Rovibration absorption lines both of ${}^{12}\mathrm{C}\mathrm{O}$ and ${}^{13}\mathrm{C}\mathrm{O}$ are observed simultaneously with the output of a $2\mu \mathrm{m}$ broadband difference frequency generated in a direct-bonded quasi-phase-matched $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ waveguide, which is a 50 mm device with a single quasi-phase-matching period that is operated at a constant temperature. The wavelength conversion efficiency and the difference-frequency generation bandwidth reach $100\%∕\mathrm{W}$ and 100 nm, respectively. The idler output bandwidth in the $2\mu \mathrm{m}$ region is obtained by group-velocity matching or phase-mismatch minimization when a $0.94\mu \mathrm{m}$ pump laser diode and a $1.55\mu \mathrm{m}$ tunable signal source are used.