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.
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 sensorcomponents 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 fibersand 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 fiberchannel can interrogate a variety of FBG sensors, which significantly reduces the size and complexity of such afiber optic system.
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.
Compared to fiber sensors, the lack of power supply at the location of electrical sensing systemsdoes 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 sensorsof 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.
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.
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).
Modernfiber sensorshave 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.
In the last decade, there were implemented many similar applications of modern fiber sensorsand 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.
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.
Nowadays fiber Bragg gratings (FBG) face several challenges in fiber optic sensor systems because of their complex installation and the high cost of interrogators provided by interrogation methods and FBG multiplexing. The thing is that such features as wavelength, time, frequency, polarizing, and spatial division multiplexing necessary for most applications also need complex tools, for instance, spectrum analyzers, spectrometers with tunable interferometers, Bragg gratings, etc.
It should be noted that one more challenge for FBG sensors include “the fact that these sensors are not addressable per se, and therefore, any spectrum overlapping leads to interrogation errors.” The possible way to overcome the problem is to use the addressed fiber Bragg gratings combined with the microwave photonics interrogation technique. The addressed FBG is considered to be a specific type of fiber Bragg gratings with two narrow notches in the reflection spectrum.
The operating principle is based on the light that passes through the FBG that has two narrow optical frequencies, herein, the difference between them is less than an optical frequency and installed in the microwave range. Such an addressed frequency does not depend on stress or temperature fields, it is also independent of fiber Bragg grating’s central frequency shifting.
The addressed FBG sensors act both as a two-frequency source and as a fiber optic sensor of the measurement system simultaneously. Thus, it is possible to develop a microwave-photonic fiber optic sensor system based on arrays of the addressed fiber Bragg gratings, if the set of address frequencies in the array is regarded as orthogonal. The addressed FBGs, in their turn, allow designing multi-addressed fiber Bragg grating structures.
The thing is that multi-addressed FBGs apply three (or more) frequency carriers, while their beatings on a photodetector create three (or more) address frequencies. The combination of address frequencies enables increasing the fiber optic sensor capacity of the measurement system as well as increasing the precision of central wavelength determination. Therefore, the multi-addressed FBGs are regarded as a specific type of fiber Bragg gratings with three (or more) narrow notches in the reflection spectrum.
The operating principle of the multi-addressed FBGs is based ion the light with three (or more) narrow optical frequencies, and the difference between them is less than an optical frequency and is placed in the microwave range. Additionally, the address frequencies set in FBG sensors does not depend on strain or temperature fields, central frequency shifting as well. Finally, the multi-addressed fiber Bragg gratings are both a multi-frequency source and a fiber optic sensor of the measurement system at the same time leading to the appearance of new applications.
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.
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.
A company from Australia offers a novel fiber optic solution that allows providing conveyor health monitoring by applying real-time data to rationalize production and on-site performance, improve occupational health, hygiene, and safety management, and implement new predictive maintenance and support capabilities to control management.
Thus, the fiber optic technology was tested in surface and sub-surface environments of some of the world’s largest mining companies and bulk material handling the equipment resulting in the accessibility of optical fiber sensing for present sale all over the world. The fact is that efficient conveyor systems are highly important because the profitability of mining companies depends fully on such fiber optic sensing systems.
Additionally, the mining industry has always a huge challenge of conveyor maintenance, and traditional sensing technologies for advanced conveyor failure detection are often precarious, subjective, they require many time and labors. The new fiber sensing system combines the technology of optical fiber detection with a sensing technology platform, “advanced signal processing algorithms and predictive analytics” enabling to acoustically monitor and check conveyor health.
The advantages of presented fiber optic solution include the provision of accurate data to maintenance technicians, site personnel, regional operational hubs, and global headquarters, the opportunity to obtain daily asset reliability reports from every conveyor, at every site worldwide due to the connection of the fiber system to
a wireless Internet.
The operation of the fiber optic system is based on the transmission process of short laser beam pulses along a single optical fiber cable installed along the length of a conveyor, while acoustic disturbances from the conveyor sensing system lead to tiny changes in the backscattered laser beam light, which is then classified into distinguished parameters.
Also, the obtained data is then processed, the following information is gathered:
the detection of a damaged ball or a broken cage in a ball race;
monitoring and “tracking idler bearings as they progressively wear”;
the prediction of potential bearing seizures and establishment of roller replacement priorities at the next maintenance shut down.
Finally, the fiber optic technology of distributed acoustic sensing is considered to be “the way of the future for conveyor health monitoring”. Such fiber optic solution successfully optimizes conveyor operation and provides essential cost savings for operators. Herewith, this fiber sensing system monitors the condition of every conveyor roller that can contain 7.000 bearings per kilometer.
The continuous development of high-temperature fiber Bragg grating technology (FBG technology) promotes a significant increase in novel applications. For instance, nowadays FBG applications include such fields as “the temperature profiling of high-temperature manufacturing equipment, monitoring of fuel combustion machinery, temperature regulation of large diesel engines in trains, as well as assessment the structural integrity of a building post-fire”.
Additionally, high-temperature FBG technology is used in oil and gas industries where the resistance to the temperatures higher than 500 °C is totally recommended. To be more precise, the sensors based on fiber Bragg gratings are able to stand temperature conditions below and above 800 °C. Herewith, the thermal stability of FBG sensors depends closely on the intrinsic thermal stability of the core-cladding materials.
