Analysis of self-pulse effect in high power fiber lasers based on seed source broadening with high-order phase modulation

The high-power fiber laser schemes based on phase modulation of random signals suffer from the interference of the self-pulse effect. We propose a spectral broadening scheme based on a single stage high-order phase modulation and flexible driving signals design. A P-tuned sequence with a bandwidth of 1.25 GHz is used as the driving signal of the phase modulator, and the optical bandwidth of the seed source can be broadened to 40 GHz after the modulation, while optimizing the shape of the spectrum. It is verified by experiments that the self-pulse effect is well suppressed under our scheme. Based on a master oscillator power amplifier fiber laser system, the output power under the same bandwidth is increased by 70% compared to using white noises as a driving signal. In addition, we analyze the influence of the signal randomness on the timing of the self-pulse effect, which is beneficial to design driving signals.