Double Crucible Method
A method of fabricating an Optical Wave guide by melting the core and clad glasses in two suitably joined concentric crucibles and then drawing a fiber from the combined melted glass.
1. Highly purified glass powders of various refractive indices are fed into the inner crucible for the core in the outer crucible for cladding.
2. The electric furnace is switched on and the materials are heated to very high temperature.
3. The material goes to molten state and the material starts squeezing through the orifice of the crucible.
4. Now the core material will start diffusing into cladding material to form an optical fiber.
5. The fiber is drawn through the bottom surface of the crucible and dopant such as thallium with high rate of diffusion in silica is used to maintain the difference in refractive index.
OPTICAL FIBER AS AN OPTICAL WAVEGUIDE
Optical fibers are used as dielectric waveguides for electromagnetic signals of optical frequencies. Figure shows the block diagram of transmission of sound along the optical fiber and conversion again to sound at the other end.
i. Sound is first converted into electrical signal by a microphone.
ii. The electrical signals modulate the intensity of light from laser.
iii. Then the information is carried along the fiber in a digital form.
Boosters or repeaters are placed at a distance of about 50km of cable to make up the signal loses occurring due to scattering and absorption.
iv. At the receiving place, a photodiode converts the digital light pulses into corresponding electrical signals.
v. The electrical signals are then converted into sound by an earphone ( receiver) Time division multiplexing system is used to transmit many thousands of telephone cells through a single optical fiber with the use of digital pulses.