We have developed an evanescent wave fiber optic biosensor which uses long fibers to facilitate the analysis of environmental and clinical samples for hazardous materials. The use of antibody/antigen binding with fluorescence-based sensing in the evanescent wave yields a sensor that is unique, adaptable, and sensitive. The variety of substances that could be detected is limited only by their antigenicity. Sensing in the real world poses several challenges that must be met. We have focused on the development of several aspects of the sensing system to transition this sensor into a field deployable device. Recent developments presented here include optimized fiber optic probe tapering, a flow chamber to facilitate sampling, and probe regeneration for repetitive analysis. Preliminary experiments assessing the potential to detect analytes in biological and environmental fluids are also presented.

A fiber optic biosensor has been developed which monitors fluorescence to detect antibody/antigen binding within the evanescent wave. The sensing region is formed by removal of cladding from the core along the distal end of a step-index optical fiber and attaching the antibody. Reducing the radius by tapering the probe overcomes the mismatch in V-number which arises between the declad, immersed probe, and the clad fiber. Ray tracing analysis of tapered probes and of combination taper probes demonstrates that the evanescent wave penetration depth increases along the length of the taper, creating a probe with increased excitation light available at the surface for stimulating fluorescence.

The conception and implementation of biosensors for monitoring of sea and river water quality are nowadays important topics in environmental research, and the monitoring of water for the presence of toxic substances, even in trace quantities, is foreseen to reach widespread use in the 90s. Because of their high sensitivity and ability to recognize a wide range of substances, immunosensors are particularly suited to this type of measurement. Optical immunosensors based on the total internal reflection fluorescence (TIRF) technique couple the selectivity of the antigen-antibody reaction to the spatial selectivity of the evanescent wave at a refractive boundary. The configuration for the TIRF sensor we have developed is based on a quartz optical fiber with a high numerical aperture.

We have analyzed and fabricated two different coupling schemes to meet the requirements for a convenient means of coupling into a planar waveguide immunosensor that is relatively insensitive to beam alignment. These are the `launch' coupler and the grating coupler. Each possesses advantages and disadvantages, depending mainly on the thickness (mode number) of the waveguide to be illuminated. For example, the launch coupler is best suited to a thick (highly multimode) waveguide and is less efficient for a thin (few mode) guide. Our experimental results verify predictions of a ray theory developed to give coupling efficiency for a variety of coupling parameters.

In modern measurement technology of biological processes and process measuring technology, optical measuring methods to determine concentration are indispensable. To make qualitative and quantitative materials analyses, attenuated total reflection (ATR) or evanescent spectroscopy can be used. A new method to calculate the evanescent absorption is discussed. Theoretical context was verified by experimental investigations about evanescent absorption on quartz rods in the visual spectral range. A sensor for IR-measurements is described.

Spectral interferometric investigations, which were carried out at thin dielectric layers with a novel fixation procedure for synthetic antigens, are reported. This work is aimed at an optical, regenerable, label-free immunosensor for antibody determinations. Most optical biosensor approaches require fixation of some kind of receptor to the sensor surface. Simple adsorption leads to washout effects and often prohibits sensor regeneration. Covalent fixation usually decreases receptor affinity significantly. In this paper we report a noncovalent fixation procedure, based on synthetic, lipid anchored peptide antigens. Fixation is achieved by strong hydrophobic interaction between the pretreated sensor surface and a lipid anchor covalently linked to the antigen. Spectral interferometry uses short coherent (white) light interference for the determination of surface and volume effects at thin films. This method has been successfully applied to hydrocarbon sensing and solid phase adsorption immunoassay.

The evanescent coupling between a side polished fiber and a high index multi-mode slab waveguide produces resonances in the wavelength response of the device. The position of these resonances is sensitive to the thickness/index of the superstrate layer. This structure could then provide either film growth monitors or refractive index sensors. The sensitivity of the device is dependent upon the parameters of the slab waveguide and we present here both theoretical and experimental results for a selection of slab indices and thicknesses. The device is capable of detecting index changes of < 4 X 10^-5 and with the use of an active material, such as lithium niobate, in the role of the overlay it offers the potential for a closed loop sensor.

