Articles about Optics and Photonics, Lasers, Fiber Bragg gratings and FBG sensors

Fiber optic sensors for extreme conditions

on June 28, 2019

1Environmental conditions, in which one or several external factors – radiation, temperature, electromagnetic field, aggressiveness, humidity, pressure, deformation – have extremely possible constant values, are considered to be extreme. Due to the rapid development of automated monitoring sensing systems in all areas of industry, the need for fiber sensors of physical quantities — temperature, pressure, acceleration, displacement, current — is increasing.

In addition to high metrological characteristics, the fiber optic sensors should have high reliability, stability, noise immunity, durability and ease of integration into micro control systems. This applies particularly to such industries as avionics, metallurgy, car electronics, heat engineering and energy, medical equipment, high-precision weapons systems.

The fiber optic sensors satisfy the requirements listed to the maximum extent. Until recently, the development of fiber optic sensors was held back mainly by two factors. Firstly, there were no cheap optoelectronic components, for example, low-noise laser diodes, highly sensitive photodiodes, passive optical fiber elements.

Secondly, due to the nonlinearity of the optical signal relative to the measured value, special signal processing algorithms are required (averaging, nonlinear processing, integral transformations), which means that a signal processor with high performance is needed.

Nevertheless, with the future development of fiber technology, the market situation began to improve; new prototypes of purely optical integrated circuits have appeared, where light carries the information, and logical operations are performed by optical fiber elements.

Before the consideration of fiber optic sensors, we should consider their principle of operation. A modern optical fiber consists of a core, through which light propagates, and a shell. Also, it is protected by a polymer sheeting outside. The fiber optic core is a thread of plastic or glass with certain additives (usually germanium) to increase the refractive index.

Fiber optic sensors are compact instruments used to track changes in various factors. Optical fiber here acts simultaneously as a sensitive element and a channel through which the received data is transmitted. Such fiber optic devices are available both in standard and in special versions for operation in harsh conditions.

Fiber optic sensors are used in a variety of technical industries. For example, they are used to controlling elements of buildings, heating mains, bridges in construction and public utilities. At the mines, the main aim of the fiber devices is warning about fire, tracking the state of working and holes. In the oil and gas industry, they demonstrate key indicators of wells and pipelines. The following types of sensors are distinguished:

Fiber optic sensors designed for use in harsh environments, at temperatures up to 400 °C, in harsh chemicals or in a vacuum, provide high accuracy and work reliably over a long service life.

Fiber optic sensors for specific purposes have optimally high performance for solving the most unusual and complex tasks: the measurement with the highest accuracy – up to 100 microns, robotic systems with regular bending of optical fibers, area monitoring, special objects or technological processes, such as the detection of semiconductor plates, labels, glass objects, liquids, etc.

The use of fiber optic sensors is economically feasible in large sites where a big number of sensing devices are required for continuous monitoring. Special models of fiber sensors are available for operation in harsh environments; these sensors are resistant to high temperatures, corroding media and able to operate in a vacuum. Depending on the principle of the fiber sensing device operation, point sensors and distributed sensors are distinguished.

Point sensors

The main component here is fiber Bragg grating. The emission reaching the fiber optic sensor from a broadband source is reflected in the form of a narrow spectral band. The rest of the light moves along the optical fiber. This fiber optic technology makes it possible to install multiple controllers along the entire length of the line, obtaining absolute responses without additional calibration. At the present time, this is the most reliable fiber sensing device.

Point sensors allow measuring:

  • temperature;
  • vibrations;
  • pressure;
  • deformations;
  • angle, etc.

Distributed sensors

2The design of distributed temperature sensors includes a couple of key components: optical fiber and interrogator. A similar fiber optic system is used in cases where monitoring is required for long lines.  

Distributed acoustic sensors can be applied to protect large objects and measure deformations. They operate on a similar principle. The difference is that the device does not measure the spectrum, but the oscillations of backscattered emission. Thanks to these data, it is possible to determine the source of the sound wave and take timely measures if unauthorized intervention occurs.

