23-02-2013, 03:52 PM
OPTICAL FIBER COMMUNICATION
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FibreOptics Material Choice?
-H.H.Hopkinsand N.S.Kapnayin 1950’s used cladding fiber:
-Good image properties demonstrated for 75 cm long fiber
[Nature 173,39 (1954)].
-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
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.
Single Mode Fibers
-With the primary degrees of freedom of core cladding
diameter and the difference of refractive indices between
them they can be optimized for attenuation and dispersion.
-Light propagation can be studied using geometrical optics.
Fabrication and Characterization of FibreOptics Based SPR
-Silver or Gold coated core of a polymer or glass clad fibre.
-Metal deposition ( e.g., using vapor deposition techniques).
-Use of suitable lithographic techniques, to fabricate periodic
optical fibrestructures such as Long-period FibreGratings (LPFG)
or Long period Waveguide Gratings (LPWG).
Glass or Polymer Optical Fibres
-Polymer of more current research interest, due to:
. Flexibility.
. Easy handling.
. High resistance to fracture.
. Perfect biocompatibility.