Engineering the Free Spectral Range Of Fabry-Perot Quantum Cascade Lasers

Quantum cascade (QC) lasers have emerged as a viable laser source in the mid-infrared spectral region due to their large wavelength design space that covers important absorption peaks of many gas species. Room temperature QC lasers offer an attractive solution for trace gas sensing applications that require fast, portable, high-sensitivity sensor systems. However, the emission of a conventional 3 mm long Fabry-Perot (FP) QC laser is typically multi-mode and thus unsuitable for many desired applications requiring a single frequency and narrow-linewidth source. In addition, the relatively low wall plug efficiencies of QC lasers result in most of the power consumed being dissipated as heat. This poses another barrier to incorporating QC lasers into portable sensor systems as considerable thermal management is needed. To overcome these barriers, this dissertation proposes and develops a number of techniques in order to obtain compact, single-mode QC laser sources.;From laser physics principles, it can be shown that shortening the cavity length will increase the free spectral range (FSR) of the resonator modes. This increased FSR in ultra-short cavity QC lasers will then increase the gain margin between the mode closest to the gain peak and the side modes. In addition, the power consumption of a QC laser scales down with cavity length. Scaling the cavity length by cleaving ultra-short QC lasers from 764 to 110 im has proven a fourfold reduction in power consumption in comparison to conventionally-sized QC lasers and demonstrated single-mode emission in continuous wave operation. The associated high mirror loss of ultra-short cavity QC lasers is mitigated with back and front-facet high-reflectivity (HR) coatings.;Examining the quantum mechanical bandstructure of QC laser designs shows sub-threshold tuning of the emission wavelength with applied electric field due to the linear Stark effect yet tuning reduces sharply above-threshold. Cavity length scaling is then used to change the threshold current and the associated threshold voltage. The increase in threshold voltage results in a higher applied electric field at lasing which results in spectral tuning of the gain peak. The insight learned from this experiment also allowed analogous gain peak tuning through facet reflectivity. Facet reflectivity tuning was performed by varying the optical feedback provided by a neutral density filter wheel into an anti-reflective (AR) coated front-facet QC laser. It is shown that the threshold voltage tunes with the effective front-facet reflectivity also resulting in gain peak tuning.;A well-established technique used in industry to attain single-mode emission is fabricating distributed feedback (DFB) gratings on top of a QC laser ridge. The DFB grating provides wavelength selectivity and threshold gain discrimination which leads to single-mode operation. A discussion of the design of a DFB-QC laser for room temperature detection of carbon monoxide (CO) and carbon dioxide (CO2) for planetary science covers pertinent topics related to DFB gratings. The designed and fabricated thick electroplated gold DFB-QC lasers were single-mode in continuous wave operation up to a heat sink temperature of 190 K. This is attributed to an ambitiously chosen Bragg wavelength being far from the gain peak. Nevertheless, the simulated Bragg period and coupling coefficient were in excellent agreement with the experimentally-determined values.;Lastly, a straightforward external cavity (EC) configuration based on a three-mirror coupled cavity is used to stabilize the wavelength of a FP-QC laser using feedback from an optical window. This EC configuration uses simple components to induce single-mode operation in an otherwise multi-mode FP-QC laser with no AR facet coating. Tuning of the emitted wavelength is provided using the Vernier effect by synchronously tuning the FP and EC resonator modes in order to maintain maximum feedback with the lasing mode. Additionally, operating FP-QC lasers in EC configurations has been shown to reduce the FP-QC laser linewidth to the MHz range. Time dependent measurements of the EC-QC laser linewidth were performed which help isolate contributions to the linewidth from current, vibration, and thermal noise sources. A DFB-QC laser with a similar wavelength using the same technique was measured for comparison and the EC-QC laser linewidth was found to be smaller at short integration times and comparable to the DFB-QC laser at medium and long integration times.