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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

Constructions Where FBG Sensors Are Applied

FBG sensors have proven to be effective and useful during their implementations into different kinds of structures. They have become a necessary part of projects that aim to operate for long periods of time. Constructions Where Fiber Bragg Grating Sensors Are Applied

Fiber Bragg Grating Sensors Applications

There are various types of FBG sensors based on the parameters they monitor: temperature, strain, displacement, etc. They have a number of advantages that make them highly effective even in the most harsh environment. Compact size, fast response, immunity to electromagnetic interference and other factors usually play a critical role in many situations.

For example, fiber optic sensors are widely used for monitoring of long structures such as tunnels and pipelines. In addition to the ability of the remote control, they provide stable operation over a long period of time. Therefore, they don’t require any additional repairs.

The other advantage of fiber Bragg gratings is multiplexing. A single optical fiber can contain multiple FBGs at the same time. This fact makes it a cost-effective instrument even for large infrastructures. This is especially true when compared to techniques where each sensor is placed separately. Due to their size and weight, fiber optic sensors can be installed in most applications.

FBG Sensors for Strategic Buildings

FBG sensors are widely applied for structural health monitoring of various existing structures including critical facilities such as bridges, strategic buildings, etc. The main problem with this type of construction is related to the appearance of cracks. Cracks can cause many problems in the operation of the construction. They are dangerous because they can be invisible to specialists and unpredictable in terms of growth rate, both of size and speed. Unfortunately, there are a lot of examples where buildings have been ruined because of this, sometimes with tragic consequences.

When it comes to bridges in particular, real-time structural health monitoring is an effective tool for structural safety. Continuous monitoring is required for safety verification especially after extreme events.

Fiber Optic Sensors in Coal Mines

The other application field where fiber optic sensors are used is coal mining infrastructure. Electronic sensors can’t provide an effective structural health monitoring due to the extreme environment in coal mines. On the contrary, in most cases, FBG sensors are chosen for their ability to operate in harsh conditions such as darkness, vibration and for their immunity to electromagnetic interference. Coal mines are in need of a modern monitoring system for the fulfillment of the operating conditions of the tunnels and the transmission of the data to the control center. Structural health monitoring provides specialists with early warning of potential coal mine failures.

Fiber Bragg grating sensors build a safety control structure health monitoring system that accurately observes the processes. It can monitor various parameters but for the coal mines the most common sensors are FBG temperature sensors and FBG pressure sensors. Fiber optic sensors are embedded in the structure itself and function during the lifetime.

FBG sensors have gained increasing interest due to their ability to alert about any tiny or critical changes. The personnel can react to the alarm and prevent the possible disaster. And if it’s already happened, the fiber optic system can help in estimating the damage.

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Structural Health Monitoring Systems Applications

Structural health monitoring systems have found a number of applications in various fields. A complex approach including several techniques is the best solution for detecting damage and structural changes.

Here are some examples of its implementation in structures where fiber optic technology is more commonly used. Structural Health Monitoring Systems Applications

Structural Health Monitoring Common Information

The most effective methods of maintenance programs include early detection of material degradation and failure. Mostly, fiber optic systems have two purposes of applications:

Structural efficiency control during the construction;
Continuous monitoring of structural efficiency under service loads.

Structural health monitoring systems consist of three subsystems:

A network of fiber Bragg grating sensors;
An FBG interrogator that records the optical reflection from each fiber optic sensor. They are placed in the necessary locations, depending on the applications and type of the structure.
A processing unit that collects all the data from the FBG interrogator, processes it and transmits it to the user.

Distributed Sensing Systems

Distributed temperature systems are often the choice for harsh environments where traditional sensors may not be suitable. When we talk about dams, tunnels and similar structures, there are conditions that greatly affect the operation of the structures and sensors. Ground movement, earth pressure, water and groundwater have an impact on the reliability and efficiency of any monitoring system.

Specialists can monitor all deformations and temperature changes through distributed sensing. Distributed fiber optic sensors are capable of measuring various parameters from thousands of points and transmitting them to a center that provides complex data on any changes.

