Optical fibre cables carry Internet traffic around the globe and provide us with broadband connections to our homes and businesses. But those same cables have an important role apart from high speed communications.
A technique known as distributed acoustic sensing (DAS) can detect changes in environment around those cables and indicate where those disturbances are occurring. For example, DAS applied to a submarine cable can detect vibrations caused by trawling and dredging or nefarious activities of states wishing to disrupt international communications.
A water company in the UK is using Openreach’s fibre broadband cables to detect vibrations caused by water leaks. AI assisted machine learning is used to pinpoint the exact location and distinguish between other causes of vibration such as traffic or roadworks.
DAS works by monitoring optical pulses as they are reflected back from natural microscopic inhomogeneities in the glass – an effect known as Rayleigh scattering. Changes in the refractive index of the fibre caused by vibrations affecting the cable create changes in the phase of the reflected signal. The time taken for the affected signal to return gives an indication of the position of the event.
Rayleigh scattering is just one of the topics explained in PTT’s online course “Optical fibre principles“. This course is an indispensable primer for those joining the telecoms sector to maintain the broadband connections we all rely on.