This review focuses on the preparation methods and wide applications of optic fiber sensors based on core-offset structure. Firstly, the classification and preparation methods of core-offset structures are introduced. Then, the applications of different types of sensors based on the core-offset structure are investigated, including the measurement of physical parameters such as temperature, strain, refractive index (RI), bending, magnetic field by Mach-Zehnder interferometer (MZI), cascaded sensor, long period fiber grating (LPFG), fiber Bragg grating (FBG), Fabry-Perot interferometer (FPI) and Surface plasmon resonance (SPR). Compared with the traditional fiber sensor, core-offset structure sensor with a non-axisymmetric characteristic has excellent sensing characteristics, especially in the recognition of bending direction and high RI sensitivity. Furthermore, the design scheme, working principle and sensing characteristics of different core-offset sensors are also briefly described. Finally, the existing challenges and development direction of optic fiber sensors based on core-offset structure is discussed and prospected.

We investigated and prepared a chirped long-period grating for high sensitivity bending measurement. The novel fiber structure is composed of the multimode fiber (MMF) with fixed length and gradually longer single-mode fiber using the continuous splicing method. The powerful ability of refractive index modulation in the MMF renders the miniaturization of the sensor. The total length of the sensor is 3.45 mm. Chirped period arrangement is adopted to improve the bending sensitivity. Through numerical calculation, the chirp coefficient is determined, and the clear resonance peaks are obtained using the wavelength scanning. The experimental results of several samples show that the maximum bending sensitivity is 53.68 n m / m − 1 at 0 − 1.803 m − 1 . With the advantages of small size and high sensitivity, the sensor is especially suitable for bending sensing with a micro-structure.

A multimode fiber chirped long-period grating (MCLPG) is proposed and studied as a sensor for simultaneous measurement of strain and temperature. This sensor consists of linear chirped multimode fiber and fixed-length single-mode fiber. The numerical analyses of the transmission spectrum and coupling mode are carried out to obtain appropriate period arrangement. The experimental results are in good agreement with the theoretical results. The chirped period arrangement induces a transmission spectrum with double peaks. Therefore, the simultaneous measurement of strain and temperature is realized by observing the wavelength shift of the two peaks. In addition, the high sensitivity of two resonance peaks provides a good resolution for the sensor.

A temporal electronic speckle pattern interferometry (ESPI) system is proposed for in-plane rotation measurement. The relationship between the rotation angle and the phase change distribution is established and the rotation direction is indicated by the sign of the partial differential of the phase change distribution. Temporal phase modulation is applied in the proposed symmetric illumination ESPI system. The phase is recovered by the temporal intensity analysis method which uses the temporal evolution history of the light intensity. The system can perform dynamic measurements and provide results in off-line real-time. Preliminary experiments were carried out with a continuously rotating target to show the feasibility and the dynamic feature of the temporal ESPI system. At present, the mean absolute error of the experiment is 0.39 arcsec.

A novel optic-fiber torsion and temperature sensor has been proposed and fabricated by sandwiching a segment of a triangular shaped single-mode fiber between two multimode fibers. This structure provides the sensitivity of torsion, which results from the destruction of the rotational symmetry of the optical fiber. Experimental results show that the polished multimode single-mode multimode (MSM) sensor has the ability of determining the twist direction and the amount of torsion with sensitivities of 0.196 nm/(rad ·m-1) and -0.18 nm/(rad ·m-1) at dip 1455 nm and dip 1505 nm, respectively. In temperature measurement, the sensitivities provided by the two dips above are 0.058 nm/°C and 0.072 nm/°C, respectively. The stability and the feasibility of the proposed sensor are experimentally verified.