Fiber Optic Sensors For Detection of Toxic and Biological Threats

Abstract

Protection of public and military personnel from chemical and biological warfareagents is an urgent and growing national security need. Along with this idea, we havedeveloped a novel class of fiber optic chemical sensors, for detection of toxic and biologicalmaterials. The design of these fiber optic sensors is based on a cladding modificationapproach. The original passive cladding of the fiber, in a small section, was removed and thefiber core was coated with a chemical sensitive material. Any change in the opticalproperties of the modified cladding material, due to the presence of a specific chemicalvapor, changes the transmission properties of the fiber and result in modal powerredistribution in multimode fibers. Both total intensity and modal power distribution (MPD)measurements were used to detect the output power change through the sensing fibers. TheMPD technique measures the power changes in the far field pattern, i.e. spatial intensitymodulation in two dimensions. Conducting polymers, such as polyaniline and polypyrrole,have been reported to undergo a reversible change in conductivity upon exposure tochemical vapors. It is found that the conductivity change is accompanied by optical propertychange in the material. Therefore, polyaniline and polypyrrole were selected as the modifiedcladding 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 showeddramatic improvement in the sensor sensitivity, when the MPD method was applied. In thispaper, an overview on the developed class of fiber optic sensors is presented and supportedwith successful achieved results.

Keywords: Biological Sensors; Chemical Sensors; Fiber Optic Sensors; Fiber Optics; Gas Sensors; Home Land Security; Sarin Detection.

Figure 1.

Switching between emeraldine base (insulating form) and emeraldine salt (conducting form) by HCl and ammonia, respectively.

Figure 2.

a) Polypyrrole structure and b) Polypyrrole structure in its oxidized form.

Figure 3.

Polyaniline film response to Ammonia and HCl

Figure 4.

Polypyrrole response to A-as deposited, B- after exposure to hydrazine, C- after exposure to hydrogen-peroxide.

Figure 5.

Polypyrrole film response to DMMP.

Figure 6.

Schematic of the optical fiber sensor design showing the modified cladding region.

Figure 7.

The general block diagram of the developed characterization method.

Figure 8.

The 2-D image and the horizontal intensity profile of the far-field pattern measured at the center before (a) and after (b) the presence perturbation.

Figure 9.

Schematic drawing of the fiber etching setup.

Figure 10.

Light intensity as the fiber cladding etched away in 16.33% HF solution.

Figure 11.

Experimental set-up for sensor testing.

Figure 12.

Sensor response to chemical vapors of HCl and NH₃. The sensing fiber was coated polyaniline by in-situ deposition. The deposition time was 30 minutes.

Figure 13.

Sensor response to chemical vapor of hydrazine (H₄N₂).

Figure 14.

Sensor response to chemical vapors of hydrogen peroxide (H₂O₂) and hydrazine (H₄N₂).

Figure 15.

Sensor response for polypyrrole coated fiber upon DMMP exposure.

Figure 16.

Sensor Response of Doped Polypyrrole for (a) HCL, (b) NDSA, and (c) ASQA dopants.

Figure 17.

Influence of dopant concentration on sensor response.

Figure 18.

Far-field ring pattern of MPD: (left) after sensor exposure to HCl vapor and (right) after sensor exposure to NH₃ vapor.

Figure 19.

Normalized intensity profiles of the 2-D ring pattern.