Phase-locked pair of terahertz quantum cascade lasers through evanescent-wave coupling

A phase-locked laser pair containing two terahertz distributed feedback (DFB) quantum cascade lasers (QCLs) has been realized via evanescent-wave coupling. The mode selection in the laser pair is dominated by the waveguide loss which can be controlled mainly by the ridge width, as well as the width of the gap between the two ridges. A judicious design enables the laser pair to stably operate on the out-of-phase supermode with a side mode suppression ratio (SMSR) of 34 dB, a peak output power of 8.06 mW, and an amplification factor of about 1.72. Due to the significant overlap of the electromagnetic fields caused by the two laser elements, the laser pair is phase-locked even when the two elements are differently biased, confirmed by the measured emission spectra and far-field beam patterns. This work represents a crucial step towards phase-locked arrays for high power single-mode terahertz radiation.