In this paper, we present systematically theoretical analysis of the high order mode (HOM) oscillation in large mode area (LMA) fiber oscillator. A detailed numerical laser oscillator model based on LMA fiber has been built. Model analysis shows that the HOM begins oscillating in fiber laser oscillator when the fiber core diameter reaches 25μm. This HOM oscillation could seriously degrade the beam quality of laser output. In order to suppress the HOM oscillation in LMA fiber laser, we put forward two fiber device models which are partially inscribed fiber Bragg gratings (FBGs) and partially doped gain fiber. Simulation proves the inhibition ability against HOM oscillation of these two devices. Our work provides a complete and concise analysis on the mode characteristics in high power fiber laser oscillator.
We report a lossless all-fiber 7x1 signal combiner, which can be used to combine more than 10 kW laser power. The measured power transmission efficiency is larger than 98.1% and power handle capability is more than 2 kilowatt (kW) for each port. When the combiner is put on a 20°C water cool plate, the average temperature rise is less than 3°C/kW. Due to the nearly lossless efficiency and good thermal performance, we can conclude that this combiner is capable of more than 10 kW power.
In this paper, we demonstrated an all fiber laser passively Q-switched by black phosphorus saturable absorber with cylindrical vector beam output. A piece of few-mode fiber Bragg grating was used as the mode-selective output coupler. The repetition rate of the pulse trains increased from 16kHz to 24.9kHz when the pump power tuned from 364mW to 460mW. The maximum pulse energy was 305.2nJ with the shortest pulse duration of 7.5μs under the pump power of 460mW.Both radial polarized and azimuthal polarized pulse output could be achieved by adjusting the polarization controllers. The purity of the cylindrical vector beam output was estimated to be over 95%.
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