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Summary form only given. Mid-infrared laser absorption spectroscopy is a promising technique for environmental trace-gas detection because many important air contaminants have strong absorption bands in that spectral region. To access the 3-5 /spl mu/m spectral region a convenient technique is difference frequency mixing in a nonlinear material, which allows the frequency shifting of readily available near infrared diode laser sources. Periodically poled LiNbO/sub 3/ (PPLN) has desirable characteristics for such difference frequency mixing, including engineerable phase matching and a high nonlinear coefficient (d/sub eff/). The use of a tapered waveguide written onto the PPLN, leads to a further enhancement of the conversion efficiency of PPLN.