A Brief review of the history and current state of the development of fiber optic chemical sensors will be presented.

The study and the development of an optical fiber sensor for entero-gastric and non-acid gastro-esophageal reflux is described. The working principle, based on the spectrophotometric properties of the bile, which constitutes the main part of the reflux, differs from the traditional measurement method, based on pH monitoring. The measuring apparatus is described as well as experimental "in vitro" and preliminary "in vivo" tests are reported.

A generic sensor for hydrogen peroxide producing metabolites was developed. The sensor consists of a small bore nylon tube fitting closely over two UV-transmitting fibers in an optrode configuration. In the case of glucose sensing, horseradish peroxidase and glucose oxidase were coimmobilized on the inside surface of the nylon tube. Glucose oxidase catalyzes the production of hydrogen peroxide from glucose and horseradish peroxidase catalyzes the reaction of hydrogen peroxide (11202) and p-hydroxyphenylacetic acid (HPA) into 6-6'-dihydroxy(1,11- biphenyl) 3,3'-diacetic acid (DBDA) which is measured by laser- assisted fluorometry. The amount of fluorescing DBDA is directly proportional to the amount of glucose present. Michaelis-Menten kinetics for soluble and immobilized enzymes were established for the H202/1-12A/HRP system to determine the range of hydrogen peroxide concentrations for which sensor linear response is observed. Sensors were found to be stable at 4°C in pH 7 phosphate buffer for a period of thirty days with no significant loss in enzyme activity. Glucose levels in buffer at physiological pH were measured and sensor linearity was established. Applicability of the sensor to the measurement of glucose levels in serum was established. Further studies will involve in vitro sensor calibration.

Fiber Optic Chemical Sensors (FOCS) permit real time, in situ environmental monitoring of selected pollutants without sample collection and at relatively low cost. The current emphasis on complex analytical laboratory based instrumentation requires sample collection and preservation (which may alter the sample) and long turn-around times. The concept of using in situ fiber optic chemical sensors (FOCS) for environmental screening or monitoring would permit real time analysis of selected pollutants without sample collection at relatively low cost. The current reliance on using complex laboratory-based analytical instrumentation designed for diagnostic analysis is not an efficient approach for routine monitoring or screening because it requires sample handling, long turn-around times, and highly skilled personnel. The development of FOCS systems would be a cost effective alternative for screening and monitoring for environmental pollutants. The scope and magnitude of the environmental monitoring needs both in the United States and globally are discussed along with the target chemicals most useful for ground water monitoring of hazardous waste sites. The rationale for different types of instrumentation for diagnostic, monitoring, and screening missions is discussed along with the different regulatory needs imposed by various American environmental laws. Performance specifications for FOCS are described. Some advantages and examples of present FOCS are given. The current status and future needs for R&D for FOCS are discussed.

An in situ, non-reversible fluorescence sensor (probe) for chloroform has been developed. It is based on a modification of the Fujiwara reaction which produces a red-fluorescent reaction product from chloroform. The reagent is stabilized with a variety of polymeric viscosity modifiers and placed in a small cell. It is covered with a membrane and placed in an aqueous chloroform solution, and the resulting fluorescence is monitored with a small precision filter fluorimeter. The design of an improved optical coupler between the analytical sample and the communication fiber is discussed.

In-situ measurement of aromatic ground water contaminants, including the benzene, ethylbenzene, toluene, and xylenes (BTEX) fraction of gasoline, has been demonstrated using fiber optic systems. A prototype field instrument has shown that this method has advantages over traditional sampling and analysis. Problems encountered and solved include coupling of the laser energy into to fiber, sensor design, and detector configuration to optimize instrument sensitivity. The effects of sensor length, corresponding to well depth, on limits of detection are presented. Effects of potential interferences, including external fluorescence quenchers, are discuss-ed. The resolution of complex mixtures is addressed, with modifications to the detector shown to be effective in separation of groups of contaminants. Instrument design considerations include the need for portability, ruggedness at field sites, and ease of operation. The modular instrument design used is shown to help solve these potential problems, while maintaining analytical sensitivity and reproducibility. Modular optical system design has also shown to be useful when modifications are made. Changes in the detector as well as provisions for multiple laser sources have allowed a flexible system to be configured to meet analytical demands as they arise. Sensor design considerations included high ultraviolet transmission, physical flexibility, resistance to breakage, and resistance to chemical and/or biological fouling. The approach to these problem areas is presented, as well as discussion of the methods used to minimize effects of fiber solarization. Results of testing the field portable prototype are presented for a variety of typical ground water analysis sites, illustrating the usefulness of this new technology in environmental monitoring.

