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The sensitivity of the mode instability threshold to different types of intensity noise

 
: Stihler, C.; Jauregui, C.; Kholaif, S.E.; Limpert, J.

:

Dong, L. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Fiber Lasers XVII. Technology and Systems : 3-6 February 2020, San Francisco, California, United States
Bellingham, WA: SPIE, 2020 (Proceedings of SPIE 11260)
ISBN: 978-1-5106-3283-7
ISBN: 978-1-5106-3284-4
Paper 1126018, 7 S.
Conference "Fiber Lasers - Technology and Systems" <17, 2020, San Francisco/Calif.>
Englisch
Konferenzbeitrag
Fraunhofer IOF ()

Abstract
In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the threshold of transverse mode instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed-or the pump intensity-noise, has a severer impact on the TMI threshold. Our experiments have shown that the TMI threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI threshold of fiber laser systems.

: http://publica.fraunhofer.de/dokumente/N-593440.html