OFC PPTs fiber materials, fabrication and signal destortion

1. Fiber Materials
In selecting materials for optical fibers, a number of
requirements must be satisfied. For example:
1) It must be possible to make long, thin, flexible
fibers from the material.
2) The material must be transparent at a particular
optical wavelength in order for the fiber to guide
light efficiently.
3) Physically compatible materials that have slightly
different refractive indices for the core and
cladding must be available.
Materials that satisfy these requirements are glasses
and plastics.

2. • The majority of fibers are made of glass consisting of
either silica (SiO2) or a silicate.
• The variety of available glass fibers ranges from
moderate-loss fibers with large cores used for short-
transmission distances to very transparent (low-loss)
fibers employed in long-haul applications.
• Plastic fibers are less widely used because of their
substantially higher attenuation than glass fibers.
• The main use of plastic fibers is in short-distance
applications (hundreds of meters) and in abusive
environments, where the greater mechanical
strength of plastic fibers offers an advantage over
the use of glass fibers.

3. Glass Fibers
• Glass is made by fusing mixtures of metal oxides,
sulfides, or selenides.
• The resulting material is a randomly connected
molecular network rather than a well-defined
ordered structure as found in crystalline materials.
• A consequence of this random order is that glasses
do not have well-defined melting points.
• When glass is heated up from room temperature, it
remains a hard solid ( several hundred degrees).
• As the temperature increases, the glass gradually
begins to soften and becomes a viscous liquid.

4. • The largest category of optically transparent glasses
from which optical fibers are made consists of the
oxide glasses.
• Of these, the most common is silica (SiO2), which has
a refractive index ranging from 1.458 at 850nm.
• To produce two similar materials that have slightly
different indices of refraction for the core and
cladding, either fluorine or various oxides (dopants)
are added.
• For example: B2O3 (Diboron trioxide),
GeO2(germanium dioxide), or P2O5 diphosphorous
pentoxide) , are added to the silica.
Glass Fibers contd...

5. • As shown in figure the
addition of GeO2 or P2O5
increases the refractive
index.
• Whereas doping the silica
with fluorine or B2O3
decreases the refractive
index.
• The cladding must have a
lower index than the core.

6. • Since the cladding must have a lower index than
the core. The examples of fiber compositions are

7. • The principal raw material for silica is sand and glass.
The fiber composed of pure silica is called as silica
glass.
• The desirable properties of silica glass are :-
- Resistance to deformation even at high
temperature.
- Resistance to breakage from thermal shocks (low
thermal expansion).
- Good chemical durability.
- Better transparency.

8. Plastic optical fibers
• The growing demand for delivering high-speed
services has led fiber developers to create high-
bandwidth graded-index polymer (plastic) optical
fibers (POF) for use in a customer premises.
• The core of these fibers is either
polymethylmethacrylate or a perfluorinated polymer.
• These fibers are hence referred to as PMMA POF and
PF POF, respectively.
• Although they exhibit considerably greater optical
signal attenuations than glass fibers, they are tough
and durable.

9. Photonic Crystal Fibers
• In the early 1990s researchers demonstrated a new optical
fiber structure.
• Initially this was called a holey fiber and later it was known
as a photonic crystal fiber (PCF) or a microstructured
fiber.
• The difference between this new structure and that of a
conventional fiber is that the cladding and, in some cases,
the core regions of a PCF contain air holes, which run
along the entire length of the fiber.
• Whereas the material properties of the core and cladding
define the light transmission characteristics of
conventional fibers, the structural arrangement in a PCF
creates an internal microstructure, which offers extra
dimensions in controlling the optical properties of light.

10. Photonic Crystal Fibers contd...
• The sizes of the holes and the hole-to-hole spacing
(known as the pitch) in the microstructure and the
refractive index of its constituent material determine
the light-guiding characteristics of photonic crystal
fibers.
• The two basic PCF categories are index-guiding fibers
and photonic bandgap fibers.
• The light transmission mechanism in an index-
guiding fiber is similar to that in a conventional fiber
as it has a high-index core surrounded by a lower-
index cladding.

11. Index-Guiding PCF
• Figure shows the two-
dimensional cross-sectional end
view of basic structure of an
index guiding PCF.
• The fibers have a solid core
surrounded by a cladding, which
contains air holes that run along
the length of the fiber.
• The core and the cladding in a
PCF are made of the same
material (for example, pure
silica), the air holes lower the
effective index of refraction in
the cladding region, since n=1
for air and 1.45 for silica.

