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Optics & Infrared Sensing

Optics & Infrared Sensing

Laser Photoacoustic Spectroscopy (LPAS)

Figure 1  Essential building blocks of the L-Pas system: Quartz tuning fork on the left and the Quantum Cascade Laser on the right.
Figure 1 Essential building blocks of the L-Pas system: Quartz tuning fork on the left and the Quantum Cascade Laser on the right.
Figure 2  A prototype design of a QCL based LPAS sensor
Figure 2 A prototype design of a QCL based LPAS sensor

PNNL has demonstrated QPAS's ability to detect gaseous nerve agent surrogates. The technique is called Quartz Laser Photo-Acoustic Sensing (QPAS) and is now ready for prototyping and field testing. The QPAS technique is based on Laser Photo-Acoustic Sensing and infrared Quantum Cascade Lasers. Laser photoacoustic sensing is an exquisitely sensitive form of optical absorption spectroscopy, where a pulsed laser beam creates a brief absorption in a sample gas, which in turn creates a very small acoustic signal. In the QPAS technique a miniature quartz tuning fork acts as a "microphone" to record the resulting sound wave. The QPAS sensor is an extremely sensitive chemical detection platform that can be miniaturized and yet is still practical to operate in field environments.

With respect to component size, several QCLs can fit on a 3 x 3 millimeter chip. And the tuning forks are identical to the kind used in wristwatches. A conceptual design for a battery-operated, prototype QPAS array sensor, which includes 10 pairs of QCLs and tuning forks, would fit into a briefcase that is 12 inches long, 12 inches wide and 6 inches high and the entire thing would weigh less than 15 pounds.

System tests were performed using diisopropyl methyl phosphonate (DIMP), which is a chemical compound similar to sarin. QPAS detected DIMP at the sub-part-per-billion level in less than one minute. The miniscule level is similar to letting one drop of liquid DIMP evaporate into a volume of air that would fill more than two Olympic-size swimming pools.

Multiple QCLs were paired with the tuning forks, producing a QPAS sensor array, allowing simultaneous examination of a single sample at many infrared wavelengths. Nearly every molecule has unique optical properties at infrared wavelengths between three and 12 micrometers, and QCLs provide access to any wavelength in this region. Additional technologies used in QPAS are a small volume thermally desorbed preconcentrator tuned for organophosphorus compounds and an advance chemometric analysis technique for discriminating between signal from target molecules and signal from interfering species.

QPAS is currently at Technology Readiness Level "four," meaning that while the technical components exist and initial testing is complete, the system still must be converted to a prototype. Part of the research was done in collaboration with Rice University, Houston, Texas, where a portion of the QPAS technology originated.

Development of the technology was funded by the Defense Advanced Research Projects Agency.

Optics & Infrared Sensing

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