Category: FBG Sensors

Compact FBG sensors control traffic density

FBG sensors for traffic control A new compact sensing system based on fiber Bragg grating technology has been designed to control urban traffic density up to 80 kph. The sensing technology includes a 2.5-m-long and a 2-cm-high fiber optic sensor made from a combination of silicone addition rubber and fiber Bragg grating put inside a carbon tube.

The design of the compact FBG sensors allows monitoring and detecting the traffic density and car crossing in a single lane. It should be noted that such a sensing system has been already tested in real traffic and demonstrated quite a high detection rate of 98.946% based on the control of 1518 vehicles of various types and sizes.

To be more precise, traffic sensing systems are considered to be special devices that determine input data and information for other transport infrastructure systems. It is possible to install traffic sensors next to, above, into, or onto a road’s surface. The fiber optic sensors control “the presence of a vehicle, vehicle speed, vehicle classification, wheelbase and number of axles, total vehicle weight, road axle load, and occupancy, all of which indicate the traffic flow quality”.

The thing is that sensing systems based on optical fibers provide the following benefits: robustness, precision, high sensitivity, electromagnetic and chemical resistance, electrical passivity, and a broad temperature operating range. Additionally, it is possible to use fiber optic sensors in hybrid systems. The operating principle of fiber sensors is based on transmitting and receiving a laser or infrared beam from an emitter to a receiver.

Another type of fiber optic sensor is distinguished which is a micro-motion one. Such a fiber sensor operates using the increasing of optical fiber attenuation. Herewith, these optical fibers have special protective coatings and they are installed into a roadway. Nevertheless, their application is experimental, and these fiber sensors are not employed in general practice.

Also, the DAS technology (Distributed Acoustic Sensor) can be used for the same purposes. For instance, DAS systems apply a single optical fiber to control multiple traffic parameters, such as detecting a car, its direction, its speed, etc. However, FBG sensors require proper protection in order to avoid damage to the sensors. The use of composite fiber protection to the FBG sensors enables us to successfully solve the current problem.

The high sensitivity of portable FBG sensors and success rates in determination vehicles are regarded as a good supposition for expanding research and future developments for monitoring automotive traffic. Nowadays researchers plan to improve the design of fiber sensors to provide a more stable sensitivity, as well as to expand the determination capabilities of the sensing system, and to produce the sensor for year-round application.

Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber optic solutions for the clients. Optromix produces a wide range of fiber optic devices, including cutting-edge customized fiber optic Bragg grating product line and fiber Bragg grating sensor systems. Moreover, Optromix 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

A new application of FBG sensors

new FBG sensors A team of researchers offers a new application of fiber Bragg grating sensors (FBG sensors) in the construction and characterization of Micro Pattern Gaseous Detector. Such a FBG sensing system includes 144 gas electron multiplier chambers of about 0.5 m2 active area each. It should be noted that FBG technology has been applied in high energy physics only for high precision positioning and re-positioning fiber sensors due to their benefits (low cost, easy installation, low space consumption). Nowadays the most common application of fiber Bragg gratings remains the use for very accurate strain measurements in material studies.

Now FBG technology is used as flatness and mechanical tensioning sensors employed to the wide GEM foils of the GE1/1 chambers. FBG sensor array allows detecting the optimal mechanical tension applied and characterizing the mechanical tension that should be applied to the foils. A new application of FBG sensors has been already tested and demonstrated great results that can determine a standard assembly procedure and possible future developments.

Fiber Bragg gratings are considered to be a type of distributed Bragg reflector, developed by a short segment of optical fiber that enables one to reflect certain wavelengths of light and transmit all others. The operating principle of FBG sensors is based on “the creation of a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror”. Thus, the FBG sensing system can be applied as a strain measurement device because changes in fiber Bragg gratings lead to various light frequency responses.

One more application of FBG sensors in GEM chamber construction includes the opportunity to be applied as a load gauge for accurate measurement of the tensile load employed to the foils, of the various layers, at the same moment. The thing is that this FBG application plays a crucial role in the case of the GE1/1 chambers because the foils are stretched through screwing nuts with a dynamometric screwdriver during their assembly procedure, therefore, it is necessary to know accurately how much to strain the foils to escape to stretch them too much.

