Tunable distributed-feedback quantum-cascade lasers for gas sensing applications

Continuously tunable single-mode emission of high performance quantum cascade (QC) lasers is achieved by application of the distributed feedback (DFB) principle. The devices are fabricated either as loss-coupled or index-coupled DFB lasers. Single-mode tuning ranges of ≈ 100 nm have been measured in both of the atmospheric windows at emission wavelengths around λ ≈ 5 μm and 8 μm. Linear thermal tuning coefficients of 0.35 nm/K and 0.55 nm/K have been obtained above 200 K for λ ≈ 5 μm and 8 μm, respectively. The side-mode suppression ratio is better than 30 dB. Pulsed single-mode operation has been achieved up to room temperature with peak power levels of 60 mW. The lasers also operated single-mode in continuous wave at temperatures above liquid Nitrogen temperature; a single-mode tuning range of 70 mn has been measured in the temperature range from 20 K to 120 K. The gas sensing capabilities of the QC-laser have also been demonstrated using both direct absorption and wavelength modulation techniques. A pulsed, room temperature, QC-DFB laser operating at λ ≈ 7.8 μm was used to detect N₂O diluted in N₂. The detection limit was found to be ≈ 500 ppb-m. In addition, the high resolution capability of the QC-DFB lasers (at 77 K) has been demonstrated via continuous, rapid-scan, direct absorption measurement of the Doppler limited R(16.5) lambda doublet of NO at λ ≈ 5.2 μm.