It is well documented that the primary limits to power scaling in fiber amplifiers are Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS), Thermal Lensing (TL), Transverse Modal Instability (TMI), and Diode Pump Brightness (DPB). These effects are well known in glass host fiber amplifiers and still garner active research in mitigating techniques for higher power scaling. In this paper, we present power thresholds for these limitations in crystalline host fiber amplifiers. We have leveraged a Coupled Mode Theory (CMT) model to simulate and analyze crystalline YAG fiber lasers for multiple dopants and with variation in step-index fiber core diameters and lengths. The dopants of interest are Ytterbium (Yb), Holmium (Ho), Thulium (Tm), and Erbium (Er). We have generated Power Scalability Maps (PSM) with varying fiber lengths and diameters which depicts the influence of the aforementioned limitations. We have leveraged a higher fidelity CMT-based model to develop comprehensive PSMs for crystalline fibers and for Yb, Ho, Tm, and Er dopants. To produce the PSMs for Tm and Er, additional considerations are required. Both Tm and Er have nonlinear energy transfer processes that make predicting the population concentration, given pump and signal intensities, challenging. We applied a specialized fit function based on a sigmoidal structure to allow analytic interpolation within the CMT-TMI model to accommodate for the complicated energy transfer effects that occur in Tm and Er. The PSMs serve as references for determining limits and potential of crystalline fibers.
Individual fiber amplifiers with increased average power while maintaining a narrow spectrum and excellent beam quality, enable power scaling of beam combined systems. To accomplish this, fiber amplifiers must contend with two well-known deleterious processes: thermal modal instability (TMI) and stimulated Brillouin scattering (SBS). Changing the fiber host material from silica glass to crystalline Yttrium Aluminum Garnet (YAG) has been reported as a potential means to increase the SBS and TMI power thresholds due to favorable material properties, mainly increased thermal conductivity and decreased electrostrictive constants. In this report, the development of numerical models to examine nonlinear effects in crystalline YAG fiber is described. Fiber simulation code previously developed at the Air Force Research Laboratory (AFRL) for silica are leveraged for crystalline gain media. Results show TMI threshold for a Ytterbium doped YAG (Yb:YAG) fiber 28 times higher than the equivalent silica fiber, and an increase in SBS threshold by over 250 times in YAG compared to silica. The investigations also include thresholds for Holmium doped YAG (Ho:YAG) and Thulium doped (Tm:YAG), which compare well with published experimental data.
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