Temperature effect on Fiber Bragg Gratings based on thermo-optic coefficients and optimization by using Apodization profile

Rumadi (2015) Temperature effect on Fiber Bragg Gratings based on thermo-optic coefficients and optimization by using Apodization profile. Diploma thesis, Universitas Al Azhar Indonesia.

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Abstract

The FBGs temperature sensor offer a high sensitivity, as well as other important advantages, such as real-time processing, long-term stability, electromagnetic interference immunity, and multiplexing capability [1]. The basic principle of FBGs sensor is the measurement of an induced shift in the wavelengthof an optical source due to a measured, such as temperature. Changes in temperature affect both the effective refractive index and grating period of an FBGs, which results in a shift in the reflected wavelength. Effect on the effective index is called the thermo-optical effect, while the effect on the grating period is called the thermal expansion effect. Compared to the thermo-optical effect, the thermal expansion effect can be ignored. In this work, we assume only thermo-optic coefficient influencing temperature effect on fiber Bragg gratings (FBGs). Based on the thermo-optic coefficients, we analyze the effect of temperature on fiber Bragg gratings. The design of fiber Bragg gratings used Coupled Mode Theory (CMT). The coupled mode equations were solved by the Transfer Matrix Method (TMM) since it was considered as good approximation to calculate the spectral response of fiber Bragg gratings. Apodization techniques are used to get optimized reflection spectrum. The simulated reflection of FBGs with different grating lengths and index change were described. The influence of index change and grating length on the full width half maximum (FWHM) of the reflected spectrum at the Bragg wavelength and the maximum reflectivity of the reflected spectrum were investigated. The increasing of index change and grating length causes the increasing of FWHM and maximum reflectivity. Temperature versus wavelength shift graph was plotted. The temperature sensitivity for silica fiber at 1550 nm was found to be 12.5 pm/0C on uniform FBGs and 13.1 pm/0C on linearly chirped FBGs. The FBGs with several types of apodization were modeled in this project. It was proven that apodization profile could be reduced the sidelobes.

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