Multimode Interference-Based Fiber-Optic Ultrasonic Sensor for Non-Contact Displacement Measurement

In this paper, ultrasonic sensor based on multimode interference in single-mode–multimode–single-mode optic fiber structure for non-contact displacement measurement is presented and investigated. By experimentally testing and optimizing the sensor structure, an ultrasonic wave with a frequency of 25 kHz can be sensitively detected with a 7 m non-contact detection distance. Displacement measurement by employing phase shift detection is conducted and the experimental results verify that a resolution 0.1 mm within the measurement range of 6 mm can be achieved for 7-m non-contact distance based on the proposed system.