Category: Fiber Bragg Gratings

Chirped FBGs and Their Common Applications

Due to high demand, fiber optic sensing continues to evolve. The most well-known technology associated with fiber optic sensors is the fiber Bragg gratings. They have become an instrument for measuring thermal, mechanical and physical parameters, like other technologies.
The FBGs differ by the method of their inscription. For chirped FBGs, one of these methods is used. Chirped Fiber Bragg Gratings and Their Applications

What are the Chirped FBGs?

Chirped FBGs are fiber Bragg gratings with a variable period lengthwise. The most common type is the linear chirped grating, where the period of the grating varies according to a linear law. Such gratings are recorded with the help of special phase masks with a variable period. In the past decades, chirped FBGs have attracted a lot of interest from specialists. This is due to their main characteristics and grating structure.

Due to their reflective index, the chirped FBGs are different from the custom fiber Bragg gratings. The refractive index profile of a grating can be modified to add other features. One such feature is a linear change in the grating period, called chirp. The reflected wavelength changes with the grating period, broadening the reflected spectrum. A chirped grating has the property of adding dispersion. This means that different wavelengths reflected from the grating will experience different delays.

Common FBGs and chirped FBGs have another significant difference. The overall spectrum is a function of the temperature or strain that is registered in each individual section of the grating. Chirped FBG sensors are capable of detecting temperature or strain changes. In this way, they can locate the events, such as hot spots or strain deformations.

Applications of Chirped FBGs

Conventional FBGs are built on the basis of the periodic modulation of the refractive index in the core of the optical fiber. They have been proven to be the top of the line grating-based type of technology.

Structural engineering;
Medical devices;
Oil and gas industry;
Nuclear monitoring, etc.

The fiber Bragg gratings are used as in-fiber mirrors or optical filters with a narrow band optical spectrum. They are commonly applied as sensitive elements in various fiber optic sensing systems for measuring strain, temperature, and other parameters.

Chirped FBGs

Chirped FBGs, as well as traditional fiber Bragg gratings, have found a variety of applications in laser technology and distributed sensing systems. All chirped FBGs have the ability to use the full length of the grating for strain or temperature change monitoring. Chirped FBGs also have their own spectral properties and ability to detect profiles. All of these key features make them in demand, and even preferred, in a number of different areas.

Their main functions are as follows:

Heat detection and localization;
Measurements of strain;
Detection and estimation of the high-pressure events;
Location of structural damage and mechanical cracks;
Velocity measurement.

For example, there are several different applications, including the following:

Transmission line monitoring;
Medical treatments;
Aerospace and automobile industry;
Structural health monitoring;
Biosensing, etc.

Temperature Measurement for Cancer Care

Chirped FBG sensors are real-time temperature measurement devices. One of them is medical thermal ablation. It is based on the hyperthermia principle. This technique is widely applied in pain relief, correction of cardiac arrhythmias, interventional cancer care. Thermal ablation is a recently developed technology for the minimally invasive treatment of tumors. It has proven to be effective.

Based on the correctly selected thermal dose, medical personnel choose the most appropriate treatment method. The higher the temperature, the more human cells get affected. This is especially true when treating cancer, where high temperatures help kill tumors. Thermal ablation is a minimally invasive procedure performed through a small skin incision or needle. This helps avoid damage to healthy tissue.

Chirped FBGs have made a significant contribution to the implementation of real-time distributed monitoring systems. They have found a number of applications including structural health monitoring (SHM) where their ability to measure temperature and strain is useful.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product line such as fiber Bragg grating sensors, for example, FBG strain 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 structural health monitoring systems and want to learn more, please contact us at info@optromix.com

Fiber Bragg Gratings for Civil and Geotechnical Engineering

Specialists have already found a series of applications where fiber Bragg gratings have shown to be profitable. FBG sensors have become a reliable distributed sensing solution for structural health monitoring. In addition to other applications, such as in the biomedical field, they are also used in civil engineering and geotechnical engineering.

All these areas have one thing in common – the environmental conditions. In such environments, it’s difficult to prevent the degradation processes such as aging and chemical effects. These factors should also be taken into account when implementing the traditional monitoring systems or equipment. In most cases, they have a short service life and require frequent replacement.

