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 (NH₃) were performed with several QC lasers, operating at either 5.2 or 8.5 μm. 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 cm^-1 in 0.6 ms can be easily achieved. Noise-equivalent absorbance of 3 × 10^-6 was also obtained without optimizing the optical arrangement.