Emerging Fibre Optic Sensor Technology For Maritime Security

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1 Emerging Fibre Optic Sensor Technology For Maritime Security ABSTRACT Paul Baker, Tim Cain Thales Underwater Systems, Thales Australia Fibre Laser based sensor technology is well suited to provide a robust solution for lightweight, ultra-thin, hydrophone arrays that can be rapidly deployed from a range of platforms to provide underwater acoustic surveillance. As a seabed array it is relevant to both maritime security and force protection. Additionally, it may be exploited for surface ship and submarine fixed and towed sonar arrays. This technology brings a number of advantages. Optical signals from the hydrophones can propagate along several kilometres of optical fibre without significant attenuation, thus enabling remote deployment of arrays from the receive electronics. It enables use of smaller winch systems, with smaller and simpler platform interface requirements, and places less demand on the overall platform power and weight budgets. Recent work has focussed on fibre laser based electro-optic hydrophone technology. In this paper we outline how the application of emerging fibre optic based sensor technology to underwater acoustic systems provides significant benefits and capabilities not previously possible with the current start-of the-art piezoelectric based sensors. INTRODUCTION Maritime Security has become an increasing concern in recent years for military, government and commercial organisations alike. The threat is multi-faceted and complex, and there are no easy solutions. Whether, the asset to be protected is a Naval vessel, an oil rig, a port or a national border, the threat could be airborne, on the surface or under the water. No single detection system can address all scenarios. The above water threat is traditionally addressed by radar, CCTV, thermal imaging cameras or a combination of all three. The underwater threat is commonly addressed by active sonar such as a Diver Detection Sonar (DDS) and Seabed Passive Low Frequency Sonar, which is used at choke points such as the US Navy SOSAS system deployed in the North Atlantic during the cold war. In the case of the underwater solutions, active sonar is expensive and requires a significant power source, the seabed passive sonar arrays based on conventional piezo-electric sensor technology require supporting electronics to be underwater and large copper cables which suffer power and signal attenuation.

2 The small size and high sensitivity of Fibre Laser Sensor (FLS) technology is ideally suited for use in lightweight, ultra-thin, seabed hydrophone arrays that can be rapidly deployed from a range of platforms to provide underwater acoustic surveillance. The technology is relevant to maritime security and force protection. A further advantage of the technology is that optical signals can propagate along several kilometres of optical fibre without significant attenuation, and can therefore make use of small diameter, light weight fibre optic cables. This opens the possibility of remotely deploying an array significant distances from the dry end photoelectronics receiver system. In the Naval domain there is an increasing emphasis on the littorals as the location for future conflicts. A rapidly deployable fibre optic seabed array has the potential for fulfilling a force multiplier role as an early warning tripwire system for task-force protection. This may either be for protection of the fleet or the rear guard defence of Special Forces put ashore by submarine. Tripwire systems also have a role to play in border security, due to the potential to remotely deploy very long arrays of multiple sensors. The advantages of a passive sonar array over an active system include the fact that they do not advertise their presence, they require much less power and infrastructure to operate and they can be deployed some distance away from the task force under protection and therefore provide an early alert to an intrusion. A BRIEF HISTORY The field of Fibre Laser Sensor Technology was pioneered in Australia by Defence Science and Technology Organisation, (DSTO), [1] [5]. Thales Australia collaborated with DSTO in this area of research under the Interactive Project Agreement (IPT) during This research was followed by the very successful Capability Technology Demonstrator (CTD) Program in This CTD leveraged extensive existing experience in conventional piezo-electric transducer, array design and materials science. The resultant Fibre Laser hydrophone design had good frequency response, depth compensation and signal to noise ratio (SNR). The array design was small in diameter (20mm) and very robust to suit the maritime environment. This eight channel array was successfully rapidly deployed and recovered at sea and demonstrated the feasibility and advantages of seabed arrays based on hydrophones which utilise Fibre Laser Sensor technology. Work has continued on the seabed array technology, focusing on a cross deck-able or portable sonar surveillance array product development. This work is expected to conclude in 2012, with an at sea trial of a sixteen channel FLS based array with a further reduction in diameter and much increased acoustic sensitivity. The inboard photo-electronics system has been improved, industrialised and packaged. Parallel work is being done to develop a Fibre Optic Towed Array (FOTA) under internal research and development funding and the FOTA CTD contract which was awarded in The design is very much in its early stage, however, together with more than 30 years of experience in conventional towed array technology, the knowledge gained during previous FLS array developments has provided a good starting position for a future fibre optic based Towed Array.