Fiber optic remote sensing is a growing area of analytical chemistry. The application of such technology promises to augment much of today's process control, in-vivo medical diagnostics and environmental instrumentation. The use of optical fibers in chemical analysis is an attractive choice for many reasons. These include immunity to electromagnetic and RF interferences, low attenuation, ruggedness, and of course suitability for in-situ on line monitoring in hazardous and/or remote environments. Fluorescence spectroscopy is an inherently sensitive technique; however, single wavelength measurements can be limited for analysis of complicated multicomponent samples. In this regard, multidimensional fluorescence can be used to obtain additional information about the sample. In multidimensional fluorescence, the fluorescent measurement involves the use of multiple luminescence parameters to increase the specificity of the measurement. In this paper, the use of a newly designed fiber optic based multidimensional fluorometer to study the effects of various environmental pollutants on the spectral characteristics of marine phytoplankton is demonstrated.

A new class of fiber optic chemical sensor is presently under development at the Pacific Northwest Laboratory. The new probe type, termed fiber optic spectrochemical emission sensor (FOSES), implements either electrical spark or radio frequency excitation (via helium plasma) of analyte species at the probe tip for remote in situ detection. The two excitation modes offer complementary chemical detection methods suitable for vapors, aerosols, or liquid samples. The new probe concept offers the potential for multi-point real-time field sampling and mapping of groundwater contamination plumes or airborne chemical contamination. This paper addresses probe designs, probe characteristics and performance, detection limits, and future concepts for multi-point sampling systems.

Liquid phase chromatography is a well known technique routinely used in analytical chemistry for assays and measurements of aminoacids 1,2. Basically, the solution is pumped at high pressure in a long capillary tube (the chromatographic column) to fraction out the constituents, is mixed to a suitable reactant (usually ninhydrine) so as to develop a spectral absorbance, and is finally analyzed in a flow cell by a colorimeter. With ninhydrine, the reaction product is DIDA (diketo-hydrindilidene-diketolhydrin diamine) which exhibits absorbance peaks at 440 nm (blue) and 570 nm (yellow) in a proportion dependent on the specific aminoacid (Fig. 1), while the amplitude of peaks is proportional to the aminoacid concentration in view of Lambert-Beer law. Besides the two measurement channels of absorbance, either of which or the sum of which is taken as the output signal, a third channel at the wavelength 690 nm at which DIDA is transparent (Ar = 0), is used internally as the reference to the first two. Thus, the colorimeter is actually a spectrophotometer with two fixed-wavelength channels, each referenced in wavelength. In this paper, we report on the design and engineering of a colorimeter aimed to medium/high performances, high reliability and low cost. Use of fiber optics as the beamsplitter of the optical channels is shown to give substantial advantages.

Spectrophotometric measurements are used in a great number of industrial processes (chemical, pharmaceutical, farm-produce...) in nuclear environment and with optical precision components. Especially the evolution of a chemical process or of an optical coating could be followed by these measurements. Spectrophotometers, using optical fibers to transport the signal out of the instrument make possible the measurement "in-situ" and in real time. The advantage of using a diode array to detect the signal is an instantaneous measurement all over the spectral range without moving parts. It allows an excellent reproductibility of the measurements. The instrument is controlled by a micro computer. The spectrophotometer will be described and its technical performs presented. An extension using optical fibers on a "classical" spectrophotometer (a H.P. one) will be also described and its technical performs with such a system presented.

Spectrophotometric behaviour of acid-base indicators immobilized on XAD-2 microspheres are reported and an accurate comparison with the properties of the dye in liquid phase is carried out. On the basis of this study phenol red is chosen as the most suitable dye for pH detection in the physiological range (7.0-7.5). The sensor model for pH measurements is described and experimental results are discussed.

A fiber optic probe has been designed and utilized for remote analysis by Raman spectroscopy. The probe is composed of a central illuminating fiber surrounded by three to six collecting fibers. The probe shell is rugged and has been designed to withstand high temperatures and pressures and corrosive solutions. Many different types of samples may be examined by Raman spectroscopy and these are discussed.

An instrument has been designed to measure residence time distribution (RTD) in an extruder in a fast semiautomatic fashion. The instrument is composed of a spectrophotometer fiber optically coupled to the exit port of the extruder. Experiments illustrate the effects of screw design and rotation speed on RTD elution profiles.

