Optical fiber sensors have gained much attention in recent years. Optical fiber based chemical sensors often use a reaction chamber within which a chemical reaction involving the sensing species occurs. A color change may result from this chemical reaction and, with light passing through the reaction chamber, the light intensity can be modulated by this color change. Consequently, this change in light intensity can be used to quantify the sensing species present. In most of these chemical sensors, either one or two optical fibers will be used. If a single fiber is used, the signal derived from the chemical reaction is relatively weak. On the other hand, if either one or two optical fibers are used, a mirror-finished surface is usually required for the reflection of light to the detector. In this research, optical fiber sensors are constructed using two different types of fibers. One is a quartz fiber and the other is a plastic fiber. The plastic fiber is more flexible and can be bent or connected with a slant surface at the top of the fiber at 45 degree(s). Two types of sensors were constructed--a temperature sensor employing a thermochromic solution and a pH sensor using a pH sensitive dye. By using the two types of fiber, a mirror-finished surface is no longer necessary. The weak signal due to the use of a single fiber is also minimized.

A new fiber-optic method for quasi-distributed remote chemical sensing is described. The technique is based on using a wavelength selective Bragg-grating radiation coupler for localized excitation and detection of fluorescent species on or near the surface of the fiber cladding. A grating can be formed in germanosilicate fiber by exposing the core, through the side of the cladding, to an ultraviolet, two-beam interference pattern. With a suitable choice of index modulation, the grating can be designed to couple light efficiently from a guided mode into a narrow radiation beam. The index perturbation also enhances the capture of the emitted luminescence. Since the gratings are blazed to optimize the coupling efficiency for a desired wavelength band, each sensing region can be designed for a specific measurement. An array of identical sensors can be sampled by optical time domaifl reflectometry.

This paper describes a diaphragm-based, multi-wavelength fiber optic pressure sensor that is suitable for combustion monitoring and control applications. Through the simultaneous optical detection of sensor temperature and diaphragm deflection, temperature induced inaccuracies are significantly reduced. Design criteria for high temperature sensors are outlined and initial experimental results are presented.

Sensors for oxygen in liquids have been commonly used for clinical chemistry applications. Optical approaches to blood gas applications have been known for some time2. The opportunity for fiber sensor applications in this area is created by familiar fiber sensor advantages: small size, dielectric isolation, and the availability of a number of sensor chemistry systems3' 4. The overall sensor system is based on a viologen compound that changes absorbance as oxidation (or reduction) occurs. This compound responds to short wavelength light by darkening; the time for a return to the clear state depends on the amount of oxygen present.

A fiber optical sensor based on time-resolved, laser-induced fluorescence is presented for PAHs in water. Over a distance of 50 m, the time decays of different 3- to 6-ring PAHs were analyzed, and for 13 PAHs the detection limits are given. For deconvolution of multi- component mixtures, chemometrical methods were applied to complete time-resolved fluorescence emission spectra.

The use of a software implemented backpropagation neural network is reported for the qualitative and quantitative analysis of the fluorescence emission spectra from multicomponent mixtures of polycyclic aromatic hydrocarbons (PAHs) in solution. Analysis of two types of data is described. First, a backpropagation network is developed to determine the component concentrations in a ternary mixture of PAHs. The input data provided to the network consists of sampled two-dimensional (intensity vs. emission wavelength) fluorescence spectra. A second backpropagation network is investigated for the analysis of three-dimensional time resolved fluorescence emission spectra for a binary PAH mixture. Both of the networks are trained to recognize preselected compounds. Each trained network is then used to evaluate unknown emission spectra and to determine the presence and relative concentration of the compounds it has learned to recognize. Results from analysis of two-dimensional emission spectra show that the trained network was able to successfully identify the individual components and their concentrations in solutions containing mixtures of anthracene, chrysene, and acenapthene. Analysis of three-dimensional time resolved fluorescence emission data showed that individual components could be resolved in mixtures of two spectrally similar components (anthracene and chrysene). Although a network could also be trained to recognize anthracene and chrysene in binary mixtures using their two-dimensional emission spectra, use of three-dimensional time decay spectra reduced the learning time required to train the network by a factor of three.

