Low receiver-bandwidth quasi-distributed ultrasonic sensing enabled by crosstalk-suppressed optical coherence domain reflectometry

The receiver bandwidth is an important factor limiting the application of quasi-distributed/distributed acoustic sensing (QDAS/DAS) systems in large-response-frequency and fine-spatial-resolution applications, particularly in ultrasonic detection scenarios. To solve the bandwidth constraint, a QDAS employing optical coherence domain reflectometry (OCDR) is proposed. If not considering the frequency shift of heterodyne reception, the requirement for receiver bandwidth in this system is determined by its acoustic sampling rate. In this study, a receiver bandwidth greater than 2 MHz suffices for reliable measurement of 1 MHz ultrasonic wave. Nonlinear frequency sweep signals are employed to suppress crosstalk between reflectors, which meets the requirements for large frequency and weak amplitude signals detection in this work. The QDAS system based on OCDR has potential applications in ultrasonic flaw detection and other related fields, and is expected to further broaden the application scope of QDAS systems.