Vibration measurement plays essential roles in machinery fault diagnosis and structural health monitoring, and vibration sensors are the most important tools for measuring equipment. Electrical vibration sensor technology is relatively mature and inexpensive. However, there are drawbacks, such as poor circuit stability, poor signal noise, and easy electromagnetic interference. In contrast, fiber Bragg grating vibration sensors have numerous advantages, such as resistances to electromagnetic interference, high and low temperature resistance, and corrosion resistance. Hence, they are widely used in aerospace, large-scale structure monitoring, industrial propulsion, and so on. Miniaturization, multidimensional measurement, and high sensitivity remain as challenges that the sensor must overcome. Therefore, in this study, a three-axis vibration sensor is designed based on a four-core fiber grating. A four-core fiber is employed as an elastic component to detect vibrations in various directions. We anticipate that the issues of sensor miniaturization, multidimensional measurements, and high sensitivity can be resolved with the aid of our structural design.

The bending sensing principle of a multi-core fiber is analyzed theoretically, and a sensor model is constructed using software. The amplitude-frequency response characteristics of the sensor model are studied using finite element simulations, and the performance of the sensor is analyzed. Finally, a four-core fiber with a diameter of 0.125 mm is identified. The grating area of the four-core fiber is 1 mm long. Brass is used for the sensor mass block, and low-density aluminum serves as the material for the sensor casing. A nickel-titanium alloy tube is also inserted outside the grating region of the four-core fiber to protect it from damage and prevent the fiber from bending when vibration is detected. The sensor is packaged using the sensor packaging platform depicted in Fig. 7 after the sensor parameters are established. This platform ensures that the sensor is packaged with the line and angular positioning accuracies both exceeding 0.1°. The amplitude is shown in Fig. 9, and the amplitude-frequency response characteristics and sensitivity characteristics of the sensor are studied.

In this study, a three-axis vibration sensor is designed based on a four-core fiber grating. A four-core fiber is used as the elastic element of the sensor. This principle is illustrated in Fig. 1. When the sensor detects external vibrations, the mass block inside the sensor is excited and vibrates, resulting in a four-core fiber bend. Inside the four-core fiber, the two cores are located on the plane in the vibration direction, with one being compressed and the other being stretched. The wavelength drifts of the two fiber gratings are the same, and their directions are opposite. Therefore, acceleration in the vibration direction can be calculated based on the real-time wavelength difference between the two gratings. Simultaneously, owing to the characteristics of the four-core fiber itself, a single four-core fiber grating can only achieve vibration monitoring in two directions. Therefore, when designing the sensor structure, the two four-core fibers are interleaved. One of the four-core fibers detects the vibration in the x- and z-directions, and the other four-core fiber detects the vibration in the x- and z-directions so that the sensor can achieve vibration monitoring in three directions. The final packaged sensor is shown in Fig. 8.

In this study, a three-axis vibration sensor based on a four-core fiber grating is designed. The size of the sensor is small: 15 mm×15 mm×15 mm. The performance of the sensor is analyzed via theory and finite element simulations, and a packaging platform and vibration test system are built to complete the packaging and performance testing of the sensor. The amplitude-frequency response and sensitivity characteristics of the sensor are tested using a sensor test system. The experimental results show that the operating frequency band of the sensor is 0?300 Hz, the characteristic frequency in the x, y and z directions is 450 Hz, and the sensitivities in the three directions are 30.5 pm/g, 32.07 pm/g, and 29.38 pm/g, respectively. The sensor designed in this study uses a four-core optical fiber as an elastic element, has a simple structure, and can be miniaturized. Simultaneously, two four-core optical fibers are combined to realize three-axis vibration monitoring in space, which has applications in remote-sensing satellites and other fields.