Efficient extraction of high pulse energy from partly quenched highly Er3+-doped fiber amplifiers.

We demonstrate efficient pulse-energy extraction from a partly quenched erbium-doped aluminosilicate fiber amplifier. This has a high erbium concentration that allows for short devices with reduced nonlinear distortions but also results in partial quenching and thus significant unsaturable absorption, even though the fiber is still able to amplify. Although the quenching degrades the average-power efficiency, the pulse energy remains high, and our results point to an increasingly promising outcome for short pulses. Furthermore, unlike unquenched fibers, the conversion efficiency improves at low repetition rates, which we attribute to smaller relative energy loss to quenched ions at higher pulse energy. A short (2.6 m) cladding-pumped partly quenched Er-doped fiber with 95-dB/m 1530-nm peak absorption and saturation energy estimated to 85 µJ reached 0.8 mJ of output energy when seeded by 0.2-µs, 23-µJ pulses. Thus, according to our results, pulses can be amplified to high energy in short highly Er-doped fibers designed to reduce nonlinear distortions at the expense of average-power efficiency.

Pump absorption, laser amplification, and effective length in double-clad ytterbium-doped fibers with small area ratio.

We use numerical simulations with the beam propagation method (BPM) and rate equations to investigate the pump absorption and amplification characteristics in double-clad ytterbium-doped fibers with small cladding-to-core area ratios, in the range 1-3. The presence of modes with low overlap with the doped region (or alternatively, skew rays) hampers the pump absorption in a circular geometry, but we find that the effect is small for area ratios of ∼2.5 or less. We derive ray-based expressions for the small-signal absorption which show similar results. However, even when the small-signal absorption scales nearly ideally with the inverse of the area ratio, the absorption in an operating amplifier is much lower, and the dependence on the area ratio much weaker, when a large fraction of the Yb-ions is excited in a small-area-ratio fiber. We derive equations which show this, and that in contrast to conventional area ratios of, e.g., 100, the fiber length depends more strongly on the required gain than on the required pump absorption. However, fibers substantially shorter than 1 m still allow for adequate pump absorption and gain. The effective length for nonlinear interactions is less affected by this, since the Yb-excitation is low where the signal power is high. Although we treat single-mode cores, the BPM amplifier simulations show there are a few percent of the signal power in cladding-guided modes with high overlap with the Yb-doped core. Nevertheless, according to our simulations, it is possible to achieve high efficiency and mode purity with a small-area-ratio circularly symmetric double-clad fiber.