We report advances made on the development of a fiber optic nerve agent sensor having its entire length as the sensing
element. Upon exposure to sarin gas or its simulant, diisopropyl fluorophosphate, the cladding changes color resulting in
an alteration of the light intensity throughput. The optical fiber is multimode and consists of a fused-silica core and a
nerve agent sensitive cladding. The absorption characteristics of the cladding affect the fiber's spectral attenuation and
limit the length of light guiding fiber that can be deployed continuously. The absorption of the cladding is also
dependent on the sensor formulation, which in turn influences the sensitivity of the fiber. In this paper, data related to
the trade-off of sensitivity, spectral attenuation, and length of fiber challenged will be reported. The fiber is mass
produced using a conventional fiber optic draw tower. This technology could be used to protect human resources and
buildings from dangerous chemical attacks, particularly when large areas or perimeters must be covered. It may also be
used passively to determine how well such areas have been decontaminated.
We report the development of absorption-based waveguide sensors for the toxic industrial chemicals hydrogen cyanide, hydrogen sulfide, and chlorine. Polymeric materials formulated as colorimetric sensors have been engineered into miniature waveguide channels. The channels have dimensions 30x0.6x0.05 mm (LxWxH) and are patterned on glass substrates using a photolithography process. Subsequent light coupling was achieved using optical fibers. Enhanced sensitivity is observed owing to the increased path length as described by the Beer-Lambert law. When the individual sensors are challenged with the IDLH concentrations of their target gases they react instantaneously with response times (T90) less than 20 seconds. When tested simultaneously as an array, a predictable level of cross interference was observed. The cross interference indicates that the inclusion of a signal processing algorithm is required to selectively resolve the analytes and reduce or eliminate false alarms.
KEYWORDS: Sensors, Fiber optics sensors, Cladding, Nerve agents, Polymers, Signal attenuation, Fiber optics, Chemical analysis, Biological and chemical sensing, Chemical fiber sensors
We report advances made on the development of a fiber optic nerve agent sensor having its entire length as the sensing
element. The optical fiber is multimode, and consists of a fused-silica core and a nerve agent sensitive cladding. Upon
exposure to sarin gas, the cladding changes color, resulting in an alteration of the light intensity throughput. The fiber is
mass produced using a conventional fiber optic draw tower. This technology could replace, or be used with, a collection
of point-detectors to protect personnel, buildings and perimeters from dangerous chemical attacks.
Fiber optic sensors that utilize evanescent field interactions as a detection mechanism have proven to be quite sensitive.
We recently reported on the development of this type of distributed sensor for toxic chemicals such as HCN, H2S, and
Cl2. The optical fibers are multimode and consist of a fused silica core and an agent-specific chemically-sensitive
cladding. Upon exposure to the corresponding challenge gas, the cladding changes color, resulting in an attenuation of
the light throughput of the fiber. These fibers were produced in long lengths using conventional fiber optic draw towers.
However, failure mechanisms, such as indicator migration, crystallization, and oxidation, decrease the lifetime of the
sensors. We report on recent progress we have made in the effort to optimize the sensor longevity with respect to these
degradation mechanisms. The optimizations include covalent attachment of the indicators with the polymer cladding
during fiber processing, and the use of antioxidants to minimize degradation.
Fiber optic sensors having their entire length as the sensing elements for chlorine or hydrogen sulfide are reported here. The chlorine fiber consists of a silica core and a chlorine-sensitive cladding, and the hydrogen sulfide fiber has a hydrogen sulfide sensitive cladding. Upon exposure to the corresponding challenge gas, the cladding very rapidly changes color resulting in attenuation of the light throughput of the fiber. A one-meter portion of the chlorine sensor fiber responds to 10 ppm chlorine in 20 seconds and to 1 ppm in several minutes. The attenuation after 10 minutes of exposure is very high, and is dependant on both chlorine concentration and fiber length. A ten-meter portion of the hydrogen sulfide sensor fiber responds to 100 ppm hydrogen sulfide in 30 seconds and to 10 ppm in 1 minute. The high sensitivity suggests that the propagating modes of the light interact strongly with the cladding, and that these interactions are massively increased (Beers Law) due to the extended sensor length. This approach will supersede the current method of having a collection of point-detectors to cover large areas.
A fiber optic hydrogen cyanide (HCN) sensor having its entire length as the sensing element is reported here. The optical fiber is multimode and consists of a pure fused-silica core and an HCN sensitive cladding. Upon exposure to HCN gas, the cladding rapidly changes color, resulting in attenuation of the fiber's light throughput. The fiber is used to detect HCN at part per million levels, which suggests that the propagating modes of light interact with the cladding. The sensitivity of the fiber as a function of sensor length and challenge concentration will be reported. Prior to exposure, the fiber attenuation measures less than 1 dB/m, making it possible to detect hydrogen cyanide on a continuous length of fiber on the scale of tens of meters. This technology could replace the need for having a collection of point-detectors to cover large areas, and hence lends itself to building and perimeter chemical detection.
A fiber optic chlorine sensor having its entire length as the sensing element is reported here. The fiber consists of a silica core and a chlorine-sensitive cladding. Upon exposure to chlorine, the cladding very rapidly changes color resulting in attenuation of the light throughput of the fiber. A two-meter portion of sensor fiber responds to 10-ppm chlorine in milliseconds and to 1 ppm in several seconds. Furthermore, response to 100 ppb chlorine is realized in minutes. The high sensitivity suggests that the propagating modes of the light interact strongly with the cladding, and that these interactions are massively increased (Beers Law) due to the extended sensor length. The sensitivity to 1 ppm chlorine gas as a function of the length of fiber exposed between 0.3-30 meters is presented. The sensitivity to concentrations of chlorine from 0.1ppm-10ppm has been determined for a fixed 2 meter length of fiber. Pre-exposure fiber attenuation measures 70 dB/km (@ 633 nm) making it possible to detect chlorine on a continuous length of fiber on the scale of one hundred meters or more using standard detection methods (e.g. laser and photodetectors). This will replace the need of having a collection of point-detectors to cover large areas.
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