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Prysmian: innovating in breakthrough fibre-optic sensing technology

Prysmian: innovating in breakthrough fibre-optic sensing technology

Fibre networks can act as sensing arrays, providing detailed, up-to-date information with no additional hardware or maintenance costs.


Distributed Fibre Optic Sensing (DFOS) is a technique that is becoming more and more relevant in monitoring critical assets and infrastructures. Thanks to DFOS, any existing optical fibre in an optical network infrastructure can become a continuous linear array of intelligent sensors that can be used to measure temperature (DTS-Distributed Temperature Sensing), strain (DSS – Distributed Strain Sensing) or vibrations (DAS – Distributed Acoustic Sensing).

Hence, the optical fibre replaces hundreds or even thousands of discrete sensors, can be monitored 24/7 from a single location, and is able to provide accurate information along tens of kilometres with no maintenance requirements.

How it works

DFOS technique is based on launching a short pulse of light into an optical fibre. Light is scattered along the fibre and a very small fraction of the injected light is scattered back towards the measuring device, known as interrogator. Since the speed of the light in the fibre is well known, the signal received can be associated to a well-defined location of the fibre. The fibre becomes a continuous sensor.

There are three main scattering mechanisms: Rayleigh, Brillouin and Raman. Rayleigh scattering is linear scattering of light at inhomogeneities (scattering centres) that are produced in the fibre fabrication process. In Rayleigh scattering, the light isn’t absorbed, just sent in a different direction than the propagation with the same wavelength. Local variations in temperature, strain or vibration change the amount of scattered light, that can be detected by the interrogator. Rayleigh is the basis of DAS. Brillouin scattering is an inelastic mechanism, where the wavelength of the scattered light is very similar to that of the injected light, but sensitive to strain and temperature changes. By measuring these frequency changes with the interrogator it is possible to determine absolute measurements of temperature and/or strain along the fibres. Brillouin scattering is the basis for Distributed Temperature and Strain Sensing (DTSS). Raman scattering is a highly inelastic mechanism that results from interaction with thermal molecular variations in the glass. For this mechanism, the intensity of the Raman back-scattered light is temperature dependent while insensitive to strain. Raman is the basis for Distributed Temperature Sensing (DTS).

These three back-scattering mechanisms occur simultaneously inside the optical fibre, and each one provides different types of information. The technique you use to interrogate the fibre determines the type of data generated. You can analyse light refractions in different ways, depending on the variable you want to measure – temperature, strain or acoustic variations. To obtain this data in a readable format, the end user needs an interface for the ‘interrogator’.

Smart Cable design

Optical cables are the way optical fibres are encapsulated to protect them from the installation and from environment. For DFOS, it is common to ‘banalize’ cable design assuming that any cable is useful for a measurement. But that’s not right. On the one hand, cable construction has a significant impact on how efficient the optical fibre can capture information from environment. On the other hand, for most scattering mechanisms such as Rayleigh and Brillouin, temperature and strain are fully coupled and there is no straightforward way of finding out which of these two factors is causing an event. Smart cable design can provide ways to decouple measuring magnitudes, and this is the field in which Prysmian has extensively work along the last couple of years, launching to the market DFOS cables suitable for multi-sensing purposes. In addition, Prysmian Group have recently launched a monitoring system for overhead transmission lines that uses the optical fibres of OPGW (the grounding cable of the HV transmission line) for detection of events like lightning strike, short-circuit, aeolian vibrations, corona effect and fire detection.

 

Josep Martín Regalado,

R&D Manager of OPGW & Specials BU, Prysmian Group

High-resolution underground maps with fibre

High-resolution imaging of near-surface structure around urban areas is challenging - but important. Modelling suggests ground shaking is amplified in the type of sedimentary basins urban areas are built on. Distributed Acoustic Sensing (DAS) allows construction of high-resolution images of structures near the surface, making DAS a valuable tool for seismic hazard mapping in urban areas. By monitoring how light pulses scatter as they travel through the cable, small strain changes in the shielding material can be measured. The authors of a recent paper on DAS, ‘Sub-Kilometre Correlation Between Near-Surface Structure and Ground Motion Measured With Distributed Acoustic Sensing’ suggest that DAS could improve urban seismic risk management, particularly in cities which already have fibre networks in place.

Sub-Kilometer Correlation Between Near-Surface Structure and Ground Motion Measured With Distributed Acoustic Sensing

Read the paper by Yan Yang , James W. Atterholt , Zhichao Shen , Jack B. Muir , Ethan F. Williams, and Zhongwen Zhan of the Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA.