Approximately $200\mu \text{W}$ of mid-infrared (mid-IR) light around $3081{\text{cm}}^{-1}$ , produced by difference frequency generation (DFG) in a periodically poled lithium niobate crystal waveguide, has been used for phase-shift cavity ring-down spectroscopy measurements. The overlapping ${{}^{12}\text{C}}_2{\text{H}}_4$ ${{}^{R}P}_0\left(14\right)$ and ${{}^{P}R}_6\left(10\right)$ and ${}^{12}\text{C}{\text{H}}_2{}^{13}\text{C}{\text{H}}_2$ ${{}^{P}Q}_3\left(10\right)$ rotational lines of the ${\nu }_9$ fundamental ethene vibrational band at $3081.0016{\text{cm}}^{-1}$ were probed in proof-of-principle experiments, and ethene detection was demonstrated with a minimum absorption coefficient of $1.4\times {10}^{-7}{\text{cm}}^{-1}$ ( $\sim 4\text{\hspace{0.17em} min}$ acquisition time). The compact DFG system, with a $>35{\text{cm}}^{-1}$ tuning range, has a considerable potential for use in trace gas detection and in molecular spectroscopy.