The Impact of Assembly Configuration on Diaphragm-Embedded Fiber Bragg Gratings Pressure Sensors

In this paper, the experimental analysis of different assembly configurations for diaphragm-embedded optical fiber sensors is proposed. A fiber Bragg grating (FBG) is embedded in a nitrile rubber diaphragm, which is positioned in different front and back supports. The difference between such supports is their contact angles with the diaphragm, which leads to differences in the strain applied in the FBG prior to the sensor application. This prestrain condition in the diaphragm influences the sensor's performance in terms of sensitivity, linearity and hysteresis. To that extent, there are 8 different configuration angles, namely 0 degrees, 1.5 degrees, 3.8 degrees, 6.8 degrees, 9.0 degrees, 11.3 degrees, 13.5 degrees and 15 degrees. The diaphragm-embedded FBG is positioned at each configuration and pressure is transversely applied on the diaphragm region (which transmits strain to the optical fiber) at constant temperature conditions. Results show a relation between the configuration angle and the sensor performance, where sensitivity and linearity presented opposite behaviors, i.e., lower angles (especially the 0 degrees angle) lead to high linearity with a coefficient of determination (R-2) of 0.992, whereas higher angles resulted in higher sensitivity with the highest sensitivity value of 13.26 pm/kPa. In contrast, the sensor hysteresis showed a peak value of 6.7% at 9.0 degrees angle with hysteresis decrease in angles higher and lower than 9.0 degrees. The smallest hysteresis of 0.2% is obtained at 1.5 degrees. The obtained results can be used as a guideline on the development of diaphragm-embedded sensors with the additional degree of freedom from the possibility of customizing the diaphragm assembly configuration to achieve predefined performance parameters.