Fiber Optic Sensor

Special Topics in Materials Science and Engineering

What are Optical Fibers?• Long, thin strands of very pure glass about the diameter of a human hair• Has an ability to transmit infrared ,UV, and visible light about confining

along its length• They are arranged in bundles (i.e. consisting of hundreds or thousands)

called optical cables and used to transmit light signals over long distances• The bundles are protected by the cable's outer covering, called a jacket

A single optical fiber• The core is thin glass center of the fiber where the light travels• Cladding is an outer optical material surrounding the core that reflects the

light back into the core• Buffer coating is the plastic coating that protects the fiber from damage

and moisture

How it works?

• Optical fiber is governed by the principle of total internal reflection (i.e. snell’s law)

• The glass core must be always dense than cladding

𝑛1 sinθ1=¿𝑛2 sinθ2 ¿

θ2=90 °𝐶

sin θ𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙=𝑛2𝑛1

𝑛1>𝑛2

How it works?

• What if transmitting flashlight beam in the hallway which is very winding with multiple bends?

• You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway

• This is exactly what happens in an optical fiber

How it works?

Advantages of Fiber Optics• Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths

of copper wire. This saves your provider (e.g. cable TV and Internet provider) and your money.

• Thinner - Optical fibers can be drawn to smaller diameters than copper wire.

• Higher carrying capacity - Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.

• Less signal degradation - The loss of signal in optical fiber is less than in copper wire.

• Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.

• Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.

Advantages of Fiber Optics

• Digital signals - Optical fibers are ideally suited for carrying digital information, which is especially useful in computer networks.

• Non-flammable - Because no electricity is passed through optical fibers, there is no fire hazard.

• Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber optic cables take up less space in the ground.

• Flexible - Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes:

• Medical imaging - in bronchoscopes, endoscopes, laparoscopes

• Mechanical imaging - inspecting mechanical welds in pipes and engines such as in airplanes, rockets, space shuttles, cars, etc.

• Plumbing - to inspect sewer lines

Materials Used• Fused Silica

- The most common due to very high transparency- Dopants are added to change the refractive index of the core and cladding

• Plastic - PMMA core is the standard used coupled with cladding of lower refractive

index polymer- Inexpensive, flexible, lightweight, and easy to handle

• Exotic fibers and light guides- these are liquid-core fibers, mid-infrared fibers, hollow optical waveguides,

photonic or microstructure fibers, and planar wave guides- used to transmit >2µm wavelength

Note: Dopants must be chosen carefully to avoid materials that absorb light or have harmful effects on the fiber quality and transparency (e.g. germanium has very low light absorption and fluorine reduce refractive index of silica)

Silica Fiber Manufacture

Making the Preform

• One approach is the inside vapor deposition method, using reacting SiCl4 (GeCl4 is added to dope) with oxygen to generate SiO2 (and GeO2 if doped) core

• One approach is to deposit the soot on the inside wall of fused silica tube

• The tube serves as the outer cladding, onto which an inner cladding layer and the core material are deposited by a modified CVD method

Inside Vapor Deposition

Making the Preform

• The chemicals react to deposit a fine glass suit, and waste gas is pumped out to an exhaust

• To spread soot uniformly along the length of the tube, the reaction zone is moved along the tube

• Heating melts the soot, and it condenses into a glass• The process can be repeated over and over to

deposit fine layers of slightly different composition, which are needed to grade the refractive index from core to cladding in graded-index fibers

Making the Preform

• Another important approach is the outside vapor deposition process, which deposits soot on the outside of the rotating ceramic rod

• The ceramic rod does not become a part of the fiber, it is just merely as a substrate

• The glass soot that will become the fiber core is deposited first, then cladding layers are deposited on top of it

• The ceramic core has a different thermal expansion coefficient than the glass layers deposited on top of it, so it slips out easily when the finished assembly is cooled before the glass is sintered to form a preform

Outside Vapor Deposition

Making the Preform

• The third main approach is vapor axial deposition, in which the rod of pure silica serves as a seed for deposition of glass soot on its end rather than on its surface

• The initial soot deposited becomes the core, then more soot is deposited radially outward to become the cladding, and new core material is grown on the end of the preform

• Vapor axial deposition does not involve a central hole

Vapor Axial deposition

Drawing Fibers

• Optical fibers are drawn from preforms by heating the glass until it softens, then pulling the hot glass away from the preform using the machine called a “drawing tower”

• The preform is mounted vertically at the top, with its bottom end in a furnace that heats the glass to its softening point

• Initially a blob of hot glass is pulled from the bottom, stretching out to become the start of the fiber which is normally discarded

• The hot glass thread emerging from the furnace solidifies almost instantaneously as it cools in open air

Drawing Fibers

Drawing Fibers

• The bare glass fiber passes through a device that monitors its diameter, then is covered with a protective plastic coating

• The end is attach to a rotating drum or spool, which turns steadily, pulling hot glass fiber from the bottom of the preform and winding plastic-coated fiber onto drum or spool

• Typically the fiber is drawn at speeds well over a meter per second

Fiber Optic Sensors

Simple probes

• One optical fiber delivers light from an external source, and a second fiber collects light emerging from the first

• Simple fiber-optic probe is used for checking parts on an assembly line

• When a part passes between them on the assembly line, it block the light

• Thus, light off indicates that a part is on the assembly line, and light on indicates that no part is passing by

Simple Probe

Optical Remote Sensing• Fiber probes can also collect light from other types of optical

sensors, in this case, the fibers function like wires attached to an electronic sensor

• One example is the liquid-level sensor, which sense when the gasoline in tank trucks reaches a certain level to prevent overfilling

• One fiber delivers light to a prism mounted at the proper level• If there is no liquid in the tank, the light from the fiber experiences

total internal reflection at the base of the prism and is directed back into the collecting fiber

• If the bottom of the prism is covered with gasoline, total internal reflection cannot occur at the angle that light strikes the prism’s bottom face, and no more light is reflected back into the fiber causing the control system to shuts off the gasoline pump

Optical Remote Sensing

Direct Intensity Modulation

• A subtle type of intensity sensor depends on microbending

• If there is no pressure on the plates, the fiber remains straight, and light passes through it

• Pressure on the plates causes microbending, in which the more pressure, the more microbending, where microbending makes light leak from the fiber core

• The more pressure, the less light is transmitted through the fiber

Polarization Sensing

Other Sensing mechanisms of Optical Fiber

• Polarization Sensing – sensing as a function of Faraday rotation (i.e. rotation of polarizes light)

• Phases or Interferometric Sensing - sensing as a function of phase change

• Wavelength sensing – sensing as a function of change in wavelength

• Length and refractive index sensor - sensing as a function of change in fiber length and change in refractive index, respectively

• Change in light guiding sensor

Smart Skins and Structures

• Fiber sensors are embedded in composites and other materials such as concrete to create smart structures or smart skins

• The goal is to create a structural element (including the skins of aircraft) equipped to monitor internal conditions

• One use of embedded fiber sensors is to monitor fabrication and curing of the composite by monitoring strain to verify that the component is not stressed excessively

• Currently, its main use is in studying properties of structures and aircraft

Smart Skins and Structures