The document discusses various techniques for fabricating optical fibers, including both liquid phase and vapor phase deposition methods. Vapor phase deposition techniques such as modified chemical vapor deposition and plasma-activated chemical vapor deposition allow for producing silica-rich glasses with very high transparency and precise control over the refractive index profile. These methods can yield fibers with very low attenuation losses under 1 dB/km and pulse dispersion below 1 ns/km, making them suitable for high-quality, large-scale fiber production.

1. 1
Fabrication of Optical Fibers
MEC

2. 2
Contents
• Materials and Practical Considerations.
• Fabrication Techniques
- Liquid Phase (Melting) Techniques.
- Vapor-Phase Deposition Techniques.
- Outside Vapor-Phase Oxidation.
- Vapor Axial Deposition.
- Modified Chemical Vapor Deposition.
- Plasma Activated Chemical Vapor Deposition.
• Fiber Drawing.

3. 3
Practical Considerations
• Good stable transmission characteristics -
long lengths, minimum cost, maximum
reproducibility.
• Availability with regard to size, refractive
indices and index profiles, operating
wavelengths, materials etc.
• Conversion into practical cables without
degradation and damage.
• Easy splicing, termination - transmission
characteristics within acceptable operating
levels.

4. 4
Waveguiding Considerations
• Variation in refractive index from core to
cladding.
• Materials with low optical attenuation, low
intrinsic absorption and scattering losses.
• Eradication of scattering centers such as
bubbles, strains and grain boundaries to
avoid scattering losses.
• Choice of fiber materials limited to glasses
(or glass-like materials), monocrystalline
structures (certain plastics).

5. 5
Considerations for Graded Index
Fibers
• Refractive index varied by suitable doping.
• Materials to have mutual solubility over a
wide range of concentrations.
• Glasses or glass like materials for low loss
fibers, monocrystalline materials not
suitable.
• Plastic-clad / plastic fibers for short-haul,
low bandwidth applications.

6. 6
Preparation of Glass Fibers
• Two stage process - glass produced and
converted into rod or preform, drawing or
pulling.
• Conventional glass refining - glass
processed in molten state (melting)
produces multi-component glass structure.
• Vapor-phase deposition produces silica-
rich glasses with melting temperatures too
high to allow conventional melt process.

7. 7
Liquid Phase (Melting) Techniques
• Preparation of ultra-pure material powders
- oxides / carbonates.
• Oxides - SiO2, GeO2, B2O2 and A2O3,
Carbonates - Na2CO3, K2CO3, CaCO3 and
BaCO3 decompose into oxides during the
glass melting.
• High initial purity essential – purification
increases cost.
• Purification - fine filtration, co-precipitation,
solvent extraction before recrystallization
and final drying in vacuum to remove
residual OH- ions.

8. 8
Liquid Phase (Melting) Techniques
• Melting high-purity, powdered, low-
melting-point glass materials to form
homogeneous, no bubble multi-component
glass.
• Refractive index variation by either change
in the composition of constituents or by ion
exchange when materials in molten phase.
• Temperatures between 900 and 1300 °C
in a silica crucible.

9. 9
Glass Making Furnace

10. 10
Glass Making Furnace
• Fused silica and platinum
crucibles used.
• Silica crucibles give
dissolution into the melt,
introduce inhomogeneities
into the glass at high
melting temperatures.
• Recommended - melting
the glass directly into a
radio-frequency (~ 5 MHz)
induction furnace.

11. 11
Radio Frequency Induction
Furnace
Melt the glass directly, Cool the
silica by gas or water flow.
Inhomogeneities reduced.

12. 12
Radio Frequency Induction
Furnace
• Melt the glass, Cool silica by
gas/water flow.
• Materials preheated to around
1000°C, exhibit sufficient ionic
conductivity to enable coupling
between the melt and the RF field.
• A thin layer of solidified pure glass
forms due to the temperature
difference b/w melt and cooled silica
crucible, protects melt from any
impurities in the crucible.

13. 13
Glass Melting Techniques
• Glass homogenized and dried by bubbling
pure gases through the melt, protection
against airborne dust particles originating
in the melt furnace/atmospheric
contamination.
• After melt processing, cooling, forming into
long rods (cane) of multi-component glass.