Alternatively, fiber optic sensors deliver modern fiber optic solutions that are able to work in extreme environments. FBGs for Civil and Geotechnical Engineering

Fiber Bragg Grating Sensors Common Characteristics

Since their first implementation, FBG sensors have been broadly deployed especially for distributed sensing of temperature, strain, and other characteristics that are particularly valuable in geotechnical and civil engineering. Fiber optic sensors have become world renowned for the features they offer. High sensitivity, immunity to electromagnetic interference are the most important parameters that are in demand.

Like traditional sensors, FBG interrogators face similar challenges. For example, they are usually required in extremely accurate equipment. That’s why there’s a high demand for maximum precision and high spectral resolution, down to picometer level. Fiber optic devices are also increasingly being used to deploy in rugged environments.

Fiber Bragg gratings are compact and can provide stable operation and durability in outdoor environments. For example, they are typically exposed to moisture influences, chemical reactions, temperature variations, etc. Distributed sensing systems should meet all the necessary requirements to ensure safety and proper operation of the entire system and to avoid the consequences of the interaction of the sensing element with the environment.

To ensure that, fiber optic systems should have robust materials and construction. In addition to complete, reliable fiber optic systems, fiber optic cables play a crucial role in creating the long-term FBG structural health monitoring systems. All fiber optic sensors should be able to withstand a certain level of thermal and mechanical loads at all times. The only way to provide customers with the best fiber optic solution is through testing of FBG sensors and application methods.

FBG Inscription Methods

Depending on the application and environmental conditions in which the FBGs are to be used, specialists can design FBG sensors for individual customer parameters.

A fiber Bragg grating is a fiber optic microstructure in which the index of refraction within the optical core changes along its length. The producing process of FBGs is usually called “writing”. Nowadays, there are three FBGs inscription methods:

The interferometric method is based on exposing a light sensitive area of the fiber to an interferometric fringe pattern to get the full grating. The pattern is achieved by illuminating an appropriate mask. In this case, the phase mask period defines the FBG period.
Continuous core-scanning method. Here, the FBG writing is provided by the motion of the translational frame where the fiber is fixed. In this technique, the period of the FBG is defined by the modulation frequency and the translation speed of the fiber.
And the last is the direct point-to-point method based on the absorption of a laser pulse. Each grating element is generated by controlling the laser parameters and moving the fiber. In this technique, the FBG period is determined by the fiber translation speed and the laser pulse repetition rate.

Applications of FBG Sensors

Fiber Bragg grating sensors are used in various fields where it is required to monitor structural health. In engineering, they are used to monitor the integrity of the entire structure and observe any deformation of its components. Such monitoring leads to the reduction of the risks and safety requirements. Fiber optic sensors are particularly useful for reinforced concrete structures because they can provide online data on the risk of corrosion.

When we talk about geotechnical and civil engineering, FBGs have also found their place for implementation.

Here are some examples of how fiber optic technology gets used:

Concrete structure monitoring;
Lateral deformation monitoring of embankment soft soil;
Pressure monitoring of tunnel’s rock and soil;
Structural health monitoring of bridges, dams, etc.

Nowadays, FBG sensors are also used to monitor landslides and slope failures and have shown good results. Distributed sensing systems have been applied to examine the landslide stability and deformation of landslides. Nevertheless, installing monitoring systems in complex environments and dealing with uncontrollable boundary conditions require careful consideration of potential issues.

Monitoring engineering and geotechnical structures with distributed sensing systems can improve profitability. Structural health monitoring methods are often preferred due to their ability to:

operate in chemically aggressive environments;
their feasibility in hard-to-reach areas of structures;
use fiber optic solutions which reduce equipment costs by utilizing a single fiber optic cable.

Overall, fiber Bragg gratings find extensive use in geotechnical and civil engineering applications. And, as with any technology, the successful implementation of fiber optic technology requires specialized experience and professionalism in designing and implementing the entire fiber optic system. Most of all, when there is a need for long-term measurements.

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FBGs for Monitoring in Hydrogen and Radiation Environment

Fiber Bragg gratings (FBGs) are commonly applied in different fiber optic devices, for instance, in fiber lasers, sensors, etc. However, because of the harsh environmental conditions, where FBGs are operating, there are several important factors that should be taken into consideration.

