Continuous-wave optical parametric oscillation tunable up to an 8 μm wavelength

High-resolution spectroscopy of molecular gases requires sources of mid-infrared laser light combining narrow linewidths and wavelength tunability. Continuous-wave optical parametric oscillators (cw OPOs) fulfill these demands; however, their mid-infrared tuning range has been limited to wavelengths below 5.5 μm so far. Here, we demonstrate the first cw OPO emitting mid-infrared light at wavelengths up to 8 μm. This device is based on a 3.5-mm-diameter whispering gallery resonator made of silver gallium selenide (AgGaSe2) pumped by a compact distributed feedback laser diode emitting at the 1.57 μm wavelength. The oscillation thresholds are in the mW region, while the output powers range from 10 to 800 μW. By changing the radial mode number of the pump wave, wavelengths of up to 8 μm are achieved. Temperature variation enables 100-nm-wide wavelength tuning. The measured tuning branches are in good accordance with the simulations. Furthermore, the latter show that whispering-gallery OPOs based on AgGaSe2 with diameters around 2 mm can generate idler waves exceeding the 10 μm wavelength.