FBG sensors: a comprehensive review

FBG sensorsA fiber Bragg grating is an optical interferometer embedded in an optical fiber. At the same time, fiber optics combined with certain substances (usually germanium) can change its refractive factor when the fiber is exposed to ultraviolet light. If such a fiber is illuminated with ultraviolet light with a specific spatial periodic structure, the optical fiber becomes a kind of diffraction grating. In other words, this optical fiber will almost completely reflect the light of a certain, predetermined range of wavelengths, and transmit light of all other wavelengths.

Application of FBG sensors

The FGB application includes the following fields:

  • Point sensors (that is able to measure deformation, temperature, pressure, tilt, displacement), embedded in composite materials and others;
  • Laser systems and amplifiers (filters, mirrors);
  • Telecommunications (dispersion compensation modules, WDM technology);
  • Research and development.

Difference between electrical and fiber sensors

For decades, electrical sensors (tensor-resistive, string, potentiometric, etc.) have been the main method of measuring physical and mechanical phenomena. Despite their widespread use, electrical sensors have several disadvantages, such as loss during signal transmission, sensibility to electromagnetic interference, the need to organize a spark-resistant electrical circuit (if there is a danger of explosion). These mentioned above limitations make electrical sensors unsuitable or difficult to use for a number of applications.

The use of fiber optic sensors is an excellent solution to these problems. In fiber optic sensors, the signal is light in the optical fiber instead of electricity in the copper wire at traditional electrical sensors.

Over the past twenty years, a huge number of innovations in optoelectronics and in the field of fiber optic telecommunications has led to a significant reduction in prices for optical components and to a significant improvement in their quality. This factor allows fiber optic sensors to move from the category of experimental laboratory tools to the category of widely used devices in various areas.

Operating principle of Bragg gratings

A fiber Bragg grating or FBG acts as a sensitive element of point fiber optic sensors, which is capable to reflect certain wavelengths of light and transmit all others.  This effect is achieved by periodically changing the refractive index in the core of the fiber optics.

When the laser light passes through an optical fiber, a part of it is reflected from the fiber grating at a certain wavelength. This peak of reflected light is registered by measuring equipment. As a result of the numerous parameters influence, the interval between the FBG bundles and the refractive index of the fiber optics change.

Consequently, the wavelength of the light reflected from the fiber Bragg grating changes. In addition, it is possible to determine the exact characteristics of the changes by changing the wavelength. In fiber optic sensors based on Bragg gratings, the measured value is converted to a Bragg wavelength offset. The recording system converts the wavelength offset into an electrical signal.

The sensing element of such FBG sensor does not contain electronic components and therefore it is completely passive, which means it can be used in the area of increased explosiveness, aggressiveness, strong electromagnetic interference. Numerous fiber Bragg gratings can be installed on a single fiber, each of which gives a response at its own wavelength. In this case, instead of a point sensor, we get a distributed sensing system with multiplexing along the wavelength.

The use of the light wavelength as an information parameter makes the FBG sensor insensitive to the long-term changes of the parameters of the source and radiation detector, as well as random attenuation of power in the optical fiber.

Common types of FBG sensors

The following types of fiber optic sensors  based on FBG technology are used for automated monitoring:

The principle of FGB sensor operation is based on the modulation of one or several properties of a propagating light wave (intensity, phase, polarization, frequency), which change occurs with a change in the measured physical quantity.

The basis of fiber-optic sensing technology is an optical fiber – a thin glass thread that transmits light through its core. The optical fiber consists of three main components: core, shell, and coating. The shell reflects the scattered light back into the core, allowing light to pass through the core with minimal loss.

It can be achieved by a higher refractive index in the core relative to the shell, resulting in a complete internal reflection of light. The outer coating protects the fiber optics from external influences and physical damage. It can consist of several layers depending on the required protection.

Benefits of fiber sensors based on Bragg gratings

The advantages of FBG sensors include:

  • Wide sensing range;
  • Possibility to integrate the FBG sensing system into the object structure;
  • Full fire and explosion safety;
  • Long-distance signal transmission;
  • Integration of several fiber optic sensors in one channel;
  • Insensitiveness to electromagnetic and radio frequency influences;
  • No need for recalibration (stable over time under constant external conditions).