Distributed temperature systems are often the choice for harsh environments where traditional sensors may not be suitable. In the case of dams, tunnels and similar structures, there are conditions that have a major impact on the operation of the facility and its sensors. Ground movement, earth pressure, water and groundwater will affect the reliability and efficiency of any monitoring system.

Structural Health Monitoring of Dams

Dam safety can have a significant impact on the surrounding environment and people. That’s why structural health monitoring of the dams is an essential part of the dam safety regime. Timely detection of defects and minimizing the effects of possible damage are the main purposes of such systems. However, specialists may face some difficulties due to the height and complex structures of the dams. Comprehensive structural health monitoring can’t be fully achieved with just a few monitoring points; it requires a complete, properly designed fiber optic system. Another problem is that some areas of the dams are inaccessible or difficult to access.

There are a number of factors that can affect dam construction. These include temperature, hydrostatic pressure, chemical reactions, etc. There are also environmental factors such as air temperature, reservoir levels, and earthquakes.

All of these circumstances can be the cause of typical dam problems such as cracking, displacement, etc. The other problem is the appearance of internal erosion, which can cause the failure of the whole mechanism in the future. This problem is hard to detect. That’s why it is important to have an effective fiber optic system in place for structural health monitoring and therefore safe operation.

Accidents in dams usually occur during the first filling or in the first few years of operation. However, there are also accidents that occur later.

Structural Health Monitoring (SHM) of Tunnels

SHM is able to provide a quick assessment of the state of health of the tunnel. It is typically used for safety improvement and optimization of maintenance and operations activities. The fiber optic monitoring system provides data on any changes in indicators that could be the cause of tunnel collapse. This makes it possible to monitor the stability of the structure and take action when it’s needed.

The construction of tunnels is based on soil conditions, the functions of the tunnels themselves, and logistical issues. Underground construction is irreplaceable, especially in cities where land resources are scarce. They are not as susceptible to the effects of natural disasters as are structures on the surface of the earth.

In the hard-to-reach or problematic areas of the tunnels, distributed monitoring systems are usually recommended. This makes it easy for staff to monitor the condition of their structures and perform necessary maintenance in a timely manner. This is especially important when it comes to fires – the biggest danger in tunnels. There are special fiber optic systems for temperature control and early warning of ignition, including fiber optic heat detectors.

Today, there are automated fiber optic solutions that provide accurate monitoring from a remote location. These fiber optic devices are often designed specifically for use in critical locations. In addition, they consider the cases that may occur, such as the use of the additional FBG sensors. If the other sensor fails, they will be able to operate.

Distributed temperature sensing is a newly introduced technique capable of complex structural health monitoring systems. Temperature and pressure control is especially important for dams and tunnels with their difficult-to-access locations and other challenges that staff usually face. SHM is the solution for these situations.

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FBG Sensors for Concrete Constructions

FBG sensors are used to monitor structures made of various materials, including concrete and metals. In particular, we are talking about structures that usually consist of metals such as aluminum, steel, copper, etc. Fiber Bragg Grating Sensors for Concrete Constructions

Concrete Constructions’ Main Issues

Reinforcement plays a major role in concrete structures. Its strength and durability can affect the appearance and growth of the cracks. One of the reasons for such a degradation is corrosion.

Corrosion occurs when a substance reacts with its surroundings, resulting in the material’s consumption or contamination by an environmental component. Corrosion can reduce the life and load-bearing capacity of a structure. Due to the chemical reactions caused by corrosion, it may change the reflection of the steel surface. The thicker the material, the more it will be subject to deformation. Here, structural health monitoring (SHM) delivers the results.

Structural Health Monitoring (SHM) is an important tool used to monitor the occurrence of corrosion. It is effectively used in constructions to detect and prevent any possible damage. Early corrosion monitoring is essential. Corrosion is dangerous because it causes high costs to repair, and it takes some time to be discovered. In the most extreme cases it can bring the collapse of the complete structure.

