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In the high-power laser system, intensity and phase fluctuations in laser beam induced by dust, defects on optical elements will grow rapidly during the propagation in laser mediums, and further lead to the small-scale self-focusing (SSSF). The traditional pinhole spatial filter is usually used to suppress SSSF effect by filtering most medium and high frequencies in the laser beams. In recent years, the angular filter based on volume Bragg gratings (VBGs) has been proposed to improve the uniformity of laser beam in near field. However, this angular filter cannot be used in plug-and-play scheme since the output beam is efficiently diffracted and the optical axis of output beam deflects compared to the incident beam through the VBGs, which will bring difficulty to the alignment of laser systems. Besides, all the medium and high frequencies over the cut-off frequency will be cleaned up and too much loss of medium and high frequencies in the filtering process may cause the decline in effective fill factor in the amplifier, leading to the reduction of energy extraction efficiency in laser amplification.
To solve the problems, a band-stop angular filter (BSF) based on VBGs and hump volume Bragg gratings was proposed. The band-stop filtering in a two-stage amplifier laser system was discussed and simulated. Simulation results show that the small-scale self-focusing effect in the laser system can be effectively suppressed with the BSF due to the control of fast nonlinear growth in the specific range of spatial frequencies in laser beams. The near-field modulation of output beam from the laser system was decreased from 2.69 to 1.37 by controlling the fast nonlinear growth of the spatial frequencies ranging from 0.6 mm-1 to 1.2 mm-1 with BSF. Besides, the BSF can be used in plug-and-play scheme and has potential applications in high power laser systems.
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