14. 14
Fiber Drawing
• Make a preform using
the rod in tube process.
• A rod of core glass
inserted into a tube of
cladding glass and the
preform drawn in a
vertical muffle furnace.
• Useful for production of
step index fibers with
large core & cladding
diameters.
• Minute perturbations
and impurities cause
very high losses.

15. 15
Double Crucible Method for Fiber
Drawing

16. 16
Double Crucible Method
• Core and Cladding glass as
separate rods fed into two
concentric platinum crucibles.
• Assembly located in a muffle
furnace capable of heating the
crucible contents to between
800 and 1200°C.
• Crucibles have nozzles in their
bases, clad fiber is drawn
directly from the melt.

17. 17
Double Crucible Method
• Index grading achieved
through diffusion of mobile ions
across the core–cladding
interface within the molten
glass.
• Reasonable refractive index
profile.
• Useful for graded index fibers,
but no precise control, not
possible to obtain optimum
near parabolic profile.

18. 18
Double Crucible Method
• Graded index fibers less dispersive than step
index fibers, do not have the bandwidth–
length products of optimum profile fibers.
• Pulse dispersion of 1 to 6 ns/km.
• Using very high-purity melting techniques
and double-crucible drawing method, step
index and graded index fibers with
attenuations as low as 3.4 dB/km & 1.1 dB/
km respectively produced.
• Such low losses not consistently obtained
using liquid-phase techniques.

19. 19
Double Crucible Method
• Typical losses for multi-component glass
fibers prepared continuously in the range 5
to 20 dB/ km at wavelength of 0.85 μm.
• Used for the production of fibers with a
large core diameter of 200 μm and above.
• Potential for continuous production.

20. 20
Vapor-Phase Deposition
Techniques
• To produce silica-rich glasses of highest
transparency & optimal optical properties.
• Starting materials - volatile compounds eg;
SiCl4, GeCl4, SiF4, BCl3, O2, BBr3 and POCl3 –
distilled to reduce concentration of most
transition metal impurities to below one part in
109 - negligible absorption losses.
• Refractive index modified through formation of
vapor-phase dopants - TiO2, GeO2, P2O5, Al2O3,
B2O3 and F.

21. 21
Variation of Refractive Index with
Dopant Concentration

22. 22
Vapor-Phase Deposition
• Gas mixtures of silica-containing compound,
doping material & oxygen combined in a vapor-
phase oxidation reaction - deposition of oxides
onto a substrate/within a hollow tube, built up as
a stack of successive layers.
• Dopant concentration varied gradually - graded
index profile.
• Hollow tube collapsed to give a solid preform -
fiber drawn.
• Flame Hydrolysis and Chemical Vapor
Deposition.

23. 23
Vapor-Phase Deposition

24. 24
Outside Vapor-Phase Oxidation
• Uses flame hydrolysis - fiber losses < 20 dB/km.
• ‘Soot’ processes developed by Hyde - silica
generated as a fine soot.
• Required glass composition deposited laterally
from a ‘soot’ generated by hydrolyzing halide
vapors in oxygen – hydrogen flame.
• Oxygen passed through silicon compound (i.e.
SiCl4) which is vaporized, removing any
impurities.
• Dopants (GeCl4 or TiCl4) added, mixture blown
through oxygen–hydrogen flame.

25. 25
Outside Vapor-Phase Oxidation

26. 26
Outside Vapor-Phase Oxidation
• Silica generated as a fine
soot, deposited on a cool
rotating mandrel.
• Burner flame reversed back
and forth over the length of
the mandrel, sufficient no. of
layers of silica deposited.
• Mandrel removed, porous
mass of silica soot sintered to
form a glass body.
• Preform may contain both
core and cladding glasses.
• Fiber drawing, collapse and
close the central hole.
soot deposition

27. 27
Outside Vapor-Phase Oxidation
• Can produce several kilometers of fiber.
• Fine control of index gradient - gas flows can be
adjusted at the completion of each traverse of
the burner.
• Fiber bandwidth–length products as high as 3
GHz-km.
• Purity of glass fiber depends on the purity of
feeding materials, OH- content due to exposure
of silica to water vapor in the flame - 50 to 200
parts per million contributes to attenuation.
• OH- impurity content reduced - use gaseous
chlorine as a drying agent during sintering.