Solution-cast membranes were incorporated into a renewable reagent, optical fiber-based chemical sensor for the determination of chlorinated hydrocarbons. Full spectra were collected over time to provide multivariate data. Some of the variables affecting the use of the sensor for multicomponent analysis are discussed.

We present a fluorescence-based sensing scheme for ammonium ion using nonactin as the receptor/carrier. The ammonium-sensitive material consists of a pvc membrane containing the nonactin, a plasticizer, and a proton carrier (lipophilic nile blue). Sensing is based on the selective extraction of the ammonium ions into the pvc membrane, and a concomitant release of a proton from the dye, contained in the pvc membrane, into the sample solution. Upon deprotonation, the dye undergoes a color change from blue to red which is detected via fluorescence intensity measurements. The sensor fully reversibly responds to ammonium ion, with detection limits varying from 0.03 to 10 mM at near neutral pH. Response times are about 1 min for membrane thicknesses of ca. 2 micrometers . The sensor is fully compatible with solid-state electronics (i.e., LED light sources and photodiode detectors) but suffers from a strong pH dependence of the signal so that the pH of the sample must be kept constant. The sensor is of potential utility in monitoring ammonium ion in wastewater and surface water samples, but may also act as a transducer in all kinds of biochemical reactions during which ammonium ion is released or consumed. Examples of biosensors for urea and creatinine are given.

A renewable-reagent fiber optic chemical sensor has been developed for measuring the partial pressure of carbon dioxide in ocean waters. A colorimetric pH indicator is delivered at a controlled flow rate through capillary tubing to the fiber tips. The fibers measure the color changes induced by CO₂ diffusion across a gas-permeable silicone membrane. Multi- wavelength detection is used to correct for system drift. Factors affecting the sensitivity and stability of the response are discussed.

This paper describes an extensive spectrophotometric study on several acid-base indicators immobilized on controlled pore glass by means of a silylation process. The spectrophotometric properties of the immobilized indicators are at variance with those of the free indicators as far as pH dependence and ranges are concerned. The differing of the spectrophotometric properties of the indicators, when the latter are immobilized on solid supports, can be justified from the point of view of steric hindrance and of the reactivity of the indicators. Particular attention was paid to the pH-sensitive bands and their position. The effect of ionic strength is also investigated. Early tests performed with optical fibers are described.

Painstaking spectrophotometric investigation was carried out on organo-metallic compounds in order to identify the most suitable one for the detection of oxygen. A bis(Histidinato)Cobalt(II) solution was chosen, characterized by an absorption change at (lambda) equals 408 nm as a function of the molecular oxygen concentration in the surrounding environment. An optical fiber system was developed, which makes use of a halogen lamp as the source and an appropriate electronic circuit as a detection system. The optrode is a very small hollow fiber membrane filled with the solution and coupled with optical fibers.

The development of fiber optic cheraical sensors over the last decade2 has involved a number of conflicting priorities. Some of the most interesting are perhaps associated with the individual disciplines involved in the development process.

