15 February 2012 Thermally induced mode instability in high power fiber amplifiers
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We present a physical model that may describe the observed phenomenon of modal instability in high power fiber amplifiers. In the power range of several hundred watts, large-mode-area, cladding-pumped, Yb3+-doped fiber amplifiers (both step-index and photonic crystal fibers), exhibit a sudden transition in the output beam profile from the fundamental mode to a higher order mode. We show how this behavior can be caused by a thermally induced mode coupling that leads to exponential gain of the higher order mode, and we implement a numerical model that quantitatively predicts the instability threshold for any large-mode-area step index fiber amplifier.
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Arlee V. Smith, Jesse J. Smith, "Thermally induced mode instability in high power fiber amplifiers", Proc. SPIE 8237, Fiber Lasers IX: Technology, Systems, and Applications, 82370B (15 February 2012); doi: 10.1117/12.906058; https://doi.org/10.1117/12.906058

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