In this era of communication and connectivity, optical-fibre sensors find many applications, some of which are discussed in this article.
For ages, sensors have been used in hazardous environments. These sensors often contain electronic components. In the past, it was a challenge for engineers to make sensors work at extremely high temperatures, such as in industrial furnaces and sintering operations. A lot of extra effort went into active and passive sensors, to keep temperature in check, as well as complex verification, characterisation and qualification of sensors—result being the innovation of optical-fibre (or fibre-optic) sensors.
Fundamentally, fibre-optic sensors make use of optical fibre as an essential element to transfer and amplify the quantity to be measured. Optical fibre is an important element in modern electronics communication such as broadband Internet. Even today, optical fibre can be used in sensor technology to increase accuracy and capability.
Why integrate optical fibre in sensor technology
Optical fibre makes use of optical principle, and is based on total internal reflection of light. It was developed in 1970 by Corning Glass Works. At the same time, GaAs semiconductor lasers were also developed for transmitting light through fibre-optical cables.
An optical-fibre cable consists of core and clading, both of which have different refractive indices. Core has a high refractive index, while clading has a low refractive index.
An optical-fibre cable receives light beam as input, which is nothing but the quantity to be measured, as sensed by input of the sensor. Input for the cable is provided by a broadband light source such as LED, or narrow-band light source such as laser diode. This light beam travels through the optical fibre without any loss. It is dispersed at an angle of 60 degrees and emitted to the target (output of the sensor).
Core may be of two types: plastic or glass.
Types of optical-fibre sensors
Optical-fibre sensors, or fibre-optic sensors, are used to measure quantities such as temperature, pressure, vibration, displacement, rotation or concentration of chemicals. These are available in tiny sizes, and can be used in remote sensing, as these require no electric power.
Optical-fibre sensors can be classified as follows:
- According to location of sensor: intrinsic fibre-optical and extrinsic fibre-optical
- Based on operating principles: intensity, phase and polarisation
- Based on application: chemical, physical and bio-medical
Intrinsic type
In this type of sensor sensing takes place within the fibre. This means that the fibre is directly affected by the measured quantity.
Extrinsic type
Here, the optical fibre is used as the information carrier. These sensors are used to measure rotation, vibration, velocity, displacement, twisting, torque and acceleration. The best example of this type of sensor is the temperature sensor inside an aircraft jet engine. It uses optical fibre to transmit radiation into a radiation pyrometer, which is located outside the engine. In the same way, these sensors are used to measure internal temperature of transformers.
A comparison between intrinsic and extrinsic optical-fibre sensors is given in the table.
Advantages are many
Optical-fibre sensors deliver the following advantages:
- Contain no moving parts or electrical circuitry; hence, are immune to all forms of electrical interferences
- No possibility of spark, so are safe to be used in hazardous sensing environments, such as oil refineries, grain bins, mining operations, pharmaceutical manufacturing and chemical processing
- No danger of electrical shocks while repairing broken fibres
- Small and lightweight
- Have excellent range, resolution and multiplexing capabilities
- Resistant to high temperature and explosive environment
Major disadvantages that need solutions
- Complex detection system
- Expensive
- Non-familiarity to end user
- Requirement of precise installation procedures
- Usable measuring system’s development is complex
Fibre-optic sensors gaining ground
It is believed that fibre-optic sensors are the best way to drive a nation’s sensor technologies. However, one must ensure that the use of these sensors is profitable at the same time. These sensors find vast applications not only in industries but also in sectors like healthcare, aerospace and marine.
Hydrophones are used for seismic and sonar applications. Hydrophones with more than 100 sensors per fibre cable have been developed. Hydrophone sensor systems are used by oil industries and in the navies of some countries.
During MRI-guided surgery, a fibre-optic microphone is used in the MRI machine.
Typical fibre-optic sensor applications can be summarised as follows:
- Measurement of physical properties, such as temperature, velocity, displacement, strain and more
- Real-time monitoring of health
- Testing of buildings, bridges, tunnels, dams, heritage structures and so on
- Night-vision cameras, electronic security systems and in measuring wheel loads of vehicles
Industrial elements—such as furnaces of all kinds, sintering operations, ovens and kilns, automated welding and more—often generate large electrical fields where conventional sensors cannot be used. High temperature processing operations, as in refractories and chemical industries, often use optical-fibre sensors. These are also used in fusion, sputtering and crystal growth processes in semiconductor industries.