According to the experiments and field results, FBGs’ characteristics can change with time. Negative environmental effects such as radiation and hydrogen effects can not only influence the parameters but also shorten the operation time of the fiber Bragg gratings. It is especially important to understand the radiation effects and other factors on essential dimensions of fiber optic systems during fiber optic technology manufacturing.

Fiber Bragg Gratings for Monitoring in Hydrogen and Radiation Environment

What are the Fiber Bragg Gratings (FBGs)?

Fiber Bragg grating is an optical fiber that has the refraction index changing along the fiber length. Due to its modulation, FBG acts as a mirror that reflects certain wavelengths and transmits the others.

FBGs have become significant tools of the fiber optic technology from the very first day of their creation. Specialists have drawn much attention to the fiber optic sensing because of its properties, for example, compact sizes, immunity to the electromagnetic interference, etc.

What are the applications of the FBGs?

Specialists have already applied fiber optic solutions in various industrial sectors: industrial and civil constructions, aerospace, etc. Fiber optic sensors are applied for measurement of different physical parameters including temperature, strain, pressure, etc. Nowadays, these abilities are effectively applied in such structures as a structural health monitoring device by fiber Bragg grating sensors.

However, implementing such a powerful instrument in difficult environmental conditions, where most tools can not be used at all, brings some troubles. Scientists have learned to deal with it and have improved fiber Bragg gratings greatly.

So, nowadays FBGs are capable of operating in high radiation conditions, as an example, for nuclear installations and space applications.

Fiber Bragg gratings (FBGs) for space

The most up-to-date fiber optic sensors can be used not only for spacecraft systems, but also for astronauts’ security and health. In space, the radiation level is very high, taking into consideration the periodic solar flares.

There is a need for an accurate system that is able to operate in radiation environments. Especially, considering the fact that it brings some troubles even to replace some equipment onboard not to tell about when a satellite is put into orbit.

That’s why fiber optic systems are highly suitable for applying in this industry and providing data about temperature, strain and radiation. Moreover, based on the received information, it can foresee possible malfunctions. They are able to operate in extreme conditions and survive during the entire mission.

These factors make all the difference in future space exploration and its colonization.

FBGs for the nuclear industry

Fiber Bragg grating sensors can be also implemented for the development of nuclear applications. It is well-known that the radiation affects the surrounding materials and consequently their features. It is about photonic and electronic components that are the most susceptible to nuclear radiation exposure.

Usually, in the nuclear industry there is no possibility to replace components or make any repairs of the already operating systems. That’s why it is required to monitor the systems’ maintenance in the radiation conditions.

There are two types of radiation that are always taken into consideration: gamma-rays and neutrons. Gamma-rays are radiated by the surrounding structures, and neutrons are relevant to the inner reactor core during its operation. FBGs can be implemented in different sensing operations. For example, monitoring of the temperature in the reactor core and observation of the underground nuclear waste storage facilities, mechanical stress measurements, etc.

FBGs in radiation environments

There are several types of FBGs that can be divided according to their diverse inscription processes, thermal and radiation resistances. In reality, fiber optic sensors can malfunction under the long-term exposure of radiation that leads to the future measurement errors. These malfunctions depend on the structure of distributed fiber optic sensors and the radiation environment.

Scientists have conducted a range of special studies to watch how fiber Bragg gratings are operating in radiation conditions. Space, high energy physics and nuclear facilities are able to apply fiber optic technology to their advantage.

Fiber optic sensors’ sensitivity to the radiation depends on their way of manufacturing and concentration. However, ionization is able to break the bonds. Moreover, there is a possibility of structural changes such as densification that causes further defects. These factors lead to the degradation of optical characteristics that are invisible to the eye.

The main effect applied in this technology is radiation-induced attenuation. It depends on various parameters including:

Harsh environments;
Qualities of the optical fibers like manufacturer process, etc;
Testing environment.

Due to the composition of the optical fibers, they will react in radiation environments differently. That’s why except for the accurate calculations, specialists take tests on the fiber radiation response in conjunction with temperature. They watch fiber optic sensors’ possible reactions before applications in the real-world environment.

Fiber Bragg gratings have many capabilities and can be applied for prevention of disasters and accidents. With a proper coating and method of inscription, FBGs are able to detect any changes and, due to it, define the concentration of the hydrogen, etc. The quick and accurate location of the resulting leakage can prevent crucial damages, unexpected expenses, and danger to human life.

