Abstract

Nanosecond optical parametric oscillators (OPO) show unique features of wavelength tunability. They can be used in particular for the spectroscopy of many molecules in the range of 3.3 μm – 3.4 μm (C-H bond absorption lines). They are well suited for LIDAR and sensing applications such as photoacoustic spectroscopy. To reduce the natural linewidth of our LiNbO₃ OPO (1 cm⁻¹), we have inserted a Fabry-Pérot interferometer (FPF) into the cavity. The thickness is optimised so that only one of its resonances lays into the OPO natural line width Δv. The width of the resonance Δv_c is related to the mirrors reflectivity. It is chosen to match the emission linewidth requirement The spacing between the FPF mirrors is tunable around 1 mm thanks to a piezoelectric stack. This allows a fine tuning of the OPO emission frequency. The mirror reflectivity is 70 %. It gives a good trade-off between the FPF spectral filtering and its transmission at resonance. To measure the signal spectral linewidth, interference rings are produced with a 0.4 cm⁻¹ free spectral range Fabry-Pérot spectrometer. They are monitored on a Vidicon tube camera (fig 1) and show that we are able to decrease the signal spectral linewidth down to 0.08 cm⁻¹.

© 2000 IEEE