Influence of temperature on the refractive index sensitivities of fiber Bragg gratings refractometers

Abstract

Abstract-Effect of cross-sensitivity between temperature and refractive index (RI) was studied using an optical sensor based on Bragg grating inscribed in an etched multi-mode fiber (MMF). RI sensitivities of different modes were used to obtain a linear response and temperature dependence in water:glycerin solutions. Results pointed out that the absolute average RI sensibility reduces from 0.97 ± 0.05 nm/RIU to 0.76 ± 0.07 nm/RIU while the temperature increases from 15 o C up to 35 o C, respectively. Effect of temperature on the RI sensitivity is related with the increasing difference between the fiber and surrounding RI, which results in lower evanescent field and, consequently, reduced surrounding RI sensitivity. Therefore, in order to improve the precision during the RI measures, the effect of temperature must be considered in the calibration curves, principally in MMF fiber grating sensors that show low RI sensitivity values.Index Terms-fiber Bragg gratings, refractive index sensitivity, temperature sensitivity, thermo-optic effect.

Towards Mode-Multiplexed Fiber Sensors: An Investigation on the Spectral Response of Etched Graded Index OM4 Multi-Mode Fiber with Bragg grating for Refractive Index and Temperature Measurement

“…By etching the fiber optic sensor-head the higher order PMG become sensitive to applied SRI variation and by inscribing the BG the higher order and lower order PMG can be discriminated at the optical detector by observing the individual Bragg reflection peaks. The advantage of this approach is that only a single sensor element would be required for the simultaneous detection of these two parameters and due to the relatively large core diameter of 50 µm (in case of the OM4 GI-MMF) the sensor elements remains stable even after etching compared to FMF (core etched down to 17 µm [8]) and SMF (core etched down to 3.4 µm [9]) approaches and thus more applicable to distributed FOSs in the field. as grating assisted mode multiplexer [1,2] or Spatial Light Modulators (SLMs) [3], and a spectral interrogator, several BG structures could be wavelength multiplexed along the same optical fiber and the additional mode-multiplexing would allow the discrimination between the different Bragg reflection of each BG structure.…”