Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads

TL;DR

This paper experimentally investigates the thermal and mode stability of high-power thulium-doped fiber amplifiers, confirming their superior heat-load tolerance over Yb-doped fibers and reporting the first observation of transverse mode instability in such systems.

Contribution

It provides the first experimental verification of high heat-load tolerance in thulium-doped fibers and reports the first observation of TMI in these amplifiers at high powers.

Findings

Thulium-doped fibers withstand heat loads over 300 W/m without TMI.

Achieved 1.15 kW average power in a thulium amplifier before TMI onset.

TMI observed at powers above 847 W during extended operation.

Abstract

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 um, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.