Temperature-insensitive optical fiber strain sensor fabricated by two parallel connection Fabry–Perot interferometers with air-bubbles

A strain sensor formed by a parallel connection of two Fabry-Perot interferometers (FPI) is proposed. The femtosecond laser is used to process a micro groove on the end face of a single-mode fiber (SMF), and then, it is welded with another SMF to form a small air bubble at the fusion point, fabricating an FPI. When the axial strain acts on the air bubble, the transverse length of the air bubble will change, causing the air cavity of the FPI to be easily deformed, and FPI can obtain high strain sensitivity. Three FPIs were manufactured with the air bubble sizes of 63, 78, and 93 µm, respectively, and the strain sensitivities of the three FPIs are 2.9, 2.0, and 1.5 pm/µε, respectively. The experimental results show that the smaller the air bubble, the higher the strain sensitivity of FPI. Since the free spectral ranges of the three FPIs are relatively similar, we, respectively, paralleled them to form two Vernier effect strain sensors, and their sensitivities are -14.9 and -14.5 pm/µε, respectively. Their sensitivities are increased by 5.1 times and 7.3 times, respectively. In addition, because three FPIs are composed of air cavities, they have very low temperature sensitivities. When they are connected in parallel, their resonance peak wavelength moves in the same direction with an increase in temperature, forming a reduced Vernier effect, and the temperature sensitivity amplification is very small. Therefore, the temperature cross-sensitivity of the sensor is extremely low and can be ignored.