Development of a Femtosecond High-Power Ytterbium-doped Fiber Laser System and Study on Spiky Spectral Modulation in Highly Nonlinear Chirped-Pulse Fiber Amplifiers

Abstract

We have developed and analyzed a femtosecond high-power fiber laser system based on chirped-pulse 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.