Low-noise fiber laser systems at 689 nm and 698 nm for strontium

We present the development and characterization of modular fiber laser systems at 689 nm and 698 nm. Two different approaches to produce low-noise, narrow-linewidth laser output are compared, based on custom Tm-doped lasers and lithium niobate waveguides. Sum frequency generation from 1970 nm and 1061 nm fiber lasers allowed us to achieve over 80 mW of 689 nm light with a measured linewidth of 12 kHz over 1 ms, whereas a frequency-tripled 2094 nm Tm-doped fiber laser demonstrated lower conversion efficiency and higher laser noise, resulting in 698 nm light with 6.5 mW of output power and a linewidth of 97 kHz over 1 ms. The stable operation of the laser system was further demonstrated through second-stage cooling of ⁸⁸Sr atoms to 4 µK without advanced frequency stabilization.