Curved Cavity Quantum Cascade Lasers and Superluminescent Emitters

Abstract

Light sources that emit in the mid-infrared (mid-IR) are of utmost technological importance due to their various applications ranging from molecular absorption spectroscopy and imaging to free-space communications and infrared countermeasures. That makes quantum cascade (QC) lasers a very important achievement of the twentieth century. Over the last 20 years, advancements in QC lasers have turned them into the most viable mid-IR light source. This dissertation advances QC emitters further by introducing several new approaches in cavity design for making monolithic widely-tunable single-mode QC lasers, low power QC lasers, as well as powerful QC superluminescent (QCSL) emitters.

It is only recently that over a milliwatt of superluminescent emission have been made possible in QC devices. These are powerful devices that support amplified spontaneous emission with low coherence length (i.e. powerful sub-threshold emission). This advancement marks a milestone for QC devices because of the difficulty in achieving large sub-threshold power due to the short non-radiative carrier lifetime of the intersubband transitions at the heart of QC devices. Powerful yet low coherence emission in these devices is necessary in the development of a first mid-IR optical coherence tomography (OCT) system for 3D imaging and spectroscopy. To meet this goal, the first part of this dissertation explores a spiral cavity design to further improve on the power emission in QCSL emitters while suppressing their lasing action by incorporating facets with a low reflectivity. The reflectivity of the back facet is then further reduced through the introduction of a passive loop cavity that replaces the anti-reflection (AR) coated wet-etched back facet. Through these two types of spiral cavity design, more than 100 milliwatt of SL power is achieved!

QC lasers on the other hand, have already come a long way. Advancements in these lasers over the last two decades have made possible the best performing QC lasers today, with watt level power output under continuous wave operation at room temperature. Despite the rapid QC laser developments, further improvements on the lasers' spectral purity and efficiency are still desired for practical implementations in QC laser based sensing systems. Toward this end, the second part of this dissertation implements asymmetric Mach-Zehnder (AMZ) interferometer type cavities in QC lasers to achieve widely tunable single-mode laser operation. By biasing the two arms of the AMZ interferometer laser cavity separately, a ten-fold improvement in single-mode wavelength tuning was achieved! Moreover, circular geometries such as cylindrical and ring cavities are also investigated for making low power (milliwatt range) QC lasers in order to enable affordable and compact sensing systems that will not saturate sensitive detectors. By employing a ring cavity geometry in conjunction with sub-wavelength scatters in a standard 8 $\mu$m QC laser design, we achieved over a ten-fold power reduction, thereby, obtaining the desired milliwatt laser emission.

The investigations carry out in this dissertations not only contribute to the advancement in QC devices' performance (in terms of spectral purity and both SL and laser power emission) but also shed light on new approaches in quantum and cavity design for future explorations.