12. • The fact that the core can be made of pure
silica gives the PCF a number of operational
advantages over conventional fibers, which
typically have a germanium-doped silica core.
• These include very low losses, the ability to
transmit high optical power levels, and a high
resistance to darkening effects from nuclear
radiation.
• The fibers can support single-mode operation
over wavelengths ranging from 300 nm to
more than 2000 nm.

13. Photonic Bandgap (PBG) Fiber
• Figure shows cross sectional
structure of PBG fiber.
• In contrast to the index guiding PCF,
here the fiber has a hollow core that
is surrounded by a cladding which
contains air holes running along the
length of the fiber.
• The function is similar to the role of
a periodic crystal lattice, which
blocks the electrons from occupying
a bandgap region.
• In a traditional PBG fiber the hollow
core acts as a defect in the photonic
bandgap structure, which creates a
region in which the light can
propagate.

14. Fiber Fabrication
• Two basic techniques are used in the fabrication of all-glass
optical waveguides.
• These are the vapor phase oxidation process and the direct-
melt methods.
• The direct-melt method follows traditional glassmaking
procedures in that optical fibers are made directly from the
molten state of purified components of silicate glasses.
• In the vapor-phase oxidation process, highly pure vapors of
metal halides (e.g., SiCl4 and GeCl4) react with oxygen to
form a white powder of SiO2 particles.
• The particles are then collected on the surface of a bulk glass
by one of four different commonly used processes.

15. Fiber drawing process

16. • The particles are then collected on the surface of a bulk
glass by one of four different commonly used processes
and are sintered (transformed to a homogeneous glass
mass by heating without melting) to form a clear glass
rod or tube.
• This rod or tube is called a preform.
• It is typically around 10–25 mm in diameter and 60–120
cm long.
• Fibers are made from the preform by using the
equipment shown in Figure.
• The preform is precision-fed into a circular heater called
the drawing furnace. Here the preform end is softened
to the point where it can be drawn into a very thin
filament, which becomes the optical fiber.

17. • The turning speed of the takeup drum at the bottom
of the draw tower determines how fast the fiber is
drawn.
• This, in turn, will determine the thickness of the
fiber, so that a precise rotation rate must be
maintained.
• An optical fiber thickness monitor is used in a
feedback loop for this speed regulation.
• To protect the bare glass fiber from external
contaminants, such as dust and water vapor, an
elastic coating is applied to the fiber immediately
after it is drawn.

18. Outside Vapor-Phase Oxidation

19. • The first fiber to have a loss of less than 20 dB/km was
made at the Corning Glass Works by the outside vapor-
phase oxidation (OVPO) process.
• First, a layer of SiO2 particles called a soot is deposited
from a burner onto a rotating graphite or ceramic
mandrel.
• The glass soot adheres to this bait rod and, layer by
layer, a cylindrical, porous glass preform is built up.
• By properly controlling the constituents of the metal
halide vapor stream during the deposition process, the
glass compositions and dimensions desired for the core
and cladding can be incorporated into the preform.
• Either step- or graded-index preforms can thus be
made.

20. • When the deposition process is completed, the
mandrel is removed and the porous tube is then
vitrified in a dry atmosphere at a high temperature
(above 1400°) to a clear glass preform.
• This clear preform is subsequently mounted in a
fiber-drawing tower and made into a fiber.
• The central hole in the tube preform collapses
during this drawing process.

21. Vapor-Phase Axial Deposition
• The OVPO process described earlier is a lateral
deposition method.
• Another OVPO-type process is the vapor-phase axial
deposition (VAD) method as illustrated in figure.
• In this method, the SiO2 particles are formed in the
same way as described in the OVPO process.
• As 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.

22. Vapor-Phase Axial Deposition

23. • As the porous preform moves upward, it is transformed
into a solid, transparent rod preform by zone melting
(heating in a narrow localized zone) with the carbon ring
heater shown in Fig.
• The resultant preform can then be drawn into a fiber by
heating it in another furnace called fiber-drawing tower.
• Both step- and graded-index fibers in either multimode
or single-mode varieties can be made by the VAD
method.

24. • The advantages of the VAD method are
(1) the preform has no central hole as occurs with the
OVPO process
(2) the preform can be fabricated in continuous lengths,
which can affect process costs and product yields
(3) the fact that the deposition chamber and the zone-
melting ring heater are tightly connected to each
other in the same enclosure allows the achievement
of a clean environment.

25. Modified Chemical Vapor Deposition (MCVD)

26. • The modified chemical vapor deposition (MCVD)
process shown was pioneered at Bell Laboratories
and widely adopted elsewhere to produce very low-
loss graded-index fibers.
• The glass vapor particles, arising from the reaction of
the constituent metal halide gases and oxygen, flow
through the inside of a revolving silica tube.
• As the SiO2 particles are deposited, they are sintered
to a clear glass layer by an oxyhydrogen torch, which
travels back and forth along the tube.

