Fiber-Optic Sensor Array for Distributed Underwater Ultrasound Sensing

Acoustic sensing in the ultrasound range is important for a variety of underwater applications, such as sonar, navigation, oceanography, marine life research, imaging and mapping of the seabed, depth measurement, and underwater acoustic communications. However, traditional acoustic point sensors have limited spatial coverage and are not practical for synchronized spatiotemporal measurements of propagating acoustic waves. Fiber-optic hydrophone arrays can overcome these limitations as they offer extended detection volume and synchronized measurements at multiple positions. However, standard interrogation techniques of reflectometric-based fiber-optic sensing arrays are limited by a trade-off between the array length and the achievable acoustic bandwidth. This article describes a theoretical and experimental study of an interrogation method which alleviates this limitation. The method was used to interrogate a Quasi-Distributed Acoustic Sensing (Q-DAS) array with a maximum length of 4 km, having a classical maximum interrogation rate of 25 kHz. The array comprised 26 weak Fiber Bragg Gratings (FBGs), out of which 9 were deployed underwater in a 20 m long test pool. The method achieved an interrogation rate of 5 MHz, which is 200 times faster than the conventional limit. It facilitated spatiotemporal tracking of ultrasound pulses, whose carrier frequencies were as high as 100 kHz. The results of this work demonstrate the potential of the method for a variety of underwater applications.