Mach-Zehnder-type quantum cascade laser
We investigate the physics of light coupling in Y-branched QCLs. Here, light starts as a single beam and is then
monolithically split into two branches. The light is reflected at the ends of the two branches and again combined
into the single stem. Through experimental analysis of cavities with different dimensions and wavelengths, we determine
phase-locking and modal-behavior so that we can influence the coherence. As a result, we can reliably fabricate Y-coupled
resonators that provide a large degree of coherence. As one application, we combine two Y junctions to form monolithic
Mach-Zehnder interferometric (MZI) lasers, as shown in the Figure to the left. The performance of these devices is comparable to those
of standard Fabry-Perot (FP) lasers with only small losses in the coupling regions.
We introduce a phase shift between the two branches by resistively heating one branch to change the refractive index.
Next, we compared the output power with that of a uniformly heated device. The additional continuous-wave
current first increases the output power as the additional carriers overcompensate the negative effect of heating. As the
heating is further increased, however, emission is reduced. Finally, constructive interference causes the MZI-laser signal
to increase again. The principle can be utilized in novel monolithic interferometric high-sensitivity sensing devices.
Demonstration of constructive and destructive interference in Mach-Zehnder QCLs