Fiber optic tech was initially developed for use in telecommunications back in the 1980s. Advances in telecom has allowed for the development of inexpensive and effective fiber optic sensors and the associated optic electronic devices. Increase in usage of FOS driven by the many performance advantages over conventional electronic sensors. Ex. Lightweight, small size, immunity to electromagnetic interference. Two basic classifications of FOS: A) EXTRINSIC – hybrid system where the fiber carries a light signal to and from a black box. The black box responds to a given environmental signal by modulating (changing) the light signal in some way. The input light signal is now different from the output light signal and a processing device can interpret the change in signal into information about the change in environment. B) INTRINSIC – all fiber system, where the fiber is both the sensor and the transport mechanism for the signal. The environmental signal impresses some information into the light beam as it travels through the fiber.
1) strain, temperature and pressure induced by the manufacturing processes for individual structures can all be monitored to make sure set parameters are not exceeded and or met. 2) At any point during the manufacture, assembly and lifetime of the structure, non-destructive evaluation can be performed using embedded sensors. Again strain, temperature are examples of parameters that can be measured. 3) Often other types of SHM sensors are integrated into a give structure, fiber optics can serve as a data transport network to support these systems. 4) Other performance and control monitoring systems such as the flight control systems on spacecraft can integrate information from structural health monitoring fiber optic sensors to make adjustments to performance of the structure.
Light is an electromagnetic wave. The speed of the wave in a medium depends on how the light ray’s electric and magnetic fields interact with the particles that make up the medium. Light travels the fastest in a vacuum since there are no particles in to interact with. Thus, the speed of light in a vacuum becomes a reference value. The ratio of the speed of light in a vacuum to the speed of light in a medium is called the index of refraction. The smallest value n can be is 1. A higher refractive index means light travels slower through the medium.A light ray traveling from one medium to another is called an incident light ray. The ray’s angle relative to the normal of the surface is called the angle of incidence. Some of the light will transmit through and some will reflect. The Law of Reflection states that the angle of the reflected ray equals the angle of the incident ray. The Law of Refraction, also known as Snell’s Law (named after the Dutch scientist Willebrord Snell), states that the index of refraction of the first medium times the sine of the angle of incidence equals the index of refraction of the second medium times the sine of the angle of the refracted ray.
The principal of fiber optics relies on total internal reflection. If a light ray from one medium enters another medium and bends 90 degrees from the normal then this is called the critical angle. If light enters a medium at an angle greater than this critical angle, then total internal reflection occurs. The light does not transmit to the other material. In fiber optics, leakage refers to how much light is refracted.
An optical fiber is composed of two components: the core and the cladding. The cladding is made of a smaller index of refraction to ensure that total internal reflection occurs in the core.Not all light rays entering the core will result in total internal reflection. The range of angles the fiber can accept and transmit light with little loss is called the numerical aperture. For a multi-mode, step-index fiber it can be defined as (equation). There are three main types of fibers. Single-mode has a very small core and thick cladding. It can only allow light that is essential parallel to its core axis. This type of fiber is good at transmission, but is limited by its small aperture. Fibers that can accept more than one mode of light are multi-mode fibers. With a graded-index fiber the index of refraction at the interface between core and cladding gradually changes, resulting in a gradual change in direction of light. In a step-index fiber the transition is abrupt and thus, a sharp change occurs. These types are listed in terms of best resolution.
When fabricating optical fibers, first you need to choose your materials. This will depend on the function you need the fiber to perform. Typical materials are glass or plastic. If you need good transmission, glass would be a good choice, since it transmits light better at longer distances. If you need a more flexible and durable fiber, like for a sensor in a harsh environment, plastic would be a good choice. To further tailor the function of the fiber you can mix in elements with the glass or fiber. These elements, called dopants, can alter the function of the fiber by increasing or decreasing its refractive index or changing its magnetic, optical, and/or electrical properties. For example, adding rare elements (Atomic numbers 57-71) change the optical and magnetic properties, while adding elements from Group VII of the periodic chart (fluorine, chlorine, bromine, and iodine) affect the transmission quality of the fiber.
