A side illuminated optical fiber sensor with three sensing points and an absorption-based indicator in the cladding was
demonstrated for the first time. This device is easy to manufacture, uses leaky modes as the signal carrier and can
measure RH in air, soil, concrete and other environments. So far, only side illuminated fluorescence sensors have been
reported. They were thought, erroneously, to have their entire signal generated by evanescent wave coupling when, in
fact, leaky modes also play an important role. This, coupled to the prevailing misconception that leaky modes propagate
for very short lengths of fiber, prevented the earlier discovery of this absorption-based configuration. A 25 cm long
fiber, with a cladding doped with an absorption dye sensitive to Relative Humidity (RH), was used in this demonstration.
The fiber was side illuminated by a broadband LED, a fraction of this light was absorbed by the cladding and the
remaining light guided to the fiber tip as low loss leaky modes. A total of three sensors, two with three sensing points
and one with two, were calibrated using a low cost photometer. The signal was linear, stable, increased with RH and had
resolutions between 0.11% and 0.25% in RH. With 5 mm diameter LEDs, devices with at least two sensing points per
centimeter of fiber can be easily fabricated resulting in sensors with a very high density of sensing points. Compared to
the prevailing axial illumination approach, the side illuminated sensor was found to be far simpler and inexpensive.
A new type of colorimeter with multiple channels was demonstrated using a side illuminated optical fiber. When different spots of a properly modified fiber are side illuminated, multiple signals are generated and guided by the waveguide: the essence of multiplexing. This configuration is simple, low cost, does not require a sensitive coating and can analyze several samples along the fiber with a single detector: the most expensive component. Since regular colorimeters use one detector per sample, our new configuration considerably lowers the cost of analyzing multiple samples. This system consists of a fiber mounted over a support, three LEDs, an LED driver, a photo diode and a read-out: to increase the signal, the fiber was tapered. For calibration purposes, six solutions of different concentrations of food dye were prepared, placed inside cuvettes along the fiber length and illuminated by the LEDs. This light passes through the solution, strikes the fiber and is guided to the detector: the darker the solution the lower the signal intensity. Several calibration curves were obtained using different light intensities: it was found that the greater the intensity, the higher the colorimeter sensitivity. This simple capability can be used to easily control the device’s sensitivity and its resolution. Although built for three samples only, this device can be modified to accommodate more. With cuvettes measuring 1 cm, it is possible to accommodate one sample per cm of fiber. Also, with minor modifications, this colorimeter can be used for fluorescence, scattering and index of refraction measurements.
KEYWORDS: Optical fibers, Absorption, Cladding, Fiber optics sensors, Sensors, Analytical research, Chemical fiber sensors, Optical engineering, Chemical detection, Chemical species
Using the weakly guiding and exact field solutions of an optical fiber, we wrote a FORTRAN program to determine the fractional power that reaches the end of an optical fiber with an absorptive cladding. We have assumed that each mode of the fiber is equally excited. This corresponds to incoherent source excitation. The results were compared to a previous approximation published by Payne and Hale in 1993. We have found that, at low V-numbers, V<20, Payne and Hale's approximation deviate by more than 20% from the weakly guiding solution. At high V-numbers, the approximation deviates by less than 10%. When compared to Payne and Hale's approximation, both the weakly guiding and exact solutions are closer to the data points obtained experimentally by Degrandpre and Burgess in 1988. Although closer than Payne and Hale's approximation, our solution still deviates from the Degrandpre and Burgess results. The difference may result from the assumption that all modes were excited equally. Another possibility was the fact that we have neglected leaky modes in our treatment.
Using the weakly guiding and exact field solutions of an optical fiber, we wrote a FORTRAN program to determine the fractional power that reaches the end of an optical fiber with an absorptive cladding. We have assumed that each mode of the fiber is equally excited. This corresponds to incoherent source excitation. The results were compared to a previous approximations published in the literature. We have found that, at low V-numbers, V < 20, Payne and Hale's approximation deviate by more than 20% from the weakly guiding solution. At high V-numbers, the approximation deviated by less than 10%. When compared to Payne and Hale's approximation, both the weakly guiding and exact solutions are closer to the data points obtained experimentally by Degrandpre and Burgess. Although closer than Payne and Hale's approximation, our solution still deviates from Degrandpre and Burgess' results. The difference may be due to the assumption that all modes were excited equally. Another possibility was the fact that we have neglected leaky modes in our treatment.
