Correlation between noise properties and coherent coupling of lateral modes in quantum cascade lasers

We report on investigations of far-field intensity patterns and angle-resolved lasing spectra emitted fromquantum cascade (QC) lasers at wavelengths around Lamda = 7.7 µm in pulsed operation. Measurements of the angle-dependent intensity noise in the far-field reveal that small driving currents lead to an incoherent superposition of multiple lateral cavity modes. Elevated currents result in a phase-coupled superposition of the light fields arising from the excited lateral modes. This superposition leads to beam steering and strong pulse-to-pulse intensity fluctuations. We present for both the incoherent and the phase-coupled superposition, a model which allows us to explain the observed far-field intensity patterns and their noise properties. The angle-resolved lasing spectra of the investigated QC lasers show distinct sets of longitudinal modes with distinctively different dependences of the lasing spectrum on the far-field angle. They can be assigned to the incoherent and the phase-coupled superposition of the lateral modes.