Interrogation techniques for FBG Sensor Arrays

FBG sensors are very suitable for sensing and data acquisition, where sensor arrays can be multiplexed using similar techniques that have been applied to fiber-optic sensors like wavelength-division multiplexing (WDM), spatial-division-multiplexing (SDM), and time-division-multiplexing (TDM) as they can be directly implemented in the fiber without changing the diameter of the fiber. This feature makes FBG sensors suitable for a wide range of applications.

The main problem with the TDM system is that the sensors must be placed sufficiently far apart because the pulse returning from the adjacent sensors must be able to reach and get detected separately. In WDM systems, different sensors have the nominal central wavelength, and other sensors are separated by a few nanometers. WDM interrogation is available in two topologies i.e., series and parallel. The parallel approach is easier to implement but the series topology allows the optical power from the sensing FBG array to be used much more efficiently than parallel topology.

The number of sensors that you can incorporate within a single fiber depends on the wavelength range of operation of each sensor and the total available wavelength range of the interrogator. Because typical interrogators provide a measurement range of 60 to 80 nm, each fiber Bragg grating array of sensors can usually incorporate anywhere from one to more than 80 sensors – as long as the reflected wavelengths do not overlap in the optical spectrum. Be careful when selecting the nominal wavelengths and ranges for the FBG sensors in an array to ensure that each sensor operates within a unique spectral range.

Major limitations in interrogating FBG sensor arrays are the cross-talk, spectral shadowing, and interference. For all the interrogation approaches, some crosstalk between adjacent sensors seems to be unavoidable. The use of a serial array of FBG sensors with the same central wavelength results in the crosstalk between sensors. The amount of light reflected by the FBG sensors located nearest to the source will affect the amount of the optical power reaching and be returned from gratings further from the source. The lower the peak reflectivity of the FBGs is, the smaller the effect is. Another source of the crosstalk in a TDM serial array of identical FBG sensors arises from multiple reflections between FBGs. This can lead to pulses arriving simultaneously at the detector having undergone a direct reflection from a sensor element and also having experienced a number of multiple reflection paths between FBGs.

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.

Fiber Optic Sensors for Borehole seismic technology system

Seismic techniques are the main techniques for the characterization of subsurface structures and stratigraphy. Borehole technology system provides the highest resolution characterization and most precise monitoring results because it generates a higher signal to noise ratio and higher frequency data than surface seismic techniques. A new generation of fiber optic borehole sensor systems has been developed based on all fiber optic data transmission and fiber optic sensor technologies. The new fiber sensors are much more sensitive and are able to record much larger bandwidth data with better vector fidelity than is possible with current seismic sensor technologies. The new sensors also can operate in most hostile environments found in boreholes such as pressure and temperature conditions. This improvement in data quality and density will generate better images and more precise monitoring results, which will allow a much improved high-resolution interpretation and ultimately better oil and gas production.

Since the borehole seismic system does not require electric power for either the optical sensors or the hydraulically operated deployment system, the entire system is intrinsically safe. The fiber-optic seismic sensor system measures the strain of the fiber between two Fiber Bragg gratings surrounding the mandrel using an interferometric measurement technique comparing the phase angle between two spaced reflections from the same light pulse traveling in the fiber. It is using a time-division multiplexing technique to transmit the dynamic fiber strain information to the interrogators. This allows the measurement of extremely small strains in the fiber. The fiber optic seismic sensor is self-standing to electric and electromagnetic interference in the borehole since the system does not require any electronics at the fiber optic sensor end. This design also makes the fiber optic seismic sensor extremely robust and able to operate in extreme environments such as temperatures up to 300 C.  All the sensors are calibrated so the optical output amplitude into absolute acceleration can be mapped. The sensors have also proved to be about 100 times more sensitive than the regular coil geophones that are used in borehole seismic systems today.

Optromix Company manufactures a wide range of sensors, that are able to feel the slightest deformation of the structures.

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.

Inclination measurement system: incline FBG sensors

Fiber Bragg Grating sensors one of the most requested fiber optic technologies have superior sensitivity and frequency specifications, making them well suited for many spheres of applications. The FBG inclinometers can be used to identify internal damage at a very early stage.  The FBG inclinometers are devices used to monitor subsurface movements through sensors designed to measure inclination with respect to vertical. When installing the FBG inclinometer casing, it is important to select the appropriate diameter. The large-diameter casing is better suited to shear zones, multiple shear zones, and slope failures. Moderate- to small-diameter casing can be used for short-term installations or slopes where smaller displacements distributed along the borehole are anticipated. Correct installation of the casing is important; and deep holes, particularly the influence of helical deformation must be considered. A conventional FBG inclinometer system consists of a plastic casing that is installed in a nearly vertical position in the ground, with a servo-accelerometer or electro-level sensor inserted into the casing to measure the local tilt of the casing in response to ground movement. The sensor element is lowered and raised, guided by grooves in the inner surface of the casing, with the tilt of the casing being recorded at fixed spatial intervals.

Incline FBG sensors have been widely used to monitor ground movements in various applications, for example, landslides, tunnels, and foundations, etc., where they provide vital ground movement information including magnitude, rate, and location. The produced information can be used for checking design assumptions and provide early warning of problems.

Another type of FBG sensors that can monitor inclination is a tiltmeter. Tiltmeters are devices used to monitor the change in the inclination of a ground surface point. The device consists of a gravity sensing transducer capable of measuring changes in inclination as small as one arc second. They are used to monitor slope movements where the landslide failure mode is expected to contain a rotational component. The advantages of using tiltmeters are their lightweight, simple operation, and relatively low cost. They may be combined with an incline FBG sensor and extensometers in what has been termed as integrated pit slope monitoring systems.

