Fiber Bragg grating for shallow landslide triggering detection

Shallow landslides pose a serious threat to structures situated in close proximity. The most dangerous are some flow-like landslides that travel at significant rates for long distances. The detaching area is the starting point for flow slides that erode and engage large amounts of soil along their path and may discharge great impact energy to engineering structures.

The detection of landslide triggers has been an important issue for many years; significant efforts have been undertaken to understand the mechanisms responsible for landslide triggering, as well as to identify the first signs of soil instability. This information will aid in the development and implementation of effective early warning systems. Among triggering, factors for a landslide are slope saturation, groundwater seepage, excavation, erosional processes, seismic action, etc. All these factors increase shear stress and pore water pressure, which in turn cause sliding surfaces due to high strain in the sliding mass. The strain increases with the approach of flow sliders; the strain increases exponentially as the landslide approaches. Soil instability follows, which causes erosion.

Therefore, the strain is the most important parameter for landslide monitoring. In recent years, fiber optic technology has attracted significant interest due to the multiple advantages that it provides. Fiber optic strain sensors are already widely used in geotechnical applications, including landslide monitoring. recently, the results of experiments involving FBG strain sensors have revealed that FBG sensors can be successfully used for measuring strain in the soil since it is possible to effectively couple the fiber cable with the soil.

One of the most attractive attributes of FBG strain sensors for landslide triggering detection is that FBG sensors do not interfere with the phenomenon that they are designated to monitor.

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.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line: fiber Bragg grating 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.

If you are interested in Optromix FBG strain sensors, please contact us at info@optromix.com

Fiber optic well monitoring

The well integrity has become a critical concern after recent events in the oil industry, such as oil spills. The interaction of a salt layer with the cement and casting for Pre-salt wells is a concern for fiber optic well monitoring and the structural integrity of the well. The development of continuous monitoring tools for well structural integrity is an ongoing task for the oil industry.

Continuous fiber optic well monitoring has the advantage of allowing the quantification of the time needed for an event. Casting integrity logging operations may provide information regarding the damage location, however, the time in the life of the well when the damage happened or the process of the well degradation can not be determined. The logging operations can only provide information on the condition of the well at a particular time, not continuously. Continuous monitoring can help to correlate well damage and events that could be the cause of the damage, like outside intervention, allowing for corrective and preventive measures to take place.

The two main parameters that need to be measured are strain and temperature. The strain in fiber optic well monitoring can indicate the strain in the casting that is caused by the creep of the salt layer. Distributed temperature sensing may be used to indicate the positioning of the cement slurry, diagnose the curing process, and indicate the cementing failures.

The sensors need to be installed outside the production casing of the production liner. The size of the sensors, therefore, is required to be small. DTS sensors, in particular, are compact and are easy to install onto any surface. However, the mounting process of the sensors needs to be delicate as the casing properties may degrade. Fiber optic well monitoring solutions shouldn’t be intrusive as the sensors could potentially cause issues, like poor isolation.

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. Optromix, Inc. is a top choice among the manufacturers of fiber Bragg grating monitoring systems. If you have any questions, please contact us at info@optromix.com

Real Time Thermal Rating systems

RTTR, or Real-Time Thermal Rating, is a method of assessing real-time operational thermal rating of the equipment, or the amount of electrical current that a power line or an electrical facility can endure before suffering critical damage. Thermal rating devices can be used to measure the temperature of overhead power lines, transformers, underground and subsea cables.

Real-time thermal rating systems rely on real-time data from environmental conditions rather than theoretical assumptions and predictions. These systems are able to not only measure the thermal ratings in real-time but also to measure the stress levels of certain areas and determine their capacity. The calculation of the thermal ratings happen on the basis of: 1) weather conditions; 2) electrical current; 3) temperature of the equipment. However, other factors may need to be considered; this depends on the environment where real-time thermal rating needs to be performed. For example, soil condition, burial depth, and configuration must be considered for temperature measurement of underground cables, the mass of the transformer and type of the cooling mechanisms for temperature measurement of transformers, etc.

The key to the real-time thermal rating system, the temperature of the asset, should be continuously measured to avoid heating of the asset to dangerous levels. Distributed temperature sensing (DTS) systems should be used for this purpose, otherwise, only predictions about the temperature can be made.

