The designs of NIR optical fiber immunosensors for the detection of biomolecules are discussed. The use of fiber optics combined with laser-excited fluorescence detection ((lambda) max equals 780 nm) and immunochemical techniques has provided the essential components for developing simpler and more practical, sensitive and selective immunosensors. The antibody is immobilized on the distal end of a surface-activated polymethyl methylacrylate sensor. As the probe is placed in different concentrations of antigenic substance, the antibody reacts with its corresponding antigen through sandwich and/or competitive immunoassays. The concentrations varied from 10 - 100 ng/ml. The near-infrared dye labeled antigen - antibody complex is excited and the emitted fluorescence is collected with a silicon photodiode detector equipped with an 820-nm bandpass filter. In order to determine various factors influencing the immunosensor's performance, the fluorescence intensity responses are obtained under a variety of conditions. The sensor response depending on the type of surface-activating reagent, surface activation period, incubation time, and other measurement conditions also are discussed.
We have used an evanescently excited fiber optic immunoassay to study the kinetic response of an immunosensor and determine its absolute sensitivity. We have estimated that when binding sites are saturated approximately 1011 antigen are bound to a fiber probe of 0.57 cm2 active area using optical techniques and without resorting to a radioassay. The number of labeled antigen for a threshold signal is approximately 109. We can easily detect fluorescently labeled antigen at subnanomolar concentrations and have measured the probe response to antigens over more than two orders of magnitude. The overall response time for this system and labeled antigen (Mw approximately equals 100,000) is on the order of 10 minutes for a 40 pM solution. At higher concentrations, a response may be obtained in a few minutes.
Fiber optic evanescent fluorosensors are under investigation in our laboratory for the study of drug-receptor interactions for detection of threat agents and antibody-antigen interactions for detection of biological toxins. In a one step assay, antibodies against Cholera toxin or Staphylococcus Enterotoxin B were noncovalently immobilized on quartz fibers and probed with fluorescein-isothiocyanate (FITC)-labeled toxins. In the two-step assay, Cholera toxin or Botulinum toxoid A was immobilized onto the fiber, followed by incubation in an antiserum or partially purified antitoxin IgG. These were then probed with FITC-anti-IgG antibodies. Unlabeled toxins competed with labeled toxins or antitoxin IgG in a dose-dependent manner and the detection of the toxins was in the nanomolar range.
The basic recognition schemes underlying the principles of standard enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay (RIA) protocols are increasingly being adapted for use with new detection devices. A direct comparison was made using a fiber optic biosensor that employs evanescent wave detection and an ELISA using avidin-biotin. The assays were developed for the detection of Ricinus communis agglutinin II, also known as ricin or RCA60. Detection limits between the two methods were comparable for ricin in phosphate buffered saline (PBS), however results in complex samples differed slightly. In PBS, sensitivity for ricin was 1 ng/ml using the fiber optic device and 500 pg/ml using the ELISA. The fiber optic sensor could not detect ricin directly in urine or serum spiked with 5 ng/ml ricin, however, the ELISA showed detection but at reduced levels to the PBS control.
In this paper, we present a new system called FOBIA that was developed and optimized with respect to automated operation of repetitive assay cycles with regenerable bioaffinity sensors. The reliability and precision of the new system is demonstrated by an application in a competitive assay for the detection of the triazine herbicide Atrazine. Using one sensor in more than 300 repetitive cycles, a signal precision better than 5% was achieved.
We discuss the molecular self-assembly on optical fibers in which a novel method for protein attachment to the sensing tip of the fiber is used. Our objective is to assemble a conjugated polythiophene copolymer as an attachment vehicle. Subsequent attachment of the photodynamic phycobiliprotein serves as the fluorescence probe element. Following our earlier experiments from Langmuir-Blodgett deposition of these polymeric materials as thin films on glass substrates, we extended the technique to optical fibers. First, the bare fiber surface is silanized with a C18 silane compound. The copolymer (3-undecylthiophene-co-3- methanolthiophene, biotinylated at the methanol moiety) assembly on the fiber is carried out presumable through van der Waals interactions between the hydrophobic fiber surface and the undecyl alkyl chains on the polymer backbone. A conjugated Str-PE (streptavidin covalently attached to phycoerythrin) complex is then attached to the copolymer via the conventional biotin-streptavidin interaction. The conjugated polymer not only supports the protein but, in principle, may help to transduce the signal generated by phycoerythrin to the fiber. Our results from fluorescence intensity measurements proved the efficacy of this system. An improved methodology is also sought to more strongly attach the conjugated copolymer to the fiber surface, and a covalent scheme is developed to polymerize and biotinylate polythiophene in situ on the fiber surface.
