Fiber optic sensing solutions for extreme conditions

FBGs for extreme conditionsElectrical 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 sensor components 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 fibers and 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 fiber channel can interrogate a variety of FBG sensors, which significantly reduces the size and complexity of such a fiber optic system.

Applications of fiber sensors

Fiber optic sensing solutions are ideal for applications where conventional electrical sensors (strain gauges, strings, thermoresistors, etc.) have proved difficult to use due to extreme conditions (long distances, EM fields, explosion protection, etc.). Since the installation and operation of fiber sensors are similar to conventional electrical sensors, it is easy to switch to fiber optic solutions. Understanding how such fiber optic systems work and the benefits of using them can greatly facilitate various measurement tasks (for example, structural health monitoring).

Benefits of FBG type

In short, the main advantages of FBG sensors include:

  • high sensitivity and performance;
  • relatively large range of measured deformations;
  • 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.

Currently, fiber optic technologies are widely used in various fields of science and technology. One of the main applications of fiber optics is the creation of portable high-sensitivity sensors. Pressure, strain, vibration, tilt, linear motion, and temperature sensors are widely applied in the industries of structural health monitoring pipelines, heating lines, power cables, mines, etc.

Application in radioactive conditions

Compared to fiber sensors, the lack of power supply at the location of electrical sensing systems does 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 sensors of 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.

Fiber Bragg grating sensors are completely immune to electromagnetic interference and are stable insulators. This makes it possible to measure high voltages up to 800 kV and high currents up to 200 kA with high accuracy (class 02s) by fiber optic sensing technology.

Application in an aggressive environment

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.

Distributed fiber optic sensors are multi-sensors: up to 10 thousand consecutive intra-fiber sensors can be used in one optical fiber (fiber optic cable) to measure physical quantities (temperature, strain, seismoacoustics, pressure, radiation, etc.). Multimode fiber optic cables allow performing remote measurements with high accuracy using borehole video cameras, and temperature fields — using pyrometers and thermal imagers.

Advantages of metrological calibration

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).

Modern fiber sensors have 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.

Today’s situation

In the last decade, there were implemented many similar applications of modern fiber sensors and 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.

How to find the best fiber optic solution?

If you are looking for reliable fiber optic sensing solutions for structural health monitoring, you should choose the Optromix company. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, Distributed Acoustic Sensing (DAS) systems, Distributed Temperature Sensing (DTS) systems. The company creates and supplies a broad variety of fiber optic solutions for monitoring worldwide. If you are interested in structural health monitoring systems and want to learn more, please contact us at info@optromix.com 

What you should know about fiber Bragg gratings

fiber Bragg gratings (FBGs)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.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) product line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, Distributed Acoustic Sensing (DAS) systems, Distributed Temperature Sensing (DTS) systems. The company creates and supplies a broad variety of fiber optic solutions for monitoring worldwide. If you are interested in structural health monitoring systems and want to learn more, please contact us at info@optromix.com

Performance of fiber Bragg gratings in radiation environments

The developments in photonics led to a communication revolution over the past two decades. The technologies developed for telecommunications are now being used in the development of sensors for a wide variety of applications. The sensors based on photonic technology are especially valuable for applications where the measurements need to be taken in harsh environments like ones encountered in space exploration and nuclear power plants. Fiber Bragg grating sensors are preferred as they are compact in size, have low power consumption, and are tolerant to environmental influences.

The sensing in FBG sensors relies on using the sensitivity of the device to changes in the refractive index of the host material. In FBG sensors the resonant condition is directly proportional to the refractive index of the waveguide. A small change in the environment, for example, a temperature drop or increase, leads to a significant change in resonance wavelength which has been used for photonic thermometry. The sensitivity of FBG sensors to small changes in the refractive index raises the question of the FBG sensor performance under harsh conditions, such as high radiation environments.

The past studies have shown that FBG sensors are fully functional for several years under exposure to radiation doses ranging from a few Gy/h to a few kGy/h. Some studies have indicated a small, but significant drifts in Bragg resonances; this suggests that the resonance wavelength redshifts with increasing dose rate, however, other studies have reported a blue shift of comparable magnitude.

Overall, FBG temperature sensors show significant peak center drift due to accumulated dose, however, the temperature sensitivity shows no changes. The changes in the measurements are assumed to be due to the complex changes in the fiber. An understanding of the changes that occur in the fiber during the exposure to radiation could enable integrated dose measurements of absorbed radiation dose with the use of appropriate correction factors.