The testing of fiber Bragg grating sensors consist of proper stretching of the GE1/1 GEM stack, then the researchers removed a single stretching screw from the chamber and replaced it with an eyelet screw with installed stainless steel wire applied to set various weights. Additionally, it was necessary to add weights in different steps until they applied load achieves in the fiber Bragg gratings facing the eyelet the same response as when stretching of the GEMs occurs by the traditional screw.

Finally, the technology of FBG sensors is regarded as a highly reliable glue-less technique that offers the correct tensioning of the three GEM foils. The correct tension across the GEM stack leads to uniform gaps resulting in the required performance of the detector. Moreover, FBG sensors operate successfully as load gauges for accurate detection of the used stretching force to GEM foils.

FBGs fiber Bragg gratings fiber optic sensors

Fiber optic sensors based on the vapor deposited conducting polymer

polymer FBG sensors A team of researchers from Australia has developed a mathematical model, which is based on energy deposition and the laser-induced damage threshold (LIDT) for low-intensity light radiation resulting in the appearance of conducting polymers in novel fiber optic sensing applications.

It should be noted that nowadays polymer features are regarded as one of the most potentials and studied CP’s, herewith, the material may find numerous applications such as light-emitting diodes (OLED), optical displays, photovoltaic devices, and fiber optic sensors.

The thing is that the poly(3,4-ethylene dioxythiophene) is almost visibly transparent in its doped state, while it converts into an opaque material (dark blue) in the dedoped state, thus, making the polymer ideal for optical applications, for instance, in electrochromic devices and optical fiber sensors.

The interaction between light and the sensing material plays a crucial role in fiber optic sensing. To be more precise, there is a reaction between the sensing material and an external stimulus, therefore, it becomes possible to interrogate and measure the change, linking the optical fiber measurement to the external stimuli.

Optical fibers are considered to be an ideal example of a way to send light to a volume or point of interest for fiber optic sensing. Usually, “optical material is coated on the side or at the tip of the fiber, which are typically inorganic material”. Numerous recent sensing applications (temperature sensors, hydrogen sensors, and polymer functionalization of exposed-core microstructured optical fibers) include fiber tip fabrication and side coating.

Thus, the researchers present a new fiber optic sensing architecture based on coating the tip of an optical fiber with the vapor-deposited conducting polymer. They confirm that such fiber optic sensors enable us to study the unique electrochemical properties of PEDOT: Tos at the sub-micron length scale. Additionally, the polymer material can be integrated with optical waveguides (optical fibers as well) resulting in a range of electro-optical devices.

Optical fibers provide such benefits as immunity to electromagnetic fields, the possibility for in-vivo, and distributed measurements with the electrochemical properties of poly((3,4-ethylene dioxythiophene). Also, the researchers prepare a technique to gradually deposit PEDOT: Tos layers with desired thicknesses on the tip of optical fibers to produce such a fiber optic sensor.

Finally, the fiber optic technology was tested, and the polymer material was deposited at the tip of cleaned and cleaved SMF-28 optical fiber. Moreover, the deposition technique is based on 4 steps: “ 1. Oxidant solution preparation, 2. Oxidant coating and splitting at the tip of the fiber, 3. VPP process and 4. Washing of unbound and unreacted monomers”.

distributed sensing fiber optic sensors fiber optic solutions

FBG sensors for measuring human body temperature

FBG sensors for temperature measurement The opportunity to measure body temperature plays a crucial role in both physiological studies and clinical investigations. New wearable sensors based on fiber Bragg grating or FBG technology offer such an opportunity in intelligent clothing for human body temperature measurement.

The main purpose of the development is the integration of FBG sensors into functional textiles to enhance the abilities of wearable fiber optic solutions for body temperature monitoring. Thus, the FBG sensing system provides a temperature sensitivity of 150 pm/°C, which is almost 15 times higher than that of a bare fiber Bragg grating.

It should be noted that fiber Bragg grating is regarded as a type of distributed FBG reflector installed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all the others. The FBG technology is used here as an inline optical filter to block specific wavelengths, or as a wavelength-specific reflector.

