Multiple conversion and optical limiting in a subharmonic-pumped parametric oscillator.

We demonstrate that multiple coexisting frequency-conversion processes can occur in an externally resonant second-harmonic generator under suitable conditions. Besides the generation of signal and idler waves by subharmonic-pumped parametric oscillation, sum-frequency mixing among the resonant subharmonic (1064-nm), signal, and idler waves was observed, leading to additional emission wavelengths around the harmonic wavelength (532 nm). The output waves both exhibit high frequency stability, with as long as 4 h of mode-hop-free parametric oscillation, and are continuously tunable over 2 GHz. Near degeneracy the parametric oscillator operates as an optical limiter for the harmonic wave.