The detection and measurement of gas properties has become essential to meet rigorous criteria of environmental unfriendly emissions and to increase the energy production efficiency. Although low cost devices such as pellistors, semiconductor gas sensors or electrochemical gas sensors can be used for these applications, they offer a very limited lifetime and suffer from cross-response and drift. On the contrary, gas sensors based on optical absorption offer fast response, zero drift, and high sensitivity with zero cross response to other gases. Hence, over the last forty years, diode laser spectroscopy (DLS) has become an established method for non-intrusive measurement of gas properties in scientific as well as industrial applications. Wavelength modulation spectroscopy (WMS) is derivative form of DLS that has been increasingly applied for making self-calibrated measurements in harsh environments due to its improved sensitivity and noise rejection capability compared to direct absorption detection. But, the complexity in signal processing and higher scope of error (when certain restrictions on operating conditions are not met), have inhibited the widespread use of the technique.
This dissertation presents a simple and novel strategy for practical implementation of WMS with commercial diode lasers. It eliminates the need for pre-characterization of laser intensity parameters or making any design changes to the conventional WMS system. Consequently, sensitivity and signal strength remain the same as that obtained from traditional WMS setup at low modulation amplitude. Like previously proposed calibration-free approaches, this new method also yields absolute gas absorption line shape or absorbance function. Residual Amplitude Modulation (RAM) contributions present in the first and second harmonic signals of WMS are recovered by exploiting their even or odd symmetric nature. These isolated RAM signals are then used to estimate the absolute line shape function and thus removing the impact of optical intensity fluctuations on measurement. Uncertainties and noises associated with the estimated absolute line shape function, and the applicability of this new method for detecting several important gases in the near infrared region are also discussed. Absorbance measurements from 1% and 8% methane-air mixtures in 60 to 100 kPa pressure range are used to demonstrate simultaneous recovery of gas concentration and pressure. The system is also proved to be self-calibrated by measuring the gas absorbance for 1% methane-air mixture while optical transmission loss changes by 12 dB.
In addition to this, a novel method for diode laser absorption spectroscopy has been proposed to accomplish spatially distributed monitoring of gases. Emission frequency chirp exhibited by semiconductor diode lasers operating in pulsed current mode, is exploited to capture full absorption response spectrum from a target gas. This new technique is referred to as frequency chirped diode laser spectroscopy (FC-DLS). By applying an injection current pulse of nanosecond duration to the diode laser, both spectroscopic properties of the gas and spatial location of sensing probe can be recovered following traditional Optical Time Domain Reflectometry (OTDR) approach. Based on FC-DLS principle, calibration-free measurement of gas absorbance is experimentally demonstrated for two separate sets of gas mixtures of approximately 5% to 20% methane-air and 0.5% to 20% acetylene-air. Finally, distributed gas monitoring is shown by measuring acetylene absorbance from two sensor probes connected in series along a single mode fiber. Optical pulse width being 10 nanosecond or smaller in the sensing optical fiber, a spatial resolution better than 1 meter has been realized by this technique.
These demonstrations prove that accurate, non-intrusive, single point, and spatially distributed measurements can be made in harsh environments using the diode laser spectroscopy technology. Consequently, it opens the door to practical implementation of optical gas sensors in a variety of new environments that were previously too difficult. / Ph. D. / The detection and measurement of gas properties has become essential to meet rigorous criteria of environmental unfriendly emissions and to increase the energy production efficiency. Although a lot of electrical gas sensors has been explored to meet these demands, they offer a very limited lifetime and suffer from cross-response and drift. On the contrary, gas sensors based on molecular spectroscopy offer fast response, zero drift, and high sensitivity with zero cross response to other gases. With the recent boom in telecomm sector, low cost diode lasers are now readily available for numerous applications. This makes them an excellent optical source for spectroscopy based gas monitoring. Hence, measurement of gas parameters using diode lasers (also known as <i>diode laser spectroscopy</i>) has become very popular over the last few decades. However, the harsh and rapidly changing conditions encountered in most industrial environments have inhibited its widespread use.
This dissertation presents novel strategies for practical implementations of diode laser spectroscopy systems. The proposed gas sensing system can simultaneously recover the concentration of a target gas and the ambient pressure at ultrahigh speed. It does not require any future calibration at installation site, which makes it quite ideal for applications like underground mine safety, monitoring combustion cycles in power plants, or monitoring leakage in natural gas pipelines. Furthermore, optical pulse generated by these diode lasers can be used to collect additional information regarding the location of gas leakage. This is demonstrated for measuring methane and acetylene gas in 60 to 100 kilopascal pressure range. Also, gas leakage location monitoring is proved by acetylene measurement from two sensor probes connected in succession along an optical fiber.
These demonstrations prove that accurate and non-intrusive measurements can be made using the diode laser spectroscopy technology even in harsh conditions. Consequently, it opens the door to practical implementation of optical gas sensors in a variety of new environments that were previously too difficult.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/84930 |
| Date | 06 March 2017 |
| Creators | Behera, Amiya Ranjan |
| Contributors | Electrical and ComputerEngineering, Wang, Anbo, Lester, Luke F., Xu, Yong, Hudait, Mantu K., Pickrell, Gary R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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