Additionally, FBG sensors offer great benefit over electronic sensing systems for the use in intelligent structures, civil engineering, harsh environmental conditions, built structural health monitoring system and other. Distributed temperature sensors based on fiber Bragg gratings measure human body temperature at fiver places: left chest, right chest, left armpit, right armpit, and at the center of the upper back. Herewith, the data information provided by these FBG sensors at five places is not the same because different parts of the human body have different temperatures.

The application of distributed temperature sensors based on FBG technology allows developing a sample of intelligent clothing and testing the heat transmission mechanism from numerous aspects. A new mathematical model of heat transmission for the human skin, the air and clothing has been created. Therefore, FBG sensors provide the theoretical basis of human temperature measurement applying intelligent clothing with distributed temperature sensors and demonstrate the implementation of optical fiber grating into the clothing.

Finally, the temperature measured by DTS systems can be useful to represent human body temperature in clinics. Nonetheless, it is planned to enlarge “research in intelligent clothing to cover the measuring and recording of real-time physiological information, such as human respiration, heartbeat, blood pressure, and other physiological signals.”

The biomedical application of wearable FBG sensors, which are non-intrusive, non-invasive, and continuously being monitored, is considered to become a highly potential monitoring and diagnostic devices. Thus, these distributed temperature sensors can determine and process physiological signals, extract signal characterization, transmit data, and have other basic options.

FBGs fiber Bragg gratings fiber optic sensors

Spacecraft structural health monitoring by fiber Bragg grating sensors

FBG sensors for spacecraft monitoring Fiber Bragg gratings or FBGs are considered to offer a markworthy application in the fastly growing aerospace industry. The FBG benefits such as phenomenal optical multiplexing make them possible to use as smart fiber optic sensors that allow them to perform structural health monitoring of airborne vehicles and maintain and lengthen their lifetime. Thus, FBG technology plays a crucial role in the spacecraft industry where tiny errors and damage can lead to death for the crew aboard.

To be more precise, fiber Bragg grating is regarded as a thin, tubular optical fiber device that includes a physical “grating” area at its core. Herewith, the core of fiber Bragg gratings is not homogeneous, it has a periodic variation in the refractive material index. The principle of FBG technology operation is based on the change in the core refractive index because of which “some light will be reflected and some will be transmitted through the tube.” Additionally, the periodic spacing of the grating influences the reflected vs transmitted wavelength of light.

FBGs demonstrate efficient operation as narrow bandwidth light filters. The FBG application includes a block of specific wavelengths and transmission of others that is similar to the mode selection that appears in laser cavities. Nevertheless, such features as pressure and strain as well as vibration and displacement also influence the wavelengths of fiber Bragg gratings, while the temperature may lead to thermo-optic effects in the silica material that conventional FBG sensors are made from. Therefore, the mentioned FBG properties allow using them as fiber optic sensors to measure various physical effects at the same time.

FBG technology opens an opportunity to use FBG sensors to determine ultrasonic and acoustic wave signals, with a unique application in structural health monitoring of aerospace vehicles. The thing is that acousto-ultrasonic detection by fiber Bragg grating sensors is highly effective in damage detection if the spacecraft is not mobile (on the ground). Herewith, an ultrasonic actuator is required to produce the ultrasonic signals. It should be noted that the damage detection by FBG sensors is very accurate and quantitative because it allows controlling both the waveform function and the repetition of measurements.

Nevertheless, the limitations in resolution and bandwidth of conventional tools applied with fiber Bragg grating sensors, for example, optical spectrum analyzers, do not provide accuracy in high-frequency detection. It is necessary to use a demodulation method to interpret the detected signals for accurate detection of ultrasonic waves. Herewith, the installation technique of the fiber Bragg gratings is also important in ultrasonic detection. The installation of the fiber Bragg grating sensors into an aerospace vehicle or craft requires the use of various techniques. For example, it is possible to place FBGs into composite materials, however, it may cause signal distortion. This is the reason why a better way is gluing FBG sensors on with some adhesive, such as epoxy.

