Reliable indirectly cooled high-power thulium fiber laser

High power thulium lasers in the kW regime face significant thermal challenges and are usually tested in experimental laboratory conditions for a limited duration only. Yet, active and passive fibers degrade owing to prolonged exposure to high temperatures and, more importantly, high humidity, especially when direct water contact is used, which is commonly found in those systems as cooling mechanism. For reliable long-term operation, we investigated an indirectly cooled high power narrow-bandwidth thulium fiber laser system. Consisting of two main amplifier stages, the gain and thermal load is split into smaller portions. One main amplifier can provide up to 500 W of output power at 2034 nm, while two consecutive main amplifiers boost it to >700 W. The beam profile is excellent, and power is limited by available pump and instabilities that are dependent on the pump power ratio of the two main amplifiers.