Development of a New Gas Sensing System Based on Terahertz Time-Domain SpectroscopyEPA Grant Number: R827122
Title: Development of a New Gas Sensing System Based on Terahertz Time-Domain Spectroscopy
Investigators: Mittleman, Daniel M. , Baraniuk, Richard G.
Institution: Rice University
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001
Project Amount: $299,817
RFA: Exploratory Research - Environmental Engineering (1998) RFA Text | Recipients Lists
Research Category: Engineering and Environmental Chemistry , Sustainability , Land and Waste Management
Description:Spectroscopic methods for the sensing and identification of gases have shown great promise, owing to their inherent non-invasive nature, relative simplicity, and high selectivity. The use of far-infrared or terahertz (1 THz = 1012 Hz, corresponding to a wavelength of ~300 µm) radiation for sensing purposes, though extremely valuable as a complement to well established mid-infrared technologies, has not achieved great success. This is largely due to the complexity of the instrumentation required for generation and detection of terahertz radiation, which renders existing spectrometers impractical for most real-world sensing applications. The objective of this research is to build a portable broadband spectrometer based on the recently developed technique of terahertz time-domain spectroscopy (THz-TDS).
Approach:The task of constructing a rugged, reliable, and portable THz-TDS system can be divided into two broad areas, both of which are addressed in this proposal. The first deals with hardware aspects, particularly the femtosecond laser system required for THz generation. The THz-TDS technique must be adapted for operation with a mode-locked fiber laser. This will require the development of new methods for THz generation, involving either new semiconductor materials or efficient frequency doubling of the fiber laser. Also, novel chirped-pulse electro-optic sensing techniques will be developed for compatibility with fiber coupling. The second involves the development of algorithms for signal processing of the THz waveforms. Methods based on wavelet processing are expected to be particularly well suited for these signals, owing to the strong resemblance of the measured waveforms to elements of a wavelet basis.
A portable and reliable far-infrared spectrometer will be built and tested. This system will be driven by a mode-locked fiber laser, and will therefore be largely insensitive to external perturbation, optical alignment issues, etc. The newly developed signal processing procedures will be incorporated into the software used to drive the real-time data acquisition system. Collection of training data on a number of gases of interest will begin. This will be useful for subsequent tests for determining the sensitivity of the system, as well as for an assessment of the progress of this project. This prototype system will be suitable for field tests in a wide range of industrial environments.
Improvement in Risk Assessment or Risk Management: The development of a commercially viable far-infrared gas sensing system will be very much complementary to the well established techniques based on vibrational spectroscopy. Whereas those systems are extremely good at sensing numerous smaller molecules, such as many greenhouse gases, they often find difficulty in identifying larger molecules, for which the vibrational ?fingerprint' region exhibits highly complex and structured spectra. The far-infrared spectra of these molecules are often far less complicated, as only the rotational degrees of freedom are involved. As a result, identification based on terahertz ?fingerprint' spectroscopy is often easier with larger polar molecules. Many candidate molecules are commonly used in industrial applications, including solvents such as acetonitrile, acetone, and trichloroethane, halogenated benzenes, and many chlorofluorocarbons. Industrial users are required to monitor the release of these species into the environment, and will undoubtedly benefit from the availability of a real-time monitoring system such as the one whose development is proposed here.