Thermal management of mid-infrared (IR) quantum cascade lasers

Semiconductor quantum cascade lasers that emit mid-infrared light in the wavelength range of 4 to 9 μm are unipolar and the laser emission is due to intersubband transitions in a repeated stack of multiple quantum wells. The thermal management of these devices is a challenge. The overheating of the active region (referred to as `core´ throughout this paper) in these lasers decreases the optical power and ultimately results in laser failure. In this work, we present a detailed finite element (FE) based numerical modeling of the thermal behavior of these devices and the measurements performed to validate the models. The studies include the effect of submount material, mounting schemes such as epi-side down or epi-side up mounting and finally, the effect of core geometry on the thermal impedance. We have also looked at various core designs such as split core. We conducted various experiments to correlate the results with the numerical modeling by measuring the thermal impedance between the laser diode´s core and the bottom of the substrate and measuring the temperature change within a pulse of a distributed-feedback (DFB) QCL which emits in a single longitudinal mode of narrow linewidth. The temperature of the active core of a DFB QCL can be determined by measuring the lasing frequency, which changes with the temperature of the active core as: v=v₀+βvT_core=v₀+βvT_submount+βvR_thP_dec, where v is the lasing wavenumber, R_th is the thermal resistance, P_elec is the electric power loading, and β is the thermal tuning coefficient. By measuring the lasing frequency as a function of time within a pump current pulse, we can determine the temperature change, and the thermal conductance of a laser structure. In the conclusion, we provide various recommendations for efficient thermal performance of these quantum cascade lasers.