This is the reason why the development of fiber optic technology with higher thermal resistance, for example, the molten core technique, is still required. Thus, it was decided to apply a circular core/cladding glass optical fiber containing a yttrium-doped aluminosilicate core and a silica cladding in FBG sensors that may withstand about 900 °C.
The following types of FBG sensors are based on the nature of refractive index modifications induced by laser irradiation. The following types of FBGs are distinguished:
The type I in fiber Bragg gratings produces a laser irradiation regime that emits an isotropic increase of the refractive index.
The type II in FBGs, in its turn, has a connection with the creation of an anisotropic index change upon irradiation, generally emitted by the presence of nanogratings, and leads to the observation of form birefringence.
Ultra-high temperature regenerated fiber Bragg gratings are able to operate above 800 °C in silica optical fibers. Therefore, these FBGs find their application in such areas as the profiling of high-temperature manufacturing equipment, dual pressure/temperature sensing for gas turbines, sodium-cooled nuclear reactors, high-temperature air flow meters for internal combustion engines and train engine temperature regulation.
Femtosecond fiber Bragg gratings are made by ultrafast laser systems usually in the NIR spectral range, resulting in their use as temperature sensors for monitoring fluidized bed combustors, as well as for radiation-resistant temperature sensors.
Sapphire fiber Bragg gratings allow achieving even higher temperature operation by using materials with higher melting points.
Researchers from the Czech Republic demonstrate a new technique that allows improving wavelength stability and tunability of semiconductor laser diodes in fiber laser interferometers due to fiber Bragg gratings (FBGs) technology. This simulation technique makes the calculation of arbitrary fiber Bragg grating (apodized, chirp, etc.) with a high level of accuracy by a combination of techniques based on layered dielectric media (LDM) and transfer matrix technology.
Thus, based on the simulations and measurements made by the commercially available FBG technology, it has succeeded in the development of a special 100 mm long fiber Bragg grating with apodization. Herewith, the researchers confirm that the new FBG technology of improved linewidth and mode-hop free tuning range of semiconductor laser systems at the wavelength 760 nm enables to increase the resolution of a laser interferometer. Therefore, the absolute fiber laser interferometer with Vertical Cavity Surface Emitting Laser (VCSEL) to easily apply the FBG system to make the wavelength parameter more stable and monitor the tuning range was produced.
Typically, two types of fiber Bragg gratings are distinguished in the used technology: FBG with a period of ~0.5 µm, and Long-Period Fiber Bragg Gratings (LPFGs) with a period from 100 to 500 µm. Additionally, it should be mentioned that their production requires different methods. Thus, the creation of fiber Bragg gratings is performing by interference patterns, while long-period FBGs are produced by side irradiation of the fiber optic components through an amplitude mask or using the fiber translating technique.
Therefore, the application of FBGs offers high thermal stability, retaining optical properties up to 500 °C. Moreover, the production technique has a dependence on such parameters as the length, the type, and the other factors of the FBG technology. It is possible to determine the necessary parameters on the basis ofthe FBG spectral profile.
Finally, the simulation technique based on the application of fiber Bragg gratings was developed and tested several types of FBGs. For example, chirped and apodized fiber Bragg gratings (FBGs with modulation of the amplitude and with modulation of the spacing). Herewith, this new FBG system provides numerous benefits in comparison to other types of FBGs.
The main benefit is suppressing the side lobes in the fiber Bragg grating spectral properties. The developed FBG system is a compact reliable one that ensures the operation in an industrial environment where the majority of optical components would be fiber-optic.
Nowadays train accidents often cause severe injuries and even cases of death. Nevertheless, researchers from China have designed new fiber optic sensors that allow measuring acceleration and vibration characteristics on trains. Additionally, this optical fiber technology can be used in a combination with artificial intelligence in order to prevent potential railway accidents and catastrophic train derailments.
To be more precise, the FBG accelerometers are able to monitor problems in the railway track or the train in real-time to detect defects before an accident happens. Such fiber optic devices identify frequencies more than double that of conventional optical fiber accelerometers, making them ideal for monitoring wheel-rail interactions.
The principle of durable FBG sensors’ operation is based on the use of no moving parts, herein, the fiber accelerometers demonstrate good operation in the noisy as well as in high-voltage environmental conditions found in the railway field of application. Moreover, the researchers confirm that their novel FBG accelerometers can be applied in other vibration monitoring applications. For example, fiber optic applications include structural health monitoring for buildings and bridges and vibration measurements of aircraft wings.
It should be noted that the researchers have been developing condition-monitoring optical fiber systems for more than 15 years using an all-optical sensing network to continuously monitor crucial railway components. It is planned that these fiber optic sensing systems help change poor efficient traditional railway maintenance routines by predictive maintenance based on actual conditions.
The developed FBG accelerometers demonstrate the following benefits that include:
immunity to electromagnetic interference;
long transmission distance;
no need for electricity.
But a need for FBG sensors for the measurement of various characteristics in railway systems still remain. Traditionally, fiber accelerometers are based on fiber Bragg gratings whose use is limited by 500 Hz vibrations. Therefore, such fiber optic technology was not suitable for the measurement of the wheel-rail interactions that are considered to be an important source of track wear.
Finally, the researchers have developed totally new FBG accelerometer based on a specific optical fiber (a polarization-maintaining photonic crystal fiber). It has been tested by installing the optical fiber device on an in-service train. Thus, the FBG accelerometer demonstrates a similar operation to the piezoelectric accelerometer, but it does not require expensive shielded fiber cables to take the effects of electromagnetic interference down.