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22 February 2007Design of refractive-index and rare-earth-dopant distributions for large-mode-area fibers used in coiled high-power amplifiers
We have numerically compared the performance of various designs for the core refractive-index (RI) and
rare-earth-dopant distributions of large-mode-area fibers for use in bend-loss-filtered, high-power
amplifiers. We first established quantitative targets for the key parameters that determine fiber-amplifier
performance, including effective LP01 modal area (Aeff, both straight and coiled), bend sensitivity (for
handling and packaging), high-order mode discrimination, mode-field displacement upon coiling, and
index contrast (manufacturability). We compared design families based on various power-law and hybrid
profiles for the RI and evaluated confined rare-earth doping for hybrid profiles. Step-index fibers with
straight-fiber Aeff values > 1000 &mgr;m2 exhibit large decreases in Aeff and transverse mode-field displacements
upon coiling, in agreement with recent calculations of Hadley et al. [Proc. of SPIE, Vol. 6102, 61021S
(2006)] and Fini [Opt. Exp. 14, 69 (2006)]. Triangular-profile fibers substantially mitigate these effects,
but suffer from excessive bend sensitivity at Aeff values of interest. Square-law (parabolic) profile fibers are
free of modal distortion but are hampered by high bend sensitivity (although to a lesser degree than
triangular profiles) and exhibit the largest mode displacements. We find that hybrid (combined power-law)
profiles provide some decoupling of these tradeoffs and allow all design goals to be achieved
simultaneously. We present optimized fiber designs based on this analysis.
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Roger Lee Farrow, G. Ronald Hadley, Dahv A. V. Kliner, Jeffrey P. Koplow, "Design of refractive-index and rare-earth-dopant distributions for large-mode-area fibers used in coiled high-power amplifiers," Proc. SPIE 6453, Fiber Lasers IV: Technology, Systems, and Applications, 64531C (22 February 2007); https://doi.org/10.1117/12.701594