Influence of facet coatings on the tunability of a 9.4 μ m external cavity quantum cascade laser

We demonstrate enhanced tuning performance of external cavity quantum cascade lasers through optimized optical feedback conditions. The influence of facet reflectivity was investigated using antireflection coatings with different residual reflectivities as well as high-reflectivity coatings. A novel six-layer antireflection coating providing a residual reflectivity of approximately 0.5% yields the most favorable external cavity feedback conditions, increasing the tuning range of a 9.4 μm quantum cascade laser from 55 cm−1 for an uncoated device to 143 cm−1 at room temperature.