A team of researchers presents a fiber optic technology based on fiber Bragg gratings (FBGs) for sensing to monitor the activity of an active volcano. The monitoring of volcanic activity plays a crucial role in better understanding and even prediction of important and potentially disruptive volcanic events, therefore, the fiber optic sensing system has to maintain harsh environmental conditions.
Nonetheless, the recording process of seismic activity now faces several difficulties concerning both discriminating between various sources of seismic wave, and the design of fiber optic sensing systems that can operate in active volcanic settings without any damages.
The team of researchers from France demonstrates the results obtained from the first high-resolution seismometer based on FBG sensors installed on an active volcano. It should be noted that the lifetime of modern fiber optic systems is quite short during their operation at high temperatures and the billowing, sulfurous, acidic gases near a fumarole.
Additionally, standard FBG sensors can fail in emergency deployment, or repair, even in pre‐eruptive phases. The operating principle of novel fiber optic sensing systems is based on interferometry forms that apply more sensitive fiber optic elements such as fiber Bragg grating resonators that enable to detect the acceleration of the ground as a change in the signal from the FBG sensor.
These fiber optic systems can be used for networking across long distances and monitoring these distance via optical fibers. The FBG sensor is considered to be “a purely optomechanical geophone that is interrogated through a 1.5-kilometer fiber optic cable by a remote, and thus it is a much safer fiber optic system down the volcano’s flank.”
Moreover, the fiber optic sensing system has been already tested and recorded tiny seismic events within the volcano for nine months. The development of new FBG sensors lasts almost a decade, the researchers use previous researches of a high-resolution optical seismometer prototype that includes a 3-kilometer fiber optic cable.
Finally, FBG sensors are regarded as highly reliable, fiber optic technology allows installing the sensors in locations that were not previously practical, providing more data about microseismic events under a volcano’s dome. The researchers claim that such fiber optic sensing systems offer more detailed information about “the fumarole signature, which helps to constrain the geometry and activity of the plumbing system of the dome”.
Fiber optic sensors of different physical quantities based on fiber Bragg gratings (FBGs) find its popularity and they are actively used in various fields of industry to solve a variety of engineering problems. The general operating principle of such fiber sensors is based on a change in the FBG wavelength under the action of external impacts. It should be noted that nowadays the population presents specific requirements to the application of assistive technology and, in particular, towards novel healthcare tools and fiber optic sensors. For instance, novel fiber sensors offer such benefits he electromagnetic field immunity, high flexibility, high sensitivity for mechanical parameters higher elastic limits, and impact resistance.
To be more precise, such benefits of fiber optic technology comply ideally with the instrumentation requirements of numerous healthcare tools and in movement analysis. Additionally, fiber optic sensors are considered to be lightweight, compact, stable to chemical substances, and they have also multiplexing capabilities. Therefore, fiber sensors are regarded as safe technology suitable for industrial, medical, and structural health monitoring applications.
The thing is that fiber optic sensors allow measuring various parameters, for example, angle, refractive index, temperature, humidity, acceleration, pressure, breathing rate, oxygen saturation, etc. It is even possible to install optical fibers in textiles for sensing applications, as well as incorporate them in composite metals, concrete and etc.
The development of fiber sensors based on fiber optic technology leads to high demands for healthcare systems, especially because of the population aging. The appearance of new medical systems results in higher demands on the fiber sensors’ performance because reliable control strategies require a robust fiber optic sensing system.
The modern fiber optic sensor should be tiny, herewith, saving its flexibility and compactness as possible. Herewith, intrusive sensing systems have higher requirements – they also need for biocompatibility, this is the reason why fiber optic technology continues developing to overcome the current challenges and provide high performance of novel healthcare systems and tools.
The increasing demands on fiber optic sensors and the fast development of the technology result in the appearance of numerous sensing systems for healthcare and medical tools. The fiber sensors enable to examine bones decalcification and strain distribution, evaluation of intervertebral disks, dental splints, cardiac monitoring, and pathologies detection.