This paper proposes a new generic technique for fiber optic chemical sensing, discusses experiments which demonstrate the technique, and briefly treats possible extensions and applications. Our results indicate a more than 100-fold increase in the coupling of energy from a chemically sensitive coating to guided modes in the fiber core, when compared with other intrinsic sensor designs. Previous reports of distributed sensors making use of chemically sensitive dyes in optical fiber claddings describe sensors which depend on the direct coupling of light between the cladding and guided modes in the fiber core. This approach, particularly when used for distributed fluorescence sensors, is limited in sensitivity since coupling takes place through the evanescent field "tails" of the fiber's core-modes, which extend only a short distance into the fiber cladding and do not correspond well with the optical properties of light in the cladding. We describe an approach which can greatly increase the amount of energy coupled from fluorescent sensor molecules in a fiber coating to guided modes in the fiber core. In this approach, fluorescent molecules of a second type are present in the fiber core, absorb the fluorescence emission from the molecules in the sensor coating, and emit their own fluorescence in the core of the fiber. The design of the fiber maximizes the amount of this secondary fluorescence that is captured and subsequently guided by the fiber. The proportionately smaller amount of coating emission which is coupled into the fiber core by evanescent field effects is also converted to secondary fluorescence. Preliminary experiments have demonstrated this "two-stage fluorescence" approach using an oxygen-sensitive dye in a permeable silicone coating placed on the outside of a plastic fluorescent-core fiber.

The estimation of partial pressure of oxygen in gaseous samples, aqueous samples and biological fluids has very important ramifications in environmental, medicinal and analytical chemistry. We have devised a fiber optic chemical sensor for the determination of oxygen concentration based on the dynamic luminescence quenching of a fluorophore bx oxygen. Ruthenium(II)tris(bipyridine), [Ru(bpy)3]4+, has been employed in our studies as the oxygen sensitive dye. The emission of Ru(bpy)32+ is centered at 610 nm and has a lifetime of 685 ns in argon purged aqueous solution. Our fiber optic chemical sensor consists of a custom built spectrometer containing argon ion laser light source, detector and associated electronics. A fiber optic cable is employed to guide light into and out of the spectrometer. A known amount of the sensing material in solution is used in a specially designed cell which has a gas permeable membrane at one end and the other end is coupled with the long cable to the spectrometer. Further research is being continued in improving the sensor chemistry and its dynamic detection range.

A new type of fiber-optic sensor for sulfur dioxide is described that is based on the inhibition of the electronic energy transfer from pyrene (the donor) to perylene (the acceptor) both dissolved in thin layer of a silicone polymer that is attached to the end of a bifurcated fiber bundle. While the donor alone is efficiently quenched by sulfur dioxide, the acceptor is not. The donor-acceptor energy transfer systems, in contrast, is extremely efficiently quenched by S02 with an Stern-Volmer constant 3-fold larger than that for the quenching of pyrene alone. The excitation light wavelength was that for pyrene (333 nm), and the fluorescence was monitored at the fluorescence of perylene (470 nm) where pyrene itself is non-fluorescent. Stern-Volmer graphs describing the quenching by sulfur dioxide are given. The results are interpreted in terms of an extremely efficient quenching of the donor-acceptor exciplex.

This paper describes the application of Langmuir-Blodgett type layers in planar optical sensors for chemical parameters including alkali ions, oxygen, halides and pH. The major advantages of LB layer based sensors are the ease of reproducible fabrication, a well-defined layer thickness, and usualy quite short response times. In addition, it offers analytical possibilities not provided by thick film techniques. As a result, potentiometric measurement of boundary potentials using electrochromic dyes have become possible. On the other hand, the type of bilayers used so far is prone to mechanical disruption, and poor signal-to-noise ratios are observed. Potential future techniques will include polymeric lipid membranes, the evanescent wave technique for gathering more intense signals, and the coupling of LB techniques to fiber optic waveguides.

Organic thin films are ubiquitous in modern science and technology. However their relatively higher propensity to beam induced damage relative to inorganic thin films, the need for molecular information, and the need to study these materials in situ mean that the vacuum based particle spectroscopies are not particularly well suited to their study. They can however be studied using the thin films themselves as the active layers of integrated optical structures. When excited with a waveguide eigenmode, for which the electric field amplitude distribution in space is well defined, the spectroscopy and optics of the structure necessarily contain information about how the chemical make-up of the film changes with depth. In optical depth profiling experiments the complete set of waveguide eigenmodes is used to excite Raman scattering from film constituents. The spectroscopic data are then used to invert a series of Fredholm integral equations of the first kind, thus yielding the molecular distribution in space. In another set of experiments the dynamics of small molecule uptake are monitored by examining the change in the resonant eigenmodes after introduction of a solution to the surface of the film.