Once you the materials have been decided, the next step is decide the fabrication process. Fibers can be created through a direct process or through a preform process.With the direct process, melt or rod forms of the core and the cladding are combined directly to form the fiber. Step-index and graded-index fibers can be created using these methods. These processes are older, but simpler. They have problems of allowing impurities to enter into the fabrication process.The preform process is a more indirect method. First a preform is fabricated. A preform is like what a spool of yarn is to a thread of yarn. It is the fiber, but on a larger scale. Preforms can be fabricated using a variety of vapor deposition processes. Once the preform is fabricated, the fibers are then obtained by placing the preform through a machine that draws the fiber out and spools it.Examples of vapor deposition processes: modified chemical vapor deposition (MCVD), plasma-enhanced MCVD (PEMCVD), outside vapor deposition (OVD), axial vapor deposition (AVD). Depends on cost, ease of fabrication, and single-mode, multi-mode, step-index, etc. Vapor depostion processes allow for precise control over fiber components.
A basic fiber sensor system consists of a light source, the optical fiber sensor, the light detector, and some electronic equipment to process the data.The light source is usually an LED or laser. The type of electronic equipment chosen depends on the type of sensing being achieved. There are several mechanisms for sensing. One of the most common sensing applications is intensity based sensing. This where you measure the change in power output when the fiber interacts with the surrounding environment. Another mechanism is evanescent field sensing. Not all light sent through a fiber is totally internally reflected. Some light will transmit into the cladding and vice versa. The light that transmit into the cladding is called the evanescent field. This light can alter the fiber modes.Another common sensing mechanism is Bragg-Grating. This is where variations in the refractive index of the core are introduced to change the output signal. Yet, another mechanism is surface plasmon resonance sensing. This is where some of the light will travel parallel to the fiber and can detect that.
1) Many advantages since the fibers are silica based and use a light beam in-lieu of electric signals. The fibers are immune to electromagnetic interference. For many electrical systems, shielding is required for the sensors, results in bulking cabling. No need for shielding with FOS, get reduction in weight and size of the sensor and signal transmission system.All passive configuration eliminates conductive paths between elements of a structure. Important where structure may be subjected to lightning strikes (such as aircraft)Reduction in energy requirement as fiber optics offer low power utilization. Chemically inert and good resistance to high temperaturesInherently small size allows for FOS to be light-weight and relatively unobtrusive when embedded in the structure.High-bandwith allows for many measuring points to be integrated into a single fiber optic channel.- High density of measuring points allows for a spatial representation of the structure being monitored.Excellent transmission capabilities allow for the measuring point to be located several kilometers from the data processing points.Electronic cabling with similar properties would be much thicker than fiber optic strands.Heavy investment / mass production of fiber optic technology has drastically reduced the cost of fiber optic strands as well as the corresponding optoelectronic devices. Fiber optic components have been refined so they are now very are reliable with a high sensitivity. Example: 1979 single mode optical fiber cost $20/m now the cost is approx $0.10/mLimitations / Challenges of FOS 1) susceptible to damage during installation and over the lifetime of the sensor. Fibers may break 2) Need technique to ensure the data from the sensor is correct and not the result of a malfunction, sensor self-validation. - possible to have sensors report on each other, so in effect we would be monitoring the sensors that monitor the structure.
SPATIAL DISTRIBUTION ResolutionMore sensors allows for a greater resolution of the monitoring of the structure. As we incorporate more and more measuring points into our monitoring system, we can create a spatial distribution of the values of those points along the length of the structure. 1) Discrete point sensor – data is taken from a single discrete point - Example: optical fiber grating can be used as a discrete measuring point to determine strain. A grating is inscribed into the fiber that only allows the passage a certain range of light wavelength. The blocked wavelength is reflected by the grating. 2) Integrated – sensor detects data along a distributed length, but all the values are integrated to form one resultant value - Example: Using a long fiber to measure elongations of a structure. Have cable with two fibers in it. Each end of the 1st fiber is attached to the structure and then prestressed in between. The reference fiber is loose within the cable.
SPATIAL DISTRIBUTION OF FOS. 3) Quasi-distributed – sensor detects discrete points along a distributed length of a single optical channel. - Essentially a series of discrete measuring points placed in series along the length of the optical channel. Could setup fiber grating strain gauges along the length of the structure. 4) Distributed – allows for measurements to be taken along a continuous line. - Can create detailed spatial distribution of the measurements along the structure. - The fiber acts as both the optical channel and the transducer. - Similar to a nervous system for an organism - Example: 2km Fiber optic cable attached to the exterior of a building with a solar panel array. Resolution could be 0.5m, allows for 4000 measurement points and is similar to a quasi-distributed sensor. However the measurements can be taken anywhere along the length of the cable. Can get full distribution of solar panel temp.
A bead/point sensor coated with 3-PEG/PU on end doped with 8-HQ that when exposed to a solution containing Al cations a change in the evanescent field occurs.