FBGs fiber Bragg grating sensors structural health monitoring

FBG sensors in Biomechanics and Rehabilitation Application

FBG sensors in biomechanics Nowadays the technology of fiber Bragg gratings is considered to be quite attractive for sensing applications in biomechanics and rehabilitation engineering fields thanks to the proposed FBG benefits such as compact size, lightweight, biocompatibility, chemical inertness, multiplexing capability, and immunity to electromagnetic interference.

Additionally, FBG technology overcomes other traditional technologies for the measurement of a range of physical parameters or for the performance of high-sensitivity biochemical analysis because fiber Bragg gratings offer a high-performance alternative to conventional sensing devices.

It should be noted that the sensing application of FBG sensors includes such areas as aeronautics, automotive, civil engineering structure monitoring, and undersea oil exploration. Nevertheless, the use of FBG sensors in biomechanics and rehabilitation is quite novel and it still requires its practicality for full-scale implementation.

The potential application of FBG technology may include the following fields: strain detection in bones, “pressure mapping in orthopedic joints, stresses in intervertebral discs, chest wall deformation, pressure distribution in Human Machine Interfaces (HMIs), forces induced by tendons and ligaments, angles between body segments during gait, and many others in dental biomechanics”.

FBG sensors demonstrate great potential for biomechanics and rehabilitation engineering thanks to their benefits, thus, making FBG technology suitable for a human body. To be more precise, fiber sensors are able to adapt to the body, so it is possible to employ them for in vivo measurement and to left for long-term monitoring.

Moreover, fiber Bragg gratings can change traditional sensing technologies such as electrical strain gauge (ESG), piezoelectric, resistive, or another solid-state sensing, both for measuring physical characteristics or for carrying out high-sensitivity biochemical analysis. The development of FBG continues, and it is possible that very soon new FBG sensors with improved characteristics appear.

If you want to obtain a highly efficient sensing system, you should choose the Optromix company. Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber optic solutions for monitoring worldwide. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, distributed acoustic sensing (DAS) systems, distributed temperature sensing (DTS) systems. The company provides a wide range of sensors: temperature sensors, strain sensors, displacement sensors, tiltmeters, accelerometers. If you are interested in FBG sensors and want to learn more, please contact us at info@optromix.com

FBG strain sensors FBGs fiber optic sensors

Space application of FBG sensors

FBG sensors for space application Fiber Bragg grating sensors are highly useful fiber optic sensing devices that help carry out the monitoring of temperature and strain characteristics even during a nanosatellite mission. FBG sensors present highly attractive reliable fiber optic solutions for process monitoring in a spacecraft. Additionally, it is possible to install these fiber sensors in composite structures or attached on their surface for structural health monitoring during the entire life cycle of a satellite.

To be more precise, space application of fiber optic sensors requires the use of two fiber Bragg gratings to measure temperature and strain characteristics during one space mission. The main aim of the mission is the validation and demonstration of the suitability and reliability of fiber optic technologies when it comes to a small area with numerous restrictions in terms of mass and power consumption.

Despite the fact that FBG sensors offer great performance in various fields of application, especially in industry, engineering and science, however, the use of fiber sensors in space application is not so popular, although fiber optics are presented more than 30 years in space but typically applied to transmit data or to guide the light for illumination purposes.

Fiber sensors based on fiber Bragg grating technology have numerous benefits such as electromagnetic immunity, the possibility of multiplexing, and weight-saving by taking copper harness away. This is the reason why fiber sensing technology is perfect for the harsh space environment.

Also, FBG sensors are better than other optical fiber sensors because their optical response does not depend on the optical power of the light source the eventual loss of energy transmitted along with the optical fiber, and of the response of the photodetector, thus, the response of the fiber sensor is considered to be wavelength-coded.

The used FBG technology has been successfully tested and the tests presented the capability and reliability of fiber optic sensors based on fiber Bragg gratings in space environmental conditions. In spite of the high restrictions in terms of mass, volume, and power consumption, FBG sensors offer an optimal performance of all-optical fiber components after their 3 years’ application in the space environment.

