Power scaling long-wavelength ytterbium-doped silica fiber lasers for frequency doubling to yellow

Ytterbium-doped silica fiber lasers and amplifiers operating at very long wavelengths were designed and demonstrated for frequency doubling to the yellow. Obtaining efficient oscillation and amplification at these long wavelengths in Yb3+-doped silica is difficult due to the low available gain and gain competition from shorter wavelengths with higher emission cross-sections. These challenges led us to develop techniques to mitigate photodarkening and maximize the suppression of amplified spontaneous emission (ASE) in silica fiber with high levels of ytterbium doping. These advances resulted in the development of an integrated fiber oscillator operating at 1150 nm with 213 mW of CW output power with a full-width half-maximum linewidth of 8 pm and a polarization extinction ratio of 21 dB.; Using additional ASE management techniques, we developed a fiber amplifier that scaled the oscillator power to 3.35 W CW and 2.25 W of average power in microsecond pulses. The CW amplifier output power was frequency doubled in a periodically poled lithium niobate (PPLN) waveguide to produce 255 mW of power at 575 nm. The output of the microsecond-pulse amplifier was frequency doubled in a bulk periodically poled near-stoichiometric lithium tantalate (PPSLT) chip to nearly 1 W of average power. In addition to generating yellow radiation, issues in scaling to higher average and peak powers in fiber amplifiers were studied. In particular, the noise characteristics of large-mode area (LMA) fiber amplifiers were investigated and the use of silicate bonding to mitigate damage to fiber ends and minimize feedback was explored.