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Spotlight on Optics


  • August 2013

Optics InfoBase > Spotlight on Optics > Segmented chirped-pulse Fourier transform submillimeter spectroscopy for broadband gas analysis

Segmented chirped-pulse Fourier transform submillimeter spectroscopy for broadband gas analysis

Published in Optics Express, Vol. 21 Issue 17, pp.19743-19749 (2013)
by Justin L. Neill, Brent J. Harris, Amanda L. Steber, Kevin O. Douglass, David F. Plusquellic, and Brooks H. Pate

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Spotlight summary: Instruments that sensitively detect and discriminate between chemicals in gases are important in modern society. Amongst many applications, they allow measuring concentrations of atmospheric pollutants and in-situ monitoring of industrial processes. Neill et al. describe a technical breakthrough that will bring commercial applications of millimeter and submillimeter spectroscopy in gas analysis a step closer to reality.

Fourier transform microwave spectroscopy (FTMW) has been performed since the 1970’s but pulses that are rapidly swept through a range of frequencies, or “chirped”, on a timescale of microseconds were first used to obtain broadband CP-FTMW (chirped-pulse Fourier transform microwave) spectra very recently, in 2006. Workers engaged in fundamental research in chemical physics in universities and national laboratories have since seized upon a new generation of instruments with enthusiasm. The first significant outcome of the work by Neill et al. is that the described segmented CP-FT technique brings the necessary instrumentation within reach of many more by achieving a major cost reduction.

It has long been known that millimeter and submillimeter radiation can be used to detect and identify gases in the atmosphere. Many of the early pioneers of microwave spectroscopy worked on the initial development of radar before moving into university or national lab research environments and using spectroscopy to study chemical physics. The absorption of millimeter wave radiation by chemicals in the atmosphere clearly had consequences for the development and effectiveness of early radar systems. Application of the very latest methods of CP-FT spectroscopy to the detailed analysis of gas mixtures will be a technically-challenging enterprise even in the modern world.

Millimeter and submillimeter spectroscopy is highly diagnostic with respect to molecular structure. The physics that connects temperature with the absorptivity of light by molecules suggests that such frequencies may be well-suited to gas analysis applications at ambient temperatures. However, typical current applications of CP-FT spectroscopy involve very low measurement duty cycles of less than 1%. Whilst these are well-matched with the needs of current experiments in fundamental chemical physics, higher rates will be needed for sensitive gas analysis. Neill et al. demonstrate a method that allows a duty cycle orders of magnitude higher than any achieved previously. This innovation will support future experiments in gas analysis likely to have considerable impact both commercially and in fundamental research.

--Nick Walker

Technical Division: Light–Matter Interactions
ToC Category: Spectroscopy
OCIS Codes: (040.6070) Detectors : Solid state detectors
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(300.6240) Spectroscopy : Spectroscopy, coherent transient
(300.6310) Spectroscopy : Spectroscopy, heterodyne
(300.6495) Spectroscopy : Spectroscopy, teraherz

Posted on August 16, 2013

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