Five Minute Fiber Expert – Hydrophones

Previously, you heard that Fibercore SM fibers are typically, skinnier, stronger and more strongly guiding than the fibers used in conventional telecommunications – but why do they need to be? Good question!

The most widespread and for Fibercore, ultimately the most important use for these fibers is acoustic sensors –sensors that detect sound waves. And whilst similar technology can be used to good effect on the land, most of the applications that we encounter involve the detection of sound waves in the water – hydrophones. Hydrophones do a wide variety of jobs. They can protect sensitive shoreline installations like oil-refineries or docks, by identifying the acoustic signatures of swimmers or speedboats. Mounted on the flanks of nuclear submarines, they can be used in conjunction with sonar to help assess depth, position and detect the presence of other vessels. They can assist the management of subsea oil reservoirs, by detecting the characteristic vibrations caused by oil flows – and arrays of hydrophones (or ‘streamers’) can even be towed behind survey ships in their quest to identify new oil reserves through the detailed seismic analysis of geological structures on or beneath the ocean floor.

Lots Of Different Applications – One Fundamental Design (courtesy of our own Dr John Wooler,who worked on hydrophones at Qinetiq before joining Fibercore).

Basically, a hydrophone sensor comprises a rod (or mandrel) encapsulated in a special, dense foam and wound with perhaps 100 m or so of singlemode optical fiber (typically one of our SM1500(X.X/80) series). When the sound waves hit the sensor, they cause the foam to expand and contract and, in doing so, the fiber is made to stretch and then relax – this modulation of the fiber changes the characteristics of the light guided within it and these changes can be analysed to determine critical characteristics of the acoustic wave.

So …

Why does our fiber need to be 80 μm (or even 50 μm) rather than 125 μm?

Because skinny fibers stretch more easily, making the sensor more sensitive and skinny fibers are under less stress when coiled and so they last longer – hydrophones in nuclear subs need to last at least 30 years.

Why does our fiber need to be much stronger than telecoms fiber – proof tested to 3% and not 1%?

Not only do these fibers have to last for 30 years or more when coiled, they are also tensioned onto the mandrel (ie pre-loaded) to ensure that they follow the compliant layer exactly as it expands and contracts.

Why does our fiber need to guide so much more strongly than a telecoms fiber?

Because, in common with many fiber sensors, designers need to get as much fiber as possible into the smallest possible volume, as a longer optical path length improves the sensitivity. Also for towed-arrays, or ‘streamers’, the individual hydrophones need to be as small as possible (typically around 10 mm diameter) in order to minimize both the turbulence they cause in the water (which could muffle-out the acoustic waves they are trying to detect) and the amount of energy it takes to drag these streamers through the water. You may have noticed that the survey ship in the picture is unusually short and wide at the stern (a bit like an iron) – that’s because it’s the kilometer or so of hydrophone streamers behind the ship that effectively complete the more usual, streamlined hull shape.

Fibercore’s product range can be found at http://www.fibercore.com/