Ubiquitous optical fiber strain sensors against a background of large-scale urbanization

FBG strain sensors in urbanization processFiber Bragg gratings are often used in strain sensing especially in such places where the environment is harsh (for instance, high-EMI, high-temperature, or highly-corrosive). Strain measurement is imperative during prototype design and testing. Strain measurements ensure that materials perform as they should and that the equipment is safe and durable. Measuring strain is crucial for testing complex structures, like aircraft, turbines, etc. There various ways in which stress can be measured, but it is widely accepted that FBG sensors are the most efficient way of strain measurement.

There are two types of applications of the fiber Bragg grating strain sensors:

  • Long-term Static Strain Sensing. The main issue in the long-term static strain test is an interest in measuring the long-term static strain of the component in question. In order to carry out the above operation, it is necessary to set up the grating on the structure, find out what the initial wavelength is, and then within a set period of time to detect and record the changes that have occurred. Also, it is possible to return to the structure, reattach, and refer back to the initial wavelength. As a result, it becomes possible to obtain the determination of what this strain is from the initial condition at that time. The ability to disconnect your monitoring instrumentation and return for results after a large amount of time such as months or even years is a very great advantage of fiber Bragg grating sensors. For instance, in the case of bridges, it is common for engineers to visit the bridge and conduct the impact testing using an impact hammer on the different parts of the bridge. This is time-consuming and even hazardous because of the height of some bridge structures. The distribution of a number of FBG sensors throughout the bridge and the attachment of the instrumentation to this bridge on a periodic basis is a much more efficient solution. This is only one of the good examples to demonstrate the effectiveness of the fiber Bragg grating strain sensors. Besides bridges, other examples of using FBG for long-term static strain testing are buildings, piers, and structures in high earthquake-prone areas.
  • Dynamic Strain Sensing. The different structures may have very-low-frequency modes, and they may also have higher modes due to the effects of wind and tide. Most earthquakes and other earth tremors are low-frequency events.  Fiber Bragg gratings can be attached to the structures and monitored for the vibrations during the earth’s tremors and earthquakes. The low-frequency dynamic strain testing can help in determining the reaction of high-rise buildings to the wind. In addition to this, FBG sensors create connections with peers and other shore structures to determine their vibrations during the ebb and flow of tides. Dynamic strain testing can also be performed on transportation vehicles like automobiles, trains, and airplanes. In addition to civil structures and vehicles, there are a number of other applications for dynamic strain testing and vibration stress testing using fiber Bragg gratings. FBG sensors can be attached to industrial machinery to determine the frequency and amplitude of the stress vibrations.  

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. Our main goal is to deliver the best quality fiber optic products to our clients. We produce a wide range of fiber optic devices, including our cutting edge customized fiber optic Bragg grating product line and fiber Bragg grating sensor systems.

If you would like to purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com or +1 617 558 98 58

 

Fiber Bragg grating sensors in smart cities

The infrastructure of modern cities is becoming increasingly more complex; it includes roads, pavements, railways, tunnels, and ducts. These structures are not only made of composite materials in which behavior under harsh environments can often be unpredictable, but they also interconnect. These systems are subjected to extreme environmental conditions, like severe winds, earthquakes, flooding, etc.; due to this, they are susceptible to catastrophic failure.

One of the main aspects of any smart city is safety. The transportation network needs to be assessed through gathering data on roadway conditions, like dangerous conditions due to roadway degradation, icing, and hydroplaning. Autonomous data gathering is vital for the safety and efficiency of the transportation network. The data gathered gives information on the inevitable roadway and infrastructure degradation over time, which enables informed decisions for life extension or timely replacement of these critical systems.

Computational tools are often not adequate for the task of condition assessment of complex infrastructure systems. These tools lack the capability to reliably predict response to extreme events. Therefore, a distributed sensor system is needed in order to ensure the safety and longevity of infrastructures in a smart city. The sensing system should also be capable of providing critical information to computational models to enable informed maintenance planning as opposed to the reactive maintenance schemes currently employed.

