Quasi-continuous wave (CW) laser output with high peak power and high energy is preferred in some industrial applications. Due to the non-linear effects and transverse mode instability, such high-peak-power laser output is difficult to achieve via monolithic fiber lasers in CW mode. For diode-pumped monolithic fiber lasers, by applying overshoot pulse modulation to the pumping diodes, we obtain a pulse laser output with a peak power much higher than that in CW mode. In this paper, it has been theoretically studied that stable pulses with the width of µs level can be generated with quasi-CW operation without distortion according to our simulation. We also experimentally demonstrate a bi-directional pumped quasi-CW monolithic fiber laser operating in CW and pulse modes. In quasi-CW mode, by applying overshoot pulse modulation to the diodes, with a frequency of 1 kHz and a pulse width of 100 µs, the peak power of the output laser reached 9713 W with an average power of 898 W and M 2 of 2.4 and 2.3 in the two orthogonal directions, respectively. To the best of our knowledge, this is the very first quasi-CW fiber laser of a 10-kW level with the M 2 level of 2.

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Random Raman fiber laser (RRFL) has been widely studied in high-power laser generation due to its special lasing characteristics. However, all previous high-power results are based on the half-open cavity. In this letter, we demonstrate an applicable high-power RRFL with the simplest structure, that is, a full-open cavity. The lasing dynamic and output characteristics are theoretically and experimentally studied. Laser source with multi-longitudinal modes can result in the random laser output from one side even in the full-open cavity. The ratio of the backward output power is mainly determined by the reflectivity of fiber ends. The experimental results show that such a simple structure can easily generate kilowatts of random laser power and is a promising setup to achieve higher output power, which is also an important platform to study the laser dynamic in high-power full-open cavity without any point-action or regular distributed reflectors.

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