Tapered fiber amplifier with high gain and output power

We present a high-power ytterbium fiber amplifier based on active tapered double-clad fiber (T-DCF) and capable of high single-pass gain. The T-DCF power amplifier seeded with a 320 mW narrow-band signal generates up to 110 W of average output power corresponding to more than 25 dB gain. The amplifier exhibits near-diffraction-limited beam quality (M ² = 1.06) at the highest output power, which was limited by the available pump power. With a broadband seed source, the amplifier produced a gain of nearly 40 dB obtained for low-signal limit of the seed. The high output power combined with high gain is achieved owing to amplified spontaneous emission (ASE) filtering and increased stimulated Brillouin scattering (SBS) threshold inherent to the axially non-uniform geometry. The amplifier operates efficiently with a wide range of input seed powers thus providing the basis for one-stage tapered amplifier which combines the functions of preamplifier and power amplifier and can be a competitive alternative to multi-stage design.