Abstract

Infrared absorption spectroscopy can be a very powerful spectroscopic technique for studying transient molecules, since it combines high resolution and high detection sensitivity. This requires the availability of suitable frequency tunable infrared sources. A CW infrared laser spectrometer will be described that is based on difference frequency mixing of two tunable high power single frequency lasers in two newly available nonlinear optical crystals: AgGaS₂ or AgGaSe₂. Such a spectrometer is capable of obtaining Doppler limited molecular spectra in the 4 to 18µm region (2500 to 550 cm^-1). The recent commercial availability of AgGaS₂ and AgGaSe₂ crystals with excellent optical transmission characteristics between 0.5 to 10 pm and 0.9 to 18µm, respectively, as well as interaction lengths in excess of 40 mm now makes it possible to generate continuously tunable infrared radiation at the 50µW level. Experiments will be described that employ two tunable CW lasers covering the range from 580 to 900 nm, and 45 mm long AgGaS₂ crystal cut for 90° phase matching. A schematic of the essential components of the difference frequency spectrometer is shown in Fig. 1. Wavelength, linewidths, output power, and temperature characteristics will also be reported. Precise computer control of wavelength scanning, wavelength calibration, and data acquisition have been implemented, based on both hardware and software techniques developed for color center and diode laser spectrometers in our laboratory. We have coupled this technology with commercially available software that is used to control one of the two tunable pump lasers (i. e., Autoscan II software than runs a Coherent Model 899-29 dye/Ti-sapphire ring laser). Continuous scans of up to 1 cm^-1 width (limited by the phase matching conditions in the crystal) using ultra-narrow linewidth (0.5 MHz or < 2 × 10^-5 cm^-1) were produced in the 1100 to 1450 cm^-1 and 1440 to 1900 cm’¹ ranges with infrared output powers of~20µW.

© 1992 IQEC