The paper reviews the area of tapered optical fibers which have significant potential as evanescent field sensing devices. They minimize some of the problems associated with any such device and can be much more sensitive to changes in fluorescence and absorption characteristics. This paper firstly discusses two evanescent field devices which are currently used and then describes the prototype tapered optical fiber device which has been fabricated to demonstrate the principal of operation. Results are presented using the dye Rhodamine 6G as the 'sensing' chemical. Problems associated with the device are considered. Two alternative tapered fiber arrangements are also presented.

We model interference effects in a D-fibre evanescent wave methane sensor and predict its influence on the performance of the system. System sensitivity in terms of minimum detectable methane concentration is expressed as a function of birefringence and spectral transmission characteristics of the D-fibre, polarisation dependent loss occurring after the sensing D-fibre, and polarisation and coherence characteristics of the light source. The sensitivity is calculated for various system arrangements and is found to agree with experimental results.

This manuscript summarizes the effort to demonstrate the feasibility of developing a field-portable Fourier Transform Infrared (FTIR) instrument that can perform a quick and accurate chemical analysis of unknown waste materials at Air Force bases without removing a sample for analysis. We report that devices containing a tapered infrared fiber optic sensor can remotely detect and quantify the range of liquid hazardous waste typically found at Air Force bases. Partial Least Squares (PLS) calibration equations were formulated and shown to accurately predict the concentration of components in a mixture with an error or +/- 0.05% volume.

Surface plasmon resonance forms the basis for several kinds of optical sensors, including immunoassays in which a local change in refractive index due to antigen binding causes a shift in the angle of minimum reflectivity from a thin metal film. Two factors will broaden the reflectivity curve: the finite size of the incident beam and any irregularities in the thickness of the metal film. In this paper, we estimate the extent of broadening due to finite beam width by using an angular spectrum approach, and we employ the numerical electromagnetic technique of finite-difference time-domain (FDTD) to analyze the effects of film roughness. It is seen that film irregularities lead to distinct changes in the reflectivity curve and to enhancement of the local electric fields at the surface of the film.

The determination of toxic metal ions in water using near-infrared compounds synthesized in our laboratories will be reported. Several near-infrared tetrasubstituted chloroaluminum 2,3-naphthalocyanine derivatives with spectral characteristics (absorbance and fluorescence) between 700 nm and 1000 nm have been used in these investigations. In the presence of metal ions the NIR dye's absorbance maximum undergoes a bathochromic shift of about 25 nm accompanied by changes in the fluorescence spectra along with molecular lifetime. The response of the NIR dye in the presence of several concentrations of toxic metal ions will be reported. The fluorescence intensity generated by the complex formed by the metal ion and the dye was monitored by (a) a modified commercially available spectrofluorometer and (b) an NIR instrument developed in our laboratories. The fluorescence intensity changes measured with the probe in the presence of metal ions can be used to construct a calibration curve for the monitoring of contaminants' metal ions in the environment. The effect of metal ions on the lifetime of the NIR dye as compared to the uncomplexed dye will be reported. The NIR instrument consists of a semiconductor laser diode, the NIR dye and a detector. The output wavelength of a 780 nm diode (used as the excitation source) matched the absorbance of these dyes and improved the detection limits of the analytes. Long term stability of the probe was investigated by a week-long period of observation. After one week the intensity varies by only 2%, suggesting suitably for long storage.

We describe the use of glass or plastic capillaries in optical chemical sensing. They possess several attractive features in that (a) they can act as a mechanical support for optically sensitive materials (coatings); (b), they represent an optical waveguide structure and enable various methods of optical interrogation; and (c) they can serve as a sample cavity of well-defined volume and are suitable for direct sampling, e.g. when drawing blood. The method of immobilization and the performance of such 'capillary sensors' is exemplified by a fast- responding carbon dioxide sensor chemistry. Its response times vary from 100 to 300 ms. Such capillary sensors have, in general, extremely short response times for three reasons, namely (a), because the two sensing layers passed by the light beam when penetrating the capillary can be made very thin; (b), equilibration of two thin layers is faster than that of a single layer of the same total thickness; and (c), the sample flow is laminar, resulting in a fast and homogeneous exchange of sample volume. We also give arrangements for optical readout devices and show that capillary sensors can overcome some of the problems of optrode membranes mounted in flow cells or on fiber tips.

