This paper provides a brief summary of the scintillation process in organic and inorganic materials, the properties and uses of fiber lightguides made from these materials. Also presented are some recent data on a new application of scintillating fiber lightguides in nuclear medicine.
In this paper, a novel optical sensor using surface plasmon resonance in a symmetrical planar lightpipe is introduced. The new design utilizes a microscope slide with beveled ends as the sensor substrate. Collimated TM polarized white light is used to interrogate the sensing surface at a single angle. Preliminary experimental results for glycerol solutions from 0.6%wt to 16%wt demonstrate a concentration sensitivity of 3.4 multiplied by 10-4 by weight. The corresponding refractive index sensitivity is estimated as 4 by 10-5.
Fiber optic gratings have the potential to be used to support a wide variety of strain measurement applications. These applications include low cost longitudinal strain measurements at temperature ranges up to 800 degrees C, high performance multiaxis strain sensing and integrated strain measurements over long distances. In many cases these applications may require the unique properties associated with fiber optic sensors and in particular fiber optic gratings including small size, the ability to be embedded in a wide variety of materials, high immunity to electromagnetic interference and ease of multiplexing. This paper reviews and summarizes ongoing efforts to apply fiber optic grating technology to a variety of applications requiring the measurement of strain.
This paper describes the design of a fiber optic sensor capable of sensing temperature and three independent components of strain simultaneously in a single, short gage length device. The sensor utilizes two fiber Bragg gratings at widely spaced wavelengths (1300 nm and 1550 nm) written at a single location in polarization maintaining optical fiber. When a broad-band light source is used to illuminate the gratings, the reflected spectrum will contain four peaks corresponding to the two polarization states for each of the two gratings. If the fiber is subjected to a change in temperature or strain, the resulting change in wavelength of the reflected peaks can be used to determine the magnitude and direction of the perturbation. In theory, the four peaks can be used to simultaneously determine the grating temperature, and three independent components of strain in the fiber.
Analysis and results of experiments concerning all-fiber passive demodulation of fiber Bragg grating sensors using a many-cycle directional coupler are reported here. The principle of operation is based on the fast wavelength response of a fused tapered 2 by 2 directional coupler pulled through many coupling cycles. The coupler's splitting ratio is then sensitive to small changes in the reflected Bragg wavelength. A design rule to allow the selection and fabrication of a coupler with the desired response has been developed, and the coupler splitting ratio linearity with wavelength is also examined. Because such directional couplers may be readily tailor-made by choosing the number of coupling cycles the coupler is pulled through during fabrication, this approach makes possible a range of sensitivities and wavelengths. Problems encountered with highly sensitive devices due to polarization dependent splitting are also discussed.
Sensing schemes using optical low coherence interferometry can offer advantages over the more conventional long coherence length interferometric techniques. These advantages are discussed as well as various techniques for single and multiplexed sensing arrangements.
This paper outlines recent progress made by Optiphase Inc. in the development of low cost fiber optic interferometric sensors. The paper's focus is on components under development, specific to interferometric fiber sensors (IFS), which aren't commercially available through normal telecommunications distribution channels.
As the market for fiber optic sensors increases, the demand for specialty optical fibers and components also increases. Today there are numerous fibers and components available to the fiber optic sensor developer, many of which have been developed by, and spun-off from, the larger telecommunications industry. This paper is a review of different optical fibers and components that are available to the fiber sensor community, and why some components are better than others for certain applications. Specific items reviewed here are: single-mode fibers, new cost effective polarization maintaining fibers, specialty fiber coatings, erbium doped fibers, bend insensitive single-mode fibers, fiber Bragg gratings and high reliability fiber couplers and taps.
Sandia National Laboratories in Livermore, Calif. and the Mechanical Engineering Department of Stanford University are involved in fiber optic sensor research and development for manufacturing process monitoring, smart materials, and other applications. Projects at Sandia and Stanford involving both embedded and surface mounted fiber optic strain and temperature sensors have demonstrated the desirability of this technology. This paper presents an overview of the fiber optic sensing capabilities at Sandia and a summary of the projects currently in progress.
The effect of input angle of polarization on the force sensitivity of embedded single mode fibers was investigated. Two types of fiber specimens were used -- one consisted of a fiber embedded with its protective plastic coating intact while the other had a fiber stripped of this coating. Plane polarized light, whose direction could be varied, was launched into the fibers and the embedded portions were compressed. The polarization changes due to birefringence effects were recorded with a pair of polarizers and photodiodes. The experiments show that the amount of polarization change due to force varied with input polarization angle. Hence, the force sensitivity of polarimetric stress sensors depended on the input angle of the plane polarized light. It was also found that the load- induced polarization changes were more significant in the stripped specimen than in the fiber embedded with coating.
The true/apparent strain ambiguity associated with structurally embedded sensors of strain can be avoided with a novel sensor that measures deformation curvature. This fiber optic sensor is intensity modulated. Suitable fiber treatment enables the sensitivity to relate selectively to deformation about a preferential axis. With the addition of an inexpensive light-loss measuring system, a device is achieved which can readily interface with a computer. In the case of structural thicknesses below 1 cm, the proposed sensor exceeds the sensitivity of resistive strain gauges, especially along the neutral axis where strain cannot be measured in bending. Since strain and curvature are functionally related, either one can usually be deduced from the knowledge of the other. Distributed curvature measurements are possible and sensors can be tailor-made for a particular vibration mode in order to achieve modal decoupling.
