Quantum cascade laser wavelength tuning due to temperature-dependent index of refraction

In this work, we study the change in the emission wavelength of quantum cascade lasers when the index of refraction of the lasing medium changes due to a change in the temperature. We find that the gain of the laser and the wavelength of the longitudinal optical modes in the laser cavity change significantly as the temperature changes. As a result, the output wavelength of QCLs changes as the temperature changes. When the wavelength of the lasing mode changes so much that one of the neighboring modes becomes the closest mode to the peak of the gain spectrum, the lasing hops to the neighboring mode. We also find that the mode-tuning and mode-hopping depend on the longitudinal length of the laser cavity. The tuning range increases for a smaller cavity, while the mode-hopping becomes more frequent for a longer cavity as the temperature increases.