Photonic Sensors: Technology and Applications for Safety Monitoring

Nowadays, photonics is an important part of the innovation-driven growth in an increasing number of fields. The application of photonics is distributed across several sectors: from optical data communications to imaging, lighting, and displays; from the manufacturing sector to life sciences, health care, security, and safety.

Photonic sensors have been designed for use in laboratories and industrial environments in order to detect a wide range of physical, biological, and chemical parameters. Photonics sensors can be defined as the set of techniques and scientific knowledge concerning the generation, propagation, control, amplification, detection, storage, and processing of the optical spectrum signals. In recent years, photonic sensors have been recognized as the technology that improves, extends across, and strengthens a wide variety of industrial sectors: healthcare, security, manufacturing, telecommunications, environment, aerospace, and biotechnology.

The photonic sensors market is classified generally by types and photonics applications. Type wise consist of fiber optic sensors, laser-based sensors, and biophotonic sensors. Application-wise, it is divided into construction, energy industry, oil and gas, military and aerospace, medical and industrial application. The photonic sensors market is diversified and fragmented. Photonic technology also has found a use for the industry, surgeries, and ordinary life besides its use in research laboratories.

The photonic sensors market is slowly moving from making general objectives to complicated tasks such as aircraft manufacturing applications that prescribed maximum precision. The usage of photonic sensing technology is increased in early detection and early-warning systems for biological hazards, structural flaws, and security threats. Sensors play a major role in the near future and, especially, in different photonic applications in the generation, distribution, and conservation of energy, as well as in the mitigation of the effects of energy production and consumption on the environment.

The photonic industry now is centering on the development of eco-efficient products that are projected to be developed and launched over the next few years. The need for enhanced safety and security solutions, better alternatives for conventional technology, and a rise in wireless sensing technology are some of the major factors that act as drivers for the photonic sensor market.

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.

FBG monitoring systems for hydraulic structures

Any sophisticated construction requires constant tracking of its technical condition to detect inconsistencies between the projected calculations and the actual working parameters and to evaluate its compliance to the established standard. Currently, the most used sensors to monitor hydraulic structures are stringer transducers that perform very well under various climate conditions, especially in the areas of deep-frozen soil. However, the increasing sophistication of modern constructions is more demanding in terms of monitoring standards. Often it is no longer sufficient to use regular sensors. Hence, a new type of monitoring with fiber optic devices was created.

The length of these constructions and its complex structure demand special fiber optic devices monitor its safety. One of the most complicated constructions to place the fiber optic solutions is a hydro-electric power station. FBG sensors can be placed in the structures where regular stringer transducers fail, for example, in long shafts and tunnels. On top of that, fiber optic solutions are completely fire-safe, resistant to electromagnetic fields and radiation, and other interferences.

Tunnels in hydro-electric power stations can be equipped with the following fiber optic devices: FBG sensors for deformation, temperature, displacement, and pressure sensors. The deformation sensor is measuring the condition of the steel reinforcing, and it embedded on its surface. The sensor is covered with the isolating material to avoid the impact of the concrete. An additional temperature sensor is installed in the same location to track temperature changes that affect deformation. A displacement sensor is used to measure the joint opening between steel compression and a ram. The pressure sensor is used to measure the water pressure in the tunnel.

Optromix offers a variety of fiber optic solutions, and we will write a tailored FBG based on your specific needs.

Monitoring loadbearing structures of large scale public facilities with fiber optic sensors

There are plenty of different fiber optic applications of FBG sensors; it can monitor a variety of structures, buildings, and other facilities. This technology is often used in construction work and to track different parameters in industrial production and other areas.

Fiber optic applications include effective monitoring of loadbearing structures in different public facilities. Most of the modern construction sites are technically sophisticated and uniquely designed. In the process of project development, certain safety measures are planned, however, it may change in time after the building utilization has started. Regular monitoring becomes crucial in this context and it may prevent different accidents. The automated monitoring fiber optic system allows tracking the technical condition of the object in real-time with minimal human involvement.

Fiber optic Bragg gratings (FBGs) are used as a sensing element in this monitoring fiber optic system. This kind of sensors doesn’t have electronic components and it makes them resistant to electromagnetic waves and fire safe. Fiber optic Bragg gratings are inscribed in the fiber with a UV laser. FBG in each sensor reflects the specific fiber optic wavelength with a spectrum width of approximately 1 nm. The grating is affected by mechanical and temperature impact and its physical change cause the shift in the fiber optic wavelength of the reflected light. The fiber optic wavelength shift is measured, which allows measuring the deformation and temperature change. Two gratings are used simultaneously to separate these parameters, where one of them is isolated from the mechanical impact. This allows tracking the temperature impact on the second grating and measuring the deformation. One optic fiber may have multiple gratings, each one of them provides a response at its own length, and the distance between gratings vary from 10 mm to several kilometers.

One of the most common methods to investigate sensors is to scan the FBG aggregate with a tunable source. The tunability range is usually around 100 nm (1500 – 1600 nm), the investigation frequency is from 1 to 500 Hz, and the number of parallel channels can be up to 8 channels.

Usually, it is enough to interrogate sensors once per 6 hours, when they are installed on a large public facility. Each sensor has 2 critical measurement levels (yellow and red), when this level is reached the optic fiber system sends out an alarm signal. The frequency of interrogation increases to 1 time per 30 minutes. Moreover, the notifications are sent out in case of malfunctioning. The software allows viewing the data collected from all the sensors as the graphs of relative deformation and temperature.

This fiber optic application of FBG sensors works perfectly well for large constructions, such as stadiums.