Types of Corrosion Monitoring Devices

Today, there are a number of inspection technologies that have been developed specifically for the purpose of detecting damage to constructions. These methods include acoustic emissions, ground penetrating radar systems, electromagnetic techniques, etc. However, they all have their shortcomings including susceptibility to electromagnetic disturbance and lack of continuous structural health monitoring.

Fiber optic sensors are an effective solution for structural health monitoring. They provide real-time monitoring and low-cost techniques that monitor and control the corrosion appearance. There are a variety of fiber optic solutions available such as fiber Bragg grating sensors, FBG interrogators, distributed sensing systems, etc.

Corrosion conditions may vary due to their structure. Therefore, there is a need for a great number of point sensors. This is where distributed fiber optic sensors really come into their own.

Fiber Optic Sensing for Structural Health Monitoring

FBG sensors are used as instruments for corrosion detection. Like any standard sensor, fiber optic sensors are built into the structure to collect all the data and transmit it to the unit. They directly or indirectly measure vibration, displacement, humidity, temperature and chemical changes. FBG sensors are divided into several types due to the parameters they measure: displacements, strain, temperature, etc. By displaying this data, they are able to predict the occurrence of corrosion and its future consequences.

Here is a list of the major features of the FBG sensors:

Portability;
High accuracy;
Simplicity;
Long life;
Easy to integrate;
Electromagnetic interference and corrosion resistance;
Ability to operate in harsh environments, etc.

FBG sensors are particularly indispensable for monitoring corrosion in the hidden areas that are usually inaccessible to specialists. Their compact size allows for high accuracy and resolution. However, fiber optic sensors do not detect corrosion appearance directly but by estimating the material layer deposited on an optical fiber. That’s why they should be located close to the structure, in the same conditions.

FBG sensors have shown promising future in interaction with the concrete structures. This expands the horizon of their applications, including critical facilities and civil engineering.

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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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Fiber Optic Sensors for Li-ion Battery Cells

Fiber optic sensors have found a number of applications where there is a strong need for temperature monitoring. The use of Li-ion batteries is no exception, and specialists have learned to use fiber optic sensors for them as well.

Temperature monitoring of Li-ion batteries is an essential aspect of their operation in the field, and the problems of temperature monitoring can be solved with the help of fiber optic sensors.

FBG Sensors for Li-ion Battery Cells

What are lithium-ion batteries?

Li-ion, or lithium-ion battery is the most common rechargeable battery that uses the reversible reduction of lithium ions to store energy. Each module consists of one or more individual cells, which are consistently connected to follow the necessary rates of voltage, energy demand and power.

Typical Li-Ion BMS includes a few thermistors to monitor on selected cells surface temperature. This means that there is a possibility that cells may heat up invisibly.

Due to the wide deployment and large number of cells in each module, there is a need for modern technologies that can monitor cell operation and prevent future catastrophic failures.

The most common problem is the significant increase in temperature and strain in the cells. The effect of high temperatures and heat production from the cell can induce a series of degradation processes, resulting in reduced capacity and performance degradation. This problem is perfectly monitored with modern distributed temperature sensing modules.

The Temperature Effect on Li-ion Batteries

Distributed temperature sensing is used because the operation of Li-ion batteries can be affected by the influence of low and high temperatures. That’s why it is necessary to maintain comfortable temperatures during operation, storage and charging.

Temperature deviations from the ideal values may occur. However, they should still be within the established operating range. In addition, the critical temperature points shouldn’t be reached. Because of the test fields, continuous use in marginal conditions leads to accelerated wear of the battery and deterioration of its technical parameters.

Cold and overheating are both undesirable and harmful for lithium-ion batteries. In cold temperatures, they can experience voltage drops, deterioration of current output and rapid discharge. However, these symptoms disappear as the temperature rises. Overheating is more dangerous to batteries. It shortens their life and reduces their performance characteristics. There is also a risk of fire in the power source.

Maintaining optimal temperature regimes during battery operation extends battery life and ensures safe use. That’s why monitoring with distributed temperature sensing is an essential element for the correct use of Li-ion batteries and the prevention of emergency situations.

