Abstract
We present a comprehensive study of the strain and temperature-sensing
characteristics of single-mode–multimode–single-mode (SMS) structures
based on the modal interference of guided modes of graded index multimode
fiber (MMF) section spliced in between two single-mode fibers. A detailed
theoretical study of the structures in terms of the refractive index distribution,
effect of dopant and their concentrations, and the variation of core diameter
has been carried out. Our study shows that for the SMS structure with a ${\rm GeO}_{2}$-doped
MMF there exists a critical wavelength on either side of which the spectrum
shows opposite spectral shift with a change in temperature/strain, whereas
for structures with a ${\rm
P}_{2}{\rm O}_{5}$-doped MMF it shows monotonic red shift
with increasing temperature/strain. It has been found that the critical wavelength
shifts toward higher wavelengths with decreasing “$q$” value/doping concentration.
Using different MMFs, both the red and blue spectral shifts have been observed
experimentally. It has also been found that the SMS structure has higher sensitivity
toward this critical wavelength. The study should find application in designing
strain-insensitive high-sensitive temperature sensors or vice versa.
© 2009 IEEE
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