3 FIBRE OPTIC ACOUSTIC ARRAY TECHNOLOGY Fibre Laser Sensor A Fibre Laser Sensor can be created by applying a Distributed Feedback (DFB) grating structure with a phase shift at its midpoint to a short length (55mm) of Erbium doped fibre having a diameter of 125µm. Such a structure forms a laser cavity that, when pumped by shorter wavelength optical radiation (980nm or 1480nm), yields a narrow line width, single longitudinal and single polarization mode output having a good Signal to Noise Ratio (SNR). The wavelength produced by the fibre laser is determined by the optical cavity (Figure 1). The acoustic sensing mechanism of a fibre laser device is provided by the fact that it will shift its operating wavelength in response to longitudinal mechanical strain, hence change in optical cavity length on the fibre. The advantages of a DFB laser include its high reliability and low power consumption and the fact that it easily lends itself to multiplexing. Figure 1: Fibre Laser Optical Cavity The key to harnessing the sensitivity of the Fibre Laser Sensor, but at the same time isolating the laser from other environmental (non-acoustic) perturbations, is in the design of the packaging of the laser, both of the hydrophone itself and the mechanical structure of the array of multiple Fibre Optic Hydrophones. Fibre Optic Hydrophone The current Fibre Optic Hydrophone design addresses all key requirements for conventional piezo-ceramic based underwater acoustic hydrophones including depth compensation, flat

4 frequency response, temperature compensation and reduced vibration sensitivity. Recent improvements in the design have been realised which provide much improved hydrophone sensitivity than previously reported [7]. The hydrophone sensitivity currently achievable (Figure 2) confirms the technology as competitive to conventional piezo-ceramic element hydrophones for a range of acoustic array applications. Figure 2: Sensitivity measurements, 16 Channel Seabed Array Fibre Optic Hydrophone Array It is possible to assemble an array of multiple Fibre Optic hydrophones onto a single optical fibre, each with a different nominal wavelength and each independently modulated by an acoustic pressure perturbation. A sixteen channel high strength, small diameter seabed array has been assembled in a continuous oil filled hose and attached to a 1.5km ruggedized fibre optic lead-in cable via a purpose designed high strength underwater fibre optic connector developed to meet the specific requirements of this demanding application. The Array includes mechanical features to securely mount the fibre laser hydrophones, provide axial strength and stiffness and radial crush resistance. The fibre is managed to prevent it from experiencing strain during deployment, operation and recovery, (Figure 3). Figure 3: 16 Channel Seabed Array on its deployment drum

5 Inboard System A functional view of the optical system architecture is shown in Figure 4. The system comprises a Wet End sub-system which includes the lead-in cable and Array of FLS hydrophones plus the Dry End sub-system which includes the opto-electronics module and the processor-display on a laptop. Optical signals from the array pass through an interferometer module through the opto-receiver module for demodulation. The demodulated sonar data are then processed and beam formed and displayed on the sonar display. Figure 4: FO Seabed Array System Architecture The inboard system can be powered from a 12V battery or the 240VAC mains. It draws very little power (around 24W for the 16 channels). It has been ruggedised and will fit neatly into a small suitcase (Figure 5), making it easily transportable, cross deck-able and with the potential to be packaged into a medium sized UUV. Figure 5: Inboard System, photo-electronics unit and processor with display