Fiber optic sensing systems provide the most efficient and economical solution. Fiber optic systems allow for the assessment of thousands of sensors in real-time on a single cable. FBG sensor systems are well-suited to the detection and recording

of critical structural response characteristics as well as environmental indicators that lead to degradation. FBG strain sensors are useful in the process of assessing the response to stressors, e.g. traffic, wind, earthquakes, blast events, support settlement, etc. Distributed acoustic sensing is ideal for the direct assessment of localized damage in steel and reinforced concrete that may occur due to seismic events, fatigue cracking, corrosion, etc. FBG sensors are also ideal for monitoring of weather conditions, as the presence of entrapped moisture in asphalt paving systems, as they are often the main cause of the rapid degradation.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line: FBG sensors, FBG interrogators, and multiplexers, Distributed Temperature Sensing (DTS) systems. We create and supply a broad variety of top-notch fiber optic solutions for the monitoring of various facilities all over the world. We provide a distributed acoustic sensing system that is much less expensive than other analogous systems present on the market.

If you are interested in Optromix distributed acoustic sensing system or Optromix FBG sensor systems, please contact us at info@optromix.com

Aerospace sensing solutions

In every infrastructure, it is important to make sure that the cracks can be detected and monitored earlier in order to avoid any unwanted incident or any deformation of structures. Recently, Fibers Bragg gratings (FBGs) are growing interest in sensing applications such as aerospace, military, structural monitoring, and many others. FBGs are very high accuracy and also high sensitivity.

Over the last two decades, the growth of air traffic has been impressive and will strongly increase in the forthcoming years. Already by 2020, it is expected that aircraft will be significantly more affordable, safer, cleaner, and quieter than at the turn of the century.

In this context, the use of composite materials is essential for the design of high-strength, lightweight aircraft structures, which may contribute significantly to the reduction of fuel consumption and pollutants without compromising flight worthiness.

Nowadays, fiber optic sensors (FOS), particularly those based on fiber Bragg gratings (FBGs), have been emerging as an increasingly interesting technology due to their distinctive advantages which include higher sensitivity, immunity to electromagnetic interference, and durability. Furthermore, their multiplexing capability offers the possibility to reduce dramatically the cumbersome wiring required by electrical strain gauges and accelerometers, traditionally employed for load monitoring.

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Fiber Optic Ship Hull Monitoring System

FBGs for ship hull monitoringThe ship’s hull, when floating or moving through the waters, is exposed to different types of forces. The magnitudes and points of those forces depend on the shape of the ship’s hull. The fiber optic ship hull monitoring system allows monitoring of not only the magnitude of strength but also its variation along the length of a continuous uninterrupted optical fiber. The distributed sensors also permit an easy and reliable comparison of a parameter at different points and the sensor cable measures at every point along the length with no “dead spots”. The ship’s integrity may be monitored in real-time by using optical fiber sensors. The collected data could be used to obtain a global condition of the ship’s hull. Ships monitoring can be relevant in the case of:

  • Strain-stress state monitoring of a ship hull
  • Cargo operations management on a tanker
  • Measurement of the draft and level in the ballast and service tanks
  • Fuel consumption monitoring system
  • Remote-controllable valves and gate valves
  • Main power auxiliaries monitoring (boilers, separators, etc.)
  • Vibration monitoring of the main ship aggregates
  • Fire detection
  • Warning solution on water entering into the cargo holds
  • Temperature measurement and control of the ship components

The mentioned system can also collect data from the hull monitoring system which can be used in the optimization of the ship’s design and operational availability. Continuous real data collection under different sea conditions and commercial operations enables the ship’s master/officers and other users interested in (ship’s owner, classification societies, ship’s insurance) to make correct conclusions about the actual ship’s hull state and about the necessary actions that have to be taken.

