Fiber optics

History of Fiber Optics

John Tyndall demonstration in 1870

Total Internal reflection is the basic idea of fiber optic

 History of Fiber optics
• During 1930, other ideas were developed with this fiber optic such as
transmitting images through a fiber.
• During the 1960s, Lasers were introduced as efficient light sources
• In 1970s All glass fibers experienced excessive optical loss, the loss of the
light signal as it traveled the fiber limiting transmission distance.

• This motivated the scientists to develop glass fibers that include a

separating glass coating. The innermost region was used to transmit the
light, while the glass coating prevented the light from leaking out of the core
by reflecting the light within the boundaries of the core.

• Today, you can find fiber optics used in variety of applications such as
medical environment to the broadcasting industry. It is used to transmit
voice, television, images and data signals through small flexible threads of
glass or plastic.
Increased bandwidth: The high signal bandwidth of optical fibers
provides significantly greater information carrying capacity. Typical
bandwidths for multimode (MM) fibers are between 200 and 600MHz-km
and >10GHz-km for single mode (SM) fibers. Typical values for electrical
conductors are 10 to 25MHz-km.

􀂃􀂃 Electromagnetic/Radio Frequency Interference Immunity: Optical

fibers are immune to electromagnetic interference and emit no radiation.

􀂃􀂃 Decreased cost, size and weight: Compared to copper conductors

of equivalent signal carrying capacity, fiber optic cables are easier to
install, require less duct space, weigh 10 to 15 times less and cost less
than copper.

􀂃􀂃 Lower loss: Optical fiber has lower attenuation (loss of signal

intensity) than copper conductors, allowing longer cable runs and fewer
repeaters.

􀂃􀂃 No sparks or shorts: Fiber optics do not emit sparks or cause short

circuits, which is important in explosive gas or flammable environments.
􀂃􀂃 Security: Since fiber optic systems do not emit RF signals,
they are difficult to tap into without being detected.

􀂃􀂃 Grounding: Fiber optic cables do not have any metal

conductors; consequently, they do not pose the shock hazards
inherent in copper cables.

􀂃􀂃 Electrical Isolation: Fiber optics allow transmission between

two points without regard to the electrical potential between them
Several applications of fiber optic

Configuration of a Fiber Optic Sensor System

Total internal Reflection
When light travels from a denser medium to a rarer
medium, it bends away from the normal. This
behavior follows from Snell's Law.
Since the angle in the second medium is greater than the
angle in the first medium, it can become as large as 90
degrees. When this occurs, the first angle is called
the critical angle and is represented as θc.
If the angle θ1 in the first medium is larger than the critical angle,
no light makes it into the second medium. This condition is
called total internal reflection (TIR). Note that some reflection
occurs for all angles of incidence θ1, but once θ1 becomes larger
than θc, all of the light is reflected from the surface.
Core: This central section, made of silica or doped silica, is the light
transmitting region of the fiber.

􀂃􀂃 Cladding: This is the first layer around the core. It is also made
of silica, but not the same composition as the core. This creates an
optical waveguide which confines the light in the core by total
internal reflection at the core-cladding interface.

􀂃􀂃 Coating: The coating is the first non-optical layer around the

cladding. The coating typically consists of one or more layers of
polymer that protect the silica structure against physical or
environmental damage. The coating is stripped off when the fiber is
connectorized or fusion spliced.
 Source and transmitters

• A basic fiber optic communications system

consists of three basic elements:
– Fiber media
– Light sources
– Light detector
 Fiber media
Optical fibers are the actual media that guides the light

There are three types of fiber optic cable commonly used

Step-index Multimode fiber

Single Mode

Plastic optic fiber

How Does fiber optic transmit light
 The loss of fiber optic
• Material obsorption
• Material Scattering
• Waveguide scattering
• Fiber bending
• Fiber coupling loss
 A Light Sources

LED (Light emitting diode) ILD (injection laser diode)

 Detectors
•Detector is the receiving end of a fiber optic link.
There are two kinds of Detectors
1. PIN (Positive Intrinsic Negative)
2. APD (Avalanche photo diodes)