Fiber Optic Sensors for Li-ion Batteries

For Li-ion batteries, fiber Bragg grating sensors have been used to obtain data on distributed temperature and strain on the battery surface in various operating conditions. Specialists use the data obtained from distributed sensing systems in real time to detect abnormal conditions and prevent battery failure.

According to the field experiments, FBG sensors have been proven to sense the temperature either in the cells or between the cells when they are placed between battery cells. As a result, fiber Bragg grating sensors are able to provide the thermal information of each cell in a battery.

Lithium-ion Battery Applications

The lithium-ion batteries are commonly used to provide uninterrupted power in data centers, factories, air and water vehicles, and other facilities. Compared with the other types of batteries, they are smaller and have a longer service life. However, there is one drawback that should be considered in its implementation. A battery contains an electrolyte that decomposes into flammable components. Today, specialists have greatly reduced the potential for explosion to the maximum, but there are still risks. That’s why fiber optic sensors are very useful for monitoring.

The commercial battery industry also includes primary batteries as one of the major segments. However, it has its peculiarities, including non-recyclable materials and hazardous components in the batteries, which also cause ecological problems.

All in all, the knowledge of the temperature distribution in a cell or between several cells is fundamental for the safety of the battery. Fiber optic sensors have proven to be suitable for both temperature and strain gradient monitoring.

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Structural Health Monitoring for the Aircraft Industry

Structural health monitoring has already found many applications in various industries and become a powerful instrument. Its purpose is measurement of the aircraft constructions’ conditions.

The real-time monitoring and review of the structure’s current state provides precise data about the state of the materials and the construction itself. The data received with the help of fiber optic technology can be applied whether by specialists that are responsible for flight safety while plane preparation or by pilots during the flights. Moreover, structural health monitoring is an up-to-date technique considering the increasing number of flying machines including drones. SHM for the Aircraft Industry

Types of Structural Health Monitoring

A correctly customized SHM system tends to increase effective implementation, extend the service life of the equipment and reduce maintenance expenses.

There are two types of structural health monitoring that are applied in the design and operation of aircraft machines over many years: common structural condition monitoring that is responsible for damage determination and examination of the aircraft parts integrity, and load monitoring.

The monitoring of structure conditions and information about loads and damage location is proved to be extremely important, especially for the aerospace and aircraft industries. Usually, load monitoring is provided by measurement of the local deformations, and damage detection that is possible due to the acoustic signals of the distributed sensing systems.

FBG Sensors for Structural Health Monitoring in Aviation

To install structural health monitoring systems specialists carry out a range of calculations and estimations regarding FBGs and their integration into a process. Thanks to the factual flight data and a number of experiments, they have defined the vibration and load characteristics in aviation. This final information has been applied later for field projects.

Fiber Bragg grating sensors are particularly effective for monitoring and measurements of temperatures, pressures and strains. A variety of qualities make FBG sensors ideally suited for applications where the accuracy is important.

In addition to high accuracy, fiber Bragg grating sensors have compact sizes and light weight. These features apply both for cables and instrumentation. Moreover, they can be produced even smaller if the construction and clients require it. Therefore, FBG sensors have often been chosen as an appropriate monitoring instrument for the aviation industry as well as for various other spheres.

One more important advantage that specialists highly appreciate is the absolute explosion safety of the fiber optic sensors. This quality can provide safe engine operation even in conditions of laying cables along fuel tanks and lines.

Another issue that usually concerns aircraft specialists is susceptibility of metals to corrosion. To solve this, there is a need for monitoring of corrosion processes in real time. That’s why the development of specialized fiber optic sensors or corrosion sensor devices that are able to track its dynamics and transfer the data about it to the center plays a crucial role for aircraft construction.

Common Applications of the FBG Sensors

In aviation fiber optic sensors can be applied for measurement of different fundamental parameters that are necessary. Due to these numbers, there can be detected any appearing changes in construction. The standard FBG sensors measure temperature, pressure and strain. In aircraft engines, they can be applied in the hot and cold parts of the engine path. Fiber Bragg grating sensors for pressure and fuel consumption, vibration, and deformation at critical points can be used to evaluate their conditions even indirectly according to the deformations of the elements.

