Study of high temperature fiber Bragg grating stability

A Fiber Bragg Grating (FBG) is an optical fiber with a periodic refractive index structure that act as a wavelength dependent filter/reflector. Most wavelengths are transmitted through the grating, but the wavelength satisfying the Bragg condition (?Bragg) will be reflected back.
The reflected wavelength satisfying ?Bragg is dependent on the fibers strain and temperature and by measuring this wavelength fiber Bragg gratings can be used as sensors for.
Sensors based on FBGs have been commercial for many years but they all have the disadvantage that they not can be used at temperatures above 250°C since that will erase the refractive index structure.
Acreo has addressed this issue by developing so-called Chemical Composition Gratings, CCGs that can withstand much higher temperatures. However, even CCGs will degrade after some time at high temperatures but today very limited amount of data exists regarding the long term stability and the mechanisms behind the degradation. Knowing the characteristics is crucial before any commercial sensors can be made. Further, more knowledge of the degradation mechanisms will give information about how to improve the stability by smart design.
The aim with the research in this Diploma work is to:
-Characterize and gain a better understanding of CCGs temperature stability over time (and even how it is chemically affected by H2O, O, H etc).
- Compare obtained data with existing theoretical degradation mechanisms, and with previous obtained data.
- Find ways how fiber design, production processes and packaging can improve stability.
In practice this means that the following steps are involved in the work:
• Learning how to handle optical fibers (cleaving, splicing etc).
• Learn the basic theories behinds FBGs and CCGs
• Actively attend in writing FBGs and especially in the thermal processing of CCGs
• Responsible for at least one long term stability test.
• Continuously logging data during the test
• Analyze and interpret collected data and compare with theories.