U.S. Army researchers have developed a chemical sensor that is designed to detect low levels of dangerous toxins in the air by their sound.
The new system updates technology based on photoacoustic effect, in which the absorption of light by materials generates characteristic acoustic waves. By using a laser and very sensitive microphones — in a technique called laser photoacoustic spectroscopy (LPAS) — very low concentrations of gases can be detected.
Traditional LPAS systems have a major drawback in that they can identify only one chemical at a time, prompting the researchers to pursue a method that would allow simultaneous detection of multiple agents. “As I started looking into the chemical/biological detection problem, it became apparent that multiple LPAS absorption measurements — representing an ‘absorption spectrum’ — might provide the added information required in any detection and identification scheme,” said Kristan Gurton, an experimental physicist at the U.S. Army Research Laboratory (ARL).
To create such a multi-wavelength LPAS system, the ARL team designed a specialized photoacoustic cell that allows different gases to flow through while modulated laser beams propagate through the cell. Each laser is each modulated at a different frequency in the acoustic range.
As gas vapors flow into the cell, a portion of the laser power is absorbed, resulting in localized heating of the gas. “Since gas dissipates thermal energy fairly quickly, the modulated laser results in a rapid heat/cooling cycle that produces a faint acoustic wave,” explains Gurton. Each laser in the system will produce a single tone, picked up by small microphones in the cell. “Different agents will affect the relative ‘loudness’ of each tone,” says Gurton, “so for one gas, some tones will be louder than others, and it is these differences that allow for species identification.”
The signals produced by each laser are separated using multiple “lock-in” amplifiers each tuned for a specific laser frequency. By comparing the results to a database of absorption information for a range of chemical species, the system can identify each of the chemicals present.
The method allows for instant identification of agents, as long as the signal-to-noise ratio, which depends on both laser power and the concentration of the compound being measured, is sufficiently high, and the material in question is in the database.
Before such a sensor could be used in the field, Gurton says, a quantum cascade (QC) laser array with at least six “well-chosen” mid-infrared laser wavelengths would need to be available. With such a set-up, the method “could be tailored for a variety of detection scenarios ranging from the obvious need to protect our soldiers during conflict to civilian applications like detecting the presence of harmful chemical gases that are difficult to detect with conventional techniques.”
A paper by the researchers entitled “Selective real-time detection of gaseous nerve agent simulants using multi-wavelength photoacoustics,” has been published in the Optical Society’s journal, Optics Letters.