In addition to engine monitoring, FBG sensors are also applied to detect deformations of wings and fuselage parts, icing conditions, loads on the landing gear during takeoff and landing, etc. Structural health monitoring is proved to be essential, considering the wide range of details for airplanes that are made of polymer composites.

Fiber optic sensors may help specialists in detecting the problems during the flight that can be caused by difficult weather conditions. Moreover, they are able to highlight the existing problems during the creation of new aircraft or detect critical points under maneuvering modes. Both these applications are important and help in preventing huge catastrophes or at least will result in reducing their number.

All in all, structural health monitoring has made a great contribution into the aviation industry, just like in other spheres. FBG sensors are proven to be cost-effective and powerful instruments that can improve operation of modern aircraft and drones.

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Structural Health Monitoring for Different Structures

This fiber optic technology has become a modern and advanced approach that attends several functions: diagnostic and simultaneous monitoring. SHM for Different Structures

Structural Health Monitoring Operation Principle

The continuous structural health monitoring gives an opportunity to accumulate data about an object’s state. Thanks to the information obtained, specialists can use it to predict possible damage in the future and extend the service life. And fiber optic solutions provide a whole system that consists of fiber optic sensors, data collecting and transmitting units, etc.

Implementation of the fiber optic system involves review of the design processes of structural elements, a change of the processes, etc. For normal operation, parameters should stay at the allowable ranges that were previously determined and recorded in the program by specialists.

In fact, the factors that can influence the structural condition can be divided into two categories:

Negative external factors that have a constant impact on the material. This includes high or low temperatures, humidity, etc.;
Force-majeure circumstances, such as floods, volcanic eruptions, earthquakes, etc.

FBG Sensors for SHM Systems

Fiber optic sensors are proven to bring benefits to specialists as a part of the structural health monitoring system, including for civil construction. Compared to the electrical sensors, fiber optic sensors offer several advantages, including resistance to electromagnetic interference that is crucial in many applications.

Fiber optic technology can transmit data over several kilometers. Their other features such as long-term stability and reliability help them to function in severe environments. FBG sensors have been examined, for example, for implications in ice and snow. Fiber optic sensors are susceptible to the mechanical and chemical impacts of the concrete constructions. Despite that, they are mainly used in concrete environments for constructions such as bridges and dams. However, FBG sensors are able to effectively operate in such conditions for a considerable time.

Still scientists are going to design and experiment with fiber Bragg grating sensors further to find their limitations in new applications. Various types of fiber optic sensors were produced to provide measurements of diverse physical and chemical characteristics. Fiber Bragg grating temperature sensors are used for structural health monitoring of civil engineering structures, as it was mentioned above. In certain concrete based building structures there is a need for FBG strain sensors or FBG moisture sensors. From the other side, concrete structures are affected by formation of cracks and moisture ingress resulting in operation failure.

Fiber Optic Sensors for Sewage Tunnels

One of the fields where fiber optic sensors are used is a structural health monitoring system for sewage tunnels. The main reason for the sewage systems’ damages is excessive loading. It may be caused by constant physical effects like corrosion, penetration of plant roots, etc. The damage can be also caused by natural disasters such as landslides and floods. The consequences of all these impacts can greatly reduce the constructions’ operation life and lead to failures in operating.

That’s the reason why fiber Bragg grating sensors are essential in structural health monitoring systems. FBG sensors are able to constantly monitor and predict these events in advance. Thus, specialists have an opportunity to prevent great damage or at least minimize the economic losses.

Thanks to the fiber optic sensors that structural health monitoring includes, this technology is proved to be cost-effective in different applications. The modern approach provides quick and simple-in-use monitoring of various kinds of structures.

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Distributed Sensing Systems for Leak Detection

The increasing role of the oil and gas industry has led to development of many related technologies including distributed sensing systems. Among other things, they have become a part of the leak detection technology that has proved to be one of the most powerful.

For development of the most cost-effective detection technology there should be a balance between three elements: well-trained staff, properly configured distributed monitoring systems and proper operations’ arrangement. By observing these conditions, the effective operation is achieved.