Investigations for a sensor application with an integrated optical (IO) interferometric arrangement are presented. One of the two waveguide arms of an IO-Mach-Zehnder- interferometer is covered with a thin layer of polysiloxane (superstrate), which is sensitive to hydrocarbons. The dielectric IO-devices are fabricated by IOT. Gases of organic compounds including halogenated and non-halogenated hydrocarbons cause a change of the polysiloxan's refractive index followed by an increase or decrease of the effective refractive index of the covered waveguide arm. The resulting phase shift between the guided light in the measuring and the reference arm depends on the detection wavelength and the concentration of gas. Using an LED as the light source the spectral interferogram becomes observable and so order and phase of the signal can be determined. The aim of this work is the development of a reversibly working, miniaturized sensor with a short response time. The advantages of spectral observation of the interference are discussed. A comparison between measured and calculated spectral interference signals is given.

The dependence of fluorescence signal in optic-fiber immunoassay detection on the index of refraction outside the fiber has been studied both theoretically and experimentally. Studies show an enhancement of factor 7 can be obtained in index matching condition.

There are a lot of optical sensors for the selective determination of ion species. Some organic compounds also have been determined by optical sensors using the enzymatic and immunological reactions. On the other hand, calixarenes are well known as novel host molecules, and specific guest ions or molecules can be incorporated inside the cavity of calixarenes. This specific recognition function of calixarene has been applied to the development of electrochemical and optical ion sensors. However, an optical sensing of organic molecules using this host-guest system is a new approach at present. In this study, a sensing membrane containing a fluorescent probe and a calixarene derivative is prepared, and it is attached on a distal end of an optical fiber. An organic compound, which specially interacts with the calixarene derivative, is optically determined. The response mechanism of the sensor is discussed.

The temperature dependence of luminescence lifetimes can be used as a temperature monitor. We describe a simple, widely applicable model for the temperature dependence of emitting species. Equations are provided for the dependence of the lifetime versus temperature as well as a figure of merit for lifetime sensors. We then explore the ramifications of this model. For example, what is the appropriate combination of rate constants and energy gap for optimizing sensor performance at different temperatures? Based on these results we show that transition metal species or phosphorescent organic molecules offer the greatest opportunities for molecular design of sensors with very high sensitivities.

Fluorescence spectroscopy has long proven to be a valuable tool in fiber optic applications. A large number of publications have addressed different fiber optic applications using fluorescent probe molecules. Since most probe molecules absorb in the UV/Vis part of the electromagnetic spectrum, the majority of these applications address the use of visible fluorophores. However, the utilization of the longer wavelength part of the spectrum may be advantageous due to its relatively low interference. Biological applications of this longer wavelength spectral region may be especially advantageous if semiconductor lasers are used as light sources. Laser diodes have all the properties of other types of lasers with the added benefits of compactness and low price. To utilize these advantages, however, new NIR absorbing probe molecules need to be developed. Certain requirements, e.g., chemical stability, presence of functional groups for binding to the fiber, etc., need to be met for using these NIR chromophores in fiber optic applications. These NIR fluorophores may be incorporated into a fiber optic probe and used for determining analytically important properties. In this paper examples of the use of NIR fluorophores are given.

2-Anthrylboronic acid (pK_a 8.8) displays decreased fluorescence in the boronate form compared with its boronic acid form, which we attribute to photoinduced electron transfer. When the boronic acid complexes to polyols such as fructose, the pK_a of the complex decreases; thus, the fluorimetrically determined pK_a of the 2-anthrylboronic acid:fructose borate ester is 5.9. These properties combined permit polyol complexation in water to be observed spectrofluorimetrically, which has not been reported previously. Fructose binds with a fluorimetrically determined K_d of 3.7 mM.

The use of irreversible indicating chemistries necessitates a constant supply of reagent to the sensor's tip. In previous work we demonstrated the delivery of reagents incorporated in an ethylene-vinyl acetate copolymer matrix. In this paper we show how a degradable polymer, poly(lactide-glycolide), can be used for sensor preparation. The use of this polymer as a matrix entrapping reagent(s) is described for two different configurations. Sensors are prepared using either composites of dispersed HPTS(1-hydroxy-pyrene-3,6,8-trisulfonate) in polymer layers or microparticles incorporating dyes.

