Fiber Optic Sensors:
The first fiber optic sensors were the flexible endoscopes developed in the first half of the twentieth century. Their development and use created a revolution in medicine that continues to this day. The modern age of fiber optic sensors was enabled by the development of extremely low-loss optical fibers in the late 1970s. A wide variety of sensor types and applications as a wide spread use for structural sensing and monitoring in civil engineering, aerospace, marine, oil & gas, composites and smart structure applications. The advantages of optical fiber sensors are intrinsically safe, passive: no need for electrical power, EMI immunity, galvanic isolation, possibility of remote, multiplexed operation, small size and lightweight, wide bandwidth, high sensitivity.
Fiber optic sensors classifications:
We have different types of fiber sensors are commercially available to measure parameters such as pressure, temperature, refractive index, displacement, gas concentration, several others. Others are at high stage of development allowing for a reliable and accurate measurement of stress and strain, electric current, vibration, sound, flow, etc.
Types of fiber optic sensors:
Measure optical phase difference between two light waves ( sagnac, micahelson, mach zehnder)
Alteration of guided light power
Measure optical resonant frequency of an optical cavity (fabry-perot), interferometers, either extrinsic or intrinsic.
Measure state of polarization of guided light wave
- SPECTRAL INTERFERENCE
Measure frequency of light wave interfering with a periodic structure (fiber Bragg grating).
Optical fiber Bragg grating sensors:
Fiber Bragg grating (FBGs) over last few years have been used extensively in the telecommunication industry for dense of wave length division de- multiplexing, dispersion compensation, laser stabilization, all at 1550 nm. In addition, FBGs have been studied for a wide variety of mechanical sensing applications including monitoring of civil structures (highways, bridges, buildings, dams, etc.), smart manufacturing and non-destructive testing (composites, laminates, etc.), remote sensing (oil wells, power cables, pipelines, space stations, etc.), smart structures ( airplane wings, ship hulls, buildings, sports equipment etc.), as well as traditional strain, pressure and temperature sensing. The main advantage of FBGs for mechanical sensing is that these devices perform a direct transformation of the sensed parameter to optical wavelength, independent of light levels, connectors or fiber losses, or other FBGs at different wavelengths. The advantages of FBGs over resistive foil strain gauges include:
- Totally passive (no resistive heating),
- Small size (can be laminated or embedded),
- Narrow band with wide wavelength operating range (can be highly multiplexed),
- Non-conductive (immune to electromagnetic interference),
- Environmentally more stable (glass compared to copper), and
- Low fiber loss at 1550 nm (for remote sensing).
- Potential for very low cost due to device simplicity and high volume telecommunication usage.
Fiber optic Bragg Grating sensor principle:
The basic principle of a fiber Bragg grating (FBG) based sensor system lies in the monitoring of the wavelength shift of the returned Bragg- signal, as a function the measured (e.g strain, temperature and force). The Bragg wavelength is related to the refractive index of the material and the grating pitch. Sensor system involving such grating usually work by injecting light from a spectrally broadband source into the fiber, with the result that the grating reflects a narrow spectral component at the Bragg wavelength, or in transmission this component is missing from the observed spectrum.
Intrinsic optical fiber sensors:
Intrinsic sensors can modulate intensity, phase, polarization, wavelength or transit time of light. Sensors which modulate light intensity tend to use mainly multimode fibers, but only mono mode cables are used to modulate other light parameters. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to 1 meter. Light intensity is the simplest parameter to manipulate in intrinsic sensors because only a simple source and detector are required. Another type of intrinsic sensor uses cable where the core and cladding have similar refractive indices but different temperature coefficients. This is used as a temperature sensor. Temperature rises cause the refractive indices to become even closer together and losses from core to increase, thus reducing the quantity of light transmitted.
Extrinsic optical fiber sensors:
Extrinsic fiber optic sensors are use an optical fiber cable, normally a multi mode one, to transmitted light from either a non-fiber optical sensor, are an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach place which are otherwise inaccessible. An example is the measurement of temperature inside aircraft jet engines by using a fiber to transmit radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of electrical transformers. Extrinsic fiber optic sensors provide excellent protection of measurement signals against noise corruption. Unfortunately, many conventional sensors produce electrical output which must be converted into an optical signal for use with fiber.