Research efforts in our department include work on novel GaAs/AlGaAs laser structures and photodetectors which could be used in fiber optic sensor applications. Brief descriptions are given of three such components: a HEMT- compatible, very low threshold diode laser; a high-gain traveling-wave amplifier or high-power, broad linewidth LED; and a MODFET photoconductor. A senior/graduate level course is also taught on guided wave optics. In this lecture/laboratory course, students do design projects on either fiber optic sensors or fiber communication systems. Two examples of student fiber sensor projects will be given: a pressure sensor and an interferometric temperature sensor.
The impedance boundary method of moments (IBMOM) for planar optical waveguides is reviewed. An extension of the IBMOM for optical fibers with truncated graded index profile is described. Results for a step index fiber show that virtually exact solutions for the modal field profile and propagation constant can be obtained with only three Legendre expansion functions. The IBMOM is applied in the design and analysis of an evanescent field optical waveguide chemical sensor which utilizes an antiresonant reflecting optical waveguide (ARROW) structure and is implemented as a Mach-Zehnder interferometer. The ARROW structure allows the use of a 5 micrometer wide guiding region for efficient coupling into a single mode optical fiber. The ARROW sensor is designed for a sensitivity of 180 degrees/cm phase change for a change of 0.05 in refractive index.
Designs of fiber optic chemical sensors are reviewed. Two basic types of sensors are described. The first is a reservoir sensor for determining chloroform based on vapor transport and fluorometric detection. The second type of sensor is a renewable reagent sensor based on vacuum aspiration of samples. Application of this sensor to the fluorometric determination of aluminum and the chemiluminescence determination of chromium is described.
Pacific Northwest National Laboratory is developing a large- area, fiber-optic chemical sensor that combines chemically selective coatings and optical spectroscopy. This is a potentially hyphenated sensing technique because of the ability to collect broadband spectroscopic information in addition to sensing the quantity of the target species. Selective compound coating of optical waveguides enables the production of chemical sensors in large lot sizes. This paper describes the progress to date to produce iodine vapor selective fiber sensors that use through the fiber absorption spectroscopy. Spectra have been collected on uncalibrated I2/N2 gas mixtures using visible light.
Thanks to the growth of the fiber optics telecommunication industry, fiber optic components have become less expensive, more reliable and well known by potential fiber optic sensor users. LEDs, optical fibers, couplers and connectors are now widely distributed and are the building blocks for the fiber optic sensor manufacturer. Additionally, the huge demand in consumer electronics of the past 10 years has provided the manufacturer with cheap and powerful programmable logic components which reduce the development time as well as the cost of the associated instrumentation. This market trend has allowed Photonetics to develop, manufacture and sell fiber optic sensors for the last 10 years. The company contribution in the fields of fiber optic gyros (4 licenses sold world wide), white light interferometry and fiber optic sensor networks is widely recognized. Moreover, its 1992 acquisition of some of the assets of Metricor Inc., greatly reinforced its position and allowed it to pursue new markets. Over the past four years, Photonetics has done an important marketing effort to better understand the need of its customers. The result of this research has fed R&D efforts towards a new generation instrument, the Metricor 2000, better adapted to the expectations of fiber optic sensors users, thanks to its unique features: (1) universality -- the system can accept more than 20 different sensors (T, P, RI, . . .). (2) scalability -- depending on the customer needs, the system can be used with 1 to 64 sensors. (3) performance -- because of its improved design, overall accuracies of 0.01% FS can be reached. (4) versatility -- its modular design enables a fast and easy custom design for specific applications. This paper presents briefly the Metricor 2000 and its family of FO probes. Then, it describes two fiber optic sensing (FOS) applications/markets where FOS have proven to be very useful.
Optical power provides a novel and often superior way of delivering power to electronic sensors and transducers. Total immunity to lightning and other electromagnetic interference comes from the use of fiber optics to provide power and data communication. The key element in any optically powered sensor or transducer is a photovoltaic power converter developed by Photonic. This device converts light into electrical energy for powering of the sensor and associated circuitry. Pertinent design issues include, choice of light source, minimization of power consumption, single vs. dual fiber, data protocol and level of integration. In addition to a discussion of these issues, a brief outlook on the future of optically powered systems is presented.
We investigated the use of wavelength compensated coupler sensors and etched optical fiber strain sensors to provide an economical on-board load indicator for passenger vehicles. Cost considerations favored the etched fiber sensor. Manufactured sensors were evaluated experimentally by straining them on a cantilever beam. For strain smaller than 600 microstrain the output of a 10 segment sensor was linear with a typical gauge factor of minus 57. Bending losses in the fiber sensor became more pronounced for larger strain. Proper weighting of the outputs of the front and back sensors on the vehicle ensures a monotonic relationship between the sensor output and load. Difference-over-sum processing minimizes the effects of sensitivity to common- mode perturbations such as temperature and source intensity changes.
Modulation of light scattering by liquid crystal droplets dispersed in a polymer matrix by an electric field forms the basis of a compact electric field sensor. A thin layer of polymer dispersed liquid crystal is interspersed between two cleaved end faces of multimode fiber. In the absence of an electric field the droplets are randomly oriented. The anisotropy of the refractive index of the liquid crystal causes light to be scattered out of the acceptance angle of the receiving fiber. As the major axis of the indicatrix of the droplets aligns with the field, the anisotropy in refractive index is lowered. The fraction of the light which is scattered is therefore reduced. In this paper we report on the properties of an electric field sensor envisaged for application to overhead transmission lines and utility substations. We discuss linearity, hysteresis, and the effect of temperature on the sensor.