Optromix Company always takes part in the shipbuilding project. As an example, Optromix installed a strain-stress state monitoring system of “Academic Tryoshnikov” Icebreaker`s Hull. Strain sensors are welded directly to the surfaces of metal structures. Sensors applied to measure the strain of the ship hull caused by ice, waves slamming, etc. They also monitor the static operational load (during cargo transportation). As a result, it increases the operational efficiency of marine facilities. The ability to register and predict the fatigue load on certain zones helps to identify and prevent emergencies and to increase the facilities’ service life.

 

Fiber Optic Sensing and Cabling Technologies

Fiber optic sensors work well in tight spots and applications with a high degree of electrical noise, but care must be taken when specifying these critical components.

Sensing part presence in machines, in fixtures, and on conveyors is an important component of industrial automation. Error-proofing assembly and controlling sequence based on the presence or absence of a part is often required. Many types of sensors are available, including inductive, magnetic, capacitive, and photoelectric. Each has its own strengths and weaknesses depending on the application.

Photoelectric sensors come with a variety of light-emission types (infrared, visible red, laser Class 1 and 2), sensing technologies (diffuse, background suppression, reflective, through-beam), and housing configurations (photo-eye or fiber optic). Fiber cabling is immune to electrical noise, and the electronics can be mounted away from the noise in a shielded enclosure.

Another very common application is small part assembly. These operations tend to be fully automated, and thus require multiple sensors to confirm part placement (seated) and assembly verification to confirm completion of an operation. Typically, the parts are moving in and out of a stage quickly on carriers or an indexing table. Because travel tolerance is minimal, precise measurement of position becomes essential.

A common issue in fiber optic installations concerns excessive flexing of the fibers. Since the fiber cables are bundles of individual fibers, they typically feel quite pliable, allowing an installer to easily bend the fibers beyond their recommended maximum bend radius. This can cause irrecoverable plastic deformation of the fibers, which will reduce the light transmission or, in the worst case, sever it entirely. The maximum bend radius, listed with all fibers, varies depending on fiber material, bundle size, and fiber dispersion in the bundle—and it must be adhered to in all cases.

Regardless of the application, machine builders must select the proper sensor technology. If fiber-optic sensors are used, amplifiers and fiber-optic heads must be carefully selected for the application to provide robust sensing performance.

Superstructure fiber Bragg gratings (sampled SFBGs)

superstructure FBGsThe FBG structure can change with the use of the refractive index, or the grating period. The grating period can be uniform or graded, and either localized or distributed in a superstructure. The refractive index has two main features, the refractive index profile, and the decline. The refractive index can be uniform or apodized, and the refractive index decline is positive or zero.

A superstructure fiber Bragg grating (SFBG), also called a sampled fiber Bragg grating, is a special FBG that consists of several small FBGs placed in close proximity to one another. SFBGs have attracted attention in recent years with the discovery of techniques allowing the creation of equivalent chirp or equivalent phase shifts. The biggest advantage of an SFBG with the equivalent chirp or equivalent phase shifts is the possibility to generate gratings with a greatly fluctuating phase and amplitude by adjusting the spatial profile of the superstructure. The realization of SFBGs with the equivalent chirp or equivalent phase shifts requires only sub-millimeter precision.

 

Superstructure (sampled) fiber Bragg gratings (SFBGs) are good optical filters for optical communication and optical sensor systems, because of their comb-like filter response. The length of SFBG is conventionally limited by that of the phase mask. However, the length of the high-quality phase mask fabricated by an interferometric technique is limited to ~ 5 cm.  The main fabrication technique for long SFBGs based on scanning phase masks and trimming relative phases between FBGs. This technique permits to fabricate of long SFBGs with short phase masks. Superstructure fiber Bragg grating is a novel fiber Bragg grating. Because it is flexible, passive, low insertion loss, and small polarization dependent. It causes great interest and enthusiasm for the people in many areas. Ambient temperature and strain can both affect the sampled grating reflection spectrum.

