Robert A. Lieberman

A distributed fiber optic sensor based on cladding fluorescence

The fiber for the sensor is formed by cladding fused silica during drawing with polydimethyl siloxane into which an organic fluorescent dye, 9, 10-diphenylanthracene, has been dissolved. Upon side illumination at a wavelength within the excitation range of the dye, the cladding fluoresces; some of this fluorescence is coupled into guided modes in the fiber core through the evanescent fields of these modes. In the presence of oxygen, fluorescent emission by the dye is diminished. For the sensor described, the rubbery liquidlike nature of the polydimethyl siloxane cladding allows rapid diffusion of gases, and the intensity of the guided fluorescence is observed to drop by 30% in less than 5 s when the ambient atmosphere changes from pure nitrogen to pure oxygen. The advantages of this sensing technique, and some of the possibilities for new sensors based on this principle, are discussed. >

Rare-earth doped optical fibers for temperature sensing

The feasibility of using a variety of rare-earth doped optical fibers for measuring spatially averaged temperatures from approximately 0 to approximately 100 degrees C over distances of 10 to 20 m is discussed. Such distributed temperature sensors would be particularly well-suited for building climate control systems and industrial processing applications. The temperature-dependent absorption spectra of 6 MCVD processed fibers containing different concentrations of Nd/sup 3+/, Pr/sup 3+/, and Yb/sup 3+/ rare-earth ions were characterized and used to determine thermally active dopant species, optimal dopant concentrations, and most sensitive operating wavelengths for use as dual wavelength distributed temperature sensors. >

A distributed fiber optic chemical sensor for hydrogen cyanide detection

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 fibers 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.

Distributed fiber optic chemical sensor for hydrogen sulfide and chlorine detection

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 sensor for nerve agent

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 sensor coatings with enhanced sensitivity and longevity

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.

Recent advances toward a fiber optic sensor for nerve agent

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 fibers 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.

Polymer waveguide sensor arrays for enhanced multichemical detection

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.

Intrinsic chemical sensor fibers for extended-length chlorine 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.

Recent progress in intrinsic fiber optic chemical sensing

An overview is given of intrinsic fiber-optic chemical sensing encompassing refractometric sensors, evanescent spectroscopic sensors, coated-fiber sensors, and core-based sensors. Specific attention is given to techniques that are being tested such as commercial fluoroimmunoassay probes, evanescent-wave sensors for monitoring composite curing, and liquid-level monitors based on refractometric sensors. Of particular interest in the realm of research are intrinsic fiber devices that employ dyes and similar intermediaries to enhance the response of the sensors. Sensors that employ such intermediaries are expected to provide greater sensitivity to target analytes. The sensor techniques presently under development are also concluded to be useful for air-quality monitoring, and specific projects are discussed that are aimed at sensing gaseous species.