What is the advantage of using fiber optic sensors in comparison with traditional semiconductor sensors in the integrated design? Consider the example of specific industries.

  • Avionics and auto electronics

In these fields of application, such advantages of fiber sensors are important as resistance to interference, the ability to operate in conditions of lower (down to –70 °C) and higher (up to 150 °C) temperatures, compact size and weight. Here optical sensors of temperature, linear and angular position, accelerometers can be used. Optical gyroscopes based on a ring interferometer are widely used in military and civil aviation.

  • Energy industry

The advantage of using fiber sensors in this area is provided by their stable thermal and electrical insulating properties, noise immunity and inertia-freedom. Fiber optic voltage transformers, current, temperature sensors can be used in this industry. Such fiber sensors can be used to develop a diagnostic system for high-voltage transformers without their taking out of service.

  • Medicine and biotechnology

The advantages of fiber optic sensors such as flexibility and small diameter of optical fiber, chemical and biological resistance, high spatial resolution are particularly inherent to this specific area.

For example, researchers from Chicago have developed a new method for diagnosing muscle disorders, which uses a thin fiber optic sensor to quickly scan muscle tissue.

Diagnosing muscle disorders often requires the use of a biopsy, which can be very painful and difficult to perform. The researchers have developed a less invasive method for diagnosing muscle diseases to solve this problem. The alternative solution is based on a miniature fiber sensor that can quickly measure the health of muscle tissue.

  • Chemical and oil and gas industry, metallurgy

In these industries, fiber optic sensors with non-contact measurement methods are in demand (beam thermometers, optical sensors for measuring gas flow, acceleration and displacement sensors), which can operate stably in aggressive and explosive environments, high temperatures, and intense electromagnetic interference.

The oil and gas industry is one of the leading industries worldwide. Due to the specifics of this industry, increased requirements are applied to it, both in terms of safety and environmental factors. The oil and gas industry is one of the leading industries in terms of inventions and the introduction of the latest monitoring and data collection systems at all stages of production because of strict requirements.

One of the fields of fiber optic application that is the main consumer of innovative and breakthrough fiber solutions is in-line diagnostics of oil and gas pipelines, it is an integral part of the transportation.

There are many sensing systems based on ultrasonic, magnetic, eddy current and other sensors. One of the modern and new directions can be identified as fiber optic technology. Fiber optic technology is not novel, but it appeared later than others in the oil and gas industry. Fiber optic sensors open up new opportunities for this area due to the superiority of certain fiber sensors by certain indicators. Fiber sensing materials can be dielectrics that do not conduct electricity and are not sensitive to electromagnetic interference, which allows their use in explosive atmospheres. Also, fiber optic sensors can operate in the environment of high temperatures and pressures, which makes it possible to install them where other sensors cannot work. Fiber optic sensors can measure almost all possible parameters. Such as temperature, pressure, magnetic field, position in space, gas concentration, vibration, distance and more. In addition to excellent measuring characteristics, fiber sensors also have other equally impressive qualities, such as high reliability, low energy consumption, small dimensions, low cost of materials for manufacturing, and long operating time.

In the oil industry, the optical fiber sensors are actively used and allow the development of fiber optic monitoring systems for the equipment involved in oil production, well construction and oilfield construction. Other sensing devices can be designed on the basis of modern fiber optic sensors for in-line diagnostics, capable of operating in extreme conditions and receiving information inaccessible to other types of sensors. In addition to the geometric data of the pipeline, devices equipped with fiber sensors based on fiber optic technology are able to obtain data on the chemical composition of petroleum and petroleum products at petrochemical facilities, since accurate and timely information on the state of petroleum products is required to optimize the process.

If you want to obtain a highly efficient sensing system, you should choose Optromix company. Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber optic solutions for monitoring worldwide. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, distributed acoustic sensing (DAS) systems, distributed temperature sensing (DTS) systems. If you are interested in FBG sensors and want to learn more, please contact us at info@optromix.com

editorFiber optic sensors for extreme conditions

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