Mahmoud A. El-Sherif

Fiber Optic Sensors For Detection of Toxic and Biological Threats

Protection of public and military personnel from chemical and biological warfare agents is an urgent and growing national security need. Along with this idea, we have developed a novel class of fiber optic chemical sensors, for detection of toxic and biological materials. The design of these fiber optic sensors is based on a cladding modification approach. The original passive cladding of the fiber, in a small section, was removed and the fiber core was coated with a chemical sensitive material. Any change in the optical properties of the modified cladding material, due to the presence of a specific chemical vapor, changes the transmission properties of the fiber and result in modal power redistribution in multimode fibers. Both total intensity and modal power distribution (MPD) measurements were used to detect the output power change through the sensing fibers. The MPD technique measures the power changes in the far field pattern, i.e. spatial intensity modulation in two dimensions. Conducting polymers, such as polyaniline and polypyrrole, have been reported to undergo a reversible change in conductivity upon exposure to chemical vapors. It is found that the conductivity change is accompanied by optical property change in the material. Therefore, polyaniline and polypyrrole were selected as the modified cladding material for the detection of hydrochloride (HCl), ammonia (NH3), hydrazine (H4N2), and dimethyl-methl-phosphonate (DMMP) {a nerve agent, sarin stimulant}, respectively. Several sensors were prepared and successfully tested. The results showed dramatic improvement in the sensor sensitivity, when the MPD method was applied. In this paper, an overview on the developed class of fiber optic sensors is presented and supported with successful achieved results.

Fiber Optic Sensors and Smart Fabrics

This paper presents novel work on developing fiber optic micro-sensors and integrating them into soldiers’ uniforms. These fiber optic sensors can be used to sense various battlefield hazards in real-time, such as chemical and biological warfare threats, above-normal field temperatures, and other hazards. The developed fiber optic sensors use multifunctional materials as modified cladding materials, which can sense various environmental conditions. Appropriate materials can be chromogenic materials, chemical or biological agent, conducting polymer and others. The sensing function is based on their ability to change the light propagation characteristics of optical fibers. Two types of materials, (1) thermochromic material, segmented polyurethane-diacetylene copolymer (SPU) [synthesized at the University of Akron (UA)], and (2) conducting polymer, polyaniline [synthesized at the University of Pennsylvania (U of P)], have been successfully used in this feasibility study to prove the concept of the design and development of the optical fiber sensors. Segmented polyurethane-diacetylene copolymer was selected as the thermochromic material for temperature sensor application. Polyaniline was chosen as the photo-chemical polymer for chemical sensor application. The developed methodology for building these sensors includes modifying the regular optical fibers, by replacing the fiber passive cladding with those sensitive materials. The integration of the sensory system into textile structures, done in collaboration with the North Carolina State University (NCSU), also showed good progress. An overview on the developed methodology and technical achievement is presented.

Development of calibration techniques for ultrasonic hydrophone probes in the frequency range from 1 to 100 MHz

The primary objective of this work was to develop and optimize the calibration techniques for ultrasonic hydrophone probes used in acoustic field measurements up to 100 MHz. A dependable, 100 MHz calibration method was necessary to examine the behavior of a sub-millimeter spatial resolution fiber optic (FO) sensor and assess the need for such a sensor as an alternative tool for high frequency characterization of ultrasound fields. Also, it was of interest to investigate the feasibility of using FO probes in high intensity fields such as those employed in HIFU (high intensity focused ultrasound) applications. In addition to the development and validation of a novel, 100 MHz calibration technique the innovative elements of this research include implementation and testing of a prototype FO sensor with an active diameter of about 10 microm that exhibits uniform sensitivity over the considered frequency range and does not require any spatial averaging corrections up to about 75 MHz. The results of the calibration measurements are presented and it is shown that the optimized calibration technique allows the sensitivity of the hydrophone probes to be determined as a virtually continuous function of frequency and is also well suited to verify the uniformity of the FO sensor frequency response. As anticipated, the overall uncertainty of the calibration was dependent on frequency and determined to be about +/-12% (+/-1 dB) up to 40 MHz, +/-20% (+/-1.5 dB) from 40 to 60 MHz and +/-25% (+/-2dB) from 60 to 100 MHz. The outcome of this research indicates that once fully developed and calibrated, the combined acousto-optic system will constitute a universal reference tool in the wide, 100 MHz bandwidth.

Fiber optic chemical sensors using a modified conducting polymer cladding

A new class ofsensors has been designed and prepared based on replacing the original cladding material on a small section of an optical fiber with a conducting polymer or other environmentally sensitive material. Vapor induced chemical interactions with the polymer result in refractive index and optical absorption changes in the polymer cladding. These changes lead to an optical intensity modulation induced within the multi-mode optical fiber. Polyaniline and polypyrrole were used as the modified cladding material on the fiber core. An in-situ deposition method was used to produce uniform thin film coatings of the electronic polymer on the optical fiber. It was found that optimization ofthe sensor sensitivity can be achieved by selecting the proper incident wavelength, excitation conditions, and optical detection technique. Chemical sensors were developed and tested for detection ofHCl and NH3 vapors along with the reducing agent hydrazine. The results clearly demonstrate that conjugated polymer coated fiber optics represent a promising new approach for the detection of volatile toxic gasses.