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Fiber Optic Solutions for fusion power plants

Fiber Optic Solutions for power plants According to the researchers from the American university, the faster specialists can detect thermal shifts the faster they can prevent disruptive quench in the HTS magnets for fusion devices in power plants. That is why scientists hope to solve this issue with the help of newly developed fiber optic solutions.

Recently, fusion became considered a safe, constant, and carbon-free energy source. The HTS magnets play a crucial role in many such programs. It increases the necessity of different instruments such as sensors and controls that help magnets to work in severe environmental conditions of a fusion power plant.

The research team had an aim to prevent quenches in power plants that are based on magnetic-confinement fusion devices. Scientists also focused on the commercialization, availability, and simplicity in the conditions of the accelerating fusion’s viability as an energy source. They aimed to create a fiber optic system that would provide minimal risks and would be robust.

Scientists used optical fibers with fiber Bragg gratings (FBGs) as a promising instrument that can measure temperature. FBG reflects just one of the wavelengths that are determined by the spacing while most of the light passes through. The reflected wavelength can demonstrate the small differences in temperature and strain. That is why the installation of fiber Bragg gratings along the fiber optic cable can help in temperature monitoring all over the length.

FBGs have been applied in many various areas for strain and temperature measurement. However, according to the researchers, they’ve never been applied for larger cables with high current densities as they have. This cable is able to handle the intense electrical and electromagnetic stresses of severe environmental conditions.

The research team designed new ultra-long fiber Bragg gratings. They behave as a long quasi-continuous FBG, but all the lengths can be meters long instead of millimeters. When the usual FBGs can monitor temperature locally, these new fiber Bragg gratings can simultaneously trace the temperature modifications along the whole cable. This fiber optic technology enables fast detection of temperature changes regardless of the heat source location. It means that the accurate location can’t be defined but the utmost importance in such systems is early detection of the problem.

As a result of the real operating conditions, the fiber optic system was able to detect small temperature changes very quickly. It was even demonstrated to be more effective than the usually applied voltage taps. Moreover, the FBG sensors’ response times could be tuned and their sensitivity became higher as quench regions expanded. All these helped to find quench events faster in comparison with voltage taps even in difficult cases.

The research team offered the fiber optic system providing the technological effectiveness and minimal technological risk of the approach. And scientists are sure that they can make a contribution to other industries where superconducting magnets are really important with the help of fiber optic technology.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product line such as fiber Bragg grating sensors, for example, fbg strain 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 structural health monitoring systems and want to learn more, please contact us at info@optromix.com

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Fiber Optic Technology as pipeline leak detection method

Fiber Optic Technology for the pipeline leak detection Special fiber optic technology can make fiber optic cable a sensing one. That solves many challenges including monitoring long units like pipelines, tunnels, or power cables. With the help of fiber optic sensors, it is more likely to find the leak in a short time. FBG interrogator sends a laser pulse through the fiber optic cable and the light comes back to the interrogator. The back scattered light delivers back acoustic, vibration, and thermal data.

Distributed sensing is widely used as an external pipeline leak detection method. Distributed sensing systems easily detect changes in temperature, noise, or vibration. Distributed sensing helps in detecting internal events, for example, liquid accumulations in gas pipelines, slugs, and flow constrictions.

Nowadays, distributed sensing pipeline leak detection software includes a wide variety of pipeline applications. The systems that apply distributed acoustic sensing (DAS) or distributed temperature sensing (DTS) are already produced in many projects across tens of thousands of kilometers of pipelines. Therefore distributed sensing systems already proved to be the most cost-effective in leak detecting.

There are standard fiber optic cables with suitable distributed sensing systems compatible with single-mode and multi-mode fibers or with a combination of them. According to the scientists, fiber optic cables with the usage of single-mode fibers are suitable for both DAS and DTS. While fiber optic cables using multi-mode fibers could enhance the DTS systems’ performance. Distributed sensing systems emphasize the detection of thermal and acoustic leak signatures. That’s why there are different types of DAS and DTS systems according to their thermal, acoustic detection capabilities, and performance requirements.

