Tunable Dual-Wavelength, Continuous-Wave, Mid-Infrared Generation Using Intracavity Difference Frequency Mixing in PPLN-Based Optical Parametric Oscillator

We report on a widely tunable, dual-wavelength, continuous-wave, mid-infrared optical parametric oscillator (OPO), pumped by a dual-wavelength fiber source that was realized by combining two fiber lasers in series. The OPO was designed as a four-mirror singly resonant cavity, in which a periodically poled lithium niobate (PPLN) crystal was used as nonlinear crystal. Two idler emissions with different wavelengths were observed, one being basically fixed at 3140 nm and the other being tunable from 3227 to 3413 nm. Under the maximum pump power, the total idler power was over 6 W and the corresponding pump-to-idler slope efficiency reached 11.4%. The generated signal beam had just one central wavelength around 1600 nm. Through theoretical analysis and experimental verification, it was analyzed that two nonlinear processes, OPO and intracavity difference frequency generation (DFG), simultaneously occurred in the resonant cavity. The experimental phenomena revealed large widened DFG gain spectrum, which was in absolute contrast to simulation results based on classical DFG theory. It indicated great potential in practical application such ultra-wide nonlinear frequency conversion.