A novel handheld Fourier-transform infrared spectrometer (FTS) system, currently in development, is described. Estimated
and measured performance data presented here are based on modeling and preliminary testing. The basic instrument will be
useful for a variety of sensing applications, including chemical agent detection. One novel aspect is a refractively-scanned,
field-widened interferometer, providing, in a miniature footprint, energy equal to a laboratory spectrometer. A second novel
aspect is the use of solid-phase extraction to concentrate airborne chemicals for infrared detection. FTS instruments provide
a powerful approach to identification of chemical and biological substances. The specificity is very high, while the
sensitivity varies with sampling interfaces and detection methods. Cost, size, sensitivity and weight have impeded the
widespread deployment of FTS systems. Cost can be reduced by a variety of means, including improved designs, mass
production, and the on-going electronics and manufacturing revolutions. Size can be addressed by the use of field widening,
which has been known for many years, though seldom used. Photoacoustic detection provides a very low-cost and relatively
sensitive sampling interface. Modeling indicates that the sensitivity can reach part per billion to part per trillion levels.
There is a critical need throughout DoD, the U.S. government, and the commercial sector for cost-effective monitoring systems to detect airborne biological warfare (BW) agents. At present, solutions for this sensor need are relatively expensive and have a high false alarm rate. Manning Applied Technology is developing a compact, portable bioaerosol sampling system for continuous monitoring of air quality, both at field locations and fixed installations. The instrument is premised on optical interrogation via a multi-step process. The first step is electrostatic concentration, to improve detection limits. An advantage of electrostatic particle concentration is the power efficiency, relative to impactors, cyclones and filter-based systems. The second step is presentation for particle analysis, which would employ one of several unique FT spectrometer designs. The advantage of spectroscopic interrogation of bioaerosol particles is the very low cost of each analysis, with no consumables required. It is thought that mid-IR and THz frequency ranges offer the best potential for accurate discrimination. The third, optional step, is archiving the collected particles for further analysis. To reduce component costs in the Fourier transform spectrometer, an optical replication process has been developed and tested, with promising results. The replication and optical testing methods are described in detail.
Manning Applied Technology is developing a series of compact and rugged interferometers with many applications, including chemical and biological agent detection. A nutating prism interferometer features high throughput, very rapid-scan capability and resistance to vibration. A novel multiplex Fabry-Perot interferometric spectrometer is especially compact and efficient, transferring about twice the modulated optical power to a detector, compared to a Michelson interferometer of equal volume. The bilithic interferometer features high throughput, intrinsic tilt compensation and compact size with the potential for high resolution measurements. For all of these interferometer geometries, digital signal processing (DSP) hardware and software are used to enhance the performance and allow rapid processing of data, including chemometric discrimination. Enhancements to system performance provided by DSP include application of Brault's data processing approach, frequency stabilization of a miniature solid state reference laser and computational correction of tilt errors. A key advantage of the DSP enhancements is compensation for non-ideal behavior of optical hardware. Many of the novel instruments described herein employ
intrinsic optic tilt compensation. Either type of tilt compensation provides greater photometric accuracy and stability. Preliminary NESR characterization is reported for two of the instruments.