The main advantage of real-time thermal rating systems is their ability to accurately measure the thermal behavior of assets, taking into consideration factors that static ratings do not. Static rating calculations are often overly conservative, therefore some power lines are often not used up to their full potential, while others are overloaded, which causes premature aging.

Thermal rating systems may be implemented into different types of power assets. Underground cable monitoring benefits greatly from introducing real-time thermal rating systems as they measure soil ambient temperature and soil thermal resistivity; these measurements help to determine actual thermal headroom to indicate unused network capacity.  

The thermal rating of the overhead power lines, depending on the actual system used, takes either the sag of the lines, the tension of the line conductor, the temperature of the line conductor, or environmental conditions into account during calculation.

Transformer load, ambient and transformer temperatures, oil temperature, and winding hot spot temperature are utilized by RTTR systems for transformers.

DTS optic sensing fibers are important for real-time thermal rating; they are installed along the length of the power cable and provide a continuous temperature profile.

If you would like to purchase DTS (Distributed Temperature System), please contact us: info@optromix.com or +1 617 558 98 58.

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.

 

Medical fiber optic sensing products and technologies

The range of medical devices incorporating optical fibers has taken a slow technological growth, with the bulk of the industry focused on endoscopy and various methods of optical power delivery for cutting, dissecting, and ablating. These technologies utilize an optical fiber’s mechanism—the ability to guide light from one location to another.

Fiber sensing technologies offer significantly advanced functionality by utilizing their inherent sensitivity to temperature, strain, and pressure. During the last five years, the medical industry has taken significant steps to adapt historic fiber-optic sensing methods to enable them to be used within in vivo environments.

The main area for recent technological developments driving fiber sensors into the medical industry has focused on minimally invasive surgery (MIS). The benefits of MIS are now well-founded, encouraging surgical-tool manufacturers to invest their money in new technology developments to pioneer new MIS procedures or to further improve existing procedures. Three exciting recently developed fiber optic sensing technologies for MIS are focused on here: haptic feedback, 3D shape sensing, and pressure sensing.

By utilizing multiple FBGs or manipulating the FBG structure, it is possible to obtain a spatially distributed strain profile. Such FBGs can be applied along the length of a surgical tool to enable haptic feedback at the regions of most concern. A prime example of this is to add haptic sensing to a grasping tool, where both the grasping and spreading forces can be measured and fed back to the surgeon to indicate how tightly they are grasping or how much force they are applying to pry tissues apart.

Fiber optic 3D shape sensing has been developed by several commercial groups to enable a dramatic reduction in the need for prolonged exposure to the visualization methods, as the optical fiber can track itself in three dimensions and thus if laid within a catheter, can recreate the shape of the catheter. This technique relies on a mixture of FBG and fiber technology, where a very special fiber has been developed specifically for this application. Optical fibers also can be optimized to be sensitive to the hydrostatic pressures experienced within the body. These new applications are being opened up by a mixture of economic desire and technology development. Specialty optical-fiber manufacturers continue to pioneer new fiber designs that medical-device manufacturers can exploit. This enables a greater diversification of medical-device product ranges and opens up new procedures that were not previously possible with minimally invasive surgery.

 

FBG structural health monitoring

FBG has been considered as an excellent sensor element, which is currently receiving more and more research interest. In order to measure strain/temperature variations with high accuracy, the ability to detect small shifts in Bragg wavelength becomes an essential requirement for an FBG sensing system.

With the construction of high buildings and bridges, etc., in recent years, the importance of structural health monitoring technologies to assess building safety is being re-examined. In previous systems using electrical strain sensors, every sensor required power supply, and the installed sensors were easily affected by electromagnetic noise, thunderstorms, etc., causing noise components in the electrical signal measured by remote sensing and presenting a risk of degraded accuracy. Optical fibers have been used as sensors to solve these problems with a focus on optical sensing technology. Since optical sensing technology does not require supplying power to the sensor itself, it offers many advantages including long life spans with excellent corrosion resistance, excellent explosion-proofs, easy remote measurement at distances of more than 10 km with no concerns about electromagnetic noise effects, etc. In addition, the characteristics of optical fibers lend them to linear and sheet designs for extreme environments, making them ideal for disaster monitoring and structural health monitoring systems.