There are many environmental and process-control sensor applications where speed and reversibility are more important than selectivity. These might include monitoring the fate and transport of contaminants in laboratory columns and microcosms, measuring hydrocarbons in waste treatment streams, mapping the extent of a plume of a known contaminant, or monitoring solvent concentrations (e.g., acetone, dichloromethane, etc.) in a manufacturing process stream. In each case, there is prior knowledge of the type of contaminant and interferences, and in many cases the target molecule is at a relatively high concentration. For such applications, the sensor should be fast and reversible to allow real-time tracking of rapidly changing concentrations; however, it does not need to be very selective and might even respond to a wide variety of compounds. We are exploring new types of sensors for these applications. The sensors are based on reversible color changes that occur when solvatochromic (SV) dyes experience polarity changes. The optical sensors are made by immobilizing the SV dye in a polymer film that is coated on the end of a clad optical fiber or on the sides (i.e., evanescent region) of an unclad optical fiber. For the sensors described here interaction of a fluorescent SV dye with analyte vapors changes the amount of light absorbed by the dye and thus the fluorescence intensity that is measured with the optical fiber. Initial experiments have focused on fuel-related contaminants (e.g., xylene, benzene), and commercial solvents (e.g., acetone, dichloromethane--DCM). This paper will focus on the xylene and DCM results.

Various schemes are proposed for realizing compact, truly integrated optical sensors based on a single technology or at least a sequence of compatible processes for their fabrication. These totally integrated sensor modules not only eliminate the need for external components and their respective alignment, but also achieve reduced size and weight, improved stability, mass- production potential, and the possibility of single chip multi-element sensor arrays. Furthermore, they lend themselves to being incorporated into sensor networks.

An integrated-optic biosensor monitors the concentration of liquid pollutants on the surface of a planar substrate composing single-mode channel waveguides. The concept uses a Mach- Zehnder interferometer structure to measure thickness and/or refractive index changes on the waveguide surface. These changes occur as pollutant molecules interact directly with the interferometer's active arm or with a hydrophobic coating on the surface of the arm. Interferometer output data were obtained for various solutions including PPM levels of benzene and toluene in water. Theoretical analysis indicated that a hydrophobic coating on the waveguide would provide sensor specificity and detect pollutants at PPB levels.

Planar waveguides fabricated from triphenylmethane cyanide dyes doped into polymer films, Ag+ diffusion, and GaFChromicTM dosimetry media are evaluated as sensors for real-time radiation monitoring. Filtered 100 keV x rays are used as a model source to compare the attenuation response of these waveguide sensors to ionizing radiation. Preliminary results indicate that all three waveguides demonstrate an increase in sensitivity over conventional dosimetry films using UV-VIS photometric detection. The fabrication, characterization, and response of each sensor are discussed.

The finite-difference time-domain method (FDTD) is a powerful numerical technique for solving Maxwell's equations in a discretized space and time grid. Its applications have up to now been in the analysis of electrically large structures in the microwave domain, and the scope of investigations has been extended only recently to the optical region. Because of computer memory limitations, the method is generally restricted to configurations which extend to the order of tens of wavelengths in three dimensions, or hundreds of wavelengths in two dimensions. Optical sensor structures are therefore of suitable size to be modeled with FDTD, and e.g., fluorescence sensor design can benefit from the use of FDTD in optimization of the waveguide structures. In general, the integration of chemical and optical design is difficult, but FDTD can bring the two design approaches closer together. One of the main advantages of FDTD is its ability to include near-field effects, such as distribution of protein molecules on the active surface of optical sensors in the model, which has been shown to be important is estimating the fluorescent excitation and collection efficiencies of molecules on surfaces. In addition, for planar structures, two-dimensional models are adequate for studying many aspects of sensor design. We applied FDTD to design planar fluorescence sensors. Excitation and emission models were analyzed for planar waveguide structures with side collection of emitted light in mind. Planar waveguides were fabricated on fused silica substrates and the characteristics of the waveguides were compared to the model. Good agreement was found with the FDTD modeling to the physical model, and based on this knowledge, an FDTD sensor model was prepared predicting good fluorescence excitation and emission side collection efficiencies.

The responses of a polymeric optical waveguide sensor and a polymer coated Lamb wave piezoelectric sensor are used to characterize the chemical interaction of an analyte with the polymer layer. Information in the sensor response about polymer swelling, pore filling, mass loading, and changes in viscoelastic properties can be used to elucidate the interaction mechanism. Additionally, using two types of sensors in a mixed array increases selectivity by providing different information about the same sample. The complementary nature of these two sensors and their response mechanisms are explored.