Bioprocess control requires reliable and continuous monitoring of nutrients and various process parameters, such as dissolved oxygen, carbon dioxide, and pH. Currently, the on-line control of these parameters is achieved electrochemically via dip-in electrodes or indirectly by off-gas analysis. Optical sensors offer an alternative to these conventional methods. Fiber- optic sensors for measuring pH, CO2 and O2 have been developed. We have demonstrated the use of a fiber-optic sensor for the continuous monitoring of fermentation pH. In this paper we discuss the use of a ratiometric method and medium calibration as a way to improve the precision of the fiber-optic sensor.
The distal tip of a 350-micrometers imaging fiber comprised of thousands of 4- to 6-micrometers fibers is coated with a thin layer of pH sensitive material. Polymerization is initiated photochemically and is combined with spin coating techniques to yield a uniform coating of polyHEMA/fluorescein on the order of 10 micrometers thick. Performance data for this fiber demonstrates it is capable of simultaneous pH measurements and near-field imaging.
Near-field optics has been applied in the nanofabrication of subwavelength optical fiber chemical and biological sensors and their operation in chemical and biological analysis. A thousandfold miniaturization of immobilized optical fiber sensors has been achieved by a near- field photo-nanofabrication technique, which is based on nanofabricated optical fiber tips and near-field photopolymerization. This technique has been further developed by multistep near- field nanofabrication and multidye probe fabrication. Multistep nanofabrication can further miniaturize optical fiber sensors, while multidye fabrication results in multifunctional optic and excitonic probes with extremely small size. These probes emit multiwavelength photons or produce excitons of different energy levels, and may have multiple chemical or biological sensitivities. The nondestructive submicrometer sensor has demonstrated its ability to carry out static and dynamic determinations of pH in intact rat conceptuses of varying gestational ages. The ability of the sensors to measure pH changes, in real time, in the intact rat conceptus, demonstrates their potential applications for dynamic analysis in multicellular organisms and single cells. The near-field interaction of photons with matter is discussed.
Integrated optical transducers with on-chip measuring variables are shown to be suitable for realizing complete miniature sensor modules. Theoretical expressions are derived and discussed that can readily be used for the design of sensors based on converting the value of a measurand into the position of an on-chip integrated optical light pointer. The emphasis is on configurations based on tapered waveguides and chirped grating couplers. The viability of this approach is demonstrated by reporting first experimental results for a novel miniature refractometer chip.
Fiber optic surface plasmon resonance sensors are used to characterize thin porous sol-gel films applied to the sensor surfaces. Techniques for coating the sensors and curing the films are discussed. Experimental and analytical methods are presented for using analysis of wavelength modulated surface plasmon resonance spectra to determine pore volume and refractive index of the films.
A Raman probe has been developed utilizing a single optical fiber as both a light pipe and an active sensing element. By coating a small segment of the surface of an exposed glass fiber core with a thin polymer film, an inverted waveguide is formed where light transmitted down the fiber is stripped out of the core and into the polymer film. The polymer coating is used both as a waveguide and as a medium for concentrating small organic molecules to be interrogated by Raman spectroscopy. The ability of the fiber optic thin-film waveguide probe to detect organic vapors is demonstrated. The utility of the probe in the detection of nonaqueous phase liquids (NAPLs) is also described.
A D-fiber evanescent field methane sensor has been reported by Culshaw et al., where a sinusoidally modulated Fabry-Perot interferometer was used for signal processing. To optimize the sensitivity, it is essential to establish a relation between output signal, methane concentration, and certain other key parameters. Previous work with open-path cells, which use a sawtooth driven Fabry-Perot interferometer, shows a noise-limited detection level of +/- 0.003% methane (1s time constant). However, no analytic expression has been reported to relate the key parameters in the system. In this paper, we report such a relation and calculate the optimal values of some key parameters.