For example, a research team from South Africa is currently applying fiber optic solutions for the detection of leaks in pipelines. Scientists investigated leak detection using fiber Bragg gratings. These were applied to measure strains and temperature on pipelines and in the ground adjacent to pipelines. With the help of fiber optic technology, they could detect water leaks by burying a fiber optic cable into a pipe trench with a new pipe or place it above an existing pipe.

The scientists hope that after a number of experiments they could implement distributed sensing systems in South Africa by monitoring the pipes and delivering the data about water leaks into a leakage detection center. So fiber optic solutions would give an opportunity to fix the leak quickly without losing massive volumes of water. According to them, fiber optic technology shows great promise as a highly effective leak detection system.

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Fiber Bragg Gratings (FBG) sensors’ applications

FBG sensors' applications The sensing technology market has a rapid growth in the last few decades. Most of all, this is explained by the main advantages like small size, environmental and electrical immunity, and distribution capabilities. The new units of FBG sensors and fiber optic cables are valuable instruments for monitoring industrial processes and infrastructure. That’s why fiber Bragg grating (FBG) sensors are applied in many different spheres.

Fiber Bragg grating (FBG) sensors are becoming more popular each year because fiber optic applications are spreading in different aspects of life and science. Some of them are security, transportation, civil engineering, medical, and etc.

Despite the diverse application space, the market driver for fiber Bragg grating sensors has been monitoring smart structures like bridges, dams, and pipelines. FBG sensors also have had an impact on the aerospace industry by controlling the temperature, vibration, strain, and other data in real-time.

There are a lot of fiber optic applications in the oil and gas industry. Fiber Bragg grating technology, fiber optic monitoring systems, and distributed temperature sensing are commonly applied for in-well temperature monitoring and exploration activities. Distributed fiber optic sensors are also widespread in the wind power industry. They are used for the measurement of stress and strain in turbine blades.

In medicine, the ability to perform all the fiber optic solutions’ benefits is very useful for operations and certain medical procedures. Fibers can be inserted in hypodermic needles or catheters. That allows for more precise positioning of the fiber optic sensors. Moreover, fiber Bragg grating sensors are applied for temperature profiling near patients’ internal organs. And finally, FBG sensors are produced for endoscopic/colonoscopic pressure profiling.

Fiber optic technology has an impact on chemical and biochemical sensing. There are bioassays based on fiber Bragg gratings as the sensing element for protein or DNA interactions.

Nowadays, because of the development of fiber optic solutions, new fiber Bragg gratings were created. They are able to cope with high temperatures and harsh environments. It is highly useful and even crucial in power plants and for combustion and jet engines.

Today FBG sensors are becoming irreplaceable tools in different fields because of their simplicity in comparison with other technologies and advantages that they provide. And according to the tendency, fiber Bragg grating sensors will continue to apply in many existing and emerging applications.

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Fiber optic sensing solutions for extreme conditions

FBGs for extreme conditions Electrical sensing systems (strain sensors, string-based, potentiometric, etc.) have been the main method of measuring physical and mechanical phenomena for decades. Despite their widespread application, electric sensing systems have a number of disadvantages, such as loss of signal transmission, susceptibility to electromagnetic interference, the need to organize an intrinsically safe electrical circuit (if there is a danger of explosion).

These inherent limitations make electrical sensors unsuitable or difficult to use for a number of tasks. The application of fiber optic sensing solutions is an excellent way to overcome these problems. The signal in fiber optic sensors is light in the optical fiber used instead of electricity in the copper wire of standard electrical sensors.

Over the past twenty years, a huge number of innovations in optoelectronics and in the field of fiber optic telecommunications have led to a significant reduction in the price of fiber sensor components and to a significant improvement in the quality of fiber optic systems. These improvements allow fiber optic sensors to move from the category of experimental laboratory devices to the category of widely used devices in such areas as monitoring of buildings and structures, etc.

The most widespread type of sensors

One of the most commonly used fiber optic sensors is considered to be fiber Bragg grating sensors (FBG). The fiber Bragg gratings in these sensors reflect a light signal whose spectral characteristic (wavelength) shifts along with changes in the measured parameter (temperature and/or deformation). During the manufacture of gratings, a region with a periodic change in the refractive index is created inside the optical fiber core, herewith, this region is directly called the FBG.

