Frequency stabilization of a novel 1.5-μm Er-Yb bulk laser to a 39K sub-Doppler line at 770.1 nm

The second harmonic output at 770.1 nm of a novel and compact Er-Yb:glass laser was frequency stabilized against the sub-Doppler linewidth of a crossover line in the ³⁹K 4S_1/2-4P _1/2 transition as obtained by saturation spectroscopy. Efficient frequency doubling, with a conversion efficiency of ~220% W ^-1, and with second harmonic power in excess of 15 μW, was achieved in a waveguide made in a periodically poled lithium niobate crystal. As measured through the analysis of the closed-loop error signal, a laser frequency instability of ~200 Hz was obtained; the Allan standard deviation of the frequency samples was below 4×10^-12 for integration times τ between 100 ms and 100 s, and reached a lowest floor level of 8×10^-13 for 20 s⩽τ⩽100 s. The measured frequency noise spectral density was in good agreement with the analysis performed in the time domain. Compared to previously published data for stabilized solid-state laser sources in this wavelength region, these results represent a significant improvement in the frequency stability