Yb-Doped Single-Mode Fiber CPA Laser System

High peak power laser systems of up to petawatt [1] are based on chirped pulse amplification (CPA) technique [2]. In the CPA laser system, ultrashort pulses from an oscillator are first stretched below the nonlinear pulse distortion threshold to reduce the peak power and the amplified pulses are recompressed at the output by a compressor. Stable ultrashort pulse oscillator is necessary for CPA laser system. CPA laser systems based on fiber amplifiers and fiber-optic components are attractive for industrial applications. Fiber lasers are compact and easy to manage and offer better stability. Ultrashort pulse generation in the mode-locking technique needs compensation of group velocity dispersion (GVD) in the laser cavity as well as an effective saturable absorber to start and stabilize the pulse train. The GVD of silica glass fiber is positive at around 1.0 μm, while it is negative at around 1.5 μm. To compensate the GVD in the cavity, a grating pair or prism pair can be used. For pulse stretching in the CPA laser system, bulk grating pairs or a conventional fiber is usually used. Pulse stretching with bulk gratings needs large air space, while a conventional fiber normally needs 1,000 m fiber to stretch a pulse from 100 fs to 500 ps. A chirped Fiber Bragg Grating (FBG) [3] also used as pulse stretcher. 10 cm-long chirped FBG is enough to stretch 100 fs pulse to 500 ps. A chirped FBG is compact, low loss, low nonlinearity, and operates as a chirped mirror. The optical period of the grating changes along the fiber length. In the chirped FBG, the position of low and high frequency ingredient of a pulse varies with Bragg wavelength. Here we demonstrate a self-starting mode-locked YDF laser that delivers femtosecond pulse train using single-mode fibers. The oscillation wavelength of mode-locked laser was tuned by inserting a slit in the cavity. Femtosecond pulse from a mode-locked oscillator was stretched by a chirped FBG. And then the stretched pulse was amplified by two single-mode YDF pre-amplifiers for the fiber CPA laser system. The CPA laser system was constructed of only single-mode fibers. The major advantage of single-mode fibers is that a high-quality single-mode beam was produced without any special precaution for higher order mode suppression, which may not be stable for long-time.