Browsing by Author "So, Stephen G."

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    Development of compact DSP based mid-infrared quantum cascade laser spectrometers
    (2005) So, Stephen G.; Tittel, Frank K.
    This thesis describes the development of digital signal processor (DSP) and quantum cascade laser (QCL) technology for compact, standalone, integrated, trace gas sensors capable of gas sensing at part-per-billion (ppb) concentration levels. Such sensitivity levels provide the ability to perform precise environmental and emissions monitoring, medical biomarker analysis, and biological/toxic agent detection. Current sensors based on tunable diode laser absorption spectroscopy (TDLAS) frequently require complex optical configurations or cryogenics to produce the proper mid-infrared optical frequencies, a full personal computer based data acquisition system to process data, and assorted nonintegrated support electronics for complete control of the system. In contrast, the developed system employs pulsed distributed feedback (DFB) QCLs to provide frequencies in the mid-infrared wavelengths targeting specific molecular ro-vibrational lines for simple direct absorption spectroscopy, while a custom DSP implementation provides integrated control and processing functions without sacrificing performance. Results show comparable sensitivities compared to traditional techniques, while simultaneously producing compact, robust, low power solutions.
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    Laser spectroscopic trace chemical sensors for environmental sensor networks and portable medical devices
    (2008) So, Stephen G.; Tittel, Frank K.
    This thesis represents the development of the first laser spectroscopy based trace-gas sensors with sensor characteristics which simultaneously satisfy low cost, handheld footprint, low power, and long term autonomous operation while still providing part-per-billion detection sensitivity and negligible interference to enable trace gas sensor networks and wearable sensors. In order to realize these demanding criteria, this work describes the development of a complete laser spectroscopic sensor platform from the ground up to determine all of the tradeoffs inherent to photonic chemical sensing, and presents a sensor platform with a configuration to meet as many application requirements as possible. Specifically, complete photonic sensor integration and design optimization (e.g. digital signal processing, low power analog, digital control technology, high speed digital design, efficient programming, infrared laser technology, mechanical design) provides sensor characteristics which are significantly improved over the current sensor technology. These sensors can permit the portable deployment of trace gas sensors and enable applications previously unattainable with any other gas sensing method. A performance comparison of the various different types of sensors measured according to these new metrics of cost, size, power consumption in addition to standard metrics (such as sensitivity and specificity) will provide a complete description of advantages and disadvantages of each trace gas sensing technique. Performance characteristics of an open-access handheld sensor platform also provide the baseline for comparison in terms of all of these new criteria. This work will also detail the development path of each major sensor component to allow new technologies to update the original modules. This thesis also describes a scalable network of high sensitivity trace gas sensors, something which has not been achieved to-date. Additionally, issues such as variable-power consumption sensor management and gas sensor data harvesting and analysis will be addressed. Several new applications will be described which may be performed with the optimized sensors which were difficult to perform previously. Finally, this thesis will extrapolate future optimal sensor configurations based on current research in MEMS, photonics, networking, integration, and sensing and will conclude with a discussion of the impact of the various advances achieved in this work.