Extrinsic optical fiber sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque and twisting.
Various fiber optic sensors have existed for many decades. However, it is only since the advent of fiber Bragg Grating (FBGs) in the mid 1990s that broad commercial acceptance began. FBG technology offers many advantages over electrical and even other optical sensors, advantages which open doors to new applications where traditional sensors either fail or cannot be used:
- Reveal the structural health of bridge, building, dam, airplane, train…
- Improve the efficiency of wind turbines, oil extraction, jet engines…
- Warn of impending dangers with a tunnel, chemical or nuclear plant…
- Enhance sensitivities of medical instruments, industrial tools…
Fiber optic sensors work within high electromagnetic fields and are immune to lighting and corrosion. They are small and light, easy to install, and for many applications, cost less than the traditional alternatives. Fiber Bragg Grating (FBGs) multiplex-able and multifunctional, meaning many sensors, can be placed along single fiber and they can be various types of sensors such as strain, temperature, acceleration, pressure, etc.
Micron optics offers fiber optic sensors, instruments and enlight sensing analysis software based on optical technologies for industries such as civil engineering, aero space, transportation and marine, oil and gas, power, and others.
Civil structure applications:
Some applications on very old structures (such as Acropolis and the duomo di Como, Italy) to very new (such as the water cube in Beijing) micron optics provide sensing solutions that deliver fast and accurate and dependable measurements. We have some bridges, buildings, tunnels and roads are using fiber optic sensors whole construction.
- Arsenal bridge at Rocks Island,
- M6 motor way Road Bridge at Lancashire, U.K.
- Sungsan bridge at South Korea,
- Hampden Bridge at Australia,
- Chiapas Bridge, Chiapas, Mexico.
- Sapgyo Grand Bridge, South Korea.
- M56 Motor way Road Bridge Cheshire, U.K.
- Manhattan Bridge, Manhattan, NY.
- Vicksburg Bridge, Vicksburg, MS.
- Stress monitoring of long spans (>1km) with hundreds of sensors.
- Tracking behavior in high stress conditions (earth quake, heavy traffic, high wind)
- Long term stress and vibrating monitoring.
- Embedded sensors in concrete beams and pilings.
- Embedded/surface sensors on cables for suspension structures.
- Surface-mounted sensors on steel components in expansion joints
- Strain sensing on steel girders
- Stress and vibration monitoring
- Footing settlement
- Temperature monitoring during curing
- Precise reservoir water level monitoring for dam flow control
- River level and flow monitoring for improved flood control
Buildings and other structures:
- Greek Public Schools, Hellinikon, Athens, Greece
- Airport terminal, Singapore.
- Underground car park, Madrid, Spain.
- Mount Rushmore, south Dakota U.S.A
- Canton TV Tower, Guangzhou, China.
- Holy Temple, Glyfada, Greece.
- Halifax Metro Centre, Halifax, Canada.
- High Rise Buildings, Singapore.
Tunnels and Roads:
- Yangtze River Tunnel, Nanjing, China.
- Hume Highway Phase I, Australia.
- Hume Highway Phase II, Australia.
- Monitoring of strains and stresses
- Crack detection and monitoring
Fiber optic sensing solutions are particularly attractive for such permanent installations.
- Components are designed to perform for decades.
- Hundreds of sensors can be multiplexed into just a few fibers.
- Different types of sensors (e.g, strain, temperature, acceleration) can be combined on the same fiber.
- Recent focus has been on making both instruments and sensors easier to install and use.
Oil & Gas:
Pipelines are part of the backbone for modern community’s lifestyle and are absolutely indispensable for transportation of water, gas, oil and all kinds of products.
Faults in these systems do not only result in service outages and financial losses but bear the potential of spillages causing environmental pollution are even disasters accident.
Due to this, governments, engineering companies and industry associations have developed design, operation and maintenances standards for pipelines based on which the number of leaks could be reduced drastically since the 60’s and early 70’s of the last century. As a result pipelines today are highly reliable and safe means of transportation.
