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A tunable continuous wave (cw) mid-infrared (MIR) laser based on difference-frequency generation (DFG) in a 1.5-cm long AgGaS2 nonlinear crystal for trace gas detection is reported. Two visible and near-infrared diode lasers were used as pump and signal sources. The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm. The phase-matching (PM) condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths. The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS2 given by Willer U, et al. (Willer U, Blanke T and Schade W 2001 Appl. Opt. 40 5439). Then the calculated PM temperatures are compared with the experimental values. The performance of the MIR-DFG laser is shown by the trace detection of the P(16) carbon monoxide (12C16O) absorption line in a laboratory-fabricated absorption cell. The enhanced sensitivity of about 0.6×10−4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell.

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Propagation of cylindrical waves in homogeneous and inhomogeneous nonlinear optical media has been investigated theoretically. The coupled-wave equations for a homogeneous and inhomogeneous nonlinear media have been investigated numerically. For homogeneous media, calculations show that phase-matching condition is not fulfilled and efficiency of output wave is very low, but for inhomogeneous media, phase-matching condition is nearly fulfilled and the generated wave amplitude of the four-wave mixing (FWM) is much larger than the homogeneous media. The effect of effective nonlinear optical coefficient [Formula: see text] on the generated wave amplitude for inhomogeneous media has been investigated.