This paper presents the results of a survey commissioned by the Optical Sensors Collaborative Association (OSCA) to identify sources and detectors in the wavelength region 250 nm to 5 micrometers , greater emphasis being placed on low cost devices e.g., LEDs and semiconductor lasers, which could form the basis of low to medium cost, general purpose instrumentation. The survey reviews both commercial availability and near term research activities and concludes that while emitters are available across the entire spectral range of interest, and indeed tunable emitters cover the entire range, few low cost devices are currently commercially available. Detectors are available to cover the entire range available, but little novelty was identified except for the inclusion of thermoelectric coolers within device packages and in the area of pyroelectrics.

A tunable fiber laser for spectroscopic gas detection is reported for the first time. The laser is based on a single-mode thulium doped fiber, which can operate at a wavelength around 1.684 micrometers , corresponding to a significant absorption line for methane. The fiber laser was pumped at 786 nm, a wavelength which is readily available with AlGaAs laser diodes and an optical threshold power of 43 mW was observed. An in-fiber photorefractive grating was used as the wavelength-selective output coupler for the laser. Simultaneous straining and heating of the grating induced a change in lasing wavelength, and a tuning range of up to 2 nm was demonstrated. This new tunable light source was configured within a methane detector and absorption spectra were recorded which demonstrate the presence of this gas. The large tuning range of the thulium fiber laser should allow the detection of many gas species with absorption bands in the wavelength region 1.65 micrometers to 2.05 micrometers .

A remote fiber optic fluorometer system which incorporates a dual wavelength UV laser excitation source is described. The system provides increased specificity for detection of multiple fluorophores without sacrificing real time sensing capability. Limitations imposed by UV transmission in fused silica fibers are discussed.

The results of detailed studies into the effects of gamma irradiation in commercially available pure silica core fibers are reported here. The absorption band centered around 630 nm was absent from the spectra of hydrogen-treated fiber. Theoretically predicted increase in attenuation due to hydroxyl (OH) ions was detected in samples exposed to high levels of radiation. The resonant fluorescence band centered around 645 nm, due to non-bridging oxygen hole centers (NBOHC), was observed in the Raman spectra of all low OH-content fibers, both un-irradiated and irradiated, but was absent from the spectra of the hydrogen enriched fibers. It would, therefore, appear that the hydrogen treatment was an effective technique in suppressing the formation of drawing and radiation induced NBOHC. Another radiation-induced fluorescence band centered at 540 nm was observed in a highly irradiated all silica (AS) fiber, and was tentatively assigned to the E'-centers. This particular band could be photobleached by 458 nm. The studies of the resonant fluorescence band detected in irradiated fibers, both plastic clad silica (PCS) and hard clad silica (HCS) fibers, were conducted using a tunable dye laser (600 nm - 650 nm). The results indicate that radiation induced NBOHC (RINBOHC) and drawing induced NBOHC (DINBOHC) are slightly different in nature.

Thin polymer films provide a convenient matrix in which to support a variety of indicator molecules that are distinguished by their ability to modulate light signals when exposed to specific analytes of interest. Using this general idea, sensors have been developed from both planar waveguides and optical fibers in conjunction with thin polymer films for a variety of process monitoring and control applications. Most of these have been concerned with pH and dissolved oxygen, but many other compounds can be similarly detected. In order to better understand the sensing process, theoretical models are developed in this paper starting from a rigorous analysis of the underlying chemical and physical processes. The validation of the model for a variety of systems is shown.

We present a novel miniature spectrometer in combination with a compact tandem optical fiber DIP probe. The system is designed to use single strand fibers to obtain high resolution spectral information for the determination of absorbance, transmission and scattering in liquids, or for measuring pH or toxic metal concentrations using immobilized indicator materials. The performance of the CCD array spectrometer in terms of spectral resolution, stray light, noise and dynamic range is shown to equal typical non-fiber analytical instruments. The mode containment optical design of the spectrometer results in excellent light throughput, and fibers as small as 50 micrometers can be used for routine applications. Tandem fiber probes were made by cutting and polishing the distal tips of two parallel fibers at a 45 degree(s) angle. A description of the tandem fiber probe (DIP probe) is presented which includes the probe's construction. Suggestions are made in geometric variations to adapt the probe to perform in other sampling tasks. Optical coupling efficiencies and refractive index effects are evaluated. Methods to construct ruggedized extension or field optical fiber cables are also discussed.

