  We report what we believe to be the first radiation-balanced fiber amplifier
- a device that provides optical gain while experiencing no temperature rise.
The gain medium is a silica fiber with a 21 um-diameter core highly doped with
Yb3+ (2.52 wt.%) and co doped with 2.00 wt.% Al to reduce concentration
quenching. The amplifier was core-pumped with 1040 nm light to create
anti-Stokes fluorescence (ASF) cooling and gain in the core at 1064 nm. Using a
custom slow-light FBG sensor with mK resolution, temperature measurements were
performed at multiple locations along the amplifier fiber. A 4.35-m fiber
pumped with 2.62 W produced 17 dB of gain while the average fiber temperature
remained slightly below room temperature. This advancement is a fundamental
step toward the creation of ultra-stable lasers necessary to many applications,
especially low-noise sensing and high-precision metrology.
