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Observation of transverse-mode instabilities in a thulium-doped fiber amplifier

: Gaida, C.; Gebhardt, M.; Heuermann, T.; Wang, Z.Y.; Stutzki, F.; Jauregui, C.; Limpert, J.


Carter, A.L. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Fiber Lasers XVI: Technology and Systems : 4-7 February 2019, San Francisco, California, United States
Bellingham, WA: SPIE, 2019 (Proceedings of SPIE 10897)
ISBN: 978-1-5106-2437-5
ISBN: 978-1-5106-2436-8
Paper 1089702, 6 S.
Conference "Fiber Lasers - Technology and Systems <16, 2019, San Francisco/Calif.>
Industrial Laser, Laser Source and Laser Applications Conference (LASE) <2019, San Francisco/Calif.>
Photonics West Conference <2019, San Francisco/Calif.>
Bundesministerium für Bildung und Forschung BMBF (Deutschland)
Fraunhofer IOF ()

We analyze the average power scaling capabilities of ultrafast thulium-doped fiber lasers in the context of available pump brightness and transverse-mode instabilities. It has been theoretically predicted that thulium-doped fiber lasers emitting at around a wavelength of 2 μm can withstand much higher thermal loads before the onset of mode instability is observed. In this work we intend to verify this theoretical hypothesis experimentally. To do so, we use state of the art laser diodes to reach a combined pump power of more than 2 kW at 793 nm. Careful thermal management of the fiber amplifier and sufficient seed power are beneficial for exploiting cross-relaxation, which allows for high optical to optical efficiencies greater than 50%, which we demonstrate in a 50/250 thulium-doped photonic crystal fiber. An average power of 1060 W after pulse compression to 265 fs duration with stable diffraction-limited beam quality is demonstrated. We have achieved a pump-limited average output power of 1.15 kW prior to pulse compression without the onset of thermal mode instability. After a longer period of operation at this power level, a degradation of the amplifier performance occurred and thermal mode instability has been observed for average powers in excess of 847 W. This is the first time, to the best of our knowledge, that thermal mode instability has been observed in a thulium-doped fiber laser. According to simulations of the amplification process, the average heat load at the threshold of thermal mode instability is at least 95 W/m, which is about 3 times higher than the typical value for ytterbium-doped fiber laser.