TECHNOLOGIES
FIBRE SENSING
Several currently known cable-based sensing technologies exist. These utilise either injected signals from the shore, known as dry-plant or remote sensing, or sensors located on the cable itself, known as wet-plant sensing. Here we look at a variety of wet and dry plant-based sensing technologies and some of their applications in relation to cable protection.
DISTRIBUTED ACOUSTIC SENSING (DAS) AND RELATED TECHNOLOGIES
Distributed acoustic/temperature/strain sensing utilises techniques where a laser source is injected into a fibre optic cable and the returned reflections are then interrogated and recorded. The technique makes use of different scattering effects found in electromagnetic spectral physics where reflections occur each time a light source hits a medium which contains particles that are smaller than the wavelength of the laser source.
There are currently two commonly used scattering effects which are used to detect changes in cables, called Raman and Rayleigh scattering.
- Raman scattering is in-elastic, which means that the wavelength and the direction of the reflected light changes.
- Rayleigh scattering is elastic, meaning that only the direction or incidence of the light source changes (not the wavelength).
By monitoring Raman and Rayleigh scattering over time, the changes to the fibre cable as a result of stretch and strain, pressure, and temperature, can be observed. As the detections occur each time a reflection occurs, each reflection point then in effect becomes a sensing point. This means that along the length of a fibre, many (sometimes hundreds of) points can be monitored at varying distances.
As the detection requires a laser signal to be injected into the cable from one side, there is no requirement for an end to end connection or session to be established across the full length of the cable for the technology to be used. This means that even if a fibre cable breaks, detections up to and including the point of the break can be detected. However, it should be noted that the sensing capabilities of these techniques reduce the further away from the source of the coherent optical signal. For example, currently the maximum distance for detection for DAS is around 200 km or until the first repeater unit of a submarine telecoms cable. The reason for stopping at the first repeater is due to built-in filters which prevent Rayleigh and Raman backscattering. Newer cable designs may allow for signals to be transported over or around the filters. These are not common and at present there are no known instances of this type of configuration, except through the use of fibre-bragg gratings which are specially built into submarine repeater units for dedicated maintenance monitoring.
Distributed Acoustic Sensors (DAS)
Distributed Acoustic Sensors utilise the Rayleigh backscattering technique to detect changes in strain along a fibre optic cable. These strain changes are typically induced by pressure changes in the medium around the cable. In water or air, pressure changes can be produced as a result of sound waves. On DWDM submarine cable systems, Distributed Acoustic Sensors can typically be used up until the first repeater unit, or a maximum of 200 km from the interrogator, to detect pressure changes, temperature changes, and breakages which affect the submarine cable the interrogator is connected to.
Optical Time Domain Reflectometry (OTDR)
Optical Time Domain Reflectometry uses the Rayleigh backscattering technique. This technique times the reflections of the laser pulse source that is injected into the cable. By measuring the time it takes for the reflection to return, events on the cable can be located. The OTDR technique is typically used by submarine cable operators to detect faults along the length of the cable. Newer cable systems typically have a specific OTDR channel built into the DWDM system, to allow for Rayleigh backscattering to be returned to the signal source for interrogation, past the Rayleigh filters built into the DWDM and submarine repeater units.
Optical Temperature Sensing (OTS)
Optical Temperature Sensing utilises the Raman scattering effect to detect temperature changes at each point a reflection occurs along the length of the cable (up until the first repeater unit). As a result of temperature changes, fibre properties change, generating different energy reflections from the coherent light source. Some variations of DAS are also able to detect temperature changes.
STATE OF POLARISATION (SoP)
Whereas DAS utilises the reflections of an injected light source into a fibre optic cable, State of Polarisation (SoP) is the study of the values that describe the polarisation state of coherent electromagnetic radiation (laser), called the Stokes vectors/parameters. Polarisation can be considered the changes to the intensity, pitch, yaw, and roll of the coherent laser light.
Dense Wavelength Division Multiplexing (DWDM) systems which carry telecommunications signals, utilise the collection of Stokes vectors in order to establish telecommunication sessions and error correct sent and received signals in submarine telecommunication systems. There are some DWDM systems which allow for the collection of these sampled variables, as all DWDM systems utilise coherent optical signals to carry telecommunications. However, most DWDM systems generate and then discard the Stokes vectors.
Another means of collecting Stokes vectors is to use a polarimeter. The polarimeter collects the parameters received from an injected laser signal, meaning that the polarimeter needs to be located on the opposite end of the cable from where the laser signal is injected. However, when used with fibre bragg gratings, which are found in the OTDR channel on a repeated submarine telecoms cable system, a polarimeter can be used to collect Stokes variables from the same cable landing station where the generated coherent laser signal is generated. This is because the reflected laser light is received on the return fibre in the fibre optic pair.
By using the fibre bragg grating based polarimeter, events on the fibre can be localised within the cable spans between repeater units.
Stokes parameters are affected by changes to the medium that the laser light travels through. This could be from sound, pressure, or handling of the fibre. Other changes can be detected such as electromagnetic fields induced around the transmission medium, or changes in the originating light source. Currently, the changes detected are an aggregate over the whole length of the transmission medium, rather than point sources like those found in DAS/DTS/DSS type sensors.
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