Monolithic Mach–Zehnder-type quantum cascade laser.

A midinfrared quantum cascade laser with Mach–Zehnder cavity and split contacts is investigated with respect to interference effects. By increasing the temperature in one of the two coupled active waveguides, the value of the effective refractive index is varied and the modal phase is shifted. As a result, destructive interference is observed within the resonator, which manifests itself in a minimum of the modulated output power. The dissipated heat is controlled by locally adding a continuous current to the drive current pulses. In the first step, thermal properties, threshold values, and far fields are analyzed and compared to a Fabry–Pérot resonator to gain insight into the physical principles of the monolithic interferometer. Based on these findings, the temperature distribution is calculated in a two-dimensional heat transfer simulation, which leads to a match between the thermal change of the effective refractive index and the condition for destructive interference; a phase shift of π between the two interfering beams is confirmed. By modulating the effective refractive index using evanescent fields instead of temperature variations, a monolithic midinfrared interferometric sensing device becomes feasible. [ABSTRACT FROM AUTHOR]