23 May 2013 High power modal instability measurements of very large mode area (VLMA) step index fibers
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High power (<0.5kW) experiments using low NA (~0.07), very large mode area (VLMA) step index fibers (SI) (with core/clad diameters: 45/375, 60/500um) and gain tailored step index (GT-SI) fibers (with doped-core/core/clad diameters: 38/60/400, 50/80/533um) are presented. In fiber amplifier experiments with multi-moded beam (M2 1.5- 3) outputs, Stimulated Thermal Rayleigh scattering (STRS) threshold is determined by comparing gain dependence of output mode quality between high power (<200W) and low power (<100W) experiments for a given fiber layout. Beam quality degradation with signal power is characterized well above the instability threshold where a saturation of the phenomena is observed. For SI fibers degree of beam quality degradation is found to be significantly worse for tighter fiber coil diameters. GT-SI fibers exhibit significantly less modal degradation compared to SI fibers. STRS instability threshold is further verified with signal power dependent multi-path interference spectrum (MPI) measurements which exhibited exponential broadening above the threshold. Strength of STRS nonlinear coupling coefficients are estimated from experimental data using a comprehensive 3-dimensional transverse spatial hole burning (TSHB) fiber MOPA numerical model, phenomenologicaly extended to include STRS.
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Doruk Engin, Doruk Engin, Wei Lu, Wei Lu, Horacio Verdun, Horacio Verdun, Shantanu Gupta, Shantanu Gupta, "High power modal instability measurements of very large mode area (VLMA) step index fibers", Proc. SPIE 8733, Laser Technology for Defense and Security IX, 87330J (23 May 2013); doi: 10.1117/12.2016106; https://doi.org/10.1117/12.2016106

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