Nanobit holographic techniques, such as high-energy electron beam lithography, allow apertures of < 50 nm diam. to be fabricated in the metal coating of SPR sensor devices. When the SPR slab waveguide is edge illuminated with a laser, the apertures act as intense scattering centers of the underlying radiation, the scattered light from which is readily detectable by conventional optical microscopy. Given the dimensions of these sub-wavelength optical sources are within an order of magnitude of those of individual biological macromolecules, it should ultimately prove possible to detect interactions between substrate/analyte and individual (labelled) biological macromolecules immobilized in the apertures. The possibility of analyzing the dynamic behavior of single biomacromolecules operating in their natural environment is discussed.

A thousandfold miniaturization of immobilized optical fiber sensors has been achieved by a near-field optical technique. This technology is based on nanofabricated optical fiber tips and near-field photoinitiated polymerization. Submicrometer pH sensors have been prepared by attaching a copolymer covalently to a silanized fiber tip surface. The sensors have demonstrated their high spatial resolving ability, excellent detection limit (zeptomoles), very fast response time (milliseconds) and good stability. The potential applications include concentration profile measurements of cytoplasms, the monitoring of fast chemical and biological reactions and other microscopic operations.

Arrays of selective sensing regions are photopolymerized on the distal tip of a single imaging fiber. Important considerations for preparing sensors include surface modification for polymer adhesion, monomer preparation, micropositioning, polymerization kinetics, and instrument response. Using the technique described in this paper, we have prepared optical sensor arrays of upwards of eight discrete sensors all disposed on a single optical fiber.

Porous glass coatings produced by the sol-gel process offer a number of advantages in optical sensor applications. In reagent-based chemical sensors they may be used to provide a robust support matrix in which analyte-sensitive dyes may be entrapped and into which the analyte may diffuse. Although distal tip and side coatings are both possible the latter, evanescent wave approach, is more advantageous. These advantages and the benefits of the sol-gel approach are illustrated by preliminary results from an oxygen sensor based on fluorescence quenching of sol-gel-entrapped ruthenium complexes. Declad multimode optical fibers coated with a thin microporous film containing either of two different oxygen-sensitive complexes were investigated. Under evanescent wave excitation the sensors exhibited repeatable quenching behavior when exposed to varying concentrations of oxygen. The oxygen sensitivity was improved by appropriate choice of ruthenium complex and film preparation. The sensors showed good signal-to-noise ratio, fast response time, and low photobleaching.

A completely reversible fiber optic chemical sensor (FOCS) for carbon monoxide (CO) has been recently developed at Physical Optics Corporation (POC). The sensor, consisting of an organometallic complex adsorbed in a short segment of porous optical fiber, demonstrated spectroscopic changes upon exposure to CO. The sensor exhibits a strong absorption peak centered at 435 nm that disappears upon exposure to CO. The absorption peak reappears as the ambient CO partial pressure is reduced. This paper reports the results from testing a FOCS for CO based on the optical transmission at this absorption peak.

The fiber optic pH sensor described is based on a colorimetric pH indicator and a pH insensitive luminescent compound co-immobilized in a hydrophilic polymer. The absorption spectra of the pH indicator and the luminescent compound overlap. Consequently, the competitive absorption between the pH indicator and the luminescent compound results in modulation of the sensor emission intensity as a function of pH in the measurement media. The feasibility of this pH measurement technique is demonstrated with on-line testing of fiber optic sensors.

Sensors based on luminescence suffer from the fact that during the operating time of the instrument changes in source intensity, light throughput, detector sensitivity, indicator quantum yield, and indicator concentration are inevitable and have to be overcome by extensive referencing and recalibration procedures. Sensors based on luminescence decay time should not suffer from these drawbacks. Decay-time sensing has relied so far on dynamic quenching, which is not well suited for pH measurements. Several other mechanisms are described in this contribution. The conditions necessary for an indicator to be useful in a decay time based pH sensing scheme are clarified and the suitability of this scheme is demonstrated.

We report on the development, characterization, and photophysics of a new fiber-optic-based sensor which uses a sol-gel entrapped recognition element. The recognition element is modified (beta) -cyclodextrin to which we have added a short tether (glycine) and a fluorophore (dansyl). This recognition element forms an intramolecular complex, and the dansyl group can include within the cyclodextrin cavity. Non-fluorescent analytes, that bind to the cyclodextrin cavity, can effectively displace the included dansyl group and result in a measurable change in signal. We report on the detection limits, dynamic range, and photophysics (i.e., transduction mechanism) of this new sensor.

