Quantum cascade lasers (QCLs) are capable of high power, tunable wavelength and single mode emission at room temperature in the mid-infrared wavelength region. These capabilities make them perfect light sources for laser based gas spectroscopy. The work described in this thesis focuses on development of QCLs suitable for selective gas sensing applications.
The thesis starts with the description of different changes to the QCL active region design. These changes were studied in order to improve laser performance while keeping the emission wavelength fixed. The proposed modifications were performed on short mid-infrared wavelength (lambda=3-4um) quantum cascade lasers based on InGaAs/AlAsSb and InAs/AlSb material systems.
The focus of this work is then moved to the description of a single mode quantum cascade laser with a third order unilateral grating. The previously unreported grating architecture that was used to achieve distributed feedback (DFB) in a QCL, as well as grating design and laser characterization are detailed in Chapter \ref{chap:uni}. The reported laser generates single mode emission with 30 dB side mode suppression ratio and a linewidth of 0.4cm^(-1). The simplified fabrication process for a third order DFB grating is developed for lambda=3.3-3.6um emission wavelength.
A different approach to achieve single mode emission in a QCL is described in Chapter 6. An external cavity QCL setup combined with the Fabry-Perot (FP) reflector is reported for the first time. The FP reflector is used to provide selective feedback that is controlled by the separation distance between two FP reflector mirrors. This external cavity arrangement allows generation of a wide spectral range and the rapid wavelength tuning capability.
Finally, the thesis is concluded with sensitive gas detection experiments. The direct absorption technique is utilized to demonstrate the 160ppmv detection of methane with the ro-vibrational absorption line located at lambda=3.3um and 1ppmv detection of nitric oxide with the absorption line located at lambda=5.3um. The experiments were performed using single mode lasers that were designed and fabricated in Sheffield.
