keywords: Femtosecond fiber laser, Chirped-pulse amplification, Self-phase modulation DOI: 10.3938/jkps.77.1135
I. INTRODUCTION Chirped-pulse amplification (CPA) is a very useful technique for the generation of ultrashort energetic laser pulses. Ultrashort seed pulses are stretched in the time domain before amplification, and the amplified pulses are compressed close to the original pulse duration. Due to the relatively low peak intensity during the amplification process in CPA, optical damage and excessive nonlinear effects in amplifiers can be avoided [1]. Since the development of CPA, femtosecond terawatt laser systems have been demonstrated routinely, and the pulse generation of multi-petawatt with tens of femtosecond has recently been reported in a Ti:sapphire laser system based on CPA [2]. Fiber lasers have been developed and used widely in many scientific and industrial applications for last two decades because of their superior characteristics, such as good beam quality, stable operation, easy maintenance and high efficiency [3]. Furthermore, thanks to the excellent property of thermal dissipation of fiber lasers ∗ E-mail:
[4], multi-kilowatt continuous-wave fiber lasers have been demonstrated with a nearly single-mode beam quality [5]. Compared to bulk-type lasers, however, fiber lasers are disadvantageous in generating high-peak-power laser pulses because the small core size and long fiber length induce many undesired nonlinear effects and even optical damage of fibers. Large
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