 

Fiber Optic Sensor Solutions: today’s field experience of sensing system

Fiber optic sensor solutions have created new opportunities in general industry applications. The fiber optic sensors (temperature, pressure, deformation, and displacement) are designed to deliver precise measurements in harsh environments and in the case of electromagnetic interference, magnetic resonance imaging, radiofrequency, microwave, and high voltage applications.

The greatest advantages of the fiber optic sensors are its inert material. It is also an optimum transducer for use in harsh environments, like in geotechnical applications.

Fiber optic sensors more and more are attracting attraction in the aerospace and defense due to resistance to chemical corrosion, high pressure, high voltage, and lighting environments. Its lightweight and small size permit weight reduction on different monitoring areas and the capacity of long-range operation still conserving high sensitivity and large bandwidth ensuring a long term reliable monitoring.

Chemistry and food are both often related to microwave radiation. Gallium Arsenide (GaAs) semiconductor crystal is non-conductive, offers high temperature operating capability, they are suitable for microwave heated installations chemical digestion under pressure and temperature conditions. A fiber optic sensor can also be used in the food industry where microwaves are used in new product development to identify precise safe and effective cooking temperatures and time.

The inherent benefits of fiber optic point sensing technology for structural health monitoring applications. In addition, they are not affected by the external environment, large temperature variation, transversal strain, or lightning.

The renewable energy market is developing quite rapidly over the last years. With wind farms or hydrokinetic systems, there is a big need for low maintenance monitoring systems that are reliable and stable for a long period. With the property of being immune to EMI and insensitive to lightning strikes, it makes it a great fit for the growing renewable energy market.

The fiber optic sensing system is the ideal solution for monitoring well pressure and temperature in thermal recovery applications like steam-assisted gravity drainage (SAGD) or cyclic steam stimulation (CSS). The FBG sensors can be used in coiled or production tubing in production and injection wells giving accurate properly measurements for reservoir surveillance, process optimization, or pump control.

In-situ and continuous monitoring of the pressure in the producing for the better understanding of the reservoir condition FBG sensors rapidly detect steam breakthrough and efficiently diagnose changes in the reservoir.  Optimization of the production process forces the oil recovery rate and reduces the operating costs associated with steam injection and oil recovery.  A fiber optic sensing solution is offered in both single and multiple-point configurations. Hence, it gives the operators a large choice of real-time monitoring from monitoring well pumps to profiling a producing well. The application “downhole pressure and temperature measurement solution” is only one example of what is possible with fiber optic technology. The fiber optic measurement solutions are custom made according to the requirements.

Phase-shifted fiber Bragg gratings (πFBGs)

Phase-shifted FBGsNowadays, the special type of FBGs whose reflection spectrum has a notch arise from a π-phase discontinuity in the center of the grating (called π-phase-shifted FBGs) attracts ground interests among researchers. Because of their highly sensitive ultrasonic detection, πFBG may provide a solution to the sensitivity problems of the FBG. By introducing a π- phase shift into a refractive index modulation of the fiber Bragg grating during its fabrication, the spectral transmission has a narrow bandpass resonance appearing within the middle of the reflection lobe of the FBG. Such an element allows reaching a very narrow transmission band of few picometers.

Fabrication of π-phase-shifted FBGs is achieved by splitting the standard FBG into two identical sub-FBGs with a half-period phase difference between them. The two sub FBGs create an interfere with each other and generate an ultra-narrow transmission window at the center of the FBG spectrum.

Due to the phase discontinuity, a πFBG can be conceptually described as a Fabry–Perot cavity formed by two FBG mirrors. When the two FBGs are highly reflective, the quality factor of the Fabry–Perot cavity is increased, leading to an extremely narrow spectral notch for highly sensitive ultrasonic detection.

Using special structures, even multiple transmission bands are possible.

The primary method used for the fabrication of π-Phase-shifted fiber Bragg grating is based on the UV laser and phase mask method. The occurrence of two peaks/dips is attributed to the refractive index modulations along with the fiber core, with the periodicity of the π-phase mask that has been observed previously in images of gratings that cause destructive interference in a reflected wave at the Bragg condition owing to the phase difference between the grating phases. Thus, the standard phase mask technique produced an alternative type of pi-phase-shifted grating at twice the design Bragg wavelength.

