Optical Fiber

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- Introduction about Optical Fibers.
Main Characteristics of Fiber Optics Communication System. Light propagation in an Optical Fiber. Mode Analysis for Single Mode Fiber. Mode Analysis for Multimode Fibers. Surface Plasmon Resonance. Optical Fiber Surface Plasmon Resonance Sensors.

Fibre Optic?
Dielectric waveguide of cylindrical geometry with core and cladding of suitable material. refractive index of core > refractive index of cladding

Main Motivation
To meet demand of increase in the telecommunication data transmission. cladding

Physical Principle
Total internal reflection (critical angle, using Snell’s law).

input light

output light

Main Advantages
      

Higher bandwidth (extremely high data transfer rate). Less signal degradation. Less costly per meter. Lighter and thinner then copper wire. Lower transmitter launching power. Less susceptible to electromagnetic interference. Flexible use in mechanical and medical imaging systems.





Main Applications
    

Telecommunications. Sensors. Fiber Lasers. Bio-medical. Automotive and many other industories.


Fibre Optics Material Choice?


H.H.Hopkins and N.S.Kapnay in 1950’s used cladding fiber: Good image properties demonstrated for 75 cm long fiber [Nature 173, 39 (1954)].

(Nobel Prize 2009)


Application found use in medicine as gastroscopes, endoscopes etc. Advent of Laser in 1960’s , but didn’t work for optical communication due to attenuation problem!. In 1964 critical theoretical suggestion by, Charles K. Kao and Charles Hockam : For long range communication system the loss limit was set to 20 dB/Km (was ~ 1000 db/Km or higher at that time!). Pure form of Silica, by reducing impurities i.e., the optical losses were not due to glass itself, but impurities in it.






Limit met by doping titanium in fused core and pure fused Silica in cladding [Appl. Phys. Lett. 17, 423 (1970)]. Today the lower limit is below 0.2 dB/KM. Plastic and Plastic–clad Silica , as well few other optical fibers materials (useful for some applications), has been invented.


Optical loss in glass as function of time.
(Source: Nagel S.(1989). Optical Fiber: The expanding medium. IEEE Circuits Devices Magaz. March, 36.)

Silica and Plastic as Fibre Optic Materials
Silica Fibers
Both core and cladding are of glass. Very pure SiO2 or fused quartz. Germanium or Phosphorus to increase the index of refraction. Boron or Flourine to decrease the index of refraction. Silica fibers mainly used due to their low intrinsic absorption at wavelengths of operation. Any other remaining impurities cause attenuation and scattering.

Plastic Fibers
Plastic core and plastic cladding. Polymethyl Methacrylate (most commonly used). Flexible and Light. Widely used in short distance applications.

Attenuation Spectrum of Silica Fibers. (Source: Miya,T.,Y.Tenuama, Thosaka, and T Miyashita , “ An ultimate low loss single mode fiber at 1.55 mm,” Electron. Letts, Vol 15, 106, 1979)

Plastic-clad Fibers
- Glass as core and plastic as cladding.

Which is better? (Plastic or Silica)
Plastic less expensive, flexible, lighter. Plastic is larger in diameter, so easy to connect across joints. Plastic is less efficient then Silica. Plastic has more attenuation, and less bandwidth making it more suitable for shorter distances.

Attenuation Spectrum of Plastic Fibers.
(Source: http://www.av.it.pt/conftele2009/Papers/31.pdf)

Main Characteristics of Optical Transmission Medium
- The ray entering the acceptance angle will be guided along the core.

- Acceptance angle is measure of the light-gathering power of the fiber. - Higher Numerical Aperature (NA) mean higher coupling from source to fiber, and less losses across joints.

Limit the optical power...
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