Fiber to planar waveguide couplers have previously been investigated as rugged 'all fiber' refractometers. Here we report an investigation of the superstate index sensitivity of the resonant wavelengths for devices using readily fabricated polymer planar waveguide overlays with and without a thin metal layer (10 nm) coated onto the top surface of the polymer. For the metal coated structure the wavelength resonances associated with the TM polarization state demonstrated strong superstrate index sensitivity while the TE resonance positions were completely insensitive. Such a feature may be useful in terms of establishing a polarization referencing technique. In addition, an active device based on electro-optic polymer was realized and shown to be capable of providing a feedback mechanism for closed loop operation.

A renewable reagent liquid core waveguide (LCW) chemical sensor which used a membrane material as both the sampling element and waveguiding component has been developed. Both aqueous and nonaqueous reagent chemistries were used. The aqueous chemistries were modified by using an ethylene glycol/water mixture (refractive index approximately equals 1.38) which allowed light to be guided inside the tubular membrane material. Several fluoropolymer membrane materials (e.g., PTFE, PFA, FEP) as well as temperature effects on waveguiding were also examined. Two different applications were used to demonstrate LCW chemical sensing. Ammonia was detected using a bromothylmol blue reagent while trichloroethylene (TCE) vapor was detected using the Fujiwara (a base catalyzed pyridine) reagent chemistry.

A new optical analytical chemical method for measuring concentrations in liquids and gases is described. The method is based on measurements of light reflection from diffraction gratings in contact with the sample. Features in the reflection spectrum are directly related to the complex dielectric constant of the sample solution.

Sol-gel materials were developed for potential fiber optic applications as films for detecting dissolved H₂S and H₂S vapor. Modification of the optical properties of gels were recognized by simply changing a processing parameter such as the type of catalyst of the introduction of a second precursor. Other properties such as chemical durability and photostability were also evaluated. Finally, the fluorescence quenching of thionin doped gels by dissolved H₂S revealed a sensitivity and reversibility in the parts-per-billion regime. After performing these preliminary steps to assess the gel's integrity, the thionin doped gel is now read for H₂S monitoring.

A single layer hydrophobic polymer membrane is presented which is optically sensitive to carbon dioxide. It is immobilized on the inner surface of a glass capillary. The sensor material is based on ion pairing between an indicator anion (D^-) and a quaternary ammonium cation (Q⁺). The ion pair (Q⁺D^-), together with a quaternary hydroxide (Q⁺OH^-), is dissolved in silicone rubber, and the resulting material is shown to be useful for optically sensing carbon dioxide over the 0 to 100 hPa (0 to 76 Torr) partial pressure range. The detection limit is approximately 0.3 hPa. The silicone rubber based sensors are capable of measuring carbon dioxide both in gases (to which they respond with a t₉₀ of 1 sec) and in aqueous sample solutions (with response times ranging from 1 to 2 min); there is no need for an additional (proton-impermeable) coating. Due to the use of silicone rubber as the polymer support, there is no cross- sensitivity to pH within the pH 5 to 9 range. We found the inner surface of glass or plastic capillaries to be most suitable as a support for the sensor chemistry. Such capillaries can act as a mechanical support, they represent an optical waveguide structure and therefore enable various methods of optical interrogation, and they can serve as a sample cavity of well defined volume, which is suitable for direct sampling, or as a flow cell. Sensitivity, which determines the degree of response of the sensor to a given analyte concentration, is a vital parameter for the performance of a sensor. In this work, it is shown that the sensitivity of carbon dioxide sensors based on ion pairing largely depends on the molar (ion pair)/(free quaternary hydroxide) ratio. We have found that the sensitivity of such optrodes was well as their degree of linearity can be adjusted by varying the (Q⁺D^-)/$Q++)OH^-) ratio.

Two evanescent wave fiber optic sensors for oxygen are reported: (i) intensity-based, and (ii) based on phase fluorimetry. Both sensors employ the quenching by oxygen of the fluorescence form a ruthenium complex trapped in the cage-line structure of a sol-gel-derived porous film on a declad section of multimode optical fiber. The sensors exhibit excellent performance using excitation from new high brightness blue LEDs and establish the viability of low-cost portable sensor devices based on the sol-gel process. The data presented for the intensity-based sensor were obtained using an all solid state excitation and detection system. Preliminary results obtained by application of the intensity- based sensor to dissolved oxygen measurement are also reported.