Optical fibers and fiber sensors are non-conductive, electrically passive, and immune to EM interference. The interrogation using a tunable high-power laser allows measurements to be made over long distances with virtually no signal loss. Additionally, in contrast to the electrical sensing system, each optical fiber channel can interrogate a variety of FBG sensors, which significantly reduces the size and complexity of such a fiber optic system.

Applications of fiber sensors

Fiber optic sensing solutions are ideal for applications where conventional electrical sensors (strain gauges, strings, thermoresistors, etc.) have proved difficult to use due to extreme conditions (long distances, EM fields, explosion protection, etc.). Since the installation and operation of fiber sensors are similar to conventional electrical sensors, it is easy to switch to fiber optic solutions. Understanding how such fiber optic systems work and the benefits of using them can greatly facilitate various measurement tasks (for example, structural health monitoring).

Benefits of FBG type

In short, the main advantages of FBG sensors include:

high sensitivity and performance;
relatively large range of measured deformations;
the best weight and overall dimensions, small size;
high noise immunity, insensitivity to electromagnetic interference, such as microwave field, spark discharge, magnetic fields, electromagnetic pulses of various nature and any intensity;
absolute electrical safety due to the absence of electrical circuits between the fiber optic sensor and the recording module;
full electrical, explosion and fire safety, high chemical resistance of sensor elements.

Extreme environmental conditions

The conditions of the environment and controlled conditions in which one or more external factors — radiation, temperature, electromagnetic field, aggressiveness, humidity, pressure, and deformation — have the maximum possible constant values are regarded as extreme.

In such conditions, primary converters of control systems for dangerous technological processes (oil production, transportation, and processing of oil and gas, nuclear power generation, storage of radioactive waste), monitoring and diagnostics systems for complex construction and engineering structures (dams, bridges, mines, etc.), and military and emergency management systems operate.

Currently, fiber optic technologies are widely used in various fields of science and technology. One of the main applications of fiber optics is the creation of portable high-sensitivity sensors. Pressure, strain, vibration, tilt, linear motion, and temperature sensors are widely applied in the industries of structural health monitoring pipelines, heating lines, power cables, mines, etc.

Application in radioactive conditions

Compared to fiber sensors, the lack of power supply at the location of electrical sensing systems does not prevent continuous remote monitoring of dangerous objects, such as nuclear power plants, in an emergency beyond design situations. For instance, the well-known events at the Japanese nuclear power plant “Fukushima-1” in 2011 were characterized by the fact that during the two weeks when the nuclear power plant was completely de-energized, there was no information from electronic sensors, which was extremely important for monitoring the technical condition of the emergency station.

Application in extreme temperatures

Problems of standard sensing systems control of tightness of tanks with liquid hydrogen, which is the fuel of modern rocket engines, has a temperature of -253 °C and very high fluidity, due to the fact that at such temperatures, most materials become very fragile, and the sensitivity of palladium sensors quickly decreases.

It is problematic to measure the pressure and dryness of superheated steam in gas generators and superheated gas in jet engine nozzles at temperatures up to + 600 °C since piezoelectric sensors quickly degrade at temperatures above + 300 °C. Modern FBG sensors of physical quantities are heat-resistant (up to +2300 °C) and cold-resistant (up to -270 °C). This provides reliable and long-term monitoring of the technical condition of high-temperature and cryogenic objects.

Operation during electromagnetic interference

Measurements of physical quantities using electrical sensing systems in conditions of high-power electromagnetic interference, including guidance on coaxial electrical cables and sensors from lightning discharges, in conditions of monitoring the patient’s pulse in a medical nuclear magnetic resonance facility, as well as measurements of high voltages and high currents in electrical engineering, are highly problematic.

Fiber Bragg grating sensors are completely immune to electromagnetic interference and are stable insulators. This makes it possible to measure high voltages up to 800 kV and high currents up to 200 kA with high accuracy (class 02s) by fiber optic sensing technology.

Application in an aggressive environment

Measurements of physical quantities of chemically aggressive media, long — term measurements of deformation of dynamically loaded objects and structures, as well as multi-sensor measurements-with the number of control points in several hundred and thousands, are also problematic for electrical sensing systems since the volume of measuring electrical cables is unacceptably increasing.

