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 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.