6 Fibre Optic Array Deployment System A deployment system has been developed which protects the minimum bend radius of the assembled array and allows rapid deployment and recovery from a RHIB (Figure 6). The deployment system can be transferred from platform to platform as a result of a quick release mechanism. For the sea trials conducted in Jervis Bay, NSW in 2008, the deployment system was designed such that it could be rapidly fitted to a standard RAN, 7.2 metre RHIB with no modifications required to the boat. To enable the RHIB to maintain its standard ocean going setup, the deployment system was fitted to the stern for array deployment and then transferred to the bow to allow recovery at the end of the operational phase of the demonstration. A deployment method was devised and applied which ensured the array was rapidly laid relatively straight on the seabed (Figure 7), with no diver intervention. Due to the very small diameter of the fibre optic cable, the deployment system demonstrated in Jervis Bay could stow and deploy up to 5km of cable from a RHIB platform, and provide a remote surveillance capability, something which is inconceivable using existing technologies. Figure 6: FO Seabed Array deployment System in use in Jervis Bay, NSW Figure 7: FO Seabed Array deployed on the seabed

7 BENEFITS OVER CONVENTIONAL ARRAY TECHNOLOGIES The key advantages of an optical system architecture (especially Fibre Laser based sensors and cables) for acoustic arrays over conventional piezo-electric analogue or digital arrays are: No electrical power distribution is required in the wet end thus removing the need for thick power cables; No preamplifiers or electronics are required in the wet end thus improving reliability and thereby reducing in-service costs and allowing smaller diameter arrays to be manufactured; Low attenuation of optical signals enabling the use of lightweight, thin fibre optic link cable several kilometres in length. Much smaller power, size and weight footprint on the deployment platform, thus opening up a greater range of platform options including Rigid Hull Inflatable Boats (RHIB), Unmanned Surface Vessels (USV) and Unmanned Underwater Vehicles (UUV). As an example of the impact this thin and lightweight technology conveys, it can be compared to the current technology used in towed array systems, (Figure 8). Conventional thin line Array section Copper based lead-in or tow cable for a conventional array FLS Array Future FLS Array Figure 8: Array technologies, physical comparison A typical towed array winch, fitted to a 3000 tonne frigate, weighs about 2.5 tonnes and has a 1.8m diameter drum. The towed array is typically 40mm or more in diameter and can weigh 1000kg. The deployment system designed for use with the fibre laser array comprises a 20kg winch and a 0.75m diameter drum that can be fitted to a 7.2 metre long RHIB (Figure 6). With the array section and lead-in cable, the entire system weighs 40kg and can be carried by two people, (Figure 9).

8 Figure 9: Winch technologies, physical comparison MARITIME SECURITY APPLICATIONS The underwater domain of Maritime Security has been addressed by increasingly sophisticated active and passive sonar systems. All these systems are currently based on conventional piezoelectric hydrophone technology. Due to the thin, lightweight and low power nature of the fibre laser based acoustic array technology, many opportunities are opened up for acoustic array applications in the Maritime Security domain that previously were unfeasible using conventional array technology. These include: Rapidly deployable seabed arrays from small vessels; Towed arrays for smaller surface ships and submarines, providing small winch systems, smaller and simpler platform interfaces and less power and weight; Submarine hull mounted arrays as an alternative to the current outboard electronics approach; Arrays for Unmanned Underwater Vehicles (UUVs) and Unmanned Surface Vessels (USVs) that demand low power and small size. Probably the most obvious and most practical application in the near term is force protection where the array would be deployed around a high value asset while in a potentially hostile harbour in a trip wire configuration. Force/Infrastructure Protection

