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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