The need for suitable remote sensors in highly radioactive defense waste storage tanks is discussed. The harsh radiological and chemical tank environment precludes the use of standard sensors because of the need for intrinsically safe systems. Potential sensor systems based on fiber-optics technologies suitable for the nuclear waste environment are identified. The need for certification standards for this type of environment is also discussed.

Spectroscopic gas sensing was carried out by using an infrared hollow waveguide as a capillary flow cell. A ZnS-coated Ag hollow waveguide is generally the most suitable selection for use as a flow cell, since it exhibits high transmittance over the wide spectral range around 10 micrometers wavelength. For the corrosive gases that cause serious damage to the ZnS film, SiO₂- or GeO₂-based glass hollow waveguides are applicable. With these low-loss hollow waveguides, CH₄, n-C₄H₁₀, NO₂, and SO₂ gases were measured successfully, and a fast response in gas detection as well as a remarkable reduction of gas consumption was demonstrated.

This paper describes initial studies that were performed to develop a personal wear badge for the accumulated exposure detection of benzene vapor. Small sections of optical fiber were coated with a membrane specific for the adsorption of benzene vapor. After exposure the fiber was placed in a Fourier transform infrared (FTIR) spectrometer and the infrared spectra of the surface bound benzene was obtained. The qualitative exposure levels were determined by peak intensities. Benzene was detected at a concentration of 100,000 ppm. Dichlorobenzene was also detectable (80,000 ppm) and could be differentiated from benzene by unique absorbance peaks. The sensor was reusable and the cumulative exposure was determined as a function of time. The sensor modifications were based on the incorporation of benzene absorbing compounds contained in a thin, high surface area porous membrane coating applied to the fiber surface. The surface coating technology was developed and demonstrated on an active fiber system. A sensor cell was designed to allow handling and direct insertion into the FTIR spectrometer.

In-situ fluorescence measurements of aromatic organic ground water contaminants do not always agree with gas chromatographic methods. Dissolved oxygen quenching of fluorescence may be an interferant in field measurements. Two standard fluorescent aromatics, quinine sulfate and naphthalene, were evaluated in this study. Over the range of dissolved oxygen concentrations expected to be encountered in the field, no effects of oxygen quenching on fluorescence of these compounds was observed. Quenching of quinine sulfate fluorescence by sodium chloride was observed using this system. Sodium chloride quenching was shown to follow the Stern-Volmer relation.

The development of a simple fiber optic probe intended for process control measurements is described. The factors which were considered and the approach to minimize errors in the measurement of absorbance also are described and some selected results presented to illustrate that these possible errors have been essentially totally removed.

The role of on-line chemical analyzers is vital to process monitoring and control and product quality. Although traditional optical filter methods such as UV-VIS, NIR, and IR have enjoyed considerable success when applied on-line, they often require inconvenient and complex sampling schemes. These restrictions can be largely eliminated by fiber optic probes as evidenced by their growing popularity. Fiber optic technology also allows remote location of full spectrum analyzers which in turn facilitates multicomponent analysis. Recently, we have developed a Raman spectrograph which utilizes fiber optic probes and a CCD detector. We have been most successful with this system when it is applied to processes in a short term, investigative role. Examples of reaction intermediates and products, contaminant identification, and process optimization are given.

A triple sensor unit consisting of opto-chemical sensors for measurement of pH, oxygen, and carbon dioxide is presented. The pH sensor and the CO₂ sensor are based on the color change of a pH-sensitive dye immobilized on a polymeric support. The resulting changes in absorption are monitored through optical fibers at one or two analytical wavelengths. The oxygen sensor is based on the quenching of the fluorescence of a metalorganic dye. The operation principle and the performance of all three sensors are described thoroughly with respect to their application in bioreactors. All three sensors are fully LED compatible. The chemical and mechanical stability, especially against common sterilization methods, are described in some detail. A calibration and measurement software comprising fit routines for the sensors and a mathematical treatment of the results are presented as well.

The reaction of oxygen with silicon tetrachloride (SiCl₄) and other glass-forming chemicals is the basis for the preparation of optical fiber preforms by the modified chemical vapor deposition (MCVD) process. Control of the concentration of the gas phase reactants is important for fabricating a preform core with the desired composition and refractive index profile. An acoustic time-of-flight (ATOF) method has been used to determine the concentration and evaluate the stability of the precursor delivery system used in the MCVD process. The measurement technique is based on accurate determination of the speed of sound which is dependent on the concentration of the binary mixture. Highly precise measurements for SiCl₄ and GeCl₄ have been obtained for a variety of oxygen carrier gas flow rates. The results obtained by the ATOF method agree within +/- 1% of the values obtained by direct gravimetric calibrations. The utility of ATOF for calibrating chemical reagent delivery systems is confirmed experimentally.