Surface plasmon resonance (SPR) biosensing promises a sensitive technique for the rapid quantization of a variety of analytes of interest to environmental sensing, chem/biological warfare detection, and medical screening. For many applications a compact, handheld sensor would be advantageous, however present implementations are not well suited to miniaturization. Using a substrate mode waveguide to excite SPR, a miniaturized SPR system has been demonstrated that is small enough to fit in the palm of a users hand. Preliminary testing has demonstrated 7 nanomolar sensitivity for an immunoassay biosensor.

This paper presents an overview of the applications of antibody-based fiber optic sensors. Special focus is devoted to antibody-based fiber optics fluoroimmunosensors developed in our laboratories to detect important pollutants and biochemicals. The fiber optics sensor utilizes antibodies covalently bound or encapsulated on the probe tip. The usefulness of fluoroimmunosensors for environmental and biomedical monitoring will be discussed.

Cyanine fluorescent dyes which can be coupled to proteins have recently become available. They are excited in the 500-700 nm range and have large extinction coefficients which make them good candidates for sensitive immunosensor applications. We evaluated three of these dyes (Cy3, Cy5, and Cy5.5) in a direct fluoroimmunoassay on evanescent wave fiber optic biosensors. The biosensors differed in laser excitation sources and emission filters in order to accommodate dye requirements. The assay consisted of rabbit anti-goat IgG immobilized fiber probes being exposed to fluorophorelabeled goat IgG (gIgG). Upon binding of the dye-conjugated protein to the fiber, a signal was generated. When using the 514 nm laser device, Cy3-gIgG gave a substantially larger signal than TRITC-gIgG. On the 650 nm laser device, Cy5-gIgG provided very good signal, while that for Cy5.5 was moderate. Cy5.5 produced an excellent response on the 670 nm laser device. Use of these dyes provides a mechanism for improving the fiber optic biosensor by changing the excitation/emission region to an area of low background for clinical and environmental samples.

An optical fibre sensor for gastric pH detection is here described, making use of plastic optical fibres (POF) as light carriers and a proper electronic system for both source driving and signal processing. The use of a suitable microprocessor and an internal buffer allows the realization of a portable and reliable device, fed by batteries. The indicators, bromophenol blue (BPB) or thymol blue (TB), are immobilized on controlled pore glasses (CPGs) fixed at the end of POFs following a proprietary process. The optrode, satisfying clinical requirements, was tested in vitro and in vivo. Precision 0.05 pH units and response time ( 25 seconds) were reached.

A novel chemical analyzer is described in which an optical fiber is inserted into a transparent capillary tube, such that the inner diameter of the tube is only a few microns larger than the outer diameter of the fiber cladding. This configuration is referred to as a torus column. When a sample volume is introduced to the torus column at a low flow rate, propagated light is mode-filtered due to a change in the critical angle at the core/clad interface, as a result of in-situ extracted chemical species. Conventionally, chemical species extracted into the cladding are sensed as a change in the transmitted light at the end of the fiber. An alternative approach, measuring this mode-filtered light directly along the side of the fiber, is reported. The new approach has a signal-to-noise advantage over the conventional approach. The result is a low volume sensor that temporally separates, as well as detects, chemical species that partition into the fiber cladding. The temporal information enhances sensor performance, providing first order information for subsequent data analysis. We have examined the modulation of the critical angle by chemical species of interest at steady-state concentrations, and as transient concentration profiles that were shifted in time. In summary, the analyzer has chemical selectivity provided by differences in the refractive index, distribution coefficient, and transient time of the concentration profile of each chemical species in a sample. The chemical analyzer should be a promising tool for process and environmental monitoring.

The use of different ultraviolet lasers for fluorescence spectroscopic detection of water pollutants with fiber optical sensors has been studied. Especially detection of small aromatic hydrocarbons via laser induced fluorescence requires short wavelength excitation. Interaction of intense ultraviolet light with the commonly used fused silica fibers leads to a decrease of fiber transmission. Some transmission affecting laser parameters have been studied. A new concept for the use of fiber optic sensors with ultraviolet excitation has been developed. This method is based on transmission of visible laser radiation through the fiber and creation of ultraviolet radiation by optical harmonic generation at the distal end of the fiber. So the unfavorable fiber behavior at short wavelengths can be avoided. Simultaneous coupling of the beam from a single laser source into several optical fibers is interesting for distributed sensor applications. In order to minimize coupling losses for these purposes we developed a new coupling scheme based on a special optical lens array.

