Application of balanced detection to absorption measurements of trace gases with room-temperature, quasi-cw quantum-cascade lasers

Distributed-feedback quantum-cascade (QC) lasers are expected to form the heart of the next-generation mid-IR laser absorption spectrometers, especially as they are applied to measurements of trace gases in a variety of environments. The incorporation of room-temperature-operable, single-mode QC lasers should result in highly compact and rugged sensors for real-world applications. We report preliminary results on the performance of a laser absorption spectrometer that uses a QC laser operating at room temperature in a quasi-cw mode in conjunction with balanced ratiometric detection. We have demonstrated sensitivities for N₂O [10 parts in 10⁶ volume-mixing ratio for a 1-m path (ppmv-m)] and NO [520 parts in 10₉ volume-mixing ratio for a 1-m path (ppbv-m)] at 5.4 μm. System improvements are described that are expected to result in a 2 orders of magnitude increase in sensitivity.