In response to the need of the U.S. Food Safety and Inspection Service, the Agriculture Research Service has undertaken a project to develop an accurate, reliable, and nondestructive sensor for detecting poultry diseased carcasses on-line at poultry processing plants. This paper presents some results of a study on the development of a nondestructive technique for the detection of abnormal poultry carcasses based on the spectroscopy of the carcasses. A diode array spectrophotometer equipped with a fiber optic probe was used to obtain optical spectra of the breasts of normal, septicemic, and cadaver poultry carcasses in visible and near-infrared regions (500 - 1100 nm). Optimal wavelengths of reflectance and interactance in the range of 500 to 850 nm were obtained for classifying the carcasses into normal and abnormal (septicemic and cadaver) classes. A back-propagation neural network model was used to develop classifiers for the classification of poultry carcasses into normal, septicemic, and cadaver classes.

Borosilicate glass substrates with etched surface gratings were fabricated via a photolithographic patterning and hydrofluoric etch procedure. Ion-exchanged waveguides supporting multiple modes were produced by exposing etched grating substrates to silver nitrate melt at elevated temperature. Coupling a collimated laser beam to an ion-exchanged waveguide via an etched surface grating results in population of all allowed waveguide modes. Each mode is angularly resolved at the output grating and is imaged with a photodiode array detector. The large electric field density associated with total internal reflection at the glass/solution interface results in extreme sensitivity of the waveguide modes to interfacial phenomenon. In addition, the individual modes are expected to exhibit differential sensitivities to interfacial perturbations due to differences in electric field strength. The described ion- exchanged waveguide evaluation system captures the response of all waveguide modes simultaneously to changes in the chemical composition of the solution phase. Multivariate statistical methods are used to characterize the complicated multi-mode waveguide response.

An improved design for fiber optic chemical sensors based on polymer swelling is applied to the detection of changes in electrolyte concentration. In this design the polymer sensing element is isolated from the fiber optics by a rubber diaphragm glued to a reflecting piece of aluminum. Changes in polymer size move the diaphragm, changing the intensity of light reflected into an optical fiber. The sensor design allows the user to adjust the distance between the optical fibers and the reflecting surface so that maximum sensitivity can be achieved. The new design is demonstrated using a bead of crosslinked strongly basic anion exchange resin as a sensing element to detect changes in electrolyte concentration.

An infrared fiber-optic neurotoxin biosensor was constructed by applying a biologically active cladding to the core of an infrared transmitting chalcogenide fiber. Binding of the surface bound receptor protein was monitored by performing infrared difference spectroscopy on the fiber-optic probe before and after its exposure to various concentrations of neurotoxin in solution. Signals measuring conformational change(s) as a result of these interactions are observed to saturate in agreement with established biochemical kinetics for the receptor. Fiber-optic components are shown to be much more sensitive than bulk optical components in performing these measurements.

Fiber optic oxygen sensors based on polynuclear aromatic hydrocarbons or transition-metal complexes require excitation light in the UV and blue region of the spectra (1-14,16,18,19). These types of oxygen sensors are generally intensity modulated and are therefore sensitive to perturbations such as bending of the optical fiber and photodegradation of the dye. Oxygen sensors based on emission decay time measurements (15,17,20,21), which would overcome these problems, have been developed; but these sensors require complicated electronics. The oxygen sensor described here is based on a ratiometric emission intensity measurement of two porphyrins, which minimizes the effect of environmental perturbations on the optical signals (23). The excitation and emission spectra of the dyes are in the green, orange, and red region of the spectra, which makes the sensor compatible with LED excitation and photodiode detection.

An interferometric technique for determination of the refractive index of liquids is described. The method is based on measurements of phase variations caused by the relative movement of an optical fiber tip in a liquid sample. The apparatus consists of two independent interferometers. A two-frequency Michelson interferometer is used to measure the liquid sample displacement in the air, while an optical fiber Mach-Zehnder interferometer measures the optical path length difference in the moving sample. The liquid sample refractive index is then derived by dividing the fringe counts obtained by both interferometers. The measurements have been performed in different liquids. With the distilled water sample, the statistical error of this method was found to be 5 X 10^-5.

