Sensing with optical fibres

Fibre optic sensors make industry safer and offer advanced sensing possibilities

By Werner Daum* (BAM), Convenor of IEC SC 86C/WG 2

Starting from the first patent of a fibre optic sensor in 1967, fibre optic sensing has become a success story throughout the world. Today, fibre optic point sensors and continuous distributed fibre optics sensor systems allow the measurement of nearly all physical quantities and contribute to higher safety levels in many industrial sectors. Monitoring of pipelines, power cables, lightweight and civil engineering structures, and of ground movements are heading the hit list of applications. Standardization in this field is of paramount relevance and brings to light some of the challenges now faced by IEC SC (Subcommittee) 86C/WG (Working Group) 2: Fibre optic sensors.

Baumer O500 Optical sensor used for object identification and positioning in the packaging industry (Photo: Baumer Group)
Baumer O500 Optical sensor used for object identification and positioning in the packaging industry (Photo: Baumer Group)

Half a century of fibre optic sensors

It is now over 50 years since the idea came up that an optical fibre could be a useful technology for sensing and measurement, leading to the first patent in this sector. From that time on, optical fibre sensing has enjoyed much success.

Today, FOS (Fibre Optic Sensor) systems are high-tech products using non-linear optical effects as well as the latest fibre optic technology equipment (e.g. optical time domain reflectometry with sub-millimetre resolution). FOS are available for mostly physical quantities (e.g. strain, temperature, pressure, electrical current) and a wide range of chemical parameters (e.g. pH value, O2 concentration in blood). They are based on standardized optical fibres used for communication purposes (single-mode and multimode fibres) or on specially designed fibres like micro-structured fibres.

Inherently safe and reliable

FOSs are immune to electromagnetic interference and do not conduct electricity, so they can be used in places where there is high voltage or flammable materials such as gases or fuels. FOS can be designed to withstand ultra-high temperatures (1 000 °C) and corrosive atmospheres as well.

There are numerous realizations of FOS, but all fit into two categories: extrinsic and intrinsic.

An extrinsic FOS simply guides the light to a sensing point where the optical signal emerges into another medium within which it is modulated. The light is then collected by the same or a different fibre after it has been modulated by the quantity to be measured and returned to a remote location for processing.

In contrast, intrinsic FOSs keep the light within the fibre at all times so that the external quantity to be measured modulates the light as it propagates along the fibre.

Modulation principles like intensity, phase, polarization, or wavelength modulation are of common use. Also the transit time of light in the fibre can be a measure for the quantity.

Due to the possibility of influencing the light transmission properties of an optical fibre locally through an external parameter, a measurement of this parameter can be realized as a function of position along the fibre. This so-called distributed measurement has emerged as an extremely important and unique advantage of fibre sensor technology. A distributed measurement of a quantity like temperature over distances up to several tens of kilometres is unique to fibre optics. Also, effective gauge lengths in the order of one meter can be achieved, and there are some which go to even shorter discrimination lengths. This unique capability opened up a novel range of application possibilities like power cable or pipeline monitoring.

FOS technology is experiencing impressive growth. Market analysts estimate that the global consumption value of fibre optic point sensors and continuous distributed fibre optics sensor systems will reach USD 4 billion in 2017. Driving sectors are the oil and gas industry, power generation and distribution, and civil engineering. Upcoming sectors are the aerospace industry and all sectors with lightweight constructions that need integrated structural health monitoring (e.g. strain monitoring of rotor blades of wind turbines).

Safety and accuracy drive FOS energy applications

The oil and gas industry is using pipelines for the transport of gases (e.g. natural gas, ammonia, and CO2) or liquids (e.g. crude oil, petrol and brine). Modern pipeline management uses distributed temperature FOS to ensure integrity, immediate leakage detection and risk mitigation. The entire downstream process and system integrity can be monitored. Temperature profiling makes it possible to detect anomalies during pipeline operation. Over long distances in remote areas, distributed fibre optic strain measurement is used to detect, for example, ground movements, landslides, or seasonal soil texture changes, which can cause a local loading of the pipeline above the designed value or a break.

Temperature monitoring in power transmission systems is an integral part of increased power flow. It is estimated that there are 30 000 transmission transformers in North America and 100 000 worldwide, all of which could run more efficiently given proper thermal management. FOS here have the additional advantage of being built into distributed sensing systems of small physical volume to be integrated directly into the transformer. Partial discharge detection by FOS is an up and coming technology in this sector.