Distributed fiber optic sensors are multi-sensors: up to 10 thousand consecutive intra-fiber sensors can be used in one optical fiber (fiber optic cable) to measure physical quantities (temperature, strain, seismoacoustics, pressure, radiation, etc.). Multimode fiber optic cables allow performing remote measurements with high accuracy using borehole video cameras, and temperature fields — using pyrometers and thermal imagers.

Advantages of metrological calibration

A serious problem of electrical sensing systems embedded in objects (in the concrete of hydraulic dams and bridges, in the pylons and walls of high-rise buildings, etc.) presents the practical difficulty of their periodic calibration (metrological verification).

Modern fiber sensors have the function of metrological self-monitoring (FMSM) due to the multimodality of the optical signal, which allows for self-calibration of fiber optic sensors in real-time without stopping the controlled processes and without verification standards.

Today’s situation

In the last decade, there were implemented many similar applications of modern fiber sensors and systems in extreme environments of nuclear, oil and gas, and aerospace industries, shipbuilding, hydraulic engineering, energy, construction, military, and natural emergencies.

Moreover, the durability of FBG sensors in these extreme conditions creates an obvious advantage of their use in the energy, oil and gas, aerospace, construction, and transport industries in comparison with non-optical types of measuring systems.

Thus, the extreme operating conditions of fiber Bragg grating sensors, for example in wells (extreme parameters, flammable, aggressive and abrasive environments) or power plants (ultra-high currents and discharges, voltages and fields, significant ionizing radiation), actually belong to the usual operating conditions of fiber optic sensors.

How to find the best fiber optic solution?

If you are looking for reliable fiber optic sensing solutions for structural health monitoring, you should choose the Optromix company. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product 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 structural health monitoring systems and want to learn more, please contact us at info@optromix.com

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What you should know about fiber Bragg gratings

fiber Bragg gratings (FBGs) Fiber Bragg gratings are currently widely used in optical fibers and light guides for compaction of channels along the wavelength, optical filtering of signals, as resonator mirrors in fiber and semiconductor laser systems, as smoothing filters in optical amplifiers, to compensate for dispersion in the main communication channels.

Another field of application of FBG technology includes its use in various measuring systems that control environmental parameters, such as temperature, humidity, pressure, deformation, and chemical content. Bragg gratings distributed along the length of the light guides allow for creating distributed acoustic systems that differ favorably from traditional complexes of the same purpose in cost and technology of production.

FBG technology for recording Bragg gratings distributed in a light guide is a key element in creating a new generation of measurement systems. Hydroacoustic antennas developed on the basis of such optical fibers, as well as systems for the protection of extended objects and systems for monitoring the condition of main pipelines, are increasingly being used abroad.

A distinctive feature of these fiber optic systems is the large extent of controlled zones, speed, and unique information capabilities. When fiber Bragg gratings are written at a standard optical fiber, a problem arises because of the fact that such a fiber has weak photosensitivity and a low saturation threshold, which is not sufficient for effective recording of gratings.

The main solution method of FBGs is to increase the concentration of germanium dioxide in the core. Other methods consist of alloying the pieces for the creating of optical fibers with such chemical elements as boron, tin, nitrogen, phosphorus, antimony together with germanium, which leads to an increase in the photorefractive power of the light guides.

Writing of fiber Bragg gratings can be classified by the type of laser system used for production, the wavelength of beam emission, the recording technique, the irradiated material, and the type of Bragg grating. Lasers used for FBG writing can be either continuous or pulsed, with a wavelength of emission from the infrared (IR) to the ultraviolet (UV) range of the spectrum.

These differences determine the spatial and temporal coherence of the optical emission sources used for writing, which, in turn, determines the choice of the appropriate method for recording fiber Bragg gratings. The main methods for FBG writing include the step-by-step method, the phase mask method, and the interferometric method.

The need to increase the speed of information transmission, associated with the development of telecommunications, increasing information flows, the growth of global information systems and databases, the expansion of the number of users, led to the fact that fiber optic system communication lines were developed using spectral multiplexing of optical channels.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product 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 structural health monitoring systems and want to learn more, please contact us at info@optromix.com

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Fiber Bragg gratings in bogie frame

FBGs for bogie frame The application of fiber optic technology as temperature and strain gauges is quite surprising in bogie frames. To be more precise, these fiber optic sensors are applied for examining the carbon fiber bogie, in addition to standard surface-mounted electrical-resistance fiber optic strain gauges.

