Low-temperature nonlinear effect of fiber Bragg gratings

Abstract

Fiber Bragg gratings (FBGs) sensor has been widely used in all kinds of detection spaces. Nonlinear effects of the fiber Bragg gratings have been observed in high-temperature conditions, however, it occurred in low-temperature as well. In this paper, we take the low-temperature experiments in the low-temperature thermostat bath, temperature range from 10°C to -80°C, the Bragg wavelength shift with the temperature decreasing linearly at the very beginning and it shows linear characteristic range from room temperature to -45°C. However with the temperature goes down continuously, the nonlinear effects emerged, the turning point temperature of the nonlinear effect is at -45.3°C. Besides, the sensitivity of the FBGs decreased as well from 8.96pm/°C to 6.72pm/°C. Considering the physical characteristic of the silica fiber, which the thermo-optic coefficient and the thermal expansion coefficient of the fused silica is not constant if temperature goes down and it shows nonlinear features, therefore we conclude the nonlinear effect at low-temperature is attributed to the thermal expansion and the thermo-optic effect of the silica fiber. Thus, we predict that appropriate doping improvements in the silica fiber can modify the linear range of FBGs which can enhance the measure precision. In addition, we find that high sensitivity FBGs has a lower temperature turning point of the nonlinear effect. The invar packaged FBGs has a sensitivity of 24.3pm/°C at room temperature. It is higher than bare FBGs’ sensitivity which is about 8.96pm/°C at room temperature. The invar packaged FBGs’ temperature turning point is at about -54.5°C, which is lower than the bare FBGs’, -45.3°C, temperature turning point. This indicates that high sensitivity FBGs can also increase the linear temperature range. The experiment results and analysis show that we can either by increasing the sensitivity of FBGs or doping in the silica fiber to modify the linear range.