We report the use of a windowless resonant spectrophone to make highly sensitive (10-9 cm-1) photoacoustic measurements of atmospheric water vapor and aerosol absorption. A tunable high-resolution cw Nd:YAG laser was used to measure the water vapor lines and aerosol absorption spectrum at 1.064 mm. The water vapor absorption lines were used to calibrate the spectrophone for the aerosol measurements. The spectrophone was also calibrated using the theoretical expression with an independent measurement of the Q of the spectrophone. The initial windowless spectrophone has the ability to make in situ real- time measurements of atmospheric absorption to an accuracy sufficient for thermal blooming calculations. The ability of the spectrophone to detect at the 40-parts-per-trillion-level of other gaseous and volatile species is discussed. The use of resonant mufflers to isolate the spectrophone from external noise is also presented.
An optical chemical sensor for dissolved carbon dioxide has been developed whose dynamic range was adjusted to CO2 partial pressures ranging from 0 to 100 hPa. The change in the pH of a buffer layer, caused by diffusion of carbon dioxide through a hydrophobic membrane, is indicated by the color change of a covalently immobilized dye and monitored through optical fibers. The sensor also incorporates an optical insulation with a resplendent pigment to increase the reflectivity and to reduce adverse effects of straylight and ambient light. Two methods for layer manufacturing (spreading and spin coating) are described. The sensor membrane is fully LED compatible. The optrode shows a promising performance with respect to chemical and mechanical long-term stability, reproducibility, and sterilizability.
The fiber-optic-based pH sensors have many advantages over the traditional pH electrodes. They are rigid, self-calibrating, and show negligible drift. In addition, they are capable of detecting pH changes with a precision of 0.001 pH units. These features make fiber optic pH sensors especially suitable for on-line industrial applications. The pH sensor consists of an indicator dye immobilized in a polymer matrix. The pH of the solution is related to the relative fraction of the protonated and the dissociated species of the dye, whose absorption spectrum changes according to the pH of the surroundings. The response of the sensor is determined by the dissociation equilibrium of the immobilized dye and its interaction with the polymer matrix. In order to have meaningful pH measurements it is important to understand the sensing mechanisms and the corresponding absorption spectra. In this paper a model is reported for the chemical reaction taking place within the membrane during the course of pH measurements. The model takes into consideration the interaction between the indicator dye and the membrane and identifies the key parameters to be used in the design of membranes for pH sensing. In addition, the absorption spectra of the dye in solution are deconvoluted and analyzed for reference purposes.
Within the atomic vapor laser isotope separation (AVLIS) program we have successfully used the laser absorption spectroscopy technique (LAS) to diagnose process physics performance and control vaporization rate. In the LAS technique, a narrow-line-width laser is tuned to an absorption line of the species to be measured. The laser light that is propagated through the sample is measured, and from this data the density of the species can be calculated. These laser systems have almost exclusively consisted of expensive, cumbersome, and difficult-to- maintain argon-ion-pumped ring dye lasers. While the wavelength flexibility of dye lasers is very useful in a laboratory environment, these laser systems are not well suited for the industrial process control system under development for an AVLIS plant. Diode lasers offer lower system costs, reduced manpower requirements, reduced space requirements, higher system availability, and improved operator safety. We report the successful deployment and test of a prototype laser-diode-based uranium vapor rate control system. Diode-laser-generated LAS data was used to control the uranium vaporization rate in a hands-off mode for greater than 50 hours. With one minor adjustment, the system successfully controlled the vaporization rate for greater than 147 hours. We report excellent agreement with ring dye laser diagnostics and uranium weigh-back measurements.
The atomic vapor laser isotope separation (AVLIS) program has been using laser absorption spectroscopy to monitor vapor densities for over 15 years. Laser absorption spectroscopy has proven itself to be an accurate and reliable method to monitor both density and composition. During this time the diagnostic has moved from a research tool toward a robust component of a process control system. The hardware used for this diagnostic is discussed elsewhere at this symposium. This paper describes how the laser absorption spectroscopy diagnostic is used as a component of a process control system as well as supplying detailed measurements on vapor densities, composition, flow velocity, internal and kinetic temperatures, and constituent distributions. Examples are drawn from the uranium AVLIS program. In addition potential applications such as composition control in the production of metal matrix composites or aircraft alloys are discussed.
We report here experimental results for a nitrogen dioxide sensor based on optical transmission through porous silica fiber. The NO2 optrode is activated by using a proprietary chemical pretreatment process on the porous silica. Light is coupled to and from the porous silica optrode via 600 micrometers diameter optical fibers. The peak response of the sensor is at approximately 420 nm and therefore can be monitored using a blue LED. The sensor demonstrated a reversible, full-range response from 0 to 10,000 ppm NO2 in a balance of nitrogen. The sensor also demonstrated a sensitivity of 10 ppm. The range and sensitivity of this sensor make it suitable for monitoring nitrogen dioxide emission from combustion stacks.
