Frequency-division-multiplexed OCDR for low receiver bandwidth quasi-distributed ultrasonic sensing with enhanced frequency response

Ultrasonic detection serves as a significant non-destructive testing method in industrial manufacturing. Quasi-distributed acoustic sensing (QDAS) offers promise for cost-effective, large-scale ultrasonic sensor arrays owing to its superior array integration capability. In this study, we implement an optical coherence domain reflectometry (OCDR) system incorporating frequency-division multiplexing to detect ultrasonic pulses propagating in metal plates via an optical fiber with weak reflectors. This strategy drastically reduces the receiver bandwidth requirement for QDAS systems. We demonstrate 1 MHz ultrasonic pulse detection over 2 km fiber with the receiver bandwidth compressed to several MHz. The proposed system significantly enhances the feasibility of QDAS for ultrasonic flaw detection, imaging, and other fields.