Oil and gas industry
The oil and gas industry often faces huge problems, and needs effective and safe solutions from time to time. For measuring temperature and pressure developed in the downhole of oil wells, ordinary sensors cannot be used. So, fibre-optic sensors can come to the rescue here.
Hydraulic fracturing is needed for the highest potential success of oil and gas wells. This can be effectively done using fibre-optic sensors.
Accurate downhole monitoring, data collection and analysis with minimal environmental impact are needed to increase the life of oil and gas wells. This can be easily achieved using fibre-optic sensors.
For permanent downhole installations, optical-fibre cables are protected inside hermetically-welded metal tubes that are typically filled with special buffer gels. Here, fibre-optic sensors can provide useful data in less time, which can bring higher automation to the oil and gas industry.
Marine industry
Epsilon Optics provides all fibre-optic strain sensing for Oracle racing. Virtually all critical structures are monitored, including hulls, foils and rigs, providing vital feedback to designers and reassurance to the sailing team. Marine applications for fibre-optic sensors include life boats, fast surface crafts, luxury yachts, high-performance racing yachts, naval vessels and submarines.
Using fibre-optic sensors as a universal medium of service
Despite the deep penetration of smart sensors and wireless sensors, fibre-optic sensors are still popular. After extensive field research, industrialists are now realising that fibre-optic sensors are the only alternative for hazardous industrial situations.
These sensors use light for measuring a quantity. Quantity is measured by modulation on intensity, spectrum, phase or polarisation of light travelling through optical fibre. It also makes use of optical sensors that convert light waves into electronic signals, which can be easily read by instruments.
Advantages of fibre-optic sensors for industrial process control instrumentation are significant. Cost of these sensors is continuously falling, while technical performance is improving. Fibre-optic sensors are ideal for use in refineries, chemical plants, power plants, oil production refineries or in any such hostile environment, since these are safe to be used in hazardous situations.
Future trends to the value chain
These days, fibre-optic sensors come with fibre-optic amplifiers, with easy-to-read digital LEDs. These digital displays provide real-time feedback. Newer sensors need significantly less wiring. For instance, in a configuration, there must be 16 sensors, all connected and sharing a single power line.
Newer sensors incorporate either a 12-bit or 16-bit CPU along with a 12-bit A/D converter that provides both higher resolution and faster response time. Certain models of sensors allow users remote access. This is especially useful in potentially hazardous environments. Using remote controllers, sensors in hostile environment can be programmed and monitored from a safe distance.
With all the flexibility and benefits, fibre-optic sensing systems can be widely used. Particularly, these find potential applications in industrial automation.
Fibre-optic sensors are mainly used in remote-sensing applications. These are resistant to electromagnetic interferences and do not conduct electricity. Hence, these find applications that involve highly-inflammable materials or high-voltage electricity.
Future trends in the fiber-optic sensors field can be briefly summarised as follows:
- Special wave guides, such as photonic crystal fibres, will enable many new sensing mechanisms and sensor configurations.
- Improved micro-fabrication technologies will continue to improve sensor performance, functionality and reliability.
- Advanced signal processing and networking technologies will enable high-density fibre-optic sensor networks.
As a part of technological initiatives, nanotechnology is slowly gaining momentum. Recent research has presented a cost-effective and rugged approach to apply nanoparticles to optical-fibre cables. Use of nanoparticles on coatings of fibre-optic cables leads to enhanced sensor response. These particles provide several advanced abilities to sensors, such as real-time monitoring, multiplexing capabilities, remote sensing, small dimensions, bio-compatibility and much more.
The main goal of these newer trends is to gain more benefits from these sensors.
Plasmotronics is yet another promising new trend in electronics. Researchers have found that plasmons can be effectively used in optical-fibre cables to enhance their capabilities. Here, plasmons are used instead of photons (light particles). Thus, fibre-optic plasmonic sensors utilise both propagation and localised surface plasmon resonance techniques.
All-in-all
Fibre-optic sensors can be effectively integrated with instrumentation functions. In this era of communication and connectivity, these find many applications. These sensors are emerging as a practical solution for many industrial applications, being faster, more efficient and effective for electronic instrumentation.
Vinayak Ramachandra Adkoli is BE in industrial production. He was lecturer in mechanical department for 10 years in three different polytechnics. Now, he is a freelance writer and cartoonist.