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 sensors, please contact us at info@optromix.com

Voltage measurements with fiber Bragg gratings

Voltage measurement is essential for many fields such as power grids, telecommunications, metallurgy, railways, oil production, etc. Voltage measurements are related to fault detection and monitoring of the state of large equipment, which is essential for these fields. As the technologies used in these fields continue to develop, voltage monitoring becomes increasingly more important to the system of fault location.

Traditional voltage-sensing devices and methods are not ideal and possess multiple drawbacks, such as:

  1. Narrow bandwidth;
  2. Large weight;
  3. A possibility of combustion under heavy working conditions;
  4. Contact sensing.

Due to the multiple drawbacks of these methods, a need for a new voltage sensor has emerged. The new method of voltage measurement needs to have good amplitude-frequency characteristics and transmission characteristics in order to build smart sensing systems.

Currently, the biggest emphasis is put on fiber optic technology. Researchers attach great importance to voltage monitoring with optical elements as they offer multiple advantages that are vital for voltage monitoring in various fields:

  1. FBG sensors are passive;
  2. They are immune to electromagnetic interference;
  3. FBG sensors are compact and lightweight;
  4. FBG sensors can be mounted on any surface;
  5. They possess a wide sensing range;
  6. FBG sensors are stable under high temperatures.

The use of FBG sensors for voltage measurements allows us to obtain the data of switching overvoltage and lightning invasion overvoltage, analyze the overvoltage incidents, and overvoltage mechanisms.

Optromix R&D team, established in 2004, has extensive experience in the development of fiber optic products and solutions, based on the advanced research work and patents of internationally recognized scientists. 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.

Fundamentals of fiber Bragg grating interrogators

The fiber Bragg grating sensors work by sending the light into the fiber, where it is reflected back from the FBGs. The light that has been reflected travels back to the photodetectors of the instrument, where it is compared to wavelength reference artifacts. During this process the fiber Bragg grating interrogator evaluates the position of the center wavelength of the FBG; this information is later converted to engineering units. The gage factor supplied with the FBG sensor helps to determine and translate the data obtained during the measurements.

The described principle is true when one FBG sensor is present on a fiber. However, if the particular application requires multiple FBG sensors, like FBG sensor pipeline monitoring, fiber optic well monitoring, FBG temperature sensing, etc., the interrogators use one of the discriminating schemes in order to discriminate between one FBG sensor and the next. There are a couple of discrimination methods that are used in FBG interrogators. The first one is referred to as time division multiplexing utilizes the known speed of light in the fiber to discern which signal is reflected from which FBG along the fiber path. Around 100 FBG sensors can be interrogated with this method.

The second method, wavelength division multiplexing, is the most utilized one. As FBG sensors are at distinctly different nominal center wavelengths from their neighbors, the FBG interrogator uses the wavelengths of the sensors to track them along with the fiber. The range of this method is largely due to the developments in fiber optic technology.

Other approaches to FBG sensor interrogating, some of which include:

  1. Broadband source, Dispersive element, Diode Array;

This method is less reliable than the aforementioned ones due to limited resolution, which is a result of the inherent limitations of commercially available diodes.

  1. Broadband source, Optical Spectrum Analyzer/Multi-line wavelength meter;

The optical spectrum analyzers are large and expensive, which makes them less desirable in a laboratory setting. They are also not able to perform optimally under some temperatures.

  1. OTDR/TDM systems;

The system cannot handle a large number of sensors on the fiber as its data acquisition rates scale down with increasing sensor counts.

  1. External Cavity Tunable Laser, Power Meter, Wavelength Meter;

External cavity tunable lasers have low speed and do not have a wide operating temperature range. Moreover, they are expensive and do not have the required mechanical robustness.

Optromix interrogators can control up to 8 optical channels. The interrogator operates with the 20 maximum sensors per channel. The device is controlled by the PC with the specialized software for sensors monitoring. The system contains a broadband source of radiation and it can carry out spectrum analysis.

If you would like to purchase an FBG interrogator, please contact us: info@optromix.com or +1 617 558 9858

Fiber optic aircraft monitoring

Fiber optic sensors are used in aircraft structural health monitoring as they possess numerous advantages for real-time monitoring like immunity to electromagnetic interferences, low weight, small size, and high bandwidth. The latter allows multiple sensors to be used simultaneously within the same system.