The phase-shifted gratings have found application in distributed feedback lasers, wavelength division multiplexing, athermal setup, or temperature stabilization, as well as to a tuning setup. Also, the π-phase-shifted FBG can be used in highly accurate wavelength references, ASE filtering, spectroscopy, and optical CDMA.

 

Tilted FBG – optical Fiber Bragg Gratings (TFBG)

Tilted FBGsThe tilted FBG is a new kind of sensor that includes all the advantages of established usage Bragg grating technology in addition to being able to excite cladding modes resonantly. This device opens possibilities for single-point sensing in hard-to-reach places.

Tilted fiber Bragg gratings are also known as slanted gratings or blazed gratings. This is a special type of short-term optical fiber gratings. TFBGs are fabricated using the same tools and techniques as standard FBGs, i. e. from a permanent refractive index change induced in doped glasses by an interference pattern between two intense ultraviolet laser beams. The boundary surface of the varied index is not vertical with respect to the fiber axis but has a certain angle. One feature of tilted FBG is that they can couple guided modes with copropagating modes or counterpropagating modes in specific wavelengths.

As we have known that tilted FBG can unite light from guided modes into radiation modes or cladding modes. The cladding modes are investigated theoretically by studying a three-layer model of optical fibers, whereas the core mode is investigated by studying a two-layer model of optical fibers. The analysis reveals that to increase the coupling of the energy transferred from the core mode to cladding modes, the cladding radius needs to be decreased. Such behavior is illustrated by studying the change in the electric field distribution and is used to enhance the sensitivity of the sensing refractive index of the surrounding medium.

Connection efficiency with the help of tilted FBGis sensitive to light polarization, various sensors, and devices based on these characteristics have been proposed or developed for a wide range of applications. TFBG sensors are using for mechanical and biochemical applications, including one-dimensional TFBG vibroscopes, accelerometers, and micro-displacement sensors; two-dimensional TFBG vector vibroscopes and vector rotation sensors; reflective TFBG refractometers with in-fiber and fiber-to-fiber configurations; polarimetric and plasmonic tilted FBG biochemical sensors for in-situ detection of a cell, protein, and glucose.

Optical fiber sensor technology and energy engineering

The power industry is one of the major markets for optical fiber sensor technologies. Most of the research and developments are currently done specifically for this sector. Fiber optic sensors have key advantages for this industry: they can monitor very lengthy facilities and it is completely resistant to electromagnetic waves.

Various new optical fiber sensor technologies are being developed right now. The hydro filter system, distributed thermal monitoring system, the current sensor (alternating and direct current), and high-frequency accelerometer are among them. Hydro filter system performs filtering processes that take place in a multileveled foundation of the facility. The real-time data obtained with the fiber optic filter, the data is analyzed and compared with the calculations done when the facility was designed. The fiber optic filter system is universal and may be used for industrial and civil engineering. Fiber optic filter controls the processes of temperature shift in closed and opened locations with limited access and in remote destinations. Moreover, fiber optic filter allows controlling several objects that have different sophistication levels with only one sensing element.

The hydro-technical cable contains the core of the modular construction with the central power element made of the dielectric rod. Optic modules are tied around the rod and an isolated copper electrical conductor is wrapped around them to ensure the constant temperature level inside the construction. This kind of fiber optic equipment allows increasing the sensitivity of the construction. Water blocking materials used inside the cable make it waterproof. Steep armor protects the fiber optic equipment from hundreds of kilos of steady-state pressure.

Monitoring boosters are also possible with the sensors of temperature and vibration. The temperature sensor is located in the liquid refrigerating agent at the top of the booster; this sensor may also serve as an alarm for when the liquid agent reaches the critical level.

There are many more different applications of optic fiber sensor technologies in the power industry. In Optromix Company we offer the highest quality product and we will deliver the fiber optic equipment according to your specific needs.