Mitigating transverse mode instability in a single-end pumped all-fiber laser oscillator with a scaling power of up to 2kW

In the power scaling of monolithic fiber laser oscillators,. thermally induced transverse mode instability (TMI) is one of the main limiting factors. The onset of TMI deteriorates the output laser beam quality and restricts the maximum achievable laser power. Most studies on. TMI. focus. on. fiber amplifiers, while reports on. TMI in fiber laser oscillators are much fewer. Here, we report an experimental study on mitigating. TMI by detuning the pumping wavelength in a single-end pumped monolithic fiber laser oscillator. The performance. of the laser oscillator is. investigated with individual 976 nm pumping, with individual 915 nm pumping, and with hybrid pumping. Experimental results show that TMI is mitigated by replacing 976 nm pumping with 915 nm pumping. In the hybrid pumping scheme, the influence of ratios of 976 nm pumping power to 915 nm pumping power on the TMI threshold is studied. Finally, by optimizing pumping power ratio, the output laser power of the monolithic fiber oscillator is enhanced to 2 kW. The M-2 factor of the output laser is similar to 1.6 and the Raman Stokes light occupies less than 1% of the total power.