Claudio Oliveira Egalon

Embeddable distributed moisture and pH sensors for nondestructive inspection of aircraft lap joints

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 aircrafts 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.

Theoretical model for a thin cylindrical film optical fiber fluorosensor

The behavior of the power efficiency, P eff , 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 P eff always increases with the difference in the indices of refraction, n core - n clad , 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 P eff is also constant. This indicates that a /λ is a new independent variable.

Injection efficiency from a side-excited thin-film fluorescent cladding of a circular waveguide

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.

Changes in the amount of core light injection for fluorescent-clad optical fiber due to variations in the fiber refractive index and core radius: experimental results

Experiments were conducted to quantify the amount of light injected from a thin film of fluorescent sources confined at the core and cladding interface of an optical fiber, also referred to as a fluorosensor. Conditions necessary for high injection of fluorescent light, previously predicted by a theoretical model, have now been experimentally verified. The results show that, for side excitation, light injection from the thin-film source into guided modes increases with the fiber diameter and the difference between the core and the cladding refractive indices, ncore − nclad.

Modeling an evanescent field absorption optical fiber sensor

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 Hales 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 Hales 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 Hales 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.

Excitation efficiency of an optical fiber core source

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.

Asymptotic approximation and first-order correction of coupled-mode equations

A reformulation of the asymptotic solution of the coupledmode equations with a periodic variation of the refractive index along the propagation length is presented. A first-order correction using the asymptotic solution and Piccard’s method are also determined. It is found that the first-order solution compares very well with the numerical solution throughout a wide range of coupling parameters. The key differences between the method presented here and elsewhere reside in the derivation of the asymptotic solution as well as in the carefull derivation of the higher order corrections.

Source polarization effects in an optical fiber fluorosensor

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.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Model of an axially strained weakly guiding optical fiber modal pattern

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 β in z 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

Phase shift of TE and TM modes in an optical fiber due to axial strain (exact solution)

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.