A periodically poled $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ (PPLN) crystal waveguide has been used to produce up to $200\mathrm{\mu W}$ of mid-infrared light around $3081{\mathrm{cm}}^{-1}$ with a wide tunability range of $\sim 35{\mathrm{cm}}^{-1}$ . Two commercial near-infrared diode lasers at $1.064\mathrm{\mu m}$ (pump) and $1.583\mathrm{\mu m}$ (signal) are mixed in a nonlinear optical crystal to achieve difference frequency generation. The $48\mathrm{mm}$ long directly-bonded quasi-phase-matched (QPM) PPLN waveguide shows a conversion efficiency of $12.3\%{\mathrm{W}}^{-1}$ . Applications in trace gas detection have been demonstrated for ethene, using multipass absorption coupled with wavelength modulation spectroscopy, and cavity enhanced absorption spectroscopy with a lock-in detection scheme: bandwidth reduced sensitivities of ${\alpha }_{\min }=8\times {10}^{-9}$ and $1.6\times {10}^{-8}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}(2\sigma )$ , respectively, have been achieved.