27. • When the desired thickness of glass has been
deposited, the vapor flow is shut off and the tube is
heated strongly to cause it to collapse into a solid rod
preform.
• The fiber that is subsequently drawn from this
preform rod will have a core that consists of the
vapor-deposited material and a cladding that
consists of the original silica tube.

28. Plasma-Activated Chemical Vapor
Deposition(PCVD)

29. • Scientists at Philips Research invented the
plasma-activated chemical vapor deposition
(PCVD) process.
• As shown in Fig., the PCVD method is similar to
the MCVD process in that deposition occurs
within a silica tube.
• However, a nonisothermal microwave plasma
operating at low pressure initiates the chemical
reaction.
• With the silica tube held at temperatures in the
range of 1000–1200°C to reduce mechanical
stresses in the growing glass films.
• A moving microwave resonator operating at 2.45
GHz generates a plasma inside the tube to
activate the chemical reaction.

30. • This process deposits clear glass material directly on
the tube wall;
• There is no soot formation. Thus, no sintering is
required.
• When one has deposited the desired glass thickness,
the tube is collapsed into a preform just as in the
MCVD case.

31. Signal Degradation in Optical Fibers
• Signal attenuation = fiber loss = signal loss)
• It determines the maximum unamplified or
repeaterless separation between a transmitter
and a receiver.
• It has a large influence on system cost.
• Signal distortion: Optical signal pulses get
broaden as they travel along a fiber.
• If these pulses travel sufficiently far, they will
eventually overlap with neighboring pulses,
thereby creating errors in the receiver output.
• This limit the information-carrying capacity of a
fiber.

32. Attenuation
• It is an important consideration in the design of an
OFC system.
• It determines the maximum transmission distance
between a transmitter and a receiver.
• Attenuation Mechanisms:
• Absorption  fiber material
• Scattering fiber material + with structural
imperfections
• Radiative losses  perturbations of the fiber
geometry

33. Attenuation Units
• As light travels along a fiber, its power decreases
exponentially with distance.
• If P(0) is the optical power in a fiber at the origin
(z=0), then the power P(z) at a distance z farther
down the fiber is
• Where
is called attenuation coefficient expressed in /km.

34. • A low loss fiber has average loss of 3 dB/km at 900 nm.
Compute the length over which –
a) Power decreases by 50 %
b) Power decreases by 75 %.
Soln. α = 3dB/km
a) Power decreases by 50 %
3= 10.(1/z).log [P(0)/P(z)]
z=1km.
a) Power decreases by 75%
3= 10.(1/z).log [P(0)/P(z)]
z=2km.

36. • When mean optical power launched into an 8 km
length of fiber is 120 μW, the mean optical power at
the fiber output is 3 μW.
Determine –
1) Overall signal attenuation in dB.
2) The overall signal attenuation for a 10 km optical
link using the same fiber with splices at 1 km
intervals, each giving an attenuation of 1 dB.

38. • Optical powers are commonly expressed in
units of dBm, which is the decibel power level
referred to 1 mW

39. Absorption
Absorption is caused by three different mechanisms:
1. Absorption by atomic defects in the glass composition.
2. Extrinsic absorption by impurity atoms in the glass material.
3. Intrinsic absorption by the basic constituent atoms of the
fiber material

40. • These are imperfections in the atomic structure of
the fiber material.
• Ex: missing molecules, high-density clusters of atom
groups, or oxygen defects in the glass structure.
• Usually, absorption losses arising from these defects
are negligible compared with intrinsic and impurity
absorption effects.
• However, they can be significant if the fiber is
exposed to ionizing radiation, like in a nuclear reactor
environment, in medical radiation therapies.
Absorption by Atomic defects

42. Extrinsic Absorption
• Extrinsic absorption occurs due to electronic transitions
between the energy level and because of charge transitions
from one ion to another.
• A major source of attenuation is from transition of metal
impurity ions such as iron, chromium, cobalt and copper.
• These losses can be up to 1 to 10 dB/km. The effect of
metallic impurities can be reduced by glass refining
techniques.
• Another major extrinsic loss is caused by absorption due to
OH (Hydroxil) ions impurities dissolved in glass.
• Vibrations occur at wavelengths between 2.7 and 4.2 μm. The
absorption peaks occurs at 1400, 950 and 750 nm. These are
first, second and third overtones respectively.

