In the late 1970s one of the first applications identified for fibre-optic sensing was the fibre-optic hydrophone. It was
recognised that the technology had the potential to provide a cost effective solution for large-scale arrays of highly
sensitive hydrophones which could be interrogated over large distances. Consequently both the United Kingdom and
United States navies funded the development of this sonar technology to the point that it is now deployed on submarines
and as seabed arrays. The basic design of a fibre-optic hydrophone has changed little; comprising a coil of optical fibre
wound on a compliant mandrel, interrogated using interferometric techniques. Although other approaches are being
investigated, including the development of fibre-laser hydrophones, the interferometric approach remains the most
efficient way to create highly multiplexed arrays of acoustic sensors. So much so, that the underlying technology is now
being exploited in civil applications. Recently the exploration and production sector of the oil and gas industry has begun
funding the development of fibre-optic seismic sensing using seabed mounted, very large-scale arrays of four component
(three accelerometers and a hydrophone) packages based upon the original technology developed for sonar systems. This
has given new impetus to the development of the sensors and the associated interrogation systems which has led to the
technology being adopted for other commercial uses. These include the development of networked in-road fibre-optic
Weigh-in-Motion sensors and of intruder detection systems which are able to acoustically monitor long lengths of
border, on both land and at sea. After two decades, the fibre-optic hydrophone and associated technology has matured
and evolved into a number of highly capable sensing solutions used by a range of industries.
Distributed feedback (DFB) fibre-laser sensors have been shown to exhibit many characteristics useful in a range of sensing applications. Uniquely, fibre laser sensors enable the wavelength division multiplexing of a number of devices while maintaining the individual sensor characteristics of extremely high strain sensitivity, high dynamic range and very wide measurement bandwidth in small diameter sensors. These factors have led us to investigate a range of applications including hydrophone arrays, accelerometer designs, embedded acoustic emission sensors and acoustic pickups for musical instruments. This paper provides an overview of the optimised DFB sensor performance, interrogation and sensor configurations along with results and a discussion of the future developments of this technology.
We describe a fibre-optic hydrophone array system architecture that can be tailored to meet the underwater acoustic surveillance requirements of the military, counter terrorist and customs authorities in protecting ports and harbours, offshore production facilities or coastal approaches. Physically the fibre-optic hydrophone array is in the form of a lightweight cable, enabling rapid deployment from a small vessel. Based upon an optical architecture of time and wavelength multiplexed interferometric hydrophones, the array is comprised of a series of hydrophone sub-arrays. Using multiple sub-arrays, extended perimeters many tens of kilometres in length can be monitored. Interrogated via a long (~50km) optical fibre data link, the acoustic date is processed using the latest open architecture sonar processing platform, ensuring that acoustic targets below, on and above the surface are detected, tracked and classified. Results obtained from an at sea trial of a 96-channel hydrophone array are given, showing the passive detection and tracking of a diver, small surface craft and big ocean going ships beyond the horizon. Furthermore, we describe how the OptaMarine fibre-optic hydrophone array fits into an integrated multi-layered approach to port and harbour security consisting of active sonar for diver detection and hull imaging, as well as thermal imaging and CCTV for surface monitoring. Finally, we briefly describe a complimentary land perimeter intruder detection system consisting of an array of fibre optic accelerometers.
Fiber optic sensors are becoming a well-established technology for a range of geophysical applications, and static pressure and temperature sensors in particular are now comparatively well developed. However, rather less attention has been paid to systems for measuring dynamic quantities such as acoustic and seismic signals. Furthermore, the very large multiplexing potential of fiber optic sensing systems has yet to be fully explored for geophysical applications. However, development of fiber optic sonar systems for military applications has proven the viability of large multiplexed arrays, and demonstrated advantages which include electrically passive arrays, long term reliability and the potential for operation in very deep ($GTR3000m) water. This paper describes the applications for large scale fiber optic sensing arrays in geophysical metrology. The main applications considered here are ocean bottom cables and streamers for marine seismic, and downwell seismic systems. Systems can require up to several thousand channels and the use of multi- component sensors, which include 3-axis geophones and hydrophones. The paper discusses the specific requirements for each application, and shows how these requirements can be met using a system approach based on time and wavelength multiplexing of interferometric sensors. Experimental and theoretical studies at DERA into the performance of highly multiplexed systems are also described, together with initial development work on fiber optic hydrophones and geophones.
We discuss theoretical and experimental in-water acoustic sensitivity measurements from a coated DFB fibre laser sensor. A noise level of -60dB re.Pa/√Hz @ 1kHz, with an acoustic responsivity reaching -10dB re.Rad/Pa was obtained with this sensor.
In recent years growing interest has surrounded the development of fiber laser sensors (FLS). This is due to their ultra high sensitivity to temperature and strain as well as their ability to be multiplexed along a single fiber using WDM techniques. It is their extreme sensitivity that has led to them being considered as acoustic pressure sensors rather than standard fiber Bragg gratings. The work presented here describes the development of an array of FLS configured as hydrophones. We discuss the design of the single mode fiber laser used throughout our system; comparing examples based upon distributed Bragg reflectors (DBR) and distributed feedback (DFB). In addition we discuss both the theoretical and experimental acoustic sensitivity enhancements obtained by the application of an elasto-plastic coating to the FLS. The array configuration is described, as is the heterodyne interrogation scheme using an unbalanced Mach-Zehnder interferometer with WDM channel selection. Results from the measurement of the minimal detectable acoustic signal of a bare fiber laser are shown to be -69 dB re.Pa/(root)Hz at 1 kHz when using a 200 m path imbalanced readout interferometer. Further gains in the sensitivity due to the application of various coatings are reported, as is a full characterization of an array of fiber laser hydrophones. Finally we discuss the future research of the FLS, and the areas in which the technology is particularly applicable.
Fibre optic hydrophones have been under development for twenty years, and have attained similar performance to the more conventional piezoelectric based transducer. However, the recent development of fibre Bragg grating (FBG) sensors makes possible an alternative approach to optical hydrophones, which offers the advantages of small sensor size and very simple manufacture