The exact field solution of a step profile fiber was used to determine the injection efficiency of a bulk distribution of sources located in the cladding of an optical fiber. Previous results for weakly guiding fibers were confirmed. We found that, for a fixed V-number, the power efficiency Peff, increases parabolicaly with the difference in the indices of refraction, ncore-nclad. Also, Peff was found to increase with the wavelength (lambda) and decrease with the core radius a.

The exact field solution of a step index profile fiber was used to determine the injection efficiency of a thin-film distribution of polarized sources located in the cladding of an optical fiber. Previous results for random source orientation were confirmed. It was found that the behavior of the power efficiency, Peff, of a polarized distribution of sources is similar to the behavior of a fiber with sources with random orientation. However, it was found that for sources polarized in either the x or y direction, Peff is more efficient.

Using evanescent wave immunosensors, a fluorescent labeled analyte may be concentrated within the active surface region by a combination of diffusion to the fiber surface and trapping of the molecule or complex by an antigen-antibody affinity reaction. With the dye B- phycoerythrin, approximately 1.5 X 10-22 moles may be sensed over a 1 mm2 surface. From this number (as determined for a particular dye and instrumentation system) and antibody affinity constants, limits to assay sensitivity can be calculated and the kinetics modeled.

The development of new infrared transmitting optical fibers with low optical losses, sufficient mechanical strength, and temperature range to meet the demanding conditions of many process environments; and the availability of improved, ruggedized low-cost FTIR spectrometers have made in situ FTIR measurements possible. This paper discusses the development of evanescent wave in situ fiber optic remote FTIR spectroscopy and its application to assaying components of biological fluids.

On-line monitoring of the relative chemical content of fluid mixture is needed for a number of industrial applications. This paper describes a lens type refractometer which may be inserted on-line without affecting fluid flow and responds to changes in the refractive index of the fluid and, therefore, to fluid composition. The refractometer includes an optical fiber that delivers light to an optical clear cylindrical fixture through which the chemical mixture flows. The cylindrical fixture/liquid combination acts as a lens whose focal length depends on the refractive index of the medium. Light delivered by the optical fiber is focused on the other side of the lends, at a position that depends on the focal length. Two optical fibers collect the focused light at positions that are selected such as to maximize the sensitivity of the refractometer over the range of interest. For example, for a binary mixture, the tip of one fiber may be positioned at the focal point of the lends when it contains 100% of one chemical species, while the tip of the second fiber may be positioned at the focal point corresponding to the second species.

We are working toward the development of an instrument for the continuous on-line measurement of the lignin concentration in wood pulp. The system will be used as a primary sensor for process control in the pulp and paper industry. The measurement is based on the fluorescence properties of lignin. The instrument will use an optical fiber probe to separate the light generation, detection, and analysis equipment from the hostile process environment. To provide the background data necessary for the design of the instrument, we have characterized the fluorescence properties of lignin bound to wood pulp. A nitrogen laser was used as the excitation source and an intensified photodetector array capable of fast grating was used to obtain time-resolved fluorescence spectra.

A diode-laser-based portable Raman spectrometer is described that uses a 2-dimensional-array detector (charge-coupled device--CCD) to simultaneously measure chemical processes at several remote points using fiber optics. Optical multiplexing with the CCD allows simultaneous measurements with little loss of sensitivity and the low-energy near-visible (NVIS) excitation (e.g., 786 to 820 nm) prevents most sample luminescence. Several fiber- optic sensor configurations optimized for particular applications, including surface-enhanced Raman (SER), are presented. Applications are described including in-situ epoxy cure chemistry monitoring in a curing oven, monitoring a distillation apparatus, and mixed waste monitoring. Some of the limitations of the system are discussed.