The principle of optodes has been shown to be efficient. Optodes are based on conventional ion-selective liquid membranes coupled with an indicator as an optical transducing agent. Calcium, sodium, potassium, and ammonium were assayed in real samples as well as the biological relevant trace elements zinc and lead in aqueous solutions. However, some kind of drawbacks were observed which can be eliminated by an improved optical measuring technique. A considerable gain in sensitivity of the optical measurements is achieved for clinical analysis of total potassium concentrations in plasma by the evanescent wave technique (ATR). For the sodium optode the analytical error is shifted towards the allowable range. Furthermore, the adsorption of biological sample components at the surface of the PVC- membrane does not influence the optical signal.

This paper describes a series of experimental results in the development of fiber optic oxygen sensor with excellent immunity to quenching by water vapor. The sensor exhibits good oxygen response (4 dB quenching when P_O(2) changes from 0 to 190 torr) identically in both 0% and 100% relative humidity environments. To develop this sensor the solution spectra of several organic fluorescent compounds were characterized for oxygen quenching efficiency. The most promising of these fluorophores were coated onto porous substrates using a proprietary process and tested for their sensitivity to oxygen and the potential interference from water vapor. The presence of oxygen quenches the fluorescence radiated by many compounds. Since water is also commonly found to quench fluorescence, a humidity insensitive oxygen sensor would be of great value in most applications. Each of the sensors was characterized for its fluorescence excitation and emission wavelengths, sensitivity for oxygen, and quantum efficiency. The most promising fluorophores, decacyclene and benzo[g,h,i]perylene, demonstrated the best overall results of the compounds tested. These fluorophores were characterized for their response to oxygen, as well as a cross- sensitivity to water vapor, carbon monoxide and carbon dioxide.

An optical fiber with a section of its cladless core immersed in a fluid uses a refractive index (RI), n, matching method to measure n and (delta) n/(delta) (lambda) (dispersion) of the fluid. Current methods of measuring n and (delta) n/(delta) (lambda) often require light transmission through the fluid; thus turbidity, which diffuses and attenuates the light, restricts RI analysis. This device, which senses the minimum light remaining within the fiber core, is immune to such limitations. Temperature, a factor in any precise measurement of n, is the only accurate measurement needed; no critical system calibration is required. Tests were performed on clear and turbid edible oil samples, and the method matched the +/- .0001 RI unit accuracy of the Abbe refractometer. A tunable laser was used to measure the oil RI within the wavelength range of 543 to 633 nm, and the data fitted to Cauchy's dispersion equation. Calculations, using (lambda) equals 589.3 nm, agree with Abbe measurements of the oil samples.

Experiments were conducted to verify a theoretical model on the injection efficiency of sources in the cladding of an optical fiber. The theoretical results predicted an increase in the injection efficiency for higher differences in refractive indices between the core and cladding. The experimental apparatus used consisted of a glass rod 50 cm long, coated at one end with a thin film of fluorescent substance. The fluorescent substance was excited with side illumination, perpendicular to the rod axis, using a 476 nm Argon-ion laser. Part of the excited fluorescence was injected into the core and guided to a detector. The signal was measured for several different cladding refractive indices. The cladding consisted of sugar dissolved in water and the refractive index was changed by varying the sugar concentration in the solution. The results indicate that the power injected into the rod, due to evanescent wave injection, increases with the difference in refractive index which is in qualitative agreement with theory.

By means of evanescent wave absorption spectroscopy of single-bilayer biological membranes coated on optical fibers, we have measured such phenomena as lipid phase transitions and incorporation into the bilayer of small antibiotic peptides. A key innovation which has resulted in a substantial increase in sensitivity is our use of a 500 micrometers diameter diamond rod as a direct optical coupling device between an IR blackbody source and the 500 micrometers optical fiber. One of the rod's ends is in direct contact with the hot SiC source, while the other abuts the cleaved end of the AsSeTe fiber. The fiber passes through a custom-built Langmuir- Blodgett trough, then into the emission port of our spectrometer where its output is collimated into the interferometer. An important property of our method of coupling light into the fiber is that it allows the excitation of a very wide range of the fiber's optical modes. This has allowed us to obtain excellent evanescent-wave spectra of liquids even using fibers with the protective plastic coating still on them.