Finally, during the whole space mission crucial attenuation proving the degradation of fiber optic components because of harsh environmental conditions (as well as radiation and vacuum conditions) has not been registered. Herewith, the used tunable laser system for telecommunications purposes also presented excellent results in the space environment. It is possible that these FBG sensors contribute to the creation of more complex sensor arrays with a more complex FBG interrogator unit that will enable better resolution and operation in a wider temperature range.

If you want to obtain a highly efficient sensing system, you should choose the Optromix company. Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber-optic solutions for monitoring worldwide. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, distributed acoustic sensing (DAS) systems, distributed temperature sensing (DTS) systems. If you are interested in FBG sensors and want to learn more, please contact us at info@optromix.com

FBGs fiber optic sensors structural health monitoring

Fiber optic inclinometer demonstrates the versatility of FBG sensors

FBG inclinometer Fiber optic inclinometer is considered to be one of the most important optical fiber devices used in numerous fields of application. For example, in geophysical engineering, the inclinometer based on the fiber Bragg grating technology (FBG) allows monitoring the angle of inclination at regular intervals along the borehole to profile the land displacement.

Current fiber optic inclinometers utilizing FBG sensors demonstrate a high level of sensitivity and angle of measurement resolution of 0.006°. It should be noted that traditional inclinometers have disadvantages when it comes to their involuntary twist during the process of installation. And optical fiber sensors were widely thought to overcome these limitations due to the absence of moving parts.

Nevertheless, the combination of traditional inclinometers with FBG technology enables us to resolve the challenge and make the fiber optic inclinometers’ operation independent of twisting, therefore, the precision remains constant. Moreover, despite numerous fiber optic techniques that measure temperature, strain, pressure, and other parameters, new fiber optic inclinometer based on a fiber’s taper and a long-period fiber Bragg grating have been recently developed.

The principle of the fiber optic device is based on the use of conventional fiber Mach-Zehnder interferometer that allows dividing the incoming light between two optical fibers and recombining it at a second junction. To be precise, in a Mach-Zehnder system, a long-period fiber Bragg grating dissipates light at its resonant wavelength from the optical fiber core into the cladding, while a second-long period FBG takes light from the cladding back into the fiber core.

The novel fiber optic inclinometer is a simpler device that includes only a single long-period fiber Bragg grating. Moreover, it uses the taper that enables to increase the core field so that part of its light is coupled into the cladding. The fiber optic inclinometer offers the following advantages in its operation:

the immunity to electromagnetic interference;
the high sensitivity;
the compact size;
the multiplexing and remote interrogation abilities.

This optical fiber device based on the interferometer was tested in the measurement of angular displacement. During the experiment, the optical fiber on either side of the taper was sheathed in capillary tubes and then twisted accurately at the taper. Thus, such a bend influences the fringe visibility, but not the location of the fringes.

Optromix is a FBG sensor manufacturer that provides top of the line FBG sensing systems suitable for monitoring of the constructions’ inclination. If you have any questions or would like to buy a FBG product, please contact us at info@optromix.com

FBG Inclinometers FBGs fiber optic sensors

FBG pressure sensors for pipeline leakage detection

FBG pressure sensors for leak detection The technology of fiber Bragg grating or FBG is widely used in numerous sensing technology fields of application. For example, today FBG pressure sensors are installed to detect the leakage process of prestressed concrete cylinder pipe. This type of pressure sensor is based on the use of a Bourdon tube with two fiber Bragg gratings attached on its outside and inside surfaces.

Such a wider sensing application is caused by an increasingly large number of attractive characteristics of FBG sensors that include intrinsic safety, immunity to electromagnetic fields, remote sensing, and large multiplexing capabilities. Therefore, the pressure parameter remains the most important physical factor in the process industry.

Nevertheless, conventional pressure sensors based on electrical train gauge, vibration wire, mechanics have several disadvantages that include the absence of the ability to withstand harsh environmental conditions with critical electromagnetic interference, adventurous chemicals, or explosion matters. Moreover, such sensing systems do not allow making pressure’s multipoint measurement and online monitoring at a long distance.