Prior work has reported on the usefulness of fiber optic sensors in the detection of dissolved aromatic organic ground water contaminants such as the benzene, toluene, ethylbenzene, and xylenes (BTEX) fraction of petroleum fuels. Our device is a laser fluorimeter using fiber optic sensors for in situ measurements. Fluorescence intensity and lifetime can be measured at any wavelength in order to compare concentrations and discriminate certain compounds. Our instrument configuration uses a pulsed Nd:YAG laser twice frequency doubled to provide 266 nm excitation light. Excitation light, and subsequent induced fluorescence, are carried to the location of interest by a pair of 600 micron core fused silica optical fibers. Fluorescence is measured using a photomultiplier (PMT). PMT output goes either into a high-speed oscilloscope for fluorescence lifetime measurements or into a gated integrator for fluorescence intensity measurements. The same system can be used to detect aromatic contaminants in the vapor phase. Phenol, toluene, and xylene have been tested in the vapor phase. Concentrations below 10 micrograms phenol per liter air are detectable. In the vapor phase, the water Raman line seen around 295 nm in aqueous solution is insignificant, allowing a greater wavelength range to be scanned by the detector. Fluorescence spectra, fluorescence lifetimes, and fluorescence versus concentration information are presented. Applications of this approach to vadose zone ground water monitoring are discussed.

An optical-fiber methane sensor that uses a correlation technique based on pressure modulation is reviewed. The sensitivity to methane was measured and the cross-sensitivity to a number of contaminant gases, such as ethane, has been determined. The performance of this system is compared with conventional gas-absorption measurement techniques in order to determine a figure of merit for the selectivity improvement.

A fiber-optic hydrocarbon vapor sensor system was developed for environmental and industrial process monitoring in spill (alarm, level) and continuous (control) modes. The transducer consists of three optical fibers made reflective and optically coupled at their distal end, and mounted in a capillary. Light from an LED is launched in the source fiber, coupled to sensing and reference fibers at their distal ends, and fed back and monitored at the source. The optical transmission of a sensing fiber section is made sensitive to chemicals to be monitored and exposed to the ambient. Sensor system design and test results are presented. Its transductance can be altered easily by varying the geometry.

Fiber optic chemical sensors are being developed for on-line monitoring of gases and liquids. The sensors utilize novel porous polymer or glass optical fibers in which selective chemical reagents have been immobilized. These reagents react with the analyte of interest resulting in a change in the optical properties of the sensor. These sensors (or optrodes) are particularly suited to in-situ detection of atmospheric trace contaminants and dissolved gases and chemicals, as may be required for environmental monitoring. Sensors have been demonstrated for low part-per-billion level detection of aromatic hydrocarbons, hydrazines and ethylene. Sensors have also been demonstrated for carbon monoxide ammonia, and humidity. Also, relevant to groundwater monitoring is the development of an integrated pH optrode system for the pH range 4 - 8, with additional optrodes for lower pH ranges.

A U-shaped fiber optic sensor for measuring refractive index is applied to the monitoring of human skin condition. Since the fiber has its cladding removed at the U-shaped sensing tip, the output includes some information about the condition of the skin to which the sensor is applied. In this paper, the relation between the sensor output and the electric resistance of the skin is investigated. Besides this subject, the applicability of the sensor is investigated for humidity measurement utilizing water evaporation time or dew-point temperature.

One of the problems in testing the performance of (fiber) optical chemical sensors is to establish a reliable and reproducible test protocol in order to determine response time, reversibility, sensor stability, and longevity. A versatile arrangement for testing a variety of sensors is presented. It is based on the flow injection analysis (FIA) technique and was used to characterize new optical sensors for pH, potassium ion, and nitrate with respect to sensitivity, dynamic range, response time, specificity, and reversibility.

Different types of optical fibers including multimode fibers, single-mode fibers, and high- birefringence and low-birefringence polarization-maintaining fibers are discussed from the point of their application with liquid crystals for sensing devices and for logic gates in optical computing.

