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 optic solutions for fire detection and temperature measurement

FBGs for fire detectionProper fire detection in the mining area is the number one priority because fire can evoke damages to valuable assets, downtime in operation, and even loss of lives. Herewith, the installation of fire protection devices can be complicated by harsh environmental conditions of the mining area because industrial equipment produces dirt, dust, humidity, and corrosive atmospheres in the production, storage, or transport of goods. Nevertheless, the application of modern fiber optic sensors based on distributed temperature sensing or DTS is considered to be a proven and efficient technology for fire detection and temperature measurement.

The fact is DTS technology provides precise temperature measurement along the length of a conveyor belt due to the use of autonomous fiber optic cables, therefore, even very long conveyor routes can be efficiently monitored. Thus, cases of fires and overheating during mining operations can be quickly and easily identified and localized to within one-meter precision by distributed temperature sensors, enabling to activate the safety countermeasures. Also, it should be mentioned that it is required the proper installation of the fiber optic sensor cables and adjustment of the alarm zones for heat detection of industrial rollers.

Nowadays mining industry applies fiber optic linear heat detection systems for these purposes, however, the new distributed sensing technology may significantly improve and increase the protection of assets in combination with the one mentioned above. The principle of DTS technology operation is based on the use of optical fibers as a distributed microphone, moreover, it means that fiber optic sensors allow measuring not only the temperature but also the acoustics. Such additional information is important because it helps to determine potential sources of danger and improves the whole purpose of fire prevention. To be precise, the DTS is highly important in hot roller detection that is the main reason for conveyor fires.

Also, it is difficult to identify the initial smoldering stage with traditional techniques, herewith, the majority of common gas detecting systems do not work properly because of high air currents. An optical fiber-based on distributed temperature sensing technology, in its turn, provides numerous benefits during both normal operation and dangerous situation that include:

  • Reliable fiber optic cable that is resistant to dirt and dust;
  • Long-distance (up to 10 km) and up to 4 measurement channels monitor several conveyors with one DTS system;
  • Practically maintenance-free;
  • Accurate localization of fires and hot rollers;
  • DTS is able to detect fires in spite of unfavorable environmental conditions.

Optromix is a DTS system manufacturer that provides top of the line distributed temperature sensing systems suitable for monitoring of commerce networks. If you have any questions or would like to buy a DTS system, please contact us at info@optromix.com

Fiber Bragg grating sensors provide groundwater flow sensing

FBG sensors for flow sensingGroundwater flow sensing is highly important when it comes to the extraction of drinking water because it allows for avoiding well clogging and pollution. Nevertheless, fiber Bragg grating sensors with optical fibers are considered to be a promising new technique for monitoring of groundwater flow.

 

The fact is that FBG sensors enable us to identify even tiny changes in temperature, pressure, and fiber shape taking into account the sensitivities influenced by the packaging. Thus, fiber Bragg grating sensors are able to create a multiplexed sensor for the groundwater flow direction and magnitude.

It should be mentioned that the fiber-optic technology of FBG sensors has numerous application range, for example, in aerospace (load monitoring and shape sensing), in civil engineering (structural health monitoring), and in the oil and gas industry (temperature and pressure monitoring).

Today the technology of fiber Bragg grating sensing is also used for groundwater flow monitoring. FBG sensors allow measuring various physical properties, strain, or temperature variations such as pressure, vibrations, and curvature of the optical fiber, herewith, fiber sensors make instant and precise measurements.

One more technique that can be used for monitoring is distributed temperature sensing, and the intended application influences directly the choice of technology – fiber Bragg grating or DTS sensing. DTS uses conventional single-mode or multimode fibers that find various applications in the telecom industry, that is why DTS fibers are available in large lengths.

Moreover, the FBG sensors are possible to be written in traditional and specialty fiber optics, and consequently, their price increases per sensor number. However, the cost of fiber interrogators that are required for the DTS sensing data collection is five times more expensive than FBG units.

Traditional fiber Bragg grating sensors already possess a sufficient temperature resolution for the measurement of groundwater temperature changes without additional packaging. In addition, this FBG system has been recently tested to demonstrate its benefits for groundwater flow monitoring.

Thus, the experiment has shown that the use of FBG sensors enables to measure the relative temperature with a precision of 0.85 ℃ for the differential measurements. Also, the used fiber Bragg grating system is able to make a multiple-sensor interrogation with the possibility of an expansion into a larger distributed sensing network.

Finally, the data results presented that fiber Bragg grating sensors can be employed as a temperature sensing system in the subsurface environment, but in this case, they did not demonstrate any benefits over the existing fiber sensing technologies. Nevertheless, multiplexed fiber Bragg grating pressure or strain sensors would possibly find their application in groundwater flow monitoring.

