A continuous-wave difference-frequency generation laser operating in the mid-infrared (3–5 μm) region for accurate line intensity measurements

In this paper we present a compact tuneable continuous-wave infrared laser, based on difference-frequency generation (DFG) using quasi phase matching in periodically poled LiNbO3 (PPLN), for accurate line intensity measurements. Several 0.1 μW of infrared radiation (tuneable in the 3–5 μm range) are obtained using a diode-pumped Nd:YAG laser (output power about 800 mW at 1064 nm, linewidth 1 kHz) together with a tuneable external-cavity diode laser (output power about 500 μW–50 mW in the 805–885 nm region, linewidth 1 MHz). Using this infrared DFG laser for absorption experiments of gas-phase molecules, both very high-resolution (1 MHz) and a high signal-to-noise ratio (up to several 1000) can be achieved in measurement time of only a few minutes, as demonstrated using absorption spectra of N2O in different wavelength regions. The present paper focuses in particular on the accuracy of the wavelength calibration and on the determination of absorption coefficients with experimental uncertainties of less than 1%.