Princeton University

School of Engineering & Applied Science

Curved Cavity Quantum Cascade Lasers and Superluminescent Emitters

Mei-Chai Zheng
Engineering Quadrangle, J401
Tuesday, March 28, 2017 - 2:00pm to 3:00pm

Mid-IR Quantum Cascade (QC) lasers and emitters are at the heart of many innovative sensing and imaging technologies. My dissertation explores the implementation of curved cavities in QC devices to enable more compact and low cost trace gas sensing systems as well as the first high resolution, hyperspectral, 3D mid-IR imaging system.
A highly sensitive trace gas sensing system requires QC lasers to emit a single wavelength of light (single-mode) with a wide tuning range. However, QC lasers realized in Fabry-Perot (FP) laser cavity, the most common type of semiconductor laser cavity, exhibit multi-mode emission. Although wavelength selectivity can be achieved in such cavities by the incorporation of a periodic grating, the need for precise periodic structures on the wavelength scale requires more complex fabrication steps, often resulting in higher costs and lower yield. By introducing curved sections into the straight FP cavities to form the so-called “Mach-Zehnder interferometer type cavities”, we achieved single mode operation in QC lasers with simple fabrication steps. Moreover, we achieved a ten-fold increase in the tuning range of these devices!
In the realm of medical diagnostics, an invasive biopsy is often needed in order to confirm the presence/absence of diseases after an initial examination with the current imaging technologies. These biopsies can be circumvented by the invention of a joint imaging/sensing system. Such a system can be achieved through mid-IR Optical Coherence Tomography (OCT) – a 3D imaging platform, where the mid-IR light emitter not only provides the spectral data for sensing, but also the spatial data for 3D imaging. Such system does not currently exist due to a lack of appropriate mid-IR light emitters that are powerful, but incoherent. These requirements mean that the emitter must achieve the same amount of optical power as a QC laser while inhibiting the laser action. Previous attempts at making these superluminescent QC emitters could only achieve a peak power in the micro-watt range at cryogenic temperatures. By taking advantage of spiral cavities that help to elongate total QC device length without compromising on the overall device footprint, we increased the power emission by three orders of magnitude!