Bridge, Australia, BridgeAt the moment, most of the sensors used in the world are electrical sensors. As it was mentioned above, in optical sensors based on fiber Bragg gratings, the signal is light passing through an optical fiber (instead of an electric current passing through a copper wire). This fundamental difference allows FBG sensors to overcome many problems typical for electrical sensors.

Features of fiber optic sensors

Optical fibers and sensors are non-conductive, electrically passive, and immune to electromagnetic interference. Monitoring with a tunable high-power laser system allows sensing over long distances with virtually no signal loss. In addition, each optical channel is able to monitor a variety of FBG sensors unlike the electrical channel, which significantly reduces the size and complexity of such a sensing system.

Optical sensing systems are ideal for use in conditions where conventional electrical sensors (strain gauge, string, thermistor, etc.) can be difficult to use (long distances, EM fields, explosion safety, etc.).  It is easy to switch to fiber optic solutions since the installation and operation of optical sensors are similar to traditional electrical sensors.

Understanding the principles of FBG operation and the benefits of Bragg grating sensor applications can greatly facilitate the solution of various problems in the field of sensing measurement (for example, monitoring of structures).

Nowadays FBG sensors are applied in various fields that require precise and fast measurements. Fiber Bragg sensing systems can be used in aeronautic, automotive, civil engineering structure monitoring, undersea oil exploration, in the mining industry, geotechnical engineering, structural engineering, tunnel construction engineering, etc.

Bragg sensors in medicine

The most promising application of FBG sensors is medicine. Now FBG technology is highly used for fiber-based biomedical sensing including biosensing, safety or security, and structural health monitoring. FBG sensors offer a new and effective way of real-time measurements. They can be applied in laser systems, medical tiny intra-aortic probes, and body sensors for biochemical analysis making.

For example, today fiber Bragg gratings apply optical-fiber sensing probes that are able to dissolve due to such ability as controlled solubility in a physiological environment. Thus,  FBG technology enables safer diagnostic of sensitive human organs and there is no need for a surgical extraction. The development of FBG continues, and it is possible that very soon new FBG sensors with improved characteristics appear.

How to choose the right fiber optic product?

If you want to obtain a highly efficient sensing system, you should choose the 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

Fiber Bragg grating sensors for bolt force status monitoring

Over the past decades, bolts support has become a widely used method of roadway roof failure control due to the easy construction and its efficient reinforcement. Moreover, bolts support improve surrounding rock structure stability. Underground coal mining has been extending to deeper levels where high-stress conditions are present; high stress causes roof subsidence and floor heaving in coal mine roadways. This can have a negative effect on the safety of the miners and construction integrity. Therefore, real-time monitoring of bolts in coal mine roadways is vital for the long-term safety and stability of the surrounding rock structures.

The systems that are used currently for real-time status monitoring of the bolts support structures are easily affected by the harsh environments and high levels of stress. Fiber Bragg grating sensing technology provides a new and effective way of real-time measurements. FBG sensors have become one of the most promising optical fiber passive device applications. Fiber Bragg grating sensors have a variety of applications and are widely used for temperature sensing (FBG temperature sensors), pressure sensing (FBG pressure sensors), acceleration sensing (FBG accelerometers), deformation sensing (FBG displacement sensors), etc.

FBG sensors have multiple advantages over traditionally used measurement equipment: FBG sensors are immune to electromagnetic interference, have a strong multiplexing ability, high reliability, and sensitivity. Fiber Bragg grating sensing technology is already used in a variety of applications, like geotechnical engineering, structural engineering, civil engineering, tunnel construction engineering.

The use of FBG sensors in engineering applications shows that fiber optic sensors can automatically acquire, and monitoring results are of great significance in roadway anchorage engineering safety and bolt support quality evaluation.

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. Our main goal is to deliver the best quality fiber optic products to our clients. We produce a wide range of fiber optic devices, including our cutting-edge customized fiber optic Bragg grating product line and fiber Bragg grating sensor systems.

