Chemical-vapor sensor uses holographic interferometry

Scientists at the University of Colorado at Boulder (Boulder, CO) have developed an interferometric approach to chemical-vapor sensing that uses dynamic holography to interrogate an array of chemically selective polymers.

Sep 1st, 2005
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Scientists at the University of Colorado at Boulder (Boulder, CO) have developed an interferometric approach to chemical-vapor sensing that uses dynamic holography to interrogate an array of chemically selective polymers.1 The interrogation of a large array of transducer elements in parallel using holography leads to what the scientists call an “odor image”-a 1-D or 2-D image of the vapor response that can be analyzed using standard image-processing techniques for high-speed identification of chemical substances.

Though interferometric techniques are extremely sensitive to vapor-induced optical changes in chemically selective materials, high-quality wavefronts are needed and the susceptibility to long-term drift is high. To reduce these problems, the researchers use dynamic holography, which also allows them to look at the vapor-absorption kinetics and gain additional sensitivity for distinguishing vapors.

Their holographic interferometer uses a 532-nm laser source, designed to have a square cross section of uniform intensity. A half-wave plate and a polarizing beamsplitter cube split the beam into a reference beam and a sensing beam. The sensing beam reflects from two corners of a glass prism (using total internal reflection) in a horseshoe shape. A glass substrate with an array of polymer patches is placed on the second surface of the prism with the polymers facing away from the prism. The polymer patches are sealed inside a vapor chamber attached to the glass substrate with an o-ring, with a pair of small tubes providing a vapor inlet and outlet to the chamber. Two lenses image the plane of the polymer arrays into a photorefractive barium titanate crystal, into which the reference beam has also been sent after reflecting from a piezoelectrically controlled mirror. The sensing beam is imaged onto a CCD array and is also incident on a low-noise photodetector to make sensitivity measurements.

By arranging the polymer patches into a 3 × 3 array and subjecting them to different chemical vapors, the polymers respond to different chemicals and the image of the particular element becomes bright at the CCD (see figure). The image fades to dark again when the vapor flow establishes a steady state. In effect, the content of the vapor is recognized by analysis of its corresponding image.


A 3 × 3 array of chemically selective polymers is exposed to various vapors including benzene (left), dichloromethane (center), and ethanol (right). If a particular polymer responds to the vapor, its bright illumination creates a pattern or chemical signature that can be observed by the holographic interferometer and used to recognize the content of a vapor.
Click here to enlarge image

The vapor-concentration sensitivity of the holographic interferometer was assessed by alternating vapor flow of various concentrations of ethyl alcohol, water, and methyl alcohol and testing the response of a poly(N-vinyl pyrrolidone) material. The chemical change of the thick polymer film has a fast surface effect and a slower bulk-diffusion effect. The vapor flow is switched between the two alcohol types and water, with clean air fed in between that serves as a reference vapor. The switching frequency is approximately 1.5 Hz; the rise time of the response is observed to be much less than 1 s. By using standard synchronous detection techniques enabled by the fast response of the interferometer, the signal-to-noise ratio of the system can be enhanced. Using a 5-s averaging time, concentration levels less than 50 parts in 106 were detected for the alcohols and water vapor.

Improving sensitivity

To further improve sensitivity, the team used an enhanced detection technique that involves rapid modulation of the phase of the reference beam and synchronous detection at the frequency of the reference modulation and at the vapor modulation frequency. In this case, concentrations could be measured down to 40 parts per billion (ppb) or 40 parts in 109 in a5-s time period. For comparison, surface acoustic wave devices can detect at the 1- to 2-parts-per-billion level, but typically require 60 s for exposure and 100 s for recovery.

Dana Anderson, physics professor at the University of Colorado, indicates the system has not yet reached limits to sensitivity set by photon-counting noise and other fundamental considerations, so there is substantial room from improvement. He and his colleagues are working to improve sensitivity and to miniaturize the interferometer. Anderson says that the polymer-array substrate is low in cost and easily removed from and replaced on the interferometer prism, which makes the system attractive for field use where one might wish to change chemical selectivity of the device according to the task and environment.

AlphaSniffer (Boulder, CO), a company founded by Anderson and Misha Plam, president and CEO, has licensed the holographic-interferometer scheme from the University of Colorado. The company is developing an air/liquid monitor for toxic compounds in the air and bacteria and pathogens in water under an SBIR (small-business innovation research) grant from NASA. The same technology can be utilized for detection of chemical and biological warfare agents and toxic industrial compounds.

Gail Overton

REFERENCE

1. Hongke Ye et al., Optics Letters30(12) 1467 (June 15, 2005).

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