43. • Figure shows absorption spectrum for OH group in
silica. Between these absorption peaks there are
regions of low attenuation.

44. Intrinsic Absorption
• Intrinsic absorption occurs when material is in absolutely
pure state, no density variation and inhomogenities.
• Thus intrinsic absorption sets the fundamental lower
limit on absorption for any particular material
• Intrinsic absorption results from electronic absorption
bands in UV region and from atomic vibration bands in
the near infrared region.
• The electronic absorption bands are associated with the
band gaps of amorphous glass materials.
• Absorption occurs when a photon interacts with an
electron in the valence band and excites it to a higher
energy level.
• UV absorption decays exponentially with increasing
wavelength (λ).

45. • The inherent IR absorption is due to interaction between the
vibrating band and the electromagnetic field of optical signal
this results in transfer of energy from field to the band,
thereby giving rise to absorption.
• Attenuation spectra for the intrinsic loss mechanism in pure
Ge is shown in Fig

46. Scattering Losses
• Scattering losses in glass arise from microscopic
variations in the material density, from
compositional fluctuations, and from structural
inhomogeneities or defects occurring during fiber
manufacture.
• As described earlier, glass is composed of a randomly
connected network of molecules.
• Such a structure naturally contains regions in which
the molecular density is either higher or lower than
the average density in the glass.

47. • In addition, since glass is made up of several oxides,
such as SiO2, GeO2, and P2O5, compositional
fluctuations can occur.
• These effects give rise to refractive-index variations
that occur within the glass over distances that are
small compared with the wavelength.
• These index variations cause a Rayleigh-type
scattering of the light.
• Rayleigh scattering in glass is the same phenomenon
that scatters light from the sun in the atmosphere,
thereby giving rise to a blue sky.

49. • Scattering loss for single component glass is given by

50. Bending Losses
• Radiative losses occur whenever an optical fiber
undergoes a bend of finite radius of curvature.
• Fibers can be subjected to two types of curvatures:
(a) macroscopic bends having radii that are large
compared with the fiber diameter, such as those that
occur when a fiber cable turns a corner.
(b) random microscopic bends of the fiber axis that
can arise when the fibers are incorporated into
cables.

51. • For slight bends the loss is extremely small and is
essentially unobservable.
• As the radius of curvature decreases, the loss
increases exponentially until at a certain critical
radius the curvature loss becomes observable.
• If the bend radius is made a bit smaller once this
threshold point has been reached, the losses
suddenly become extremely large.

52. Microbending
• Microbending is a loss due to small bending or distortions.
• This small microbending is not visible. The losses due to this are
temperature related, tensile related or crush related.
• The effects of microbending on multimode fiber can result in
increasing attenuation (depending on wavelength) to a series of
periodic peaks and troughs on the spectral attenuation curve.
• These effects can be minimized during installation and testing.
Fig. illustrates microbending.

53. Macrobending
• The change in spectral attenuation caused by macrobending is
different to microbending.
• Usually there are no peaks and troughs because in a
macrobending no light is coupled back into the core from the
cladding as can happen in the case of microbends.
• The macrobending losses are cause by large scale bending of
fiber. The losses are eliminated when the bends are
straightened.
• The losses can be minimized by not exceeding the long term
bend radii..

54. Core and Cladding Losses
• Since the core and cladding have different indices of
refraction hence they have different attenuation
coefficients α1 and α2 respectively.
• For step index fiber, the loss for a mode order (v, m)
is given by
• For lower order modes the expression reduces to

55. • For graded index fiber
• The loss for a given mode is expressed by
• where p(r) is the power density of that mode at r

56. Signal Dispersion in Fibers
• An optical signal weakens from attenuation
mechanisms and broadens due to dispersion effects
as it travels along a fiber.
• Eventually these two factors will cause neighboring
pulses to overlap.
• After a certain amount of overlap occurs, the
receiver can no longer distinguish the individual
adjacent pulses and errors arise when interpreting
the received signal.

58. • Material dispersion: arises due to the variations of
the refractive index of the core material as a function
of wavelength. Material dispersion also is referred to
as chromatic dispersion, since this is the same effect
by which a prism spreads out a spectrum.
• Waveguide dispersion: causes pulse spreading
because only part of the optical power propagation
along a fiber is confined to the core.

59. Factors Contributing to Dispersion
• Group Delay
• Material Dispersion
• Waveguide Dispersion
• Dispersion in Single-Mode Fibers
• Polarization-Mode Dispersion

60. Characteristics of Single-Mode Fibers
• Refractive-Index Profi les
• Cutoff Wavelength
• Dispersion Calculations
• Mode-Field Diameter
• Bending Loss