special topics

1. Fiber Optic Sensor Special Topics in Materials Science and Engineering

2. 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

3. 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

4. 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>𝑛2 𝑛1sinθ1=𝑛2sinθ2 θ2=90°𝐶 sinθ𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙=𝑛2𝑛1

5. 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

6. How it works?

7. 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.

8. 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. Fiberoptic 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

10. used to transmit >2µm wavelengthNote: 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)

11. Silica Fiber Manufacture

12. 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 GeO2if 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

13. Inside Vapor Deposition

14. 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

15. 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

16. Outside Vapor Deposition

17. 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

18. Vapor Axial deposition

19. 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

20. Drawing Fibers

21. 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

22. Fiber Optic Sensors

23. 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

24. Simple Probe

25. 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

26. Optical Remote Sensing

27. 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

28. Polarization Sensing

29. 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

30. 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

31. Smart Skins and Structures