Several kinds of optical fiber probes for measuring liquid concentration are presented in this paper. All the probes are designed for industrial pollution applications. Experimental results with analyses and comparisons are given.
A core-based intrinsic fiber optic absorption sensor has been developed for the detection of volatile organic compounds. The sensor can detect organics in aqueous solutions or in the vapor phase without a chemical reaction. The distal ends of transmission and receiving fibers are connected by a small section of an optically clear silicone rubber. The silicone rubber section acts both as a lightpipe and as a selective membrane into which the analyte molecules can diffuse. Absorption spectra obtained in the nearinfrared (NIR) provide qualitative and quantitative information about the analyte. Water, which has strong broadband absorption in aqueous solutions of the NIR, is excluded from the spectra due to the hydrophobic properties of the silicone rubber. In a stirred solution, the sensor reaches equilibrium in approximately 10 minutes. The current limit of detection is 1.0 ppm for TCE in an aqueous solution.
A distributed fiber optic moisture sensor based on intrinsic changes in the optical properties of the cladding is reported. A 10-meter-long fiber sensor was fabricated that demonstrated a response to humidity in less than 5 minutes. The humidity-sensitive cladding was fabricated on-line during fiber draw by continuously coating a multimode glass core fiber with a polyvinyl acetate cladding, in which a water-sensitive indicator had been dissolved. The indicator was a solvatochromic dye that showed a pronounced hypsochromic shift in its absorption spectrum in the presence of water. The moisture response of the sensor was monitored by measuring changes in the optical attenuation of the fiber in the region between 580 nm and 650 nm. This spectral region facilitates the use of commercially available solid state optoelectronic devices such as LEDs, laser diodes, and PIN photodiode detectors, in order to produce a low-cost, compact, lightweight humidity sensor.
An optical sensor has been developed that can conveniently measure the depth of organic liquid layers floating on aqueous solution. Layer depth can be measured with an accuracy of +/- 0.14 mm. The dynamic range of the sensor is 0.3 mm to 20 mm. The method is robust and is designed for use in an industrial environment.
This paper describes results using a nonabsorbing wavelength as an internal reference to correct for nonanalyte-related signal changes and long-term drift in an absorbance-based fiber optic sensor. A renewable-reagent sensor developed for seawater pCO2 (CO2 partial pressure) was repetitively calibrated over a 12-day period and the drift in the calibration was studied. The results indicate that although the internal reference significantly improves the long-term performance, it does not fully correct for all sources of drift over the 12-day period.
An intrinsic fiber optic environmental sensor has been developed for on-line monitoring of oxygen gas and dissolved oxygen (DO). In this O2/DO sensor, a highly stable compound [Tris-(4,7-Diphenyl-1, 10-phenanthroline) Ruthenium (II) complex] is synthesized and selected as a chemical indicator for oxygen. A hybrid matrix is designed by a sol-gel process and used as a stable substrate for the immobilization of Ru compound. The microporous nature of the nondensified sol-gel coating, in which the photochemical dye is immobilized, provides a unique local structure that is environmentally stable and immune to photochemical bleaching and chemical leaching. The doped hybrid material is then coated on a porous optical fiber substrate used as the sensing component. The oxygen penetrates into the interconnective porous core and interacts with the immobilized Ru compound in which in-line dynamic luminescence quenching takes place. The use of Ru(ph2phen)3[2+] as a highly stable, reagent sensing indicator and also the use of the sol-gel coating technique for the immobilization and incorporation of Ru compound to porous core optical fibers have resulted in the development of an intrinsic O2/DO sensor with high sensitivity, reproducibility, and long-term stability.
This presentation focuses on mechanical and electro-optical design considerations embodied in VOtectTM -- an infrared fiber optic sensor for volatile organic compounds. Presently, the VOtectTM system is configured for remote detection of hydrocarbon vapors associated with gasoline and other internal-combustion fuels. Using commercially available zirconate glass optical fibers, the sensor exploits the overlap of absorption spectra due to carbon-hydrogen stretching vibrations between 3.3 and 3.6 microns, with the optical output of an infrared HeNe laser operating at 3.39 microns. Compensation for position-dependent fiber bending losses is achieved using 1.15-micron radiation simultaneously emitted by the laser source. Initial laboratory evaluations of the VOtectTM system indicates detection sensitivities well below the lower explosion limits for petroleum distillates, indicating the usefulness of the sensor for petrochemical safety applications. The sensor is intrinsically safe (e.g., explosion-proof), since no electrical power is required at the probe tip. Preliminary sensor optical power budget calculations indicate that the zirconate fiber optic umbilical, which connects the sensor probe to the electro-optical detection system, can be as long as several hundred meters. Calibration data for a variety of hydrocarbons indicate linear relationships between ln(V/Vo) and vapor concentration, suggesting that the sensor should prove useful for on-line, real-time process control applications.
