High-resolution (Doppler-limited) spectroscopy using quantum-cascade distributed-feedback lasers

Lasing characteristics were evaluated for distributed-feedback quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the QC lasers’ suitability for use in high-resolution spectroscopic applications, including Doppler-limited molecular absorption and pressure-limited lidar applications. By use of a rapid-scan technique, direct absorbance measurements of nitric oxide (NO) and ammonia $\left({\mathrm{NH}}_3\right)$ were performed with several QC lasers, operating at either 5.2 or $8.5 \mathit{\mu }\mathrm{m}$. Results include time-averaged linewidths of better than 40 MHz and long-term laser frequency reproducibility, even after numerous temperature cycles, of 80 MHz or better. Tuning rates of $2.5 {\mathrm{cm}}^{-1}$ in 0.6 ms can be easily achieved. Noise-equivalent absorbance of $3\times {10}^{-6}$ was also obtained without optimizing the optical arrangement.