Fabrication and analysis of enhanced thermal stability and high-sensitivity turnaround point long-period fiber grating.

This paper presents a what we believe is a novel method to fabricate turnaround point long-period gratings (TAP-LPGs) possessing enhanced thermal stability and high sensitivity. It is shown by analysis and by experiment that LPG resonance in photosensitive fibers can be controlled partially by UV fluence and thermal annealing. TAP-LPGs with enhanced thermal stability were fabricated by following three steps: (I) finding grating period versus writing UV fluence for TAP operation; (II) writing gratings at a relatively higher period with higher fluence, in which case the resonance is out of phase; (III) controlled annealing so that the postannealed LPG operates at/near TAP. The thermal stability is enhanced. The average temperature sensitivity of dual peak resonance measured for a typical TAP-LPG in the temperature interval of 70°C-240°C is about 2.3 nm/°C. This study will be useful for the development of high temperature TAP-LPG sensors.

Study of the nonuniform behavior of temperature sensitivity in bare and embedded fiber Bragg gratings: experimental results and analysis.

This paper presents an experiment and analysis on the factors affecting nonlinear evolution of Bragg wavelength with change in temperature in typical bare and embedded fiber Bragg grating-based (FBG) temperature sensors. The purpose of the study was to find the constants in the function required to evaluate temperature from Bragg wavelength shift. The temperature sensitivity of bare FBGs was found to increase with temperature elevation, and is different for FBGs written in different fiber types. The average temperature sensitivity increased by about 20% when the bare FBG temperature was elevated from 25°C to 525°C. The average temperature sensitivity of the embedded FBG sensor, investigated in the temperature range of 30°C-90°C, was a factor of 2-3 times larger than for bare FBG, depending on its fastened length with the substrate. Analytically, it is shown that the nonuniform behavior of temperature sensitivity in bare FBGs is the result of both the thermal expansion effect of the fiber and the temperature derivatives of the effective refractive index. The strain transfer and temperature coefficients of thermal expansion of the substrate affect the nonuniform behavior of temperature sensitivity in embedded FBG sensors.