20 ns smooth pulse generation in single-mode actively Q-switched double-clad Yb-doped fiber laser

Master Oscillator - Power Fiber Amplifier (MOPFA) configuration is a standard configuration for pulsed fiber laser systems used in material processing applications. To improve the marking process efficiency, it is benefit to increase the output peak power of these lasers, which can be obtained by reducing the oscillator pulse width. Q-switched fiber lasers are a priori good candidates for this purpose. However, in these systems, the longitudinal distribution of the gain medium and the length of fiber components determine a cavity length longer than in bulk lasers. One consequence is strong intensity variations along the fiber which can leads to multipeaks generation [1]. On the other hand, to decrease pulse duration, the pump power must be increased and this leads to detrimental nonlinear effects. To avoid these problems, the power density is usually reduced by using large mode area fibers but with a detrimental effect on the spatial beam quality [2]. In this paper, we theoretically investigate the design of an all-fiber, acousto-optically-modulated single-mode ytterbium-doped double-clad fiber laser, in order to achieve the minimum pulse duration with energies on the order of 100 µJ. These results are then used for experimental realizations. We demonstrate, for the first time to the best knowledge of the authors, in Q-switch single mode architecture, 20 ns pulse width generation with smooth pulse shape, at 20 kHz repetition rate.