Mitigation of transverse mode instability by heat-load modulation

Abstract

We present the first experimental realization of a new mitigation strategy for TMI based on controlling the phase shift between the modal intensity pattern and the thermally induced refractive index grating. If specific modulation parameters are applied while pulsing the seed and/or pump radiation, the direction of energy transfer is forced from the higher-order modes into the fundamental mode. In this way, the fiber amplifier can operate at an average output power significantly higher than the TMI threshold with a diffraction-limited beam profile. A stable beam profile is observed at an average output power that is 83% higher than the TMI threshold of the free-running system, with an intra-burst average power that is 4.15 times higher than the TMI threshold.

2 × 4.5 kW bidirectional output near-single-mode quasi-continuous wave monolithic fiber laser

“…As a result, there is less heat accumulation, reducing the likelihood of thermal refractive index grating-induced TMI. Additionally, recent studies have shown that small-duty cycle pump modulation on the order of kHz can inhibit TMI 31 , 32 . Therefore, controlling SRS intensity is particularly important for high-power QCW fiber lasers and TMI is not considered in our simulation.…”