Sensors have become instrumental in various practical applications across numerous fields, allowing for detecting different changes in the system or the environment. In particular, fiber Bragg grating (FBG) sensors are excellent candidates for sensing various physical quantities, including temperature and strain, owing to their remarkable properties like small size, high accuracy, and low energy consumption. Typically, FBG can be described as distributed Bragg reflector constructed on optical fiber and can reflect particular light wavelengths while transmitting others. Consequently, the grating period, refractive index, wavelength of the reflected light and the overall sensor performance are significantly affected by the change in the external environment conditions.
Effective application of FBG sensors requires high strain measurement capabilities. Generally, strain sensing in FBG sensors involves integrating the gratings with the structure surface. This allows for the measurement of the strain by sensing the change in the central wavelength of the gratings. However, the discrepancies in the elastic modulus and the corresponding differences between the gratings and the structure may compromise the strain measurement accuracy. Therefore, it is recommendable to accurately calibrate FBG sensors to enhance the strain measurement precision. To date, several strain calibration methods have been developed. Unfortunately, most of these methods are based on either strain gauge or beam structure and do not account for the strain gauge strain and the bending deformation of the beam, leading to poor calibration accuracy.
To address these challenges, a team of researchers from the North University of China: Mr. Ruoshui Tan, Dr. Chen Chen, Mr. Yongqiu Zheng, Dr. Jiamin Chen and Dr. Liyun Wu developed a new optical lever-based strain calibration method for accurate and direct calibration of FBG sensors. The authors aimed at overcoming the challenges and limitations associated with the conventional strain calibration techniques based on strain gauge and beam structure to enhance the calibration accuracy. The research work is currently published in the journal, Optical Fiber Technology.
In their approach, an optical lever which was basically a calibration instrument was utilized to measure the displacement with the assistance of optical amplification. Due to the higher temperature sensitivity of the sensor, a temperature control module was used to control the thermoelectric cooler. On the other hand, through a combination of theoretical analysis and strain experiments, the authors calibrated the strain of the FBG sensor at a room temperature of 26 °C using the optical lever. The principle for calibration and strain sensing were discussed in detail. Lastly, different factors affecting the strain calibration process, such as temperature, were investigated.
The results demonstrated the high FBG sensor calibration accuracy of the proposed method. Compared to the conventional methods, it overcame the problems of low stress transfer and sensitivity. Moreover, a strain calibration of 1.13 pm/µɛ was reported for the FBG sensor at room temperature, representing an accuracy of 99.9987%. Even though the temperature exhibited minimal effects on the calibration accuracy, it did affect the relationship between strain and the central wavelength attributed to the instability of the temperature control. It was worth noting that increasing the telescope-reflector distance enhanced the strain sensitivity of the sensor.
In summary, a simple and robust FBG strain calibration method based on optical amplification was reported. Through a comprehensive analysis of the optical lever and strain sensing principles coupled with strain calibration experiments, the authors successfully demonstrated the superior performance of the proposed method in terms of accuracy, precision and sensitivity properties. This allowed for the calibration of the FBG strain at different temperatures. In a statement to Advances in Engineering, the authors explained their study enable future enhancement of FGB calibration by adopting different strategies such as temperature compensation.
Tan, R., Chen, C., Zheng, Y., Chen, J., & Wu, L. (2021). High-precision calibration method for fiber Bragg grating strain sensing based on an optical lever. Optical Fiber Technology, 61, 102392.