Thulium-doped high-power Fiber Amplifier for <500 ps directly gain-switched Seed Diodes

In this publication, we present a linearly polarized MOFPA (Master Oscillator Fiber Power Amplifier) setup for generating and amplifying short pulses in the ps- to ns-regime at wavelengths around 2 µm to >20 W of average power. The Master Oscillator consists of a directly gain-switched seed diode laser at a center wavelength of approx. 1950 nm which is capable to generate pulses with durations down to 50-70 ps by separating the gain spike and suppressing the trailing edge by using specially designed driver electronics. The subsequent fiber amplifier is formed by three cascaded amplifier stages based on polarization-maintaining Thulium-doped active fibers pumped by multimode pump laser diodes around 800 nm. The realized setup achieves a total gain of >60 dB, which leads to a spectrally filtered output power of >20 W with slightly broadened pulse durations <350 ps due to nonlinear effects, for example when an input pulse duration of 222 ps is used. Since our fully monolithic fiber amplifier system consists of single-mode and LMA fibers, the output beam quality is nearly diffraction limited. Due to the separately variable pulse durations and pulse repetition rates, our here presented approach offers technical and economic advantages over commercially available laser systems with comparable pulse durations, which are largely based on mode-locked oscillators.