With distributed monitoring systems, the pipeline management leads to the fast detection of any accidents, thus, reducing the extent of damage, as well as unforeseen expenses. Distributed Sensing Systems for Leak Detection

The Most Common Classification of Distributed Sensing Systems

Distributed sensing is a continuous monitoring technique which operates across the whole fiber optic cable in real time.

In contrast with the conventional sensors that were previously applied, distributed sensing systems use the whole optical fiber instead of the separate sensors that measure parameters in certain points. For this reason, fiber optic systems are easier to install even in heavy ambient conditions. With fiber optic cable, specialists have no need in defining particular locations where sensors should be placed. This fact makes the monitoring process more cost-effective.

The distributed sensing systems are different due to the data they require. So, there are distributed acoustic sensing (DAS) or distributed temperature sensing (DTS).

Distributed temperature sensing (DTS) uses a fiber optic cable that serves as linear temperature sensors. Eventually, specialists get the full temperature profile for the required period.
Operation principle of the distributed acoustic sensing (DAS) is similar to the DTS system. It applies a fiber optic cable as well, that provides the data. However, instead of registering temperature changes, they are relying on the acoustic and vibration changes in fibers.

Applications of the Leak Detection Systems

Leak detection fiber optic systems are installed to get the information about any appearing changes in parameters and leak possibilities. The distributed monitoring systems are able to find leaks almost instantly.

Here are a few examples where fiber optic sensing systems have found their applications:

One of the last well-known developments connected to fiber optic sensors is the monitoring of offshore Arctic fields. Pipelines are constructed for safe transportation of oil and gas avoiding any leaks. However, there are high risks of their deformation because of the severe environmental conditions like the effects of ice gouging, permafrost thaw settlement, or physical damage. The absence of fiber optic monitoring systems may result in severe environmental, and economic impacts.
Distributed sensing systems can be also applied for liquid sodium leaks monitoring. Fiber optic sensors can be implemented into various nuclear power plants and other facilities. Electromagnetic immunity, ability to resist high temperatures and operating in radioactive environments make fiber Bragg grating sensors a powerful instrument in monitoring.
Distributed monitoring systems are also used at wastewater treatment plants. The leak can lead to dangerous consequences such as spillage of contaminated fluids and therefore environmental damage.

Main Purposes of the Fiber Optic Technology Applications in Leak Detection

Fiber optic sensing systems bring many benefits while using. As a result, they have become useful instruments and can be applied for different purposes. Firstly, distributed monitoring systems are usually used for monitoring any changes that take place over long distances of the pipelines. Fiber optic sensors can monitor flow rates, temperature, pressure, ground strain, etc. All measured physical parameters are sent to the center where all the data is displayed with the help of the software.

Due to the received data from fiber optic monitoring systems, specialists are able to watch any changes or conduct the necessary calculations. In case of any changes, the operator or security personnel gets a signal that there are changes and sees their accurate site. So specialists can quickly respond to any changes and prevent any further possible damage.

The other purpose of distributed sensing monitoring is leak detection. Fiber optic monitoring systems can be installed both on buried and unburied pipelines. They can detect the precise location of the leakage in a short time. Moreover, they can discover other accidents like ground disturbances, theft, manual and machine excavation, etc.

Fiber optic monitoring systems designed for the oil and gas industry can also help in optimizing the lifetime of the wells. They are used as integrity monitoring instruments of the storage tanks, process vessels and other equipment. Considering all the information, specialists can make a decision on the maintenance programs as a means of extending the service life.

Due to all these possible applications, distributed sensing systems have found their place in the gas and oil industry. Fiber optic sensing systems have proved to be cost-effective in measurements of main parameters in pipes in the conditions of severe environmental and industrial equipment.

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Distributed Sensing for Seismic Monitoring Systems

We have already published a range of articles referring to different applications of fiber optic technology in various industries and fields. Geophysics has also applied distributed sensing as a powerful instrument for structural health monitoring, such in, the oil and gas industry, tunnel safety monitoring, etc.

