Two single-mode diode lasers and AgGaS(2) as a nonlinear medium are applied for difference frequency generation (DFG) in the mid-infrared spectral range between 4.9 and 6.5 mum. Phase matching is achieved by either temperature tuning or angle tuning of the crystal. Experimentally measured sets of input wavelengths lambda(s) and lambda(p), the resulting DFG wavelength lambda(i), and corresponding phase-matching temperatures or angles are compared with the calculated values derived by use of different Sellmeier equations and coefficients and temperature-dispersion equations dn/dT. Our results show that only specific combinations of previously published Sellmeier equations, coefficients, and temperature-dispersion equations are suitable for exact calculations of phase-matching parameters. These combinations reproduce our experimentally obtained phase-matching temperatures and angles with an accuracy of better than 5% and are therefore of fundamental interest for the design of a mid-infrared DFG spectrometer with AgGaS(2) as a nonlinear medium.

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The generation of tunable laser radiation in the spectral range between 4 and 10 µm is of general interest for the spectroscopy since most of the fundamental vibrational modes of molecules are within this spectral region [1]. There exist several conventional tunable cw-laser systems, like lead salt-diodes, color center-, optical parametric-, and CO or CO2 side band lasers. However, they need cryogenic cooling, do not cover all spectral regions that are of general interest for the spectroscopy and in most cases they are very complicated and complex laser sources. This limits the possibilities for practical applications of mid-infrared absorption spectroscopy, e.g. in the trace analysis of environmental pollutants. Therefore there is still a lack in simple, compact, portable, and rugged mid-infrared laser sources. In this context diode-lasers and difference frequency generation (DFG) in new nonlinear media such as AgGaS2 have drawn considerable interest in the construction of compact cw laser spectrometers [2,3].

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