Two distributed fiber optic sensors for use in the prevention and monitoring of corrosion in aircraft are described. These sensors, based on optical fibers that are intrinsically sensitive to either water or changes in pH, will alert maintenance personnel to the presence of water in lap joints and other inaccessible critical areas. Furthermore, the sensors can also locate precisely where the moisture infiltration has occurred. In a typical application, a sensor fiber would be embedded in a lap joint along the bottom panel of an aircraft's body, or on a wing, where water is likely to collect. Changes in the optical transmission through the fiber can be monitored either periodically or continuously to determine the extent of water penetration.
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 a 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.
A numerical model was developed to calculate the modal phase shift of a circular step index profile weakly guiding optical fiber under axial strain. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each mode change. In consequence, the phase of each mode is also
modified. A relationship for the modal phase shift is presented. This relation is applied to bpth single-mode and two-mode fibers to determine the sensitivity characteristics of strained fibers. It was found that the phase shift is strongly dependent on the core refractive index nco. It was also found that it is possible to design fibers that are insensitive to axial strain. Practical applications of strain-insensitive fibers are discussed.
Axial strain may be determined by monitoring the phase shift of a single mode optical fiber. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each fiber mode change. In consequence, the phase term, (beta) (Zeta) , is also modified. In this paper, a relationship for the phase shift, due to axial strain, of the hybrid modes HE1n and EH1n is presented using the exact field solution. This relation is applied to single mode fibers in order to determine phase sensitivity characteristics.
Axial strain on an optical fiber can be determined by monitoring the phase shift of a variety of optical fiber sensors. The exact analytical solutions for optical fields that propagate in a circular core optical fiber are used here to determine the phase shifts that occur for TE and TM modes due to axial strain. Whenever an optical fiber is stressed, the optical path length, the index of refraction, and the propagation constant of each fiber mode change. In consequence, the modal phase term βz of the fields is shifted by an amount ΔΦ. In certain cases, it is desirable to control the phase sensitivity to make fibers that are either more or less sensitive to strain. We show that it can be accomplished by choosing appropriate fiber parameters.
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.
Temperature may be determined by monitoring the modal phase shift of an optical fiber. In this paper we present the results of a numerical model that has been developed to calculate the phase shift of a weakly guiding optical fiber due to thermal strain. Whenever an optical fiber is subjected to temperature changes, the optical path length, the index of refraction and the propagation constants of each fiber mode change. In consequence, the modal phase term, (beta) inL, of the fields is also modified. A relationship for the modal phase shift is presented. This relation is applied to both single mode and two mode fibers in order to determine the sensitivity characteristics of fibers that are subjected to temperature changes.
A numerical model has been developed to calculate the modal phase shift of circular step index profile weakly guiding fibers under axial strain. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each mode change. In consequence, the phase of each mode is also modified. A relationship for the modal phase shift is presented. This relation is applied to both single mode and two-mode fibers in order to determine the sensitivity characteristics of strained fibers. It was found that the phase shift is strongly dependent on the core refractive index, n(co). It was also found that it is possible to design fibers which are insensitive to axial strain. Practical applications of strain insensitive fibers are discussed.
The exact field solution of a step-index profile fiber was used to determine the injection efficiency of a thin-film distribution of polarized sources located in the cladding of an optical fiber. Previous results for random source orientation were confirmed. The behavior of the power efficiency, Peff, of a polarized distribution of sources was found to be similar to the behavior of a fiber with sources with random orientation. However, for sources polarized in either the x or y direction, Peff was found to be more efficient.
Axial strain may be determined by monitoring the phase shift of modes of a variety of optical fiber sensors. In this paper, the exact solution of a circular optical fiber is used to calculate the phase shift of the TE and TM modes. Whenever an optical fiber is stressed, the optical path length, the index of refraction, and the propagation constants of each fiber mode change. In consequence, the modal phase term, beta(ln)z, of the fields is shifted by an amount Delta phi. In certain cases, it is desirable to control the phase shift term in order to make the fiber either more or less sensitive to certain kinds of strain. It is shown that it can be accomplished by choosing appropriate fiber parameters.
Axial strain can be determined by monitoring the modal pattern variation of an optical fiber. The results of a numerical model developed to calculate the modal pattern variation at the end of a weakly guiding optical fiber under axial strain is presented. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each fiber mode change. In consequence, the modal phase term βinz of the fields and the fiber output pattern are also modified. For multimode fibers, very complicated patterns result. The predicted patterns are presented and an expression for the phase variation with strain is derived
The exact field solution of a step-index profile fiber was used to determine the excitation efficiency of a distribution of sources in the core of an optical fiber. We have compared previous results of a thin-film cladding source distribution to its core source counterpart. The behavior of power efficiency Peff with the fiber parameters was examined and found to be similar to the behavior exhibited by cladding sources. It was also found that a core source fiber is two orders of magnitude more efficient than a fiber with a bulk distribution of cladding sources. This result agrees, qualitatively, with previous ones obtained experimentally.