9 The USS COLE bombing in October 2000 highlighted the vulnerability of Naval ships when in port to asymmetric threats such as small boats. Also, divers can either swim or use diver delivery vehicles to approach Naval vessels almost undetected. There is currently a number of existing active sonar systems in the market for diver detection giving detection out to about 800m. However, there may be operational scenarios whereby a passive approach may be preferable, for instance if a larger perimeter detection range is desired or if keeping the diver unaltered to his detection is preferable, or where transmitting on active sonar sources may be banned from operation in some ports. Passive diver detection utilising FLS hydrophone technology is currently being evaluated. Similarly, offshore oil and gas infrastructure can be protected with the use of a trip wire style FLS sea bed array. Another potential application includes perimeter protection of archaeological or sensitive ecological sites from unauthorised visitors. Border Security The FLS based array technology lends itself to the border protection application. By acting as a trip wire laid on the seabed a very large distance can be surveyed around the clock. The array can be permanently deployed at known choke points or on transit paths for smugglers, pirates or illegal entry or departure routes of people and vessels. The fibre optic cable can be brought back to land for signal processing and monitoring many kilometers from the array or connected to a moored buoy with RF or satellite communications links. Harbour Defence Entrances to ports and harbours can easily be acoustically monitored both for surface and underwater movements using an FLS seabed array. The acoustic signature of vessels can be correlated with radar and/or video or thermal camera images. UUV Surveillance Array UUV s are increasingly being used as a covert means to survey seabeds for mines either ahead of a fleet or to monitor mine laying activities. The FLS based small diameter array with its low drag, low power demand and compact photo-electronics system could add useful additional surveillance capability to the UUV, enabling it to also monitor shipping and submarine movements. CONCLUSIONS FLS based acoustic arrays have proven advantages over conventional piezo-electric based arrays. These advantages include it s inherently small size and low power needs, which have lead to the development of small footprint arrays, handling equipment and an inboard system. These advantages lend themselves very well to a number of possible applications in the Maritime

10 Security domain. The most promising of which include Force Protection, Border Security, Harbour Defence and UUV Surveillance Arrays. A small diameter seabed array with good sensitivity has been demonstrated in 2008, and a small diameter surveillance array with more sensors will be demonstrated in the near future. A towed array prototype is expected to be demonstrated in ACKNOWLEDGEMENTS This work was carried out as part of the Fibre Laser Sensor Capability Technology Demonstrator Program, Thales Research and Development funding programs and the Fibre Optic Towed Array Capability Technology Demonstrator Project and has benefited from close collaboration with DSTO. The Thales team involved in the development of the FLS hydrophones, array, deployment and inboard systems included the following key personnel: Ian Bedwell, David Jones, Barry O Keefe, David Mann, Daniel Mercer, Harley Boggis, Peter Harvey and Fabio Souto. REFERENCES 1. Foster.S, Tikhomirov.A, Hardy.G, Milnes.M, Van Vetzen.J, (2004), Ultra-thin all photonic hydrophone array research at DSTO, Proc. 33 rd meeting TTCP Mar TP9 Newport. 2. Foster.S, Tikhomirov.A, Milnes.M, Van Vetzen.J, Hardy.G, (2005) A fibre laser hydrophone, Proc. SPIE 5855 (OFS 17, Brugge), pp Tikhomirov.A, Foster.S, Milnes.M, Van Vetzen.J and Hardy.G, (2003), Acoustic and vibrational response of a DFB fibre laser sensor, COIN/ACOFT Conference Proceedings, R. Tucker and T. Aoyama (ed.), pp , Melbourne, Australia. 4. Goodman.S, Tikhomirov.A and Foster.S, (2008), "Pressure compensated distributed feedback fibre laser hydrophone", Proceedings of the 19th International Conference on Optical Fiber Sensors (OFS-19), Perth, Australia. 5. Foster.S, Tikhomirov.A, Englund.M, Inglis.H, Edvell.G and Milnes.M, (2006), ``A 16 Channel Fibre Laser Sensor Array'', Proceedings of the 18th International Conference on Optical Fiber Sensors (OFS-18)}, Cancun, Mexico. 6. Bedwell.I, Jones.D, (2010), Fibre Laser Hydrophone Performance IEEE OCEANS Cain.T, Bedwell.I, Jones.D, Hodder.B, Mercer.D, Boggis.H, Foster.S, Tikhomirov.A, Goodman.S, (2010), Fibre Laser Sensors for Underwater Acoustic Surveillance Arrays Mast 2010