Intrinsic fiber optic chemical sensor for the detection of dimethyl methylphosphonate

We report the early stage development of an intrinsic fiber optic sensor to detect the presence of nerve agent sarin simulant dim- ethyl methylphosphonate (DMMP). The sensor design is based on the modified cladding or coating approach. Conducting polymer polypyrrole is the chemo-optic transducer, i.e., is used as a modified cladding mate- rial. Sensitivity to approximately 134 ppm of DMMP is demonstrated in the developed sensor, with a sensor response of 20 mV and a response time of 2 sec. Morphology characterization of the polypyrrole is per- formed by scanning electron microscopy. Selectivity study of the devel- oped sensor is presented by exposing the sensing element to other gases like acetone and ammonia. Influence of temperature and humidity on the developed sensor is investigated, along with ambient aging of polypyrrole films.

A NOVEL FIBER OPTIC SYSTEM FOR MEASURING THE DYNAMIC STRUCTURAL BEHAVIOR OF PARACHUTES

A novel approach has been developed for real-time characterization of a parachute during inflation. Two techniques were applied to measure static and dynamic stresses in a parachute canopy fabric material and suspension lines: the modal power distribution (MPD) technique and the fiber Bragg grating (FBG) technique. Several tests were conducted under various loading conditions, using a bi-axial tensile tester. The MPD technique was used to measure the transverse stresses in the parachute canopy fabric material and was applied using two experimental set-ups: one using a CCD camera and the other using a photo-detector. The FBG technique was used to measure the axial stresses in the parachute canopy fabric material. The tests were performed using the spectrum analyzer set-up with the bi-axial machine. Finally, a drop test was performed on the parachute canopy fabric material using the MPD technique with the photo-detector set-up.

Intrinsic optical-fiber sensor for nerve agent sensing

A novel chemical-sensing technique to detect the nerve agent sarin stimulant dimethylmethylphosphonate (DMMP) is presented. This technique uses a combination of doped polypyrrole as an active chemical material coated on an optical fiber to form an intrinsic fiber-optic sensor. Sensitivity of up to 26 ppm of DMMP with response time of a few seconds is demonstrated. Influence of three different dopants, i.e., 1,5 naphthalene disulphonic acid, anthraquinone 2 sulphonic acid, and hydrochloric acid is investigated for sensor response and sensitivity. Two polymer processing techniques, i.e., in situ deposition and monomer vapor phase deposition is investigated for optimal polypyrrole morphology for DMMP sensitivity. The influence of substrate nature, i.e., hydrophilic and hydrophobic, on sensor sensitivity is studied. Organophosphate specific binding sites have been created in polypyrrole structure using Cu/sup 2+/ ions to enhance DMMP response. The selectivity issue is addressed by testing the sensor in the presence of other gases like ammonia, water vapor, and acetone which influence the electronic properties of polypyrrole.

Optical response of a sapphire multimode optical sensor for ceramic composite applications

The main objective of this study is the development of an embedded fiber optic sensor for testing ceramic composites in a very high temperature environment. The sensing element is an optical grade sapphire fiber operating on the principle of spatial modulation in a multimode waveguide. In order to employ this waveguide as a stress sensor, optomechanical testing has been performed to examine the optical response to external stresses. Several tests, including tension, micro-bending, and lateral compression, are in progress. These tests will establish the basis for using embedded optical sensors for characterization of ceramic composites in real environment. The principles of operation and experimental investigations on the microbending tests are presented in this paper. The results show that the developed sensor can be applied for stress monitoring as well as displacement measurements in a very high temperature environment.

Fiber optic neurotoxin sensor

Nerve agents are chemicals that attack the central nervous system. A release of a nerve agent has the potential to rapidly affect a large number of people. The majority of nerve agents belong to a class of compounds called Organophosphates. The presence of these compounds cannot be sensed by hydrolysis. A combination of a conducting polymer and optical fiber provides with an effective medium to sense organophosphates. This paper describes the development of an optical fiber sensor to detect the presence of organophosphate dimethylmethylphosphonate.

Development of on-fiber optical sensors utilizing chromogenic materials

On-fiber optical sensors, designed with chromogenic materials used as the fiber modified cladding, were developed for sensing environmental conditions. The design was based on the previously developed on-fiber devices. It is known that the light propagation characteristics in optical fibers are strongly influenced by the refractive index of the cladding materials. Thus, the idea of the on- fiber devices is based on replacing the passive optical fiber cladding with active or sensitive materials. For example, temperature sensors can be developed by replacing the fiber clad material with thermochromic materials. In this paper, segmented polyurethane-diacetylene copolymer (SPU), was selected as the thermochromic material for temperature sensors applications. This material has unique chromogenic properties as well as the required mechanical behaviors. During UV exposure and heat treatment, the color of the SPU copolymer varies with its refractive index. The boundary condition between core and cladding changes due to the change of the refractive index of the modified cladding material. The method used for the sensor development presented involves three steps: (a) removing the fiber jacket and cladding from a small region, (b) coating the chromogenic materials onto the modified region, and (c) integrating the optical fiber sensor components. The experimental set-up was established to detect the changes of the output signal based on the temperature variations. For the sensor evaluation, real-time measurements were performed under different heating-cooling cycles. Abrupt irreversible changes of the sensor output power were detected during the first heating-cooling cycle. At the same time, color changes of the SPU copolymer were observed in the modified region of the optical fiber. For the next heating-cooling cycles, however, the observed changes were almost completely reversible. This result demonstrates that a low-temperature sensor can be built by utilizing the chromogenic SPU copolymer as the modified cladding material.