If you are interested in Optromix FBG sensors, please contact us at info@optromix.com


The use of FBG sensors in intelligent textiles

Medical textiles are an exciting idea that has the potential to significantly improve people’s lives. The use of textiles for medical purposes has a long history, and they are still used in the healthcare industry today. Traditionally, the textile industry was confined to the production of fibers and fabrics; modern technology allows the applications of textiles to expand. Recently, important developments have been made in enhancing the capabilities of textiles to respond to environmental stimuli. These textiles are called smart textiles; they possess the properties of conventional textiles and also carry additional functional values.

Smart textiles correspond with a wide area of products, as well as studies, that enhance the usefulness and functionality of conventional fabrics by using woven or non-woven structures that connect with the environment or users. The structures have the capability to sense environmental conditions.

There are multiple kinds of smart textiles that are used nowadays, one of them being E-textiles, or electronic textiles. E-textiles include smart health monitoring devices and sensors within the clothing that sense and record the basic bodily functions like body temperature, respiration, the functioning of a heart, etc. The data collected by the sensors is then transmitted to healthcare centers or hospitals. Fiber optic technology is widely incorporated in smart textiles in the form of sensors. Fiber optic sensors are used due to multiple advantages that they present, like low zero drift, large bandwidth, flexibility, good accuracy, immunity to electromagnetic interference, etc. Over the years fiber optic sensors have also become cheap and easily accessible.

Different fiber Bragg grating sensors are used in a variety of smart fabrics. Some of the FBG sensors that are used in smart fabrics include:

  • Stretch-sensitive sensors;

FBG strain sensors are used for monitoring and sensing body parameters. The sensors are used for determining respiration, movement, blood pressure, heart rate.

  • Temperature sensing;

FBG temperature sensors provide temperature monitoring on skin surfaces

as well as the near-body environment. These sensors are often incorporated in combat clothing.

  • Sensing harness;

FBG strain sensors and FBG displacement sensors are placed to measure abdominal and thoracic movements caused due to breathing activity without corrupting signals or overlapping signals.

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line: fiber Bragg grating sensors, FBG interrogators and multiplexers, Distributed Temperature Sensing (DTS) systems. We create and supply a broad variety of top-notch fiber optic solutions for the monitoring of various facilities all over the world.

If you are interested in Optromix FBG sensors, please contact us at info@optromix.com

Fiber bragg grating sensors for structural health monitoring in harsh conditions

Piezoelectric transducers have been traditionally used in structural health monitoring. These devices convert pressure to an electric voltage. The transducers are typically reliable in normal working conditions, however, they become less accurate in harsh environments, like high temperatures – over 300 degrees Celsius – or highly corrosive environments.

With the development of laser technologies the popularity of fiber Bragg grating sensors have been increasing due to their numerous advantages over traditional means of measuring strain, pressure, and temperature: 1) FBG sensors work reliably under very high temperatures because of the optical fiber material – fused silica; 2) fiber Bragg grating sensors are immune to electromagnetic interference; 3) the sensors are not susceptible to corrosion; 4) FBG sensors are very small and can be easily mounted on any surface. The development of fiber-optics technology will enhance structural health monitoring in harsh conditions, such as nuclear power plants and modern superheaters.

The following types of FBG sensors are used in structural health monitoring:

  1. FBG temperature sensors;

The main advantages of fiber Bragg grating temperature sensors are their reliability and accuracy under high temperatures, absolute temperature measurements, fast response, immunity to electric sparks. The fast response rate is particularly useful in cases where temperature shifts are extreme.

2. FBG strain sensors;

FBG sensors are the best strain sensors at the moment due to multiple advantages that are inherent to all FBG products. These sensors can determine structure parts that are stressed.  

3. FBG displacement sensors.

Fiber Bragg grating displacement sensors can detect shifts in structures and the appearance of cracks.

There are many applications of fiber Bragg grating sensors for structural health monitoring. For example, one of the possible uses of FBG sensors is pipeline monitoring. P19 pipelines, that are used in the power industry, are usually used to transmit corrosive high-temperature, high-pressure steam. The integrity of the pipes may deteriorate over time leading to eventual failure of the steam transmitting system.

If you would like to purchase FBG (Fiber Bragg Gratings) sensors, please contact us: info@optromix.com or +1 617 558 98 58