The behavior of the power efficiency Peff of an optical fiber with a bulk distribution of source in its cladding is analyzed. The expressions derived make use of the exact field solution of a cylindrical fiber, whereas previous work has made use of the weakly guiding approximation. Although more complicated and harder to interpret, the formulas presented allow the analysis of the power injection efficiency of fibers with arbitrary differences in indices of refraction. The results obtained are relevant to the design of more efficient optical fiber fluorosensors. The conclusions follow. Previous results using the weakly guiding approximation were confirmed. However, we have found that Peff does not always increase with the V number; it does, however, increase with the difference in the indices of refraction, ncore - nclad, and the wavelength λ. It also decreases with the fiber core radius a. Finally, a new independent variable, a/λ, was found.
The exact field solution of a step index profile fiber was used to determine the injection efficiency of a thin-film distribution of polarized sources located in the cladding of an optical fiber. Previous results for random source orientation were confirmed. It was found that the behavior of the power efficiency, Peff, of a polarized distribution of sources is similar to the behavior of a fiber with sources with random orientation. However, it was found that for sources polarized in either the x or y direction, Peff is more efficient.
The exact field solution of a step profile fiber was used to determine the injection efficiency of a bulk distribution of sources located in the cladding of an optical fiber. Previous results for weakly guiding fibers were confirmed. We found that, for a fixed V-number, the power efficiency Peff, increases parabolicaly with the difference in the indices of refraction, ncore-nclad. Also, Peff was found to increase with the wavelength (lambda) and decrease with the core radius a.
The behavior of the power efficiency, Peff, of an optical fiber with a thin-film source distributed in the core/cladding interlace is analyzed. The expressions derived make use of the exact field solution of a cylindrical fiber, whereas previous work has made use of the weakly guiding approximation. Although more complicated and harder to interpret, the formulas presented allow the analysis of the power injection efficiency of fibers with arbitrary differences in indices of refraction. The results obtained are relevant to the design of more efficient optical fiber fluorosensors. In this work, some results using the weakly guiding approximation were confirmed. However, in contrast to the weakly guiding work, we have found that Peft does not always increase with the V number. It was found that Peff always increases with the difference in the indices of refraction, ncore - nclad, and varies only slightly with the wavelength, λ, and the fiber core radius, a. Finally, by varying a and λ in such a way as to make their ratio, a/λ, constant, it was found that Peff is also constant. This indicates that a/λ is a new independent variable.
Axial strain may be determined by monitoring the modal pattern variation of an optical fiber. In this paper we present the results of a numerical model that has been developed to calculate the modal pattern variation at the end of a weakly guiding optical fiber under axial strain. Whenever an optical fiber is under stress, the optical path length, the index of refraction and the propagation constants of each fiber mode change. In consequence, the modal phase term of the fields and the fiber output pattern are also modified. For multimode fibers, very complicated patterns result. The predicted patterns are presented, and an expression for the phase variation with strain is derived.
The exact field solution of a step index profile fiber was used to determine the excitation efficiency of a distribution of sources in the core of an optical fiber. Previous results of a thin- film cladding source distribution are compared to its core source counterpart. The behavior of Peff with the fiber parameters was examined and found to be similar to the behavior exhibited by cladding sources. It also was found that a core source fiber is two orders of magnitude more efficient than a fiber with a bulk distribution of cladding sources. This result agrees, qualitatively, with previous ones obtained experimentally.
The efficiency of core-light injection from sources in the cladding of an optical fiber is modeled analytically by means of the exact field solution of a step-profile fiber. The analysis is based on the techniques by Marcuse (1988) in which the sources are treated as infinitesimal electric currents with random phase and orientation that excite radiation fields and bound modes. Expressions are developed based on an infinite cladding approximation which yield the power efficiency for a fiber coated with fluorescent sources in the core/cladding interface. Marcuse''s results are confirmed for the case of a weakly guiding cylindrical fiber with fluorescent sources uniformly distributed in the cladding, and the power efficiency is shown to be practically constant for variable wavelengths and core radii. The most efficient fibers have the thin film located at the core/cladding boundary, and fibers with larger differences in the indices of refraction are shown to be the most efficient.
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