Railway transport is an important part of China’s transportation strategy and is still in the high-speed construction stage, with its total operation mileage reaching 1.59×10⁵ km. As the direct carrier of train operation, the inside and surface of rails will have a variety of defects and damages due to long-term operation and vibration shock. What’s worse, this will directly endanger the safety of trains and passengers if defects or damages are not found and repaired in time. At present, piezoelectric sensors are usually employed to detect ultrasonic rail damage and each of them needs a power line connection, which makes it difficult to achieve multi-point multiplexing detection. In contrast, optical fiber Bragg grating (FBG) has the unique advantages of electromagnetic interference resistance, easy reuse, and corrosion resistance, which makes it suitable for damage detection of long-distance structures such as steel rails in harsh environments. To detect ultrasonic waves, we propose and design a high-sensitivity FBG ultrasonic sensor based on a coupled dual-slant cone structure. Meanwhile, the optimal size of the sensor is analyzed and determined, with the sensor object made. The dynamic characteristics of the sensor are tested experimentally, with a new approach provided for long-distance structure ultrasonic detection.

First, the optimal sensor size is obtained. Based on the mechanism of FBG and ultrasonic waves and the principle of slant conical energy accumulation, the slant conical sensor is modeled and solved by COMSOL simulation software 6.0. The amplitudes of ultrasonic signals detected by non-conical, positive conical, and slant conical structures are analyzed and compared respectively. The influence of slant conical structure parameters β, d, H, and i on the sensor performance is studied in detail. Then, three-dimensional (3D) printing technology is adopted to make the entity of the sensor device. After that, a narrow-band light source demodulation system is built to detect ultrasonic signals of different frequencies. Finally, the performance of PZT, bare FBG, and mono-clinic cone sensing devices is tested and compared.

Sensor structure size is analyzed and optimized. The verification effect of angle β on the performance of the slant cone shows that when β=25°-35°, the pressure and x-axis displacement at FBG have larger values. The effect of the bottom diameter d on the performance of the slant cone shows that when β=30° and d=10 mm, the maximum pressure and displacement are 707 Pa and 1.7×10^-7 mm respectively. The effect of base height H on the performance of the slant cone shows that when β=30°, d=10 mm, and H=12.5mm, the pressure and displacement have maximum values of 880 Pa and 2.4×10^-7 mm respectively. Meanwhile, it is found that the slope i of the slant cone is also an important factor affecting the structure energy concentration. When i=1.138, the pressure at the tip of the cone is the maximum. By comparing the maximum and minimum values of different parameters, we find that each parameter has a great influence on the sensor performance. The maximum pressure is 880 Pa, and the minimum pressure is 47 Pa, with a difference of about 18.6 times. The maximum displacement is 2.4×10^-7 mm, the minimum is 1.1×10^-8 mm, and the difference is about 21.8 times. It is determined that the ideal size of the sensor is β=30°, d=10 mm, H=12.5 mm, and i=1.138. In this case, the cone has a strong ultrasonic focusing ability and a greater effect on ultrasonic detection improvement by FBG. A narrow-band light source system is built to detect sinusoidal ultrasonic signals. The experimental results show that the dual-slant cone FBG sensor has strong resolution and the ability to detect ultrasonic waves in the range of 20-150 kHz, and the response voltage has a good linear relationship with the drive voltage. In the detection frequency range of 40-60 kHz, the detection amplitude is 90-230 mV, the signal-to-noise ratio is 10-19 dB. The signal-to-noise ratio characteristics of bare FBG, single-slant cone ultrasonic sensing device, piezoelectric sensor, and dual-slant cone ultrasonic sensing device are compared. The test results show that the dual-slant cone and single-slant cone FBG ultrasonic sensors have a higher signal-to-noise ratio of ultrasonic signals than piezoelectric sensors in the frequency range of 40-70 kHz. The noise of piezoelectric sensors is relatively small at about 1-2 mV, which can achieve a high signal-to-noise ratio. In detecting sinusoidal ultrasonic signals above 80 kHz, the signal-to-noise ratio reaches a maximum of 30 dB, and the signal-to-noise ratio is higher than that of the ultra FBG sensing device. Compared with piezoelectric sensors, the slant cone FBG ultrasonic sensing device has significant advantages in detecting frequencies of 40-70 kHz.

To enhance the sensitivity and reuse of FBG ultrasonic sensors, we propose an FBG ultrasonic sensor based on a focusing coupling slant cone structure. Based on the analysis, a physical sensor prototype is fabricated and the dynamic properties of the sensor are experimentally tested. The research results reveal that the encapsulated dual-cone FBG ultrasonic sensor significantly improves the sensitivity of FBG in detecting ultrasonic waves within the frequency range of 20-130 kHz, and the detection amplitude is increased by about 21 times at 50 kHz. Additionally, the dual-cone sensor has remarkable characteristics of double-end output and strong reuse, which is suitable for applications in long-distance structural damage monitoring such as rails and bridges.