It should be noted that the intrinsic pressure sensitivity of an FBG sensor is considered to be about 3.04 pm/MPa, which is quite a low index for the practical pressure measurement. Despite numerous ways proposed to increase the sensitivity factor of FBG pressure sensors, such as the installation of FBG in polymer, soldering metal-coated FBGs on a free elastic cylinder, and embedding the FBG fiber to a diaphragm, all these techniques propose relatively complex structures which are difficult to produce.

The thing is that the prestressed concrete cylinder pipe is widely used for water transportation in different areas, for example, municipal, industrial, and plant piping systems. Thus, the role of FBG pressure sensors for security monitoring and reliability management of huge cylinder pipes remains very important because now the sensing technology still faces numerous challenges.

Nevertheless, the use of a Bourdon tube in FBG pressure sensors as the spring element allows detecting spring elements. Herewith, the measurement sensitivity increases and becomes 1.414 pm/kPa in a range from 0 to 1 MPa while the correlative coefficient is 99.949%. That is the reason why the development of a totally new pressure sensor is planned, which probably finds new FBG sensing applications of pressure quasi-distributed measurement or online monitoring in industry and manufacture fields.

If you want to obtain a highly efficient sensing system, you should choose the Optromix company. Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber optic solutions for monitoring worldwide. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, distributed acoustic sensing (DAS) systems, distributed temperature sensing (DTS) systems. If you are interested in FBG sensors and want to learn more, please contact us at info@optromix.com

FBG pressure sensors FBGs fiber optic sensors

Absolutely new optical gauge allows cutting well surveillance costs

FBG sensors for optical gauge Fiber optic technology is highly promising now, in spite of the fact that fiber optic solutions found their place in numerous fields of application, the optical fiber sphere still continues developing. Thus, recently a group of researchers from the UK demonstrated a novel optical pressure-temperature gauge of compact size.

The optical gauge is considered to enlarge the fiber optic application, especially when it comes to a well surveillance area. Moreover, this optical device allows cutting all the costs and offering a new alternative to conventional downhole fiber sensors and other electronic logging tools.

Also, it should be mentioned that it is possible to combine the optical gauge with other devices, for example, with a simple deployment mechanism and distributed sensing technology, adding new properties and offering savings of 50-90% compared to traditional logging techniques.

A fiber optic gauge is a special tool that is based on fiber Bragg gratings without using electronics, external connections, or power supply equipment. Herewith, optical gauges provide the following advantages that include:

Immunity to electromagnetic fields;
Potential application in highly explosive environments;
Ability to withstand high vibration loads without any mechanical failure of sensor material;
Reduced number of connection lines and consequently, less impact on test purposes;
Low interconnection is required because of the possibility to include numerous sensors with different fiber Bragg grating wavelengths in one optical fiber.

The novel optical gauge can be installed in the fiber optic line and used in harsh atmospheres. Moreover, this fiber optic device is able to measure up to 10,000 psi (689 bar) and 150°C (302°F), optionally enlarged to 20,000 psi (1,379 bar) and 300°C (572°F).

Herein, the size of the optical gauge is about 10 cu mm, that is why the device can be easily implemented in various optical sensors and even installed in narrow access wells to log data during descent and to capture bottom hole temperature and pressure over time.

Finally, the optical pressure-temperature gauge is able to operate several months without human control, streaming real-time, live well information along with the optical fiber back to the surface. Usually, fiber optic sensors are applied in complex surveillance systems that have a high cost, sometimes they need either batteries or electrical power as well as special equipment compared to the novel optical gauges. Nevertheless, FBG sensors remain number one in the market today due to their lightweight, simple installation and relatively low cost.

If you want to obtain a highly efficient sensing system, you should choose the Optromix company. Optromix is a manufacturer of innovative fiber optic products for the global market. The company provides the most technologically advanced fiber optic solutions for monitoring worldwide. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line such as fiber Bragg grating sensors, FBG interrogators and multiplexers, distributed acoustic sensing (DAS) systems, distributed temperature sensing (DTS) systems. If you are interested in FBG sensors and want to learn more, please contact us at info@optromix.com

FBGs fiber Bragg gratings fiber optic sensors