The Confederation bridge in Canda 1) The concrete box-girder bridge has a 100 year design service life and is located over iced-ocean water. 2) Fiber optic sensors using the fiber bragg grating principle were used. Alters the index of refraction at a very precise pitch over a short length of the fiber. Light at the Bragg wavelength is reflected, and all other light passes through the grating. A strain at the grating changes the index of refraction and the pitch and causes a shift in the Bragg wavelength. An instrument reads the reflected light and can determine the strain based on the wavelength. 3) Strain measurements are taken at critical points of bending for this structure.
The FOS operates on simple concepts that have been rigously developed by the telecommunications industry.FOS can be used in a wide variety of functionsFOS are generally more expensive than conventional mechanical sensors, but they are also more reliable and rugged. Can have automated measurement process.Still not readily accepted in civil structures since the application of the technology is so new.
Fiber optic sensors in sHM UC Davis ECI 223 Aileen vandenberg Randy Presleigh April 18, 2011
Introduction to FOS Fiber optic technology history Two basic classes of sensors: 1) Extrinsic Hybrid 2) Intrinsic All Fiber
Fiber optic sensors in SHM Four potential functions: Monitoring external manufacturing process parameters Embedded sensors, non-destructive evaluation Serve as data-link network to support other SHM systems Compliment performance monitoring / control systems
Physics of Optical Fibers Index of Refraction The ratio of the speed of light, c, in a vacuum to the speed of light, v, in a material. Notes
A higher refractive index indicates light travels slower through the medium.
n = 𝐶𝑣
The Law of Reflection The angle of reflectionis equal to the angle of incidence. The Law of Refraction, a.k.a. Snell’s Law n1 sin θ1 = n2 sin θ2
Physics of Optical Fibers Total Internal Reflection When the angle of incidence is greater than the critical angle. The Critical Angle of Incidence When the angle of refraction is equal to 90°. Principal of fiber optics
Basics of an Optical Fiber Components of optical fiber Types of fibers To achieve total internal reflection, ncladding < ncore Numerical Aperture Describes a range of angles the fiber can accept or transmit light with little loss. Note: This is for a multi-mode, step-index fiber.
Fabrication of Optical Fibers Step 1: Materials
fos performance attributes Advantages over conventional electronic sensors: 1) Silica based material 2) Multi-plexing 3) Cost Limitations / challenges of FOS In general, have a rugged, high band-with, low cost system.
fos performance attributes Sensor spatial distribution resolution 1) Discrete point Ex: Fiber grating strain gauge 2) Integrated Ex: Long gauge fiber
fos performance attributes Sensor spatial distribution resolution 3) Quasi-distributed Ex: Series of fiber grating strain gages 4) Distributed Ex: Temp sensor along building face
Applications of Optical Fiber Sensors Corrosion Sensing Sensor head Dope with fluorescent products
Strain Sensing Confederation Bridge in Canada - Service life / environment - Embedded FBG sensors - Measure strain in concrete
Conclusion and Questions Simple and rugged Costs Versatile QUESTIONS ? Fiber optic afro?
References 1. Adding dopants to a fiber to change or add characteristics. [cited 2011 April 15]; Available from: http://www.paradigmoptics.com/pof/custompof.html. 2. Fabrication of an Optical Fibre. 30 May, 1997 [cited 2011 April 15]; Available from: http://www.vislab.uq.edu.au/photonics/fibres. 3. AL-Zu’bi, R.e. Optical Fiber Fabrication & Measurements. [cited 2011 April 15]; Available from: http://dar.ju.edu.jo/mansour/optical/Fiber%20Fabrication%20and%20measurments.htm. 4. S. C. Warren-Smith, H. Ebendorff-Heidepriem, S. Afshar, G. McAdam, C. Davis, and T.M. Monro, Corrosion Sensing of Aluminum Alloys Using Exposed Core Microstructured Optical Fibers, Institute of Materials Engineering Australasia, 2009. 5. Overview of Fiber Optic Sensors. [cited 2011 April 15]; Available from: http://www.bluerr.com/images/Overview_of_FOS2.pdf. 6. Fiber Optics. [cited 2011 April 16]; Available from: http://www.eliteavi.com/blog/dallasaudio-video-integration/fiber-optics/. 7. José Miguel López-Higuera, Luis Rodriguez Cobo, Antonio QuintelaIncera, and Adolfo Cobo, Fiber Optic Sensors in Structural Health Monitoring, Journal of Lightwave Technology, Vol. 29, NO. 4, FEBRUARY 15, 2011