Optical fibers of 125 micrometers in diameter or 250 micrometers with a coating layer are perfect for this aim. The thing is that the optical fiber is improved to produce fiber Bragg gratings (FBG) in the fiber, efficiently producing a number of semi-reflective mirrors over short but equal intervals.

The operating principle of the FBG system is based on the reflection of the signal (a small amount of the signal at each semi-reflective mirror) when the light is transmitted through a fiber Bragg grating. Herewith, “the originally reflected wavelengths (without the influence of strain) from each Bragg grating are compared to the reflected wavelengths when the structure is loaded.”

It should be noted that in the case of FBG deformation by strain, the spacing between the semi-reflective mirrors is either enlarged (tension) or decreased (compression). Therefore, the change combined with the efficient refractive index and the period of the fiber Bragg gratings leads to a shift in the reflected central Bragg wavelength.

The thing is that the wavelength size demonstrates the strain magnitude. Nevertheless, there is the same effect that happened with temperature change, while the temperature effect is over 10 times the strain effect that is why the fiber optic technology needs to correct for temperature.

The researchers present the techniques applied to compensate for temperature where the fiber Bragg grating is placed close to the end-face of a cleaved optical fiber. The fact is the optical fiber with FBG is put in a capillary tube where one end is fused to the fiber, well away from the grating, and the opposite end is sealed. Finally, the FBG system responds only to temperature.

Nonetheless, it is not enough only to install several strain gauges into the bogie and link them to the instrumentation either. Ir is required to choose the proper fiber, for instance, bend-insensitive optical fibers are suitable. These are optical fibers where the diameter of the core includes 9.5-micrometer fibers with 4.5 mm long fiber Bragg gratings.

Additionally, it is necessary to properly install FBG systems to the bogie so as to act as a homogeneous part of the structure. Fiber Bragg gratings provide such benefits as efficient strain gauge transfer, capable to accommodate localized variations in the surface topology of the composite.

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FBG accelerometers for railway structural health monitoring

FBG accelerometers for railway monitoring New improved accelerometers based on fiber Bragg grating or FBG technology by fiber optic sensors allow performing railway structural health monitoring in both frequency and acceleration range. Such FBG accelerometers provide such advantage as immunity to electromagnetic interference, herewith, the fiber accelerometer offers multiplexed data information along very long lengths of a railway or pipeline for the in situ single-headend measurements of such parameters as vibration, strain, temperature, and fault locations or other challenges.

Thus, distributed sensors based on fiber optic technology are considered to be a better sensing device for structural health monitoring, compared to other sensing systems. It should be noted that FBG accelerometer systems are able to perform “perimeter security and other applications where events span frequencies <700 Hz and acceleration shock values of <30 G with sensitivities of about 16 pm/G” among various fiber optic sensing systems.

Nevertheless, the application in railway structural health monitoring requires optical fiber sensors that offer a higher level of acceleration peak value and broader frequency ranges. The developed optical fiber accelerometer offers the following parameters: >40 G acceleration values, 8 pm/G sensitivity for frequency values up to 1 kHz and the device is operable up to 2.5 kHz. The developed fiber optic sensor was already tested in a railway application and demonstrated successful results.

Additionally, it is possible to change the optical fiber cross-section to make the stress-induced measurement optimized, depending on the required parameters. For example, this FBG accelerometer has a commercially available length (about 0.35 m), a 15 mm diameter, 50/50 splitter (3 dB coupler), a 56 g stainless-steel mass (25 mm diameter, 15 mm height).

The principle of FBG accelerometer operation is based on the lateral forces cause birefringence changes that directly correlate with force parameters, such as vibration. Therefore, the FBG accelerometer installed on a moving train enables to monitor of the reflection spectrum in real-time to determine various problems with the track of a railway such as cracks, corrugations, or weak points.

Finally, the developed microstructured optical fiber used in new types of FBG accelerometers increases the level of measurement sensitivity of available systems up to 5X, and it is planned to apply the optical fiber to manufacture additional FBG accelerometers for field tests. Herewith, the FBG accelerometer is regarded as a highly important sensing device in an all-optical fiber sensing network that offers a great amount of data information resulting in efficient structural health monitoring of railways.

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