Continuous-wave 355-nm laser source based on doubly resonant sum-frequency mixing in an external resonator

Using doubly resonant sum-frequency mixing between 1064 and 532 nm in an external resonator, we obtained 186 mW of continuous-wave ultraviolet output at λ = 355 nm with more than 20% conversion efficiency from the total incident power onto the mixing cavity. The resonator mirrors are shared by both the 1064- and the 532-nm beams, yielding an optimum mode overlap between the two resonating modes. To maintain the simultaneous resonance, we locked the external resonator to the single-frequency green laser and locked the 1064-nm laser to the resonator. This technique can be applied to cw sum- or difference-frequency mixing of any two wavelengths.