For decades, electrical sensors have been used down hole to measure pressure, temperature, and flow. Sensors are used to maximize production while minimizing risk to humans, the environment and the production equipment. Fiber optic sensors are now in use where traditional electrical sensors perform poorly or not at all. fiber optic sensors typically last longer in these harsh environments, provide more reliable data, offer an attractive small size, and perform in situations where actual sensor readings must be many tens of kilometers away from the well head. Fiber Bragg Grating (FBG) optical sensors are particularly attractive as pressure sensors and in long arrays with many temperature sensing points.
Other applications related to oil and gas where FBG sensors are ideal include monitoring pipelines, off-shore platforms, storage facilities, tankers, and refineries. Passive fibers with long range capability provide explosion proof monitoring of these critical assets.
Fiber optic sensors are using many customers in Oil and Gas industry including:
> BP, Petro bras,
> Statoil Hydro,
> Weather ford.
Power and Wind Turbines:
Fiber optic sensors for remote monitoring in high power electronic machines, such as generators, transformers, present some difficulties due to the insulation and EM immunity requirements. Such problems can be solved using optical fiber sensors. Sensors allow to measurements and remote control without electric wires. These features along with compactness, reliability and counterbalance the more complex technologies and higher costs of traditional piezoelectric sensors. The optical sensors used in wind turbines also with focus on fiber optic sensors. The fiber optical sensors can be made out of glass, they are not conducting and have advantages over electric sensors since they are immune to EMI and lighting which is of great advantage in wind turbines for offshore use. Optical fiber sensors is possible to place a sensors in the blades that measures load e.g., strain. Light weight and unobtrusive to the structure, improves EMI and lighting proof, FBG technology benefits for wind blades, 28 sensors on two channels on two 3 mm reinforced fibers. The user interfaces can be accessed through a web browser allowing for remote connections to the FBG blade monitoring system from any computer with access to the internet.
In addition to providing absolute measurement, low loss, high bandwidth, and excellent resolution and range, fiber optic sensors are immune to electromagnetic interference. Only non conductive optical fibers run to and from the sensors. FBG based strain gauges, temperature gauges, and accelerometer perform perfect within these strong electromagnetic fields. In addition, networks of hundreds of fiber optic sensors can be monitored with just one interrogation instrument, providing feedback to control systems and warnings of impending failures to engineers. Now the wind turbine monitoring places are in U.S.A and some other places.
Fiber optic sensors are light and small. Aircraft engineers often used hundreds or thousands of sensors for each application. Today, most fiber optic sensors for aerospace applications are used in ground tests and design. However some aircrafts are already flying with networks of fiber optic sensors on board. The long term vision is that all new aircrafts will fly with Fiber Bragg Grating (FBGs) optical sensors. The FBG sensors provide will improve safety, prolong the life of airframes, and reduce maintenance and improve the in flight efficiency of engines.
Fiber Bragg Grating sensors test structural integrity of wings fuselage components, performance of engines, load on landing gear, and icing of wings. FBG sensors for strain, temperature, and acceleration come in rugged packages that are easy to install. Passive operation of FBG sensors is an advantage when sensors and fiber leads must run through fuel tanks. Since, even hundreds, dozens of sensors can be multiplexed on a single fiber.
Fiber optic sensors are using many customers in aerospace industry including;
- EADS – European Aeronautic Defense and Space Company,
- GE Aerospace & GEAE – General electronic aircraft engines,
- NASA – National Aeronautical and Space Administration,
- Northrop Grumman,
- Lockheed Martin,
Railway operators and departments of transportation responsible for networks of highways need quantitative information provided by sensor systems. Fiber optic sensors work particularly well for this application because thousands of fiber optic sensors can be multiplexed on just a few fibers ranging over tens of kilometers. Additionally they can be integrated with existing structure embedded in new ones, and with their excellent corrosion and lighting resistance, they can last for decades. Measuring the infrastructure is important, but railroads are now monitoring both the rails and trains that ride them. The result is both improved safety and more optimized use of maintenance.