In our ongoing work developing a reversible optical chemical sensor (OCS) for CO, we have produced a greatly simplified and improved sensor design. Chemically impregnated porous optical fiber has been replaced with impregnated porous Vycor glass; a battery powered blue LED has replaced a compact Hg lamp and power supply; optical fibers are no longer necessary; The Vycor acts as both chemical transducer and optical waveguide. The CO OCS now consists of five components: a 9 V battery and a blue LED; a transition metal impregnated porous Vycor matrix/waveguide; a short pass interference filter or a colloidally colored glass bandpass filter; and a large area, resin coated Si photodetector. The CO OCS responds reversibly to the presence of [CO] in both air and in N₂, over the 40 to 950 ppm range at STP. The sensor has shown a lower limit of detection of approximately 40 ppm at STP. In terms of the transmitted intensity, the sensor is very slow in responding to very slow changes or buildups in the [CO]_ambient. Yet, in the presence of rapid changes in the [CO]_ambient, the sensor displays a 100% time constant of 60 to 70 seconds irrespective of the delta [CO]_ambient. The new design simplifies sensing greatly in that optical fibers (and the various problems associated with their use), bulky light sources, and extremely fragile porous optical fibers have been supplanted by small, durable, inexpensive, commercially available components. The sensor could find application in environments in which the [CO] can or does change rapidly.

Surface-enhanced Raman spectroscopy (SERS) results in million-fold enhancements of the Raman signal. SERS has recently been recognized as a promising platform for reversible, rapidly responding sensors. Techniques for monitoring ions, aromatics, and chlorinated ethylenes in aqueous solutions have been demonstrated. High specificity is achieved because of the distinct Raman signal of each analyte. A major obstacle to implementing this technology is the lack of appropriate SERS substrates compatible with optical-fiber excitation and collection. Typically, SERS spectra are collected from a monolayer of analyte over an area the size of a laser spot or slit image. The minuscule quantity of analyte results in SERS spectra with modest signal-to-noise ratios. We report a new technique for fabricating island-film SERS surfaces over larger areas. This technique is intrinsically compatible with optical fibers. The SERS signal will increase in direction proportion to the area or volume of analyte observed.

We have developed a method to prepare powdered phosphors which can serve as chemical sensors for a variety of gases. These sensors indicate the presence of the gas by changes in the photoluminescence (PL) intensity relative to N₂ as a reference gas. Specifically, we have found that the PL of the CdS:Te phosphor increases by as much as 50% when exposed to ammonia and decreases by 50% when exposed to sulfur dioxide. This phosphor also responds to a lesser degree to O₂ and water. We have also explored other common phosphors based on ZnS, Zn_xCd_1-xS, CaS, and SrS. The ZnS containing phosphors do not show any changes in PL upon exposure to either ammonia or sulfur dioxide while the CaS phosphors respond to sulfur dioxide and water.

Fiber optic chemical dosimeters are being developed for use in the remote detection of toxic rocket propellant vapors, (hydrazine and its derivatives, and nitrogen tetroxide) that are used at Air Force and civilian rocket launch sites. The dosimeters employ colorimetric indicators that react selectively and irreversibly with the propellant vapors to yield chemical compounds that absorb laser light launched into a fiber optic network. The dosimeters are fabricated by dispersing the reagent within either a porous cladding or a porous distal end coating, that is prepared by a low temperature sol-gel technique. Remote field- scale detection of hydrazine vapor in a few hundreds of ppb-min integrated dose regime has been demonstrated with a network that is approximately equals 1 kilometer in length and the use of a low power (10 mW) diode laser. We have also assembled a computer model of a multimode fiber optic dosimeter network for prediction of the operational capabilities of a multiplexed system containing 100 dosimeters. The model was encoded in both spreadsheet and BASIC formats. It was used to evaluate the performance of a field-scale, remote fiber optic detection system incorporating discrete chemical vapor dosimeters in serial, parallel, or hybrid serial/parallel topologies. Additionally, we have begun exploratory work utilizing chemical reagents that react reversibly with hydrazine vapor to develop hydrazine vapor concentration sensors that could be deployed in a similar fashion on a remote fiber optic network to detect hydrazine vapor in the ppb regime.