טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAbir Jonathan
SubjectCorrelation between Optical Attenuation and Fatigue
Damage Evolution in Silver-Halide Optical
Fibers
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Emeritus Eli Altus
Dr. Vadim Regelman
Full Thesis textFull thesis text - English Version


Abstract

Silver-Halide optical Fibers (SHF) transmits light in the infrared region (2-20µm) being used in material processing applications. SHF are manufactured in various compositions (AgClxBr1-x, 0<x<1) having different optical and mechanical properties.

Bending capability of SHF is critical in medical procedures where it repeatedly bends in elastic and plastic regimes.

SHF spectral measurements of transmittance during fatigue experiments shows three stages of optical degradation related to accumulation of surface cracks: A. no significant optical loss or wavelength sensitivity, B. moderate optical loss and wavelength sensitivity and C. significant optical loss and moderate wavelength sensitivity. The major goal of this thesis is to determine a correlation between scattering defects diameter and surface density with optical transmission properties in B and C regions.

Finding the correlation between fatigue damage and optical transmission may allow estimating the fibers service life and used as a health monitoring sensor.

Light scattering from surface cracks depends on wavelength, refractive index and the scatterer properties: shape, size and density. Wavelength is controllable and refractive index is known. The only unknowns are the scatterer properties.

A simple “opto-fatigue” model is constructed by assuming:

         Scattered rays beyond a critical angle are treated as a loss.

         Crack density is low enough consider the scattering effect to be independent on cracks location and order.

         The scattering is linear.

         The cracks are much smaller than the fiber optic diameter.

Beer-Lambert law describes the transmission in a scattered media by two parameters: the scatterer’s optical Cross Section Area (CSA) and density. We defined a relative transmission function-γ of two different wavelengths, which relates to the optical CSA ratio. This ratio is independent on scattering centers density but depends only on crack shape and dimensions. Assuming a certain crack shape a typical crack dimension can be estimated without any fitting parameters and directly compared with the experimental data. The optical CSA was calculated based on the crack dimensions, and then used to estimate the density of the cracks directly from the measured transmission values. Mie scattering theory analytical solution for sphericale and Infinite Cylinder shaped scattering centers was used to calculate the CSA. The model was further elaborated assuming various size distributions of scattering centers: a uniform sized crack and a Weibull probability density function. The calculated size(s) and density were plotted for each bending cycle, thus new information about the micro-damage fatigue process was obtained -distance between cracks and crack phases-“nucleation” and crack growth.