Fiber optic sensors are using in both rail tracks and trains to monitoring the system, to know the location, speed, and weight of passing trains, in HONG KONG it will using now
- Rail Track Monitoring, Hong Kong.
- Commuter Train Monitoring, Hong Kong.
Fiber optic sensors are using for improve safety, reliability and efficiency and transform conventional system into “smart railways”. The FBG sensors are used to monitor many important railway sub-systems such as:
- Axle counters,
- Anti-derailment monitor,
- Train load detectors,
- Continuous rail crack detectors
- And to monitor the instantaneous vibration signatures of passing train at selected locations.
Allows maximum network capacity optimization of electricity utilization and effective detection of potential operational hazards to enhance overall service safety and quality. The real time monitoring system is fully operational and currently in service and provides information on:
- Stresses experienced during service, both static and dynamic, under different operational conditions,
- The loading and traffic status of the passenger cars,
- Temperature induced stresses and deformations on rails and carriages,
- Temperatures in and around axles and wheel breaks,
- Dynamic axle vibrations due to corrosion and bearing wear,
FBG sensors can be readily installed on the left and right rails (for the two wheels of the same axle) at low cost to measure the important parameters railway engineers monitoring the anti-derailment ratio of passing trains are able to detect imbalances on the two sides of rail wheels. The FBG sensors are epoxied on the rail tracks and are connected by armored optical cables to an optical fiber backbone that runs along the entire route of the rail tracks.
The application involves the development and implementation of an optical fiber sensor system that incorporates hundreds of Fiber Bragg Grating (FBGs) in multiple trains. Monitoring of the commuter trains braking components
- Prevent and flag brake problems
- Identify operation inconsistencies
Under carriage cable routing and monitoring of weld points for transformers and other train equipment. The FBG sensors measure wide strain and temperature ranges well beyond he technical requirements the application demands. Sensors multiplexing allow installation time savings when compared to traditional electrical sensors. The FBG sensors are fixed inside and outside of the train at various points monitoring.
- Motor bearing temperature monitoring,
- Axle box strain monitoring
- Motor winding temperature monitoring
- FBG sensor on train sole bar
- Sensors on train charger, under carriage welding point
- Sensors on train rectifier
Six FBG sensors installed on trailer car and the same number of sensors was installed on the motor of the car of train. For the trailer car, three sensors were positioned at the corners of the window, with one at the bottom steel bar, while the other two were at the top surface of the car. Multiplexing large number of FBGs over long distance is particularly important for condition monitoring applications in railways. FBG arrays for the measurement of temperature, strain, tilt angle, and acceleration of train wagons using optical fiber sensors.
Marine applications are also same similarities to the transportation systems like railways. They too require large number of sensors in large, corrosive environments. But these often have added requirements. For example LNG tankers need sensors to detect leakage, but the sensors must pose no explosion risk. FBG sensors are passive.
The fiber optic sensors are used to monitoring the system will give a warning when stress and bending moment levels and acceleration of ship motion approach levels which require corrective action such as escaping the area, reducing the speed or changing the heading of the vessel.
The new fiber optic technology allows any number of sensors to be fitted anywhere in the ship for uninterrupted recordings of actual degree of stress. We can monitor the real time display of each sensor. In ships, temperature sensors, flooding sensors, like different types sensors are used in marine applications.
The Alinghi’s has successfully implemented optical fiber sensors on their boats. The data has proven invaluable both for design validation, performance development and for real-time monitoring. FBG sensors have survived the almost daily sailing of international American’s cup class winner for last five years. Where fiber optic sensors provide to monitoring of the boat’s performance, for using sensors easy to monitor
- Wind speed, 2. Wind direction 3. Wind swirls
4. Temperature 5. Temperature gradients 6. Water roughness
7. Water currents 8. Mass of the boat 9. Drag of the boat in the water
10. Drag of the boat in the air.
Typically there are two primarily types of FBG sensors: “Strain” and “temperature” sensors. The fundamental parameters that are measured using FBGs are temperature and strain. In order to correctly more complex information from a sail, such as curvature, shape, twist, flapping etc.