Miniature Twist/Rotation Fabry Perot Sensor Based on a Four-Core Fiber

Abstract

This paper presents a miniature Fabry Perot twist/rotation sensor. The presented sensor consists of a single lead-in multicore fiber, which has four eccentrically positioned cores, a special asymmetrical microstructure, similar to a truncated cylinder, and an inline semi reflective mirror, all packed in a glass capillary housing. The perpendicular cut lead-in multicore fiber and the inline semi reflective mirror form four Fabry-Perot cavities. The optical path length of each Fabry-Perot interferometer is defined by the distance between mirrors, refractive index and twist/rotation angle of the microstructure in relation to the core positions in the lead in multicore fiber. Optical paths of Fabry-Perot Interferometers are modulated by a structure’s twist/rotation, change of structure length, or change of temperature. Each of these parameters modulate the optical path length of the individual interferometers in their own separate fashion, thus allowing independent measurements of twist/rotation, length/strain and temperature.

Advances in Multicore Fiber Interferometric Sensors

“…The reported four different structures of MCF interferometric sensors exhibit their own advantages and disadvantages. For example, MCF-based FPIs are able to achieve a very tiny size of the sensor head (less than 200 μm [ 13 , 14 , 24 , 31 , 32 , 34 ]), and each core can be used to fabricate an individual sensor, which can measure differential responses in distinct cores separately [ 13 , 14 , 24 , 31 , 32 , 33 ]. However, the manipulation of an individual core in the MCF is still a technical challenge, thus it is difficult to fabricate intrinsic FPIs in different cores of MCF.…”

“…In MCF-based Fabry–Perot interferometric sensors, FPIs are usually discrete and parallel in different cores of a weakly coupled MCF [ 13 , 14 , 30 , 31 , 32 , 33 ]. Instead of using a single FPI, it is able to achieve vector sensing by measuring several parallel FPIs simultaneously in a single MCF, such as bending direction [ 13 , 30 ], torsion [ 31 ], flow direction [ 32 ], etc.…”

“…In MCF-based Fabry–Perot interferometric sensors, FPIs are usually discrete and parallel in different cores of a weakly coupled MCF [ 13 , 14 , 30 , 31 , 32 , 33 ]. Instead of using a single FPI, it is able to achieve vector sensing by measuring several parallel FPIs simultaneously in a single MCF, such as bending direction [ 13 , 30 ], torsion [ 31 ], flow direction [ 32 ], etc. For example, in 2017, Yang Ouyang et al proposed a directional bending sensor with the structure of a dual air-cavity FPI, which was fabricated by using MCF and a dual side-hole fiber [ 13 ], whose structure is shown in Figure 4 a.…”

“…In addition to bending sensing, parallel FPIs can also be used to measure tiny torsion angles by utilizing the twist-induced cavity length variation. For example, in 2022, Jing Zhang et al proposed a torsion sensor where four FPIs were made up of vertically cleaved four-core fibers and an angled mirror, and the sensor was demonstrated to be capable of sensing torsion with a range of ±90° and a resolution of 0.01° [ 31 ]. As shown in Figure 5 , the cavity length of each FPI will change, respectively, when torsion is applied to the four-core fiber and the spectral characteristics of each FPI can be analyzed utilizing a four-core fan-out unit.…”