Power generation by offshore wind turbine farms is very common. The generated power is transmitted by underwater power cables to onshore distribution locations. The temperature of such power cables provides, at minimum, condition monitoring information. Temperature monitoring shows how cables are responding to load and allows the load to be managed according to the actual temperature of the cables.

Export and interconnector cables in shallow water may experience dramatic strains affecting their temperature under the same load. A cable may be surrounded by cool water one day and covered with meters of mud the next, or buried and then exposed, which will give a dramatically different operating environment. Damage from fishing and shipping activities may result in damaged insulation and an unusual temperature event. By monitoring the entire cable length, changes in the cable's environment and condition can be detected and acted upon.

Dam if you don't install FOS!

Early warning before ground or dam movements can save lives and avoid important material or property damages by preventing geotechnical hazards. The appropriate geotechnical monitoringby distributed fibre optic strain or deformation sensors in landslide-sensitive areas or in dams can detect soil movement before catastrophic failures happen. Such failures are always preceded by slow and small ground movement which can be detected prior to the complete failure of the slope or dam, and which could allow adequate measures to be taken in order to protect the area.

The use of FOS in structural health monitoring of civil engineering structures (e.g. bridges, tunnels, towers and buildings) allows the detection of local deterioration, damage, destruction, and partial collapse, but also highly localized strain contributions along sensors associated to the development of cracks in structures. For example, crack detection is performed within a spatial resolution in the order of 0,5 m, and cracks in the sub-millimetre range can be efficiently detected.

Standards support introduction of innovative products

A shortage of International Standards is hindering a breakthrough of this innovative sensor technology and obstructing the comprehensive use of FOS. Technical authorities all over the world are hesitant to approve such sensors on a routine basis for safety relevant applications due to the lack of standards.

Motivated by demands from several industrial sectors, IEC SC 86C: Fibre optic systems and active devices, re-established in 2010 its WG 2: Fibre optic sensors, which is now preparing and maintaining International Standards and specifications for FOS. These Standards cover performance and interface characteristics, as well as terminology, test methods, reliability and environmental attributes. At present SC 86C/WG 2 brings together some 50 experts from 16 countries. Representatives from all major economic areas of the world, manufacturers, users from different application fields, and research institution guarantee high quality and impartiality of the developed standards.

Industry to benefit from new Standards

Industry and equipment manufacturers take advantage of these Standards by using them as guides for specifying the measurement performance and reliability of their products in a uniform, recognized way. In addition, standards help applicants to select, install and operate FOS based on harmonized procedures. Furthermore, Standards create confidence in this technology and its reliability, and supports applicants to get approval of relevant authorities with respect to the use of FOS in new fields or safety relevant fields of application. Moreover, standards help reduce expenditures by avoiding costly single-system validations.

To accommodate the complexity of technology and the wide variety of applications, a hierarchical structured series of Standards was created. IEC 61757-1:2012Fibre optic sensors: Generic specifications, builds the foundation of this series. Two other Standards under development, IEC 61757-2-1, Fibre optic sensors: Strain measurement - Strain sensors based on fibre Bragg gratings, and IEC 61757-3-1, Fibre optic sensors: Temperature measurement - Distributed sensing, which address sectional and family specifications together with product and details specifications, will complete this hierarchical structure.

Although the different applications and operating principles of various FOS represent a real challenge for proper standardization, IEC SC 86C/WG 2 is proving it can provide valid Standards to the FOS market, thus promoting its consistent development.

*Dr Werner Daum is Head of division sensors, measurement and testing methods and Head of the Department of non-destructive testing at BAM, the German Federal Institute for Materials Research and Testing. Dr Daum received the IEC 1906 Award in 2012.

Baumer O500 Optical sensor used for object identification and positioning in the packaging industry (Photo: Baumer Group) Baumer O500 Optical sensor used for object identification and positioning in the packaging industry (Photo: Baumer Group)
Fibre optic EFPI (extrinsic Fabry-Perot interferometer) scanner (Photo: BAM) Fibre optic EFPI (extrinsic Fabry-Perot interferometer) scanner (Photo: BAM)
FOS are used for structural health monitoring of civil engineering structures, such as bridges or dams FOS are used for structural health monitoring of civil engineering structures, such as bridges or dams