We summarize the performance of mode-filtered, Yb-doped fiber amplifiers seeded by microchip lasers with nanosecond-duration pulses. These systems offer the advantages of compactness, efficiency, high peak power, diffraction-limited beam quality, and widely variable pulse energy and repetition rate. We review the fundamental limits on pulsed fiber amplifiers imposed by nonlinear processes, with a focus on the specific regime of nanosecond pulses. Different design options for the fiber and the seed laser are discussed, including the effects of pulse duration, wavelength, and linewidth. We show an example of a microchip-seeded, single-stage, single-pass fiber amplifier that produced pulses with 1.1 MW peak power, 0.76 mJ pulse energy, smooth temporal and spectral profiles, diffractionlimited beam quality, and linear polarization.
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.
We report results from Yb-doped fiber amplifiers seeded with two microchip lasers having 0.38-ns and 2.3-ns pulse durations. The shorter duration seed resulted in output pulses with a peak power of > 1.2 MW and pulse energy of 0.67 mJ. Peak power was limited by nonlinear processes that caused breakup and broadening of the pulse envelope as the pump power increased. The 2.3-ns duration seed laser resulted in output pulses with a peak power of >300 kW and pulse energy of > 1.1 mJ. Pulse energies were limited by the onset of stimulated Brillouin scattering and ultimately by internal optical damage (fluences in excess of 400 J/cm 2 were generated). In both experiments, nearly diffraction-limited beam profiles were obtained, with M 2 values of < 1.2. Preliminary results of a pulse-amplification model are in excellent agreement with the experimental results of the amplifiers operating in the low-to-moderate gain-depletion regime.