Some of the well-known technologies in optical fiber sensing rely on measuring changes in the frequency of Brillouin backscatter occurring in optical fibers to determine structural deformations and temperature changes. Another method uses a Fiber Bragg Grating (FBG) forming a diffraction grating at the optical fiber core as a sensor to measure changes in the center wavelength of the optical spectrum reflected from the FBG sensor as an index of the amount of strain impressed on the fiber and temperature changes.  Since FBG sensor monitors are used mainly for natural disaster and structural health monitoring they are designed to be convenient for installation, small, and lightweight. Additionally, to be able to measure small strain and temperature changes quickly, they require a high responsivity of better than 1 kHz as well as better measurement accuracy than commercially available FBG sensor monitors and our previously developed model.

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.

 

A tunable fiber Bragg grating (FBG) optical filters

FBGs in optical filtersIn order to make the fiber Bragg grating tunable (it means controlling the reflected wavelength), the Bragg grating period must be controllable. This is achieved by one of several methods. For example, the application of a stretching force elongates the fiber, which thus changes its period (mechanical tuning). Mechanical tuning of the FBG results in a faster and large response in terms of wavelength shift.  The application of heat elongates the fiber, thus changing its period (thermal tuning). Changing the temperature of the FBG results in a slow wavelength shift; hence, a small tuning range is achieved.  FBG final applications are in fiber dispersion compensation, in gain flattening of erbium-doped fiber amplifiers, and in add-drop multiplexers/demultiplexers. However, the fiber used to make an FBG should be free of imperfections as well as microscopic variations of the refractive index.

Tunable FBG is a valuable option for all applications that require flexibility in center wavelength (and/or frequency). The tuning setup consists of the mechanical assembly where the FBG is applied to. The mechanics induce strain to the FBG, shifting its center wavelength homogeneously and chirp-free.

Common FBGs have the flat top response, low cost, and low insertion loss that meets the requirement of add/drop FBG multiplexers in the optical WDM fiber transmission systems. However, a great deal of research interest in FBGs involves the property of being tunable in both the Bragg wavelength and bandwidth. Recent studies have shown large tuning ranges of 110 nm in the Bragg wavelength and more than 10 nm in the 3-dB bandwidth for uniform FBGs. Given such desirable tunability, FBG is becoming a very flexible and “smart” optical fiber component. Bandwidth-tunable FBG optical filters have been studied in many applications, such as tunable dispersion compensation, phased array antenna, and temperature-independent fiber grating sensing.

The important parameters of light sources for FBG interrogators are optical power, tuning range, tuning speed, and continuous tuning capabilities. Among the tunable laser sources are DFB lasers, multi-section distributed Bragg reflector lasers, and external cavity lasers.

Bragg mirror (reflector) features and technologies

FBGs in Bragg mirrorA Bragg mirror (also known as distributed Bragg reflector / dielectric mirror) is a type of mirror consist of composite thin layers of dielectric material, usually settled on a substrate of glass or other optical material. By careful choice of the type and thickness of the dielectric layers, one can project optical coating with specified reflectivity at various wavelengths. Bragg mirrors can be adapted to obtain any reflectivity between 0% and nearly 100% for a specific target wavelength.

Different technologies can be used to fabricate Bragg mirrors. First are the thin-film coating technology, fabricated, for example, by electron beam evaporation or with ion beam sputtering. These technologies are used as laser mirrors in solid-state bulk lasers. Second are Fiber Bragg gratings, including long-period fiber gratings, which are often used in fiber lasers and other fiber devices. They can be fabricated by irradiating a fiber with spatially patterned ultraviolet light. Next, are lithographic methods for semiconductor Bragg mirrors. They are used, for example, in surface-emitting semiconductor lasers and semiconductor saturable absorber mirrors, but also as separate optical components.

There are also different types of Bragg reflectors used in other waveguides, based on, for example, corrugated waveguide structures that can be fabricated via lithography. Such kind of gratings is used in some distributed Bragg reflector or distributed feedback laser diodes.

There are some advantages of using Bragg mirrors:

  1. Low-loss, if the dielectric material is transparent to the wavelength under study. That means it can exhibit ultra-high reflectance, useful for a high-Q cavity.
  2. Broadband. It can be as broad as metal, at a much higher reflectance.
  3. Foreseeable reflection phase.

Bragg mirrors found their applications in laser cavity end mirrors, hot and cold mirrors, thin-film beam splitters, and the coatings on new-day mirrorshades.