A fiberoptic sensor (FOS) has been developed. In recent years , progress has been made in the understanding the theory of laser absorption and emission and the electronic components and fiberoptic necessary to make up a useful instrument. The technique has been an analytical method of major importance in the areas of geochemistry , biomedical and blood gas analysis. According to the type of variables to be measured , the wide range classes of biomedical optical fiber sensor can be defined. Biomedical sensors can be used for oxygen saturation , pH, P02 , PCO2 measurement.

The measurement of pH in industrial chemical process is well established. Since pH can affect reaction rates, system corrosion, and/or water quality, a number of on-line electrodes have been commercialized. However, these devices necessarily contain a porous membrane (usually glass) to allow the flow of hydrogen ions which provides the electrical potential for measurement. In a recent application to a geothermal brine solution, we were unable to eliminate electrode fouling, even with considerable sample conditioning. As an alternative, we designed and constructed an in-line pH instrument based on the spectroscopic absorption of acid-base indicator dyes. The instrument introduced bromocresol green into the stream via a static mixer and measured the ratio of the characteristic acidic and basic absorption bands using an in-line fiber optic cell. Details of the design and its application are presented.

LPE growth at elevated temperatures has been used to fabricate high-quality InAsSbP and InGaAs p-n structures, emitting light in the wavelength range 2.5 - 4.7 micrometers . New IR LEDs and photodiodes based on InAsSbP and InGaAs have been tested and used for detection of CH4, CO2, and other gases. Transmittance spectra measurements have been carried out in the 3.1 - 3.5 micrometers spectral range using a 1 X 15 LED array, diffraction grating, and photodetector. The wavelength range used is optimized for near infrared transmittance analysis of hydrocarbons and some other gases.

An investigation of the suitability of monomode fluoride glass fibers for the sensing of fluid concentration by evanescent wave absorption in the mid-infrared spectral region is reported. The test system consisted of a HeNe laser ((lambda) equals 3.39 micrometers ), a polished semi- coupler, and a cooled PbSe detector. Evanescent wave absorption in isopropanol was measured for a range of cladding overlayer thicknesses. Results are in broad agreement with the predictions of a theoretical model based on a planar waveguide structure under similar conditions. Calculations based on this model indicate that more elaborate sensor designs are required in order to achieve sufficient sensitivity for the sensing of gaseous species.

A fiber optic chemical sensor has been designed for groundwater and vadose zone monitoring of volatile halogenated hydrocarbons. The principle of detection is a quantitative, irreversible chemical reaction that forms visible light absorbing products. This absorption is measured remotely using fiber optics. Modifications of our previous sensor design have resulted in lower detection limits and increased durability. In this paper we describe the measurement system and present the new sensor design along with calibration data and preliminary field test results.

Time-resolved luminescence spectroscopy is an effective method for the determination of a wide range of uranyl concentrations in aqueous samples. We have applied this technique to the development of a remote-sensing device using fiber optic cables coupled with a microflow cell to probe for uranyl in aqueous samples. This sensor incorporates a Nafion membrane through which UO22+ can diffuse into a reaction/analysis chamber containing phosphoric acid, a reagent that enhances the uranyl luminescence intensity and lifetime. With this device, anionic and fluorescing organic interferences could be eliminated, allowing for the determination of uranyl over a concentration range of 10-4 to 10-9 M.

Easy-to-use and cost-effective fiber optic sensor modules serving as an interface between fiber-optic sensors and signal processing circuitry are of paramount importance for broad acceptance of fiber optic sensing. Fully integrated PCB-mountable modules which incorporate all the functions necessary to drive amplitude modulated or wavelength encoded fiber optic sensors are discussed. They contain LEDs emitting on different wavelengths, wavelength division multiplexers receivers, and star-couplers to realize sensor networks.

Antigen binding properties and storage stability of immunoglobulin G (IgG) thin films deposited onto the quartz core of optical fibers are described.