FIBER FABRICATION
Two basic techniques in fabrication of all glass optical waveguides.
- Vapor-phase oxidation process
- Direct melt methods
Direct Melt Methods
-Optical fibers are made directly from the molten state of purified components of
silica glasses.
Vapor –phase Oxidation Process
-Highly pure vapors of metal halides react with O2 to form white powder of SiO2
particles.
-The particles are then collected on the surface of a bulk glass and are sintered
to form a glass rod.
-This rod or tube is called a preform.
-Typically 10-25mm dia and 60-120cm long.
-Fibers are made from preform using the following equipment
Outside Vapor –phase oxidation
-Loss less than 20dB/km
-A layer of SiO2 particles called SOOT is deposited from a burner onto a rotating
graphite.
-The glass SOOT adheres to this bait rod and layer and layer, a cylindrical porous
glass preform is build up.
-By controlling the constituents of the metal halide vapor stream during the
deposition process, the glass dimensions desired for the core and cladding can be
incorporated into the preform.
-When the deposition process is completed the mandral is removed and porous
tube is then virtified in a dry atmosphere at high temperature to a clear glass
preform.
- This preform is mounted in a fiber drawing tower and made into a fiber.
Vapor-phase Axial Deposition
- SiO2 particles are formed in the same way.
-These particles emerge from the torches they are deposited onto the end surface
of a silica glass rod which acts as a seed.
-A porous preform is grown in the axial direction by moving the rod upward.
-The rod is also continuously rotated to maintain cylindrical symmetry of the
particle deposition.
-As the porous preform moves upward, it is transformed into a solid, transparent
rod preform by melting with the carbon ring heater.
Modified Chemical Vapor Deposition
-Pioneered at Bell laboratories
-Low loss graded index fibers
-The glass vapor particles , arising from the reaction of metal halide gases and
oxygen flow through inside of silica tube.
-As the SiO2 are deposited, they are sintered to clear glass layer which travel
back and forth along the tube.
-When the desired thickness of glass rod have been deposited, the vapor flow is
shut off and tube is heated strongly to cause it to collapse into solid rod preform.
-The fiber is drawn from this preform rod , core consists of vapor deposited
material and cladding consists of original silica tube.
Plasma Activated Chemical Vapor Deposition:
-Philips research invented
-Similar to MCVP
- A non isothermal microwave plasma operating at low pressure initiates the
chemical reaction.
-With the silica tube , a moving microwave resonator operating at 2.45GHz
generates a plasma inside the tube to activate the chemical reaction.
-This process deposits clear glass material directly on the tube wall.
-No SOOT formation , no sintering is required.
-When desired glass thickness is deposited , the tube is collapsed into a preform.
Double Crucible Method:
-Silica, Chalgenide and halide glass fibers can be made.
-Glass rods for the core and cladding materials are first made separately by
melting mixtures of purified powders.
- These rods are used as feedstock for each of two concentric crucibles.
-The inner crucible contains the molten core glass and outer contains cladding
glass.
-The fibers are drawn from the molten state through orifices in the bottom of the
two concentric crucibles in a continuous production process.
-Attention must be paid to avoid contaminants arise from furnace environment and
from the crucible.
MECHANICAL PROPERTIES OF OPTICAL FIBER
Fiber strength and durability
-Optical fibers exclusively fabricated from silica or compound of glass.
-Materials are brittle and exhibit perfect elasticity until their breaking point is
reached.
- Bulk material strength of flawless glass is quite high and estimated for individual
materials using the relationship
St= (‫ע‬pE/4la)1/2
St= theoritical cohesive strength
‫ע‬p= Surface energy of the material
E= Youngs modulus of the material
la=atomic space/bond distance
-Bulk material strength reduced by the presence of surface flaws within the
material.
-In order to treat surface flaws in glass, the griffith theory is normally used.
-This theory assumes that surface flaws are narrow cracks with small radii of
curvature at their tips.
-It postulates that the stress is concentrated at the tip of the crack which leads to
crack growth.
-Stress intensity factor
Ki=SYC1/2__________1
S= macroscopic stress on the fiber
Y= constant
C= depth of the crack
-Critical stress intensity factor where fracture occurs
Kic=(2E‫ע‬p)1/2______________2
-Combining 1 and 2
S= [2E‫ע‬p/Y2C]1/2
-A primary protective coating is usually applied , mechanically induced flaws can be
minimized.
- Flaws also occur due to chemical and structural changes.
-Another effect is stress corrosion, occurs bec of molecular bonds are attacked by
water