Nowadays cases, when people meet polar bears, have dramatically increased especially in arctic areas because the environment continues rapidly transforming. Thus, there is a need for a structural health monitoring system that allows detecting bears to decrease the number of such meetings. Distributed acoustic sensing (DAS) is an ideal technology to perform this task.
DAS systems are used as an intrusion detection system that is able to operate in environments where temperatures fall to -70 C. Usually, the installation of the distributed acoustic sensing system takes place on the ground, that is why DAS system performance requires its testing in the snow. The DAS has been already tested at similar conditions (deep snow and extreme cold), herewith, people helped to imitate polar bears walking near the distributed acoustic sensing system.
It should be noted that standard DAS system consists of the following components:
a sensing fiber optic cable that can be stretched over long distances;
a laser central processing unit (DAS).
The operating principle of fiber optic technology is based on mechanical vibrations that undertake fiber impingement, leading to laser beam backscatter and therefore, allowing researchers to measure the signal required.
The researchers planed to test several opportunities provided by the DAS system: the ability of optical fiber to maintain extreme temperatures, the suitability of distributed acoustic sensing to snow coupling. The performance of the DAS system has been tested at the temperature of the -70C in the MTS environmental control chamber, and it demonstrated good results.
“Launch boxes with 2200m of spooled fiber optic cable were applied on either side of the 150m of distributed acoustic sensing cable to imitate a field deployment of 4.5km.” The researchers pay careful attention to four separate test temperature ranges provided by the DAS system, they even calculate shoe surface area combined with the weight of the participants in order to learn human foot pressure and connect it with polar bear foot pressure to classify bears of different sizes.
Additionally, DAS also allows differentiating humans from polar bears. The thing is that polar bears generally walk with 3 points of ground contact, while people need only one point. Even though the feet of polar bears are quite large, researchers can easily offer similar foot pressures.
Finally, the test results of distributed acoustic sensing demonstrate that fiber optic cables can maintain extreme cold temperatures in the arctic regions, where the temperature is required not to disturb optical fiber performance. The DAS system detects signals at depths of at least 0.65m in the unprocessed data.
Distributed fiber optic strain gauges are known for their crucial advantages compared with traditional measurement techniques, for instance, inductive displacement transducers, etc. To be more precise, fbg strain sensors are corrosion resistant, dielectric, and insensible to electromagnetic radiation.
It should be noted that any part of optical fibers is applied as the sensing element for the fiber optic strain sensors, herewith, the measurement is not limited by a specific section. Moreover, fbg strain sensors provide one more benefit that includes the opportunity of installing the optical fiber into the building material matrix.
Such a fiber optic sensing system allows detecting strain within concrete elements, which can include data about the curing and load behavior. The thing is that distributed fiber optic strain gauges play an important role in massive concrete structures, for example, foundations or concrete roads, therefore, these fbg strain sensors demonstrate the structural and loading conditions.
Herewith, such a fiber optic technology enables to combine the quality management of posttreatment and health monitoring. The operating principle of fbg strain sensors is based on the use of optical fibers, the core of which detects the strain. Also, it is necessary to pay careful attention to two techniques that influence the deformations of the fiber optic sensors: “slippage can occur between the fiber cladding (the so-called coating) and surrounding substrate; depending on the coating material, the cladding cannot wholly transfer the strain from the substrate to the fiber cladding and the core.”
Finally, optical fiber coatings are also important, therefore, different teams of scientists have analyzed their effect on strain transfer during matrix measurements. For instance, such a technique as Brillouin scattering shows thatfiber optic cables have lower strain values in the matrix than the reference technique. Herewith, there are the strain transfer rates of embedded FBG sensors in mortar prisms.
The fiber optic technology has been already tested. The team installed the optical fiber into a reinforcing bar, which was later installed in the concrete. Herewith, a brass frame and optical fiber with a single-layer polyimide coating allow researchers to install the fiber in small concrete specimens and obtain similar values to the reference sensing measurements.
Additionally, distributed fiber optic sensors demonstrate higher values than the standard strain gauge measurements on the surface. The researchers claim that the concrete is required to be quite cured to provide the strain transfer in the fiber optic sensors. However, the acrylate coating has higher strain losses.
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.