It is anticipated that air traffic will grow significantly in the forthcoming years. Most modern airplanes make use of composite materials instead of conventional aluminum alloys, therefore durability and safety issues arise due to the complexity of composite materials. The different materials that compose them react differently to different environmental changes, which results in unpredictable behavior. The systems that enable the opportunity to monitor the aircraft in real-time are essential for safety and reliability improvement and reduction of maintenance costs. Fiber optic aircraft monitoring is a promising way of improving efficiency and operation costs.

FBG sensors have already found a wide range of applications. Fiber Bragg grating sensors are used in the monitoring of composite structures during on-ground aircraft testing; some airplanes are already equipped with FBG sensors that provide real-time data during a flight. The data provided by the sensors is valuable for local damage detection, and fiber optic aircraft monitoring has proven to be beneficial to the field. The integration of FBG sensors into the composite materials would enable the monitoring of the material during its whole life cycle.

Among multiple types of sensors optical sensors, namely FBG sensors, are of particular interest for aircraft monitoring due to their high sensitivity and durability. The multiplexing capability of FBG sensors reduces the weight of the wiring. Moreover, fiber Bragg grating sensors are significantly less expensive than other sensors.

However, the technology is not fully developed yet and more effort is still needed to bring it to a mature level. Among the challenges of fiber optic aircraft monitoring is the development of methods to monitor structural parameters over large structures.

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. If you would like to buy FBG sensors, please contact us at info@optromix.com

Tilted FBG – optical Fiber Bragg Gratings (TFBG)

Tilted FBGsSpecialized sensors known as Tilted Fiber Bragg Gratings (TFBGs) expand the capabilities of Fiber Bragg Grating (FBG) technology. In a range of applications TFBGs display unique optical properties and improved performance due to their capability to activate cladding modes through using tilted grating planes in regard to the fiber axis. In addition to maintaining all the well-established benefits of FBGs, this structural innovation makes TFBGs valuable in fiber-optic technology by allowing single-point sensing in challenging areas.

How Tilted Fiber Bragg Gratings (TFBGs) work

A short-period optical fiber grating is known as tilted fiber Bragg grating (TFBG), sometimes it’s referred to as a blazed or slanted grating. The same methods applied to create Fiber Bragg Gratings (FBGs) are also employed to manufacture TFBGs, featuring an interference pattern formed by ultraviolet laser beams to alter the refractive index of doped glass.

Unlike ordinary FBGs, TFBGs include grating planes tilted at a specified angle relating to the fiber axis. Not only does this tilt angle improve coupling between guided modes and either counterpropagating or copropagating modes at specific wavelengths, but it also affects the grating period which influences mode coupling. As a result, TFBGs provide complex interactions between core, cladding, and radiation modes, resulting in a transmission spectrum with many resonance peaks, making them ideal for sophisticated sensing applications.

Mode coupling and sensitivity enhancement

Enhancing sensing capabilities requires Tilted Fiber Bragg Gratings (TFBGs) to transfer light from guided core modes into radiation or cladding modes. In order to perceive this process, the core mode is investigated by a more straightforward two-layer model, while cladding modes are studied by a theoretical model that considers the fiber as a three-layer structure. The sensor susceptibility to fluctuations in the refractive index of the surrounding medium is improved by decreasing the cladding radius since it enhances the energy transfer from the cladding modes to the core.

Furthermore, a variety of TFBG sensors suited to a broad range of applications can be designed due to the high sensitivity of their coupling efficiency to light polarization.

Tilted Fiber Bragg Gratings (TFBGs) advantages

  • Multi-functionality: TFBGs are adaptable instruments for sensing applications since they can combine several light modes to react to multiple measurements.
  • High sensitivity: TFBGs can obtain great sensitivity, especially for measuring temperature and strain because of their efficient use of mode coupling effects.
  • Diverse applications: Their efficacy and versatility are demonstrated by their usage in a variety of domains, including optical communications, structural health monitoring, medical diagnostics, and environmental sensing.

TFBGs applications

TFBG sensors are used for monitoring and measuring mechanical changes as well as in biological areas. They include one-dimensional TFBG devices like vibroscopes, accelerometers, and micro-displacement sensors, as well as two-dimensional TFBG instruments like vector vibroscopes and rotation sensors. Reflective TFBG refractometers are available in-fiber and fiber-to-fiber configurations, whereas polarimetric and plasmonic TFBG biochemical sensors can detect cells, proteins, and glucose levels in situ.

Being an important step forward in optical fiber technology, tilted fiber Bragg gratings (TFBGs) combine distinctive structural characteristics with a wide range of beneficial applications. They are vital in current technological advancements owing to their mode coupling qualities and high sensitivity.