Lithium-ion batteries are widely used in a variety of fields due to their high energy density, high power density, long service life, and environmental friendliness. However, safety accidents with lithium-ion batteries occur frequently. The real-time safety monitoring of lithium-ion batteries is particularly important during their use. The fiber Bragg grating (FBG) sensors have some additional advantages over conventional electrochemical sensors, such as low invasiveness, electromagnetic anti-interference, and insulating properties. This paper reviews lithium-ion battery safety monitoring based on FBG sensors. The principles and sensing performance of FBG sensors are described. The single-parameter monitoring and dual-parameter monitoring of lithium-ion batteries based on FBG sensors are reviewed. The current application state of the monitored data in lithium-ion batteries is summarized. We also present a brief overview of the recent developments in FBG sensors used in lithium-ion batteries. Finally, we discuss future trends in lithium-ion battery safety monitoring based on FBG sensors.

Fiber Bragg grating (FBG) has been widely used in many fields for the measurement of temperature, strain, and other parameters, but single-mode FBG has a cross-sensitivity problem between temperature and strain. In this paper, a new scheme is proposed to solve the problem by using parallel core-cladding FBGs, whose grating areas are located at the interface region of the core and cladding of the fiber. The sensing principle of parallel core-cladding FBGs for the simultaneous measurement of temperature and strain is theoretically analyzed. The femtosecond laser point-by-point method of the parallel core-cladding FBGs is experimentally investigated. Two parallel FBGs, namely core FBG and cladding FBG, are formed efficiently and conveniently by a single time inscription. Two transmission peaks have been observed, which are excited by the core mode and the cladding mode, respectively. The experiment results demonstrated that the proposed compact sensor can detect the temperature with sensitivity of 11.043 pm/°C and strain with sensitivity of 1.2101 pm/με up to 1.114×10 ³ με, and the relative errors of temperature and strain are less than 5.8% and 8.3%. The proposed parallel core-cladding FBGs are much more compact as one FBG element compared with the conventional structure with two cascaded FBGs or parallel FBGs. The parallel core-cladding FBGs can overcome the cross-sensitivity problem of single-mode FBG, which has great application value and potential in fiber sensing fields for dual-parameter.

Spin-exchange relaxation-free atomic spin gyroscope (SERF gyro) has ultrahigh theoretical sensitivity and is regarded as one of the most potential atomic gyroscopes. To investigate its rotation response, a high linear atomic spin precession detection module was developed firstly by modifying the fiber reflective Sagnac interferometer for atomic spin precession detection and operating it in closed-loop mode. The SERF gyro based on K-Rb- ²¹ Ne comagnetometer was built and tested and appeared obvious nonlinear response feature. The nonlinear rotation response model was deduced by assuming the rotation-induced nuclei spin procession detecting as an in-situ magnetometer inside the SERF gyro and the fitting curves are in good agreement with the experimental results. The optimized pump power for maximum sensitivity and measurable dynamic range were analyzed theoretically and demonstrated experimentally. The investigation results show that the nonlinear rotation response is an intrinsic property of SERF gyro and its measurable dynamic range is limited due to the saturation characteristic of the sum of pumping and relaxation rate. This indicates that SERF gyro is propitious to a position gyroscope with high sensitivity.

A polarization-maintaining fiber Bragg grating (PM-FBG) is a crucial component of fiber optic sensors, and the inscription method associated with it has garnered significant interest. Compared to alternative technologies, the femtosecond laser point-by-point inscription method offers greater flexibility. However, it is still unclear how the controllable factors of the femtosecond laser point-by-point inscription process relate to the spectral characteristics of PM-FBG. The main factors that affect the spectral properties of PM-FBG inscribed via the femtosecond laser point-by-point method, including the pulse energy, grating length, fiber type, and grating apodization, are investigated experimentally and theoretically in this article. An experimental investigation was conducted to examine the relationship between the pulse energy and side lobe suppression ratio (SLSR), and the optimal pulse energy was approximately 30 nJ to obtain the highest SLSR. The theoretical model of the full width at half maximum (FWHM) is presented, and it is shown that the FWHM tends to saturate with grating lengths greater than 8 mm. There is a linear relationship between the distance of the two reflection peaks and the birefringence of the polarization-maintaining fiber (PMF), regardless of the birefringence induced by the inscription process. The oblique apodization method can decrease the SLSR of the two reflection peaks, but it also broadens their FWHM. This study provides compelling evidence to support the improvement in the spectral performance of PM-FBGs inscribed via the point-by-point femtosecond laser inscription method.