Publications

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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 μm2 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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We demonstrate direct diode-bar side pumping of a Yb-doped fiber laser using embedded-mirror side pumping (EMSP). In this method, the pump beam is launched by reflection from a micro-mirror embedded in a channel polished into the inner cladding of a double-clad fiber (DCF). The amplifier employed an unformatted, non-lensed, ten-emitter diode bar (20 W) and glass-clad, polarization-maintaining, large-mode-area fiber. Measurements with passive fiber showed that the coupling efficiency of the raw diode-bar output into the DCF (ten launch sites) was {approx}84%; for comparison, the net coupling efficiency using a conventional, formatted, fiber-coupled diode bar is typically 50-70%, i.e., EMSP results in a factor of 2-3 less wasted pump power. The slope efficiency of the side-pumped fiber laser was {approx}80% with respect to launched pump power and 24% with respect to electrical power consumption of the diode bar; at a fiber-laser output power of 7.5 W, the EMSP diode bar consumed 41 W of electrical power (18% electrical-to-optical efficiency). When end pumped using a formatted diode bar, the fiber laser consumed 96 W at 7.5 W output power, a factor of 2.3 less efficient, and the electrical-to-optical slope efficiency was lower by a factor of 2.0. Passive-fiber measurements showed that the EMSP alignment sensitivity is nearly identical for a single emitter as for the ten-emitter bar. EMSP is the only method capable of directly launching the unformatted output of a diode bar directly into DCF (including glass-clad DCF), enabling fabrication of low-cost, simple, and compact, diode-bar-pumped fiber lasers and amplifiers.