28. 28
Outside Vapor-Phase Oxidation
• Batch process, limit on the use for volume
production.
• Removal of mandrel - cracks due to stress
concentration on the surface of the inside
wall.
• Refractive index profile has a central
depression due to collapsed hole when
fiber drawn.

29. 29
Vapor Axial Deposition
• Process developed by Izawa et al.
• Continuous technique for the production of
low-loss optical fibers.
• Flame hydrolysis - end-on deposition onto
a rotating fused silica target.
• Deposited on the end of the starting target
in the axial direction.
• Spatial refractive index profile may be
achieved.

30. 30
Vapor Axial Deposition
• Vaporized constituents
injected from burners, react
to form silica soot by flame
hydrolysis.
• Deposited on the end of the
starting target in axial
direction, forms a solid
porous glass preform.
• Preform growth in axial
direction, pulled upwards at
the growth rate.

31. 31
Vapor Axial Deposition
• Initially dehydrated by heating
with SOCl2, then sintered into
solid preform in a graphite
resistance furnace at 1500 °C.
• Can be adapted for continuous
process- resultant preforms can
yield more than 100 km of fiber.

32. 32
Modified Chemical Vapor
Deposition
• Inside vapor-phase
oxidation (IVPO)
technique.
• Vapor-phase reactants
(halide and oxygen) pass
through a hot zone,
homogeneous.
• Glass particles formed
during this reaction travel
with the gas flow and are
deposited on the walls of
the silica tube.
• Tube may form the
cladding material.

33. 33
Modified Chemical Vapor
Deposition
• Tube may form cladding, is merely a supporting
structure which is heated on the outside by an
oxygen–hydrogen flame to between 1400 and
1600 °C.
• A hot zone is created which encourages high-
temperature oxidation reactions, reduce the OH-
impurity concentration.
• Hot zone moved back and forth along the tube
allows particles to be deposited layer-by-layer,
gives a sintered transparent silica film on the
walls.

34. 34
Modified Chemical Vapor
Deposition
• Film up to 10 μm thick, uniformity maintained by
rotating the tube.
• Graded refractive index profile created by
changing the composition of layers as glass is
deposited.
• When sufficient thickness formed by successive
traverses of the burner for cladding, vaporized
chlorides of germanium (GeCl4) or phosphorus
(POCl3) added to the gas flow.
• Core glass formed by deposition of successive
layers of germanosilicate or phosphosilicate
glass.

35. 35
Modified Chemical Vapor
Deposition
• Cladding acts as a barrier, suppresses OH- diffusion
into the core glass.
• After deposition temperature is increased to
between 1700 and 1900 °C, tube then collapsed to
give a solid preform, then drawn into fiber at 2000 to
2200 °C.
• Reduced OH- impurity contamination, deposition
occurs within an enclosed reactor - very clean
environment.
• Gaseous and particulate impurities may be avoided
during both layer deposition and preform collapse
phases.
• Suitable for mass production.

36. 36
Plasma-activated Chemical Vapor
Deposition
• Developed by Kuppers and Koenings, involves
plasma-induced chemical vapor deposition
inside a silica tube.
• Oxide formation by a nonisothermal plasma at
low pressure in a microwave cavity (2.45 GHz)
which surrounds the tube.
• Volatile reactants introduced into the tube react
heterogeneously within microwave cavity, no
particulate matter formed in the vapor phase.

37. 37
Plasma-activated Chemical Vapor
Deposition

38. 38
Plasma-activated Chemical Vapor
Deposition
• Reaction zone moved back
& forth along the tube by
control of microwave cavity,
circularly symmetric layer
growth formed.
• Tube rotation not
necessary, deposition
virtually 100% efficient.
• Film deposition can occur at
s 500 °C, high chlorine
content cause expansivity,
film cracking, hence tube
heated to around 1000 °C
using a stationary furnace.

39. 39
Plasma-activated Chemical Vapor
Deposition
• Thin layer deposition, formation of up to 2000
individual layers.
• Graded index profiles, optimum near-parabolic
profile.
• Low-pulse dispersion less than 0.8 ns/km,
attenuations between 3 and 4 dB/km, at a
wavelength of 0.85 μm.
• Large-scale production, preparation of over 200
km of fiber, high deposition efficiency.

40. 40
Summary of Vapor-Phase
Deposition Techniques

41. 41
Thank You