Optical Interferometric Force Sensor Based on a Buckled Beam

This paper reports a novel extrinsic Fabry-Perot interferometer (EFPI)-based fiber optic sensor for force measurement. The prototype force sensor consists of two EFPIs mounted on a stainless steel rectangular frame. The primary sensing element, i.e., the first EFPI, is formed between the endface of a horizontally-placed optical fiber and a stainless steel buckled beam. The second EFPI, fashioned between a longitudinally-placed optical fiber and a silver-coated glass beam, is arranged to demonstrate the amplification mechanism of the buckled beam structure. When the sensor is subjected to a tension force, the pre-buckled beam will deflect backward, resulting in a longitudinal/axial displacement of the pre-buckled beam. The axial displacement is further transferred and amplified to a horizontal/vertical deflection at the middle of the buckled beam, leading to a relatively-significant change in the Fabry-Perot cavity length. A force sensitivity of 796 nm/ ${N}$ (change in cavity length/Newton) is achieved with a low-temperature dependence of $0.005 {N}$ /°C. The stability of the sensor is also investigated with a standard deviation of ± 5 nm, corresponding to a measurement resolution of ±0.0064 N. A simulation is conducted to study the axial displacement and stress distribution of the sensor when it is subjected to a tension load of 250 N. It is demonstrated that the maximum stress of the sensor is tremendously reduced attributed to the buckled design, enabling a long service life cycle of the force sensor. The robust and simple-to-manufacture force sensor has great potential in structural health monitoring, robotics control, and oil/ gas refining systems.