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. The company creates and supplies a broad variety of fiber optic solutions for monitoring worldwide. If you are interested in FBG sensors and want to learn more, please contact us at info@optromix.com

Geophysical application of distributed acoustic sensing

DAS in geophysicsDistributed acoustic sensing or DAS system is a relatively new technology that finds numerous fields of application and has several benefits such as low cost, complete coverage, and repeatability for data collection. The principle of DAS system operation is based on the use of an optical fiber cable instead of geophones in conventional systems for acoustic recording.

Compared to traditional geophones, the DAS system demonstrates more advantages during its applications. For example, DAS technology employs a slim optical cable, due to which there are no limits in a horizontal well or ultra-slim well. Also, the cost of fiber optic cables is lower than geophones’.

Moreover,  the installation process of optical fiber cables is pretty easy and can include other fiber optic sensors such as distributed temperature sensing and distributed pressure sensing. Herewith, the cables offer continuous data transmission and the DAS system can be employed in the exploration well, production well, and observation well.  

In spite of the mentioned benefits, nowadays distributed acoustic sensing has some limitations, for example, the low S/N ratio, channel depth uncertainty, poor transverse fiber sensibility that are an obstacle for future applications. The most frequent DAS applications include:

  • vertical seismic profile or VSP;
  • microseismic measurements;
  • well and reservoir surveillance;
  • hydraulic fracturing monitoring, and diagnostics.

Despite the fact that distributed acoustic sensing technology is a highly promising prospect in the oil and gas industry, it requires several improvements for more stable operation. The most crucial problem is the development of the S/N ratio that can be solved by reducing the noise floor, using a stronger acoustic source.

It should be mentioned that distributed acoustic sensing data are connected with the strain amplitude, therefore, this factor allows to enlarge the S/N ratio by coherent stacking and correlation.

Also, removal of the random noise in distributed acoustic sensing can be made with the help of bandpass and median filters. In addition, it is very important to process a huge amount of data quickly, since DAS measurements are excessive. Herewith, DAS technology is often combined with multi-functional optical fiber sensors and the obtained measurements allow making the ambiguity lower.

Optromix is a DAS system manufacturer that provides top of the line distributed acoustic sensing systems suitable for monitoring of commerce networks. If you have any questions or would like to buy a DAS system, please contact us at info@optromix.com

Quasi-distributed fiber optic sensor based on fiber Bragg gratings

Quasi-distributed FBG sensorsAt the present time distributed optical fiber sensors based on Raman, Brillouin, and Rayleigh scattering play a crucial role in the area of monitoring due to their ability to measure strain or temperature distributions. Nevertheless, weak signals within optical fibers cause numerous challenges, such as limited spatial resolution, low measurement speed, high system complexity, or high cost of the system. That is why new technology that replaces the previous one and is called a quasi-distributed fiber optic sensing based on fiber Bragg gratings is developed now.

The fact is that over the last decades, fiber optic sensors have become one of the fastest-growing and promising areas of modern sensing technologies. Thus, distributed fiber optic sensing has been successfully employed in numerous strain- or temperature-sensing applications because of the multiple benefits of many sensing points and a long sensing range. Nevertheless, weak signals create new difficulties in the monitoring process.

The use of fiber Bragg grating (FBG) sensors is the only solution to the problem because FBG sensors have stronger reflection signals within an optical fiber. As a result, it is possible to use the technology for obtaining numerous physical and chemical parameters from discretized local points of a few millimeters along with a single optical fiber. Moreover, these fiber optic sensors based on fiber Bragg grating can be easily multiplexed for assurance of high measurement speeds.

Unfortunately, at the moment fiber Bragg grating sensors have several drawbacks. The first is the limit of fiber optic sensors for FBG interrogation systems based on multiplexing. Thus, the maximum available number of sensors is considered to be a few tens or less. The second drawback of FBG sensors is the relatively high cost of the writing of periodic grating that is needful for the assurance of high reflectivity at different Bragg wavelengths. This is the reason why fiber optic sensors based on fiber Bragg grating are not suitable for mass production.

Herewith, a novel real-time quasi-distributed fiber optic sensor with weak fiber Bragg gratings allows increasing the number of FBG sensors up to 31 for the successful and efficient process of interrogation. Such a system does not require special maintenance, has a long lifetime because its most components are optical fibers and semiconductor tools that can be easily found in the optical industry.

Finally, this FBG system offers the following advantages that can find numerous applications in the near future:

  • high linearity;
  • high speed;
  • high stability against harsh environmental factors such as vibration, temperature, and humidity.

Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber optic solutions for the clients. Optromix produces a wide range of fiber optic devices, including cutting-edge customized fiber optic Bragg grating product line and fiber Bragg grating sensor systems. Moreover, Optromix is a top choice among the manufacturers of fiber Bragg grating monitoring systems. If you have any questions, please contact us at info@optromix.com