Imagine a line of security guards standing 10 m apart guarding a 40 km section of a border or perimeter. Now imagine that these 4000 guards never get hungry or tired and are invisible. That's how QinetiQ of the UK asks potential customers to think about OptaSense – a fibre-optic acoustic sensor where every 10 m length of the optical fibre acts as an independent element capable of reporting any disturbance.

"We originally looked at this technology for perimeter and border protection," OptaSense product manager David Hill told OLE. "But other relevant uses include monitoring for third-party interference or damage on oil and gas pipelines, as well as on optical telecommunication fibres and networks."

The product works on a similar principle to conventional optical time delay reflectometers (OTDRs) that are used to characterize optical fibres. An OTDR injects a series of optical pulses into the fibre under test and monitors the returning pulses.

In the case of OptaSense, the optical fibre is buried in the ground and when a propagating seismic signal passes through the fibre it displaces the fibre by a minute amount – many orders of magnitude less than a millimetre.

"This minute displacement creates a strain on the fibre and we measure this dynamic strain," explained Hill. "OptaSense takes an OTDR to a significantly greater level. We send a conditioned pulse down the fibre and look at changes on that pulse when it returns. It can tell you in much greater detail what the strain was at every location along the fibre."

A new use for dark fibre

QinetiQ has a long history of developing fibre-optic sensors, in particular acoustic sensors such as hydrophone arrays for military applications. When QinetiQ started to diversify into commercial applications, one area it began investigating was intruder detection and perimeter monitoring.

The firm's next move was to team up with UK company Sensoptics, a fellow expert in fibre-optic sensors. After a highly successful partnership, the end result is today's OptaSense product that combines Sensoptics' specialist optical knowledge with QinetiQ's experience of acoustic signal processing.

OptaSense is made up of three elements: a standard singlemode telecommunication fibre that acts as the sensing element, a Cobalt interrogation unit produced by Sensoptics that houses the optical technology, and a graphical user interface and processing capability that allows the customer to interact with the system.

"You can use any existing fibre that is available," commented Hill. "We suggest that it should be dark fibre optimized for wavelengths around 1550 nm. If there is no fibre available, it can be installed using standard direct burial cable installation techniques and does not require any special ground preparation. In soft ground, a ploughed installation of 10 km per day is not uncommon."

Hill adds that although the company quotes a maximum fibre length of 40 km, longer deployments are possible. "40 km is half the typical distance between repeater stations in a telecommunications network," he said. "If you want to go longer you can, but then you start to trade-off performance with the noise floor increasing and the sensitivity decreasing."

Intruder alert

The Cobalt box uses the optical fibre as its sensing element. The system monitors the entire length of the fibre continuously and can detect, classify and locate (DCL) any number of simultaneous disturbances anywhere along the fibre with a resolution of 10 m.

Crucially, the signal extracted from each 10 m section of the fibre is unaffected by the vibration on any other section of the fibre, including those immediately adjacent. Hill adds that multiple simultaneous disturbances do not reduce the system's ability to DCL other activity along the fibre.

But just how sensitive is the system? "The performance depends on the type of soil surrounding the fibre," explained Hill. "However, we can typically detect a person walking when they are 5–10 m away from the buried fibre. We can detect vehicles that are 50 m away from the fibre and diggers/other larger machinery that are between 300 to 500 m away. These are conservative estimates."

This aspect of the system makes use of QinetiQ's signal-processing expertise. Cobalt automatically learns the normal background level of vibration along each 10 m section of fibre and then sets appropriate amplitude thresholds for each location. "The system looks at the acoustic signal from the event to classify the disturbance," said Hill. "To date, we can classify a vehicle and a person but we will be increasing this library considerably over time. We are using our SONAR processing capabilities to process the 4000 output channels from Cobalt simultaneously."

According to Hill, a 40 km length of optical fibre has a frequency response of 0, or DC, to around 1.25 kHz, and this upper limit reaches 5 kHz for a shorter 8 km fibre. "The majority of seismic signals tend to be at low frequencies of sub-100 Hz, with the ground attenuating all of the higher frequencies" he added. "OptaSense is ideal for seismic-type measurements."

User experience

Hill and QinetiQ are keen to stress that the system is simple to operate thanks to an intuitive graphical user interface that presents everything that you need to know in three windows.

The first window displays an aerial photo or an ordnance survey map of the area and details the precise route of the fibre such that when an event is triggered OptaSense can tell the user the exact GPS coordinates of the disturbance.

The second window contains two histograms – one showing the activity along the entire length of the fibre and one showing a selectable zoomed length of the fibre. Each histogram displays the amplitude of the signal detected on every 10 m length of fibre. The third and final window gives an alert history showing what has been detected, when and where.

In addition to the automatic alarm, the system's operator can also put on a pair of headphones and listen to the sounds detected at any position along the fibre. For example, the user could hear the characteristic sounds of someone walking or a pneumatic drill.

One additional point is that the output acoustic signals from Cobalt are available via a TCP/IP Ethernet connection although the Cobalt system can also be configured and monitored remotely over the internet.

Testing and potential applications

The Cobalt interrogation unit has been through a rigorous testing process, with evaluators including the US Department of Defense and US telecoms giant AT&T, and is now available as an off-the-shelf product. Hill recalls working with AT&T, where its intended use was cable protection.

"AT&T attached OptaSense to one of its live telecom cables in Atlanta, Georgia, to monitor for any third party interference occurring in the surrounding area," he explained. "Sensoptics was monitoring the system remotely over the web from the UK and one day saw a huge amount of activity along the fibre route. They phoned AT&T and advised them to go to the exact GPS coordinates and they found builders digging foundations to a house about 3 m from a main telco trunk cable."

According to Hill, the four primary industries that could benefit from deploying OptaSense are security for perimeters and borders, oil and gas, telcos and transportation.

"In the oil and gas industry, third party interference could be a farmer digging up a pipe or intentional interference where people drill into the pipe and attempt to steal the oil or gas," he explained. "But OptaSense could also be used to detect leaks or ringfence storage facilities to provide intruder detection."

With the price of copper rising, one emerging area is protection against theft. "The rail and telco industries have a big problem with copper cable theft," said Hill. "It is organized crime in the rail industry where people would go to a remote site and remove up to 500 m sections of the cable from ducting running alongside the track. Fortuitously, the cable ducts that contain the copper cable increasingly include a telco cable as well. We can use some of the dark fibre in the telco cable to immediately detect when someone is tampering with the copper cable."

For further information, see www.QinetiQ.com/OptaSense.

• This article originally appeared in the February 2008 issue of Optics & Laser Europe magazine.