Chlorinated hydrocarbons such as trichloroethylene and methylene chloride are common contaminants in soils at polluted sites. The chemical characterization of contaminated soils as a precursor to remediation is important. Raman spectroscopy is especially useful for the selective determination of a broad range of compounds. A fiber-optic Raman probe suitable for use in a soil environment has been constructed and tested. The intensity of the Raman signal of the chlorinated hydrocarbons trichloroethylene and methylene chloride has been measured in a variety of standard and nonstandard soil and sand samples. The effect of the soil parameters (opacity, particle size, etc.) on the intensity of the Raman signal has been investigated. The general implications for spectroscopic fiber optic chemical sensors used in a cone penetrometer system are discussed.
A carbon tetrachloride sensor is presented that is based on a novel approach that involves quenching of the fluorescence emission from the second excited state (So $IMP S2 transition) of a dye, congo red, and an air purge mechanism. The Stern-Volmer plot for the sensor response towards carbon tetrachloride in air is linear over the investigated concentration range, 0 - 5 mM, with a Ksv value of 84.85 M-1. The detection limit was found to be 47 (mu) M CCl4 in air. It is suggested that the operating fluorescence quenching process is diffusion controlled and collisional in nature. The sensor is fully reversible with an analytical response time < 25 s and recovery time < 2 s. The environmental applications of this sensor that is capable of providing a reference signal, while in-situ, are discussed.
To satisfy the need for a gasoline leak sensor with faster detection of fuel leaks and a much faster recovery after detection, an optical approach has been developed. This approach uses differences in refractive indices to discriminate gasoline from air or water. The sensor is designed to detect an 1/8-inch layer of gasoline, even when it is floating on water, but it will not trigger on a gasoline layer less than 1/32 inch thick. The basic sensor design is described including techniques used to stabilize the output over a range of required operating conditions.
Currently there is a great deal of interest in the development and use of fiber optic chemical sensors for characterization of contaminated waste sites. Development of remote, in-situ sensors for rapid determination of the presence, and concentration of hazardous materials will significantly reduce site remediation costs. The state-of-the-art technology for assessing site contamination is the cone penetrometer system. This system consists of a 2-1/2 ton truck, a hydraulic ram, and a steel tube. The steel tube, which is generally 1-3/4 inches OD and 1 inch ID, has a sharp tip on one end. To begin site characterization the penetrometer tube is placed into the hydraulic ram then the tube is pushed into the ground. Sensors are mounted in the penetrometer tube to measure contaminants in the surrounding soil and ground water. This system has several distinct advantages over conventional drilling techniques. Additionally, site characterization can be performed much quicker than standard drilling techniques. Fiber optic chemical sensors are readily applicable towards use in cone penetrometer systems since they are small in size and can report real time, in-situ results. Some fiber optic chemical sensors have been deployed and tested in the cone penetrometer system.
Recently, we have described a fiber optic biosensor specific for zinc that transduces the presence of the metal as a shift in the emission of a fluorescent sulfonamide inhibitor that binds to a metalloenzyme, human erythrocyte carbonic anhydrase, if and only if the metal is present. Due to fiber photoluminescence and calibration issues, we decided to determine if comparable results could be obtained by sensing based on fluorescence lifetime changes, as recently described by Demas, Wolfbeis, Lakowicz, and others. Results of these experiments are included, as well as a discussion of the dynamic range of the method.
Simplicity, cost efficiency, and higher fidelity compared to the single-channel design are the main advantages stimulating the recent interest on fiber optic intensity-ratio sensors. Optical and electronic designs of refractometric sensors of this kind are discussed here. Two optical schemes of retroreflective probes with one input and two output fibers are proposed. The probe sensitivity to liquid refractivity changes in the interval 1.33 < n < 1.41 is studied by computer simulation. An originally developed one-ADC-based signal processing circuit providing digital display of the intensity-ratio determined n-values is described as well.