By means of special modified multicrucible technology multimode and near monomode original fibers of two to nine separated cores have been manufactured. Special topology of crucibles have been used to obtain circular-like shapes of cores and required separation between them. Fibers of low attenuation and low crosstalks between channels have been obtained.

A new type of single mode fiber optic sensor is described which possesses broad application to various sensor configurations. The sensor uses a potted fused tapered biconical fusion coupler as a strain transducer element. Changing or imposing conditions of internal stress near the fusion joint cause the output fiber coupling ratio to change. The overall excess loss of the coupler remains constant, only a change in output coupling ratio is observed. This characteristic is highly desirable for feeding the inverting and noninverting inputs of an electronic differential amplifier through application of a photoconductive device. A variety of geometrical configurations allow a coupler sensor to be designed as a differential pressure sensor with a wide range of applications. The sensor exhibits no observable one-over-f noise from over 10K Hertz to the steady state. Pressure oscillations of extremely low amplitude and frequency allow applications to various geophysical and chemical sensors.

Fiber optic waveguides based on porous glasses offer excellent sensitivity and specificity for remote chemical sensing. This paper examines the effect of temperature on the response of a porous fiber optic humidity sensor. Cobalt chloride is used as the colorimetric reagent, permeated into the porous segment of the fiber. The humidity response of the fiber has been evaluated over a temperature range of 25°C to 90°C. Temperature effects appear to result both from changes in the extinction coefficient of cobalt chloride as well as differences in the sensing probe's equilibrium constants. The basic temperature dependence of the humidity sensor has been modeled in an effort to provide compensation for the measurement probe in a real world environment.

One of the very critical steps in the manufacturing process of VLSI (Very Large Scale Integrated) circuits is the reflow of PSG and BPSG (Phosphosilicate Glasses and Borophosphosilicate Glasses). The ref/ow consists in increasing the temperature during a given time to obtain a viscous deformation of these silica based glasses. So far, the only way to measure the effective result of the reflow on the layer was a destructive test. In this paper, we present a new system which uses an original non-destructive measurement of the degree of reflow. The principle of this method is to monitor the diffraction pattern evolution of one or several corrugated gratings located on the wafe

A step-indexed optical fiber is tapered to a diameter of one-third of the original diameter by heating under tension. Some of the cladding in the tapered region is removed by etching; and, this region is immersed in refractive index liquids in the range 1.33 to 1.65. The percentage of the light transmitted by the tapered section decreases as the index of refraction increases. Changes in index of refraction of 6x10-5 are measureable. The refractometer sensitivity may be optimized in the different index ranges by measuring the intensity in the various modal groups.

An all-fiber spectroscopic probe based on an evanescent wave sensing mechanism is described. the influences of refractive index, fiber length, fiber bend radius and the polymer cladding are evaluated.

Experimental results are reported for an unclad multimode optical fiber whose lateral surface is immersed in dilute fluorescein solution. This system provides a physical basis for fiber optic fluoroimmunoassay independent of immunochemical binding and kinetics. The excitation of fluorescein by evanescent wave interaction and subsequent detection of fluorescence from the optical fiber endface by the reciprocal process was studied. A systematic variation of optical launch conditions and cladding refractive index was completed. These results are compared with a semi-classical optical model which has been developed for this purpose. This predictive model yields results which for the first time are fully quantitative. Good agreement has been obtained between the experiments and the model. Additionally, sensitivity to 10 -9 molar solution has been obtained.

A novel optical waveguide refractometer is reported suitable for on-line and in situ monitoring of index of refraction. The device exhibits theoretical sensitivity dramatically higher than the sensitivity of the previously reported fiber optic refractometers. Based on the Bragg effect in the periodically corrugated dielectric waveguide, the refractometer can detect variations in the index of refraction of the medium in contact with the waveguide. It is found that more than two orders of magnitude higher theoretical sensitivity can be accomplished with the Bragg-type refractometer compared to the one based on the reflection principle.

The presence of a chemical may be sensed by its effect on the evanescent field of an optical wave; the evanescent field may conveniently be established through use of an optical waveguide. We have applied three techniques to the problem of estimating the sensitivity of a variety of guide structures to the presence of a chemical species. The results for an asymmetric guide show that for a given mode, sensitivity is hinly dependent on the guide thickness. Additionally, the effect of buffer layers (fouling) at the waveguide surface has been examined and results indicate the need for careful control of the surface conditions.