Seismic monitoring has been no exception. Resistance to electromagnetic disturbance, cost-effectiveness and possibility of implementing into hard-to-reach regions or places that can be harmful for human health are undeniable advantages of distributed sensing systems. Distributed Sensing for Seismic Monitoring Systems

What is Distributed Sensing Technology?

Distributed sensing is a technology that provides continuous measurements in real-time. Compared to traditional sensors, placed at certain points, distributed sensing acts as a sensing element along its entire length because the whole fiber optic cable is applied.

Due to the exploitation of the entire fiber optic cable all over its length, this method is considered to be one of the most cost-effective tools that can be placed in severe conditions.

Operation Principle of the Distributed Sensing Systems

In simpler terms, DAS systems as a part of the distributed sensing technology consist of several components including fiber optic cable and an attached optoelectronic device – FBG interrogator. The interrogator sends short pulses of the pulsed laser light into fiber optic cable. Then the backscattered light moves back up the fiber to the FBG interrogation unit. Due to the time that the laser pulse takes, there can be found the relation backscatter event and a fiber distance.

The FBG interrogator is connected to a processing unit that processes and stores the received data. This unit gets the raw data and transfers it into the information that is displayed in the program.

The software provides the visualized analysis that includes the previous data as well. In case there are any differences, fiber optic system alarms specialists. For instance, the system can display the location of the fiber optic cable on the map and highlight locations where the rates have changed or exceeded acceptable limits.

What’s the Difference Between DAS and DTS Systems?

Traditionally, distributed sensing systems are divided into distributed temperature sensing and distributed acoustic sensing.

The main difference between DTS and DAS systems is the type of signals they get to provide analysis. DTS systems are sensitive to temperature changes, while DAS is sensitive to acoustic vibrations. Therefore, their operation principle is also different. Due to the fiber qualities, the performance of DTS systems is able to stay at the necessary high injected pump power level. At the same time, DAS systems don’t require such high pump power as DTS systems because of the Rayleigh scattering.

DAS Systems and Their Applications in Seismic Monitoring

If we are talking about seismic monitoring, distributed acoustic sensing is used. DAS systems measure any vibrations that can’t be detected by DTS technology.

The goal of all seismic monitoring systems is warning before any accident occurs. Their effectiveness depends on the accuracy of the obtained data, lifetime and length. That’s why there is a necessity in a continuous operating system that is capable of working in severe environments and over long distances.

DAS systems were implemented as seismic monitoring systems due to the above-mentioned advantages as well as low cost, no special maintenance, robustness, etc. Therefore, distributed acoustic systems have provided new capabilities for seismic monitoring.

Field Projects Where Sensing Systems Were Applied

If we are talking about sensing systems, in most cases fiber optic sensors were more effective in comparison with the conventional electronic ones. That provides unrivaled performance, especially in critical applications.

For example, two years ago the specialists started a range of experiments in the Arctic and the Arctic Ocean. This is the first case when DAS systems were installed in the Arctic. Scientists wanted to get the analysis of the seafloor seismic activities and other processes under Arctic sea ice with a distributed acoustic sensing system. Fiber optic cable catches the vibrations about all the changes of the ocean 24/7.

The other project with DAS systems has also allowed scientists to see them in other severe environmental conditions. Since they are installed mostly in the ground, specialists have monitored their performance in snow. The most concerning parameters for them were the snow depth and severe frosts.

Finally, in the conditions of decreasing temperatures the reduction in background noise and better signal-to-noise ratio was noticed. In fact, the lower temperatures, the better results distributed acoustic sensing has provided. Cold temperatures don’t make fiber optic performance difficult or interfere with accuracy of the received data. Moreover, fiber optic cable is able to transfer signals at snow depths of at least 0.65m.

Distributed sensing as a tool for seismic monitoring systems has proved to be effective in detection of any seismic shocks and flow changes. All in all, DAS provides capabilities for seismic monitoring of the near surface.

DAS systems distributed acoustic sensing distributed sensing distributed sensing systems DTS systems FBG interrogators fiber optic cable