Transverse Mode Instabilities in kW-class Tm-doped Fiber Amplifier

Transverse Mode Instabilities (TMI) are a thermally-induced nonlinear effect that results in a fluctuating output beam profile unsuited for most applications. Since their first observation in Yb-doped amplifiers [1], TMI have manifested as major limitation in scaling the average output power of high-power fiber lasers. In 2021, TMI have been observed in Tm-doped fiber amplifiers as well [2,3]. Recently, Anderson et al. presented 1.1 kW at 1950 nm [3] and 905 W at 2040 nm [4] in Tm-doped fiber amplifiers in 20/400 µm geometry, both pumped with diode lasers of a wavelength around 794 nm. Their experiments with 25/400µm Tm-doped fibers were TMI-limited around 600-700 W [4] and dependencies on the active fiber length were investigated [4]. In our contribution we will present the dependence of the TMI-threshold on the pump wavelength, as has been done for Yb amplifiers [5]. Furthermore, we will show that Tm-doped fiber amplifiers in 25/400µm geometry can be scaled TMI-free beyond 700 W.