  We have demonstrated a frequency-stabilized tunable 318.6 nm ultraviolet (UV)
laser system for the single-photon 6S1/2 - nP (n = 70 ~ 100) Rydberg excitation
of cesium atoms. The 637.2 nm laser produced by single-pass sum frequency
generation from two infrared fiber lasers is offset locked to a high-finesse
ultra-low expansion (ULE) optical cavity placed in ultra-high vacuum using the
electronic sideband locking technique. The generated 318.6 nm UV laser via
cavity-enhanced second-harmonic generation can be continuously tuned over 4 GHz
by indirectly changing modulation frequency on the electro-optic phase
modulator while the whole laser system remains locked. We analyze the tuning
range mainly depends on the modulator bandwidth and the tunable range of the
seed laser. The locking scheme offers a method to compensate the frequency
difference between the reference frequency and the goal frequency to a desired
excited state, and has huge potential in precision spectroscopic experiments of
cold atoms.
