This paper reports on a trace gas monitor using a dual wavelength coherent light source. The laser spectrometer is based on difference frequency generation (DFG) techniques in a periodically poled lithium niobate (PPLN) crystal, naturally having two laser sources (i.e., signal and pump laser sources) in it. A newly proposed idea is to use residual signal DFB-LD power to tune to NH3 absorption lines around at 1.536 μm when the DFG output wavelengths are accessed at 3.46 μm. In the experiments, the signal DFB-LD was carefully tune the wavelength guided by HITRAN database in order to select appropriate mid-IR absorption lines of NO2, while the DFB-LD can detect simultaneously NH3 absorption lines. As results, both gaseous species were detected successfully with the same multi-pass optical cell in one DFG spectrometer scheme. Spectroscopic data both in near-IR and in mid-IR region, simulating flue gas condition are discussed in detail.