Design considerations and experimental results are presented for the fabrication and characterization of grating couplers in gradient effective index waveguides used for realizing miniature integrated optical sensor modules. Hard dielectric substrates such as glass and fused silica have been structured by photolithographic techniques, while hot embossing has been used for the grating fabrication in plastic substrates. The gradient effective index waveguides were produced by shadow mask evaporation techniques in one single deposition step in a rotating substrate holder arrangement. Optical diffraction measurements, atomic force microscopy, and a newly developed method based on performing spatially resolved grating coupler resonance angle measurements are shown to be valuable tools for the characterization of the waveguides and the grating couplers. The experimental results indicate that the deposition of high-index waveguiding films such as Ta2O5 and TiO2 on previously structured substrates is a viable technique for the cost-effective fabrication of integrated optical sensor chips.
A thermoelectric cooled, purge-and-trap device (TEC) has been developed to enhance the utility of fiber optic VOC sensors. This simple device consists of a thin copper radiator sandwiched between two thermoelectric coolers. Two water-cooled, copper heat sinks are used to maintain the hot side surfaces of the thermoelectric coolers at 5 degrees C, allowing internal surface temperatures of the radiator to be controlled between 5 degrees C to -43 degrees C. This paper describes the use of the TEC with a reversible, fiber optic, carbon tetrachloride sensor. The sensor by itself has a detection limit of around 1,000 ppmv carbon tetrachloride in nitrogen. When the sensor is used with the TEC, vapor phase concentrations down to 100 ppmv can be measured.
A recent structure of a guide is presented in which the guiding medium is filled with a chiral material and separated from the dielectric clad by a parabolic cylindrical boundary. The eigenvalue equation and the cutoff condition of such a guide have been derived. A possible application of the present analysis may be in the description of curved slab dielectric-chiral guides.
The paper introduces an all-fiber version of the hydrostatic pressure sensor with a chiral nematic liquid crystal film acting as a sensing element. The liquid crystalline film was obtained from a new class of liquid crystalline materials dispersed in a polymer matrix. As light sources, two laser diodes operating at red and infrared wavelengths were used, which were coupled to a compact liquid crystalline optrode. The optrode was placed inside a standard thermally stabilized high-pressure chamber designed to sustain pressures up to 500 MPa. Insertion of the optrode into the chamber was accomplished due to a fiber optic leadthrough system. The all-fiber liquid crystalline hydrostatic pressure sensor offers high response to pressure and, depending on the liquid crystalline optrode used, can be preset for a required range of pressure. Potential areas of applications include pipelines and mining instrumentation, process-control technologies, and environmental protection.
In support of the Lawrence Livermore National Laboratory's (LLNL's) Uranium Atomic Vapor Laser Isotope Separation (U-AVLIS) Program, a laser atomic absorption spectroscopy (LAS) system has been developed. This multilaser system is capable of simultaneously measuring the line densities of 238U ground and metastable states, 235U ground and metastable states, iron, and ions at up to nine locations within the separator vessel. Supporting enrichment experiments that last over one hundred hours, this laser spectroscopy system is employed to diagnose and optimize separator system performance, control the electron beam vaporizer and metal feed systems, and provide physics data for the validation of computer models. As a tool for spectroscopic research, vapor plume characterization, vapor deposition monitoring, and vaporizer development, LLNL's LAS laboratory with its six argon-ion-pumped ring dye lasers and recently added Ti:Sapphire and external-cavity diode- lasers has capabilities far beyond the requirements of its primary mission.
This paper describes a remotely controlled, in-situ pH sensor by measuring the absorption change of an optical waveguide loaded with benzopurpurin 4B solutions. The theoretical evaluation is given for the sensitivity of TE and TM mode. The advantages of this sensor are as follows: (1) a liquid flow indicator avoids any irreversible effects; (2) waveguides generate a uniform evanescent field to provide a long stable reaction length. The TE mode shows better sensitivity than that of the TM mode for this sensor. It has a sensitivity of +/- 0.01 pH and a dynamic range of pH value 2.38 to 6.28.
Use of optical fiber sensors and gyroscopes require a number of in-line components. One important device is a polarizing element. The design of a fiber polarizer using a biconical taper is reported. The model performances for the operating wavelength of 1.3 micrometers are characterized by an extinction ratio of 40 dB or more, with a loss of the desired polarization (throughput loss) of about 2 dB.