Comparison and Optimization on Transverse Mode Instability of Fiber Laser Amplifier Pumped by Wavelength-Stabilized and Non-Wavelength-Stabilized 976 nm Laser Diode

We present an investigation of the effect of pumping wavelength on the threshold of transverse mode instability (TMI) in high-power ytterbium-doped fiber laser. By using a fiber laser amplifier with a bi-directional pumping structure, we obtain the TMI thresholds under wavelength-stabilized and non-wavelength-stabilized 976 nm laser diodes (LDs) pumping scheme. The results show that the non-wavelength-stabilized LD pump has a higher TMI threshold compared to the wavelength-stabilized LD pump. By optimizing the operating current of the non-wavelength-stabilized 976 nm LD, a ∼4kW output laser with near single-mode beam quality (M
²
∼1.3) is achieved, with a slope efficiency of ∼84.4%. The analysis yields that when the pumping wavelength deviates from the 976 nm peak, the TMI threshold can be increased. This reveals that optimizing pump source wavelength has great potential for suppressing TMI effect in the power scaling of fiber lasers.