PIN
APD
 Idea of Modulation
• When sending information by an optical
fiber, the information must be encoded or
transformed somehow into information that
capable of being transmitted through a
fiber. The signal needs to be modulated.
There are two types of modulation Analog
and digital.
 The advantages of fiber optic over
wire cable
• Thinner
• Higher carrying capacity
• Less signal degradation
• Light signal
• Low power
• Flexible
• Non-flammable
• Lightweight
 Optical Fiber
• Definition: An optical fiber is a cylindrical
wave guide made of transparent dielectric,
(glass or clear plastic), which guides light
waves along its length by total internal
reflection.
• It is very thin like human hair, approximately
70µm or 0.003 inch diameter.
• The thin strand of a metal is called a wire and
a thin strand of dielectric materials is called a
Fiber.
Fiber Core- Schematic
• Structure: A practical optical fiber is cylindrical
in shape.
• Core:
 The innermost cylindrical region is the light guiding
region known as the core.
 In general the diameter of the core is of the order of 8.5
µm to 62.5 µm
• Cladding:
• The core is surrounded by a coaxial middle region known
as the cladding.
• The diameter of the cladding is of the order of 125 µm.
• The refractive index of cladding ( n2) is always lower than
that of core (n1)
• Light launched into the core and striking the
core-to-cladding interface at angle greater
than critical angle will be reflected back into
the core.
• When the angles of incidence and reflection
are equal, the light will continue to rebound
and propagate through the fiber.
• Buffer:
 The outermost region is called the sheath or a
protective buffer coating.
 It is a plastic coating given to the cladding for extra
protection.
 This coating is applied during the manufacturing
process to provide physical and environmental
protection for the fiber.
 The buffer is elastic in nature and prevents
abrasions.
 The coating vary in size from 250 µm or 900 µm.
 Necessity for Cladding
• The actual fiber is very thin and light entering a bare
fiber will travel along the fibre through repeated
total internal reflections at the glass-air boundary.
• However , bare fibers are used only in certain
applications.
• For use in communications and some other
applications, the optical fibre is provided with a
cladding.
• The cladding maintains uniform size of the fibre,
protects the walls of the fibre from chipping, and
reduces the size of the cone of light that will be
trapped in the fibre.
The cladding performs the following important
functions:
• Keeps the size of the fibre constant and
reduces loss of light from the core into the
surrounding air.
• Protects the fibre from physical damage and
absorbing surface contaminants
• Prevents leakage of light energy from the fibre
through evanescent waves.
• Prevents leakage of light energy from the core
through frustrated total internal reflection.
• Reduces the core of acceptance and increases
the rate of transmission of data.
• A solid cladding, instead of air, also makes it
easier to add other protective layers over the
fibre.
 Total Internal Reflection
• A medium having a lower refractive index is
called rare medium while a medium having
higher refractive index is known as denser
medium.
•
• when a ray of light passes from denser
medium to rare medium, it is bent away from
the normal in the rare medium.
• Snell’s law is
• where θ1 is the angle of incidence of light ray
in the denser medium
• θ2 is the angle of refraction in the rare
medium .
1. If θ1 ∠ θc , the ray refracts into the rare medium
2. If θ1 = θc , the ray just grazes the interface of
rarer-to-denser media.
3. If θ1 > θc , the ray is refracted back into the
denser medium
• The phenomenon in which light is totally
reflected from a denser –to-rare medium
boundary is known as total internal reflection.
• The rays that experience total internal reflection
obey the laws of reflection.
• Therefore, the critical angle can be determined
from Snell’s law.
When
Therefore, from equation, we get

when the rare medium is air , μ2 =1 and writing μ1

= μ
we get
Classification of optical fiber
 Classification based on refractive index profile
• Step index fibres: The refractive index of the core is
constant along the radial direction and sudden falls to
a lower value at the cladding and core boundary
• Graded index(GRIN) fibres: The refractive index of the
core is not constant but varies smoothly over the
diameter of the core
• It has a maximum value at the centre and decreases
gradually towards the outer edge of the core.
• At the core-cladding interface the refractive index of
the core matches with the refractive index of the
cladding
• The refractive index of the cladding is constant.
•
 Classification based on the modes of light propagation

• (a) Single mode fibre (SMF)

• (b) Multimode fibre (MMF)
• Single mode fibre (SMF) has a smaller core diameter
and can support only one mode of propagation.
• Multimode fibre (MMF): A multimode fibre has a
larger core diameter and supports a number of
modes.
• There is one more mode which is also multimode is
Graded index(GRIN) fibre.
 Classification based on materials
• This classification deals with the materials
used for core and cladding.
• The optical fibres, under this consideration are
classified in to three categories.
1. Glass/glass fibres (glass core glass cladding)
2. Plastic/plastic fibres (plastic core with plastic
cladding)
3. PCS fibres (polymer clad silica)
 The three types of fibers
1)Single mode step index fiber
• structure
• Structure: A single mode step index fibre has
a very fine thin core of diameter of 8μm to
12μm
• It is usually made of germanium doped silicon.
The core is surrounded by a thick cladding of
lower refractive index.
• The cladding is composed of silica lightly
doped with phosphorous oxide.
• The external diameter of the cladding is of the
order of 125μm.
• The fibre is surrounded by an opaque
protective sheath.
• The refractive index of the fibre changes
suddenly at the core-cladding boundary.
• The variation of the refractive index of a step
index fibre as a function of radial distance be
mathematically represented as
• Propagation of light in SMF
• Light travels in SMF along a single path that is along
the axis as shown in figure
• It is the zero order mode that is supported by a SMF
• Both Δ and N A are very small for single mode fibres.
• This small value is obtained by reducing the fibre
radius and by making Δ to be small.
• The low N A means low acceptance angle.
• Therefore, light coupling into the fibre becomes
difficult.
• Costly laser diodes are needed to launch light into
SMDF.
2) Multi mode step index fiber
• structure
• Structure:
• The figure shows (a) its R.I. profile, (b) Ray paths
(c) typical dimensions.
• A multimode step index fibre is very much similar
to the single mode step index fibre except that its
core is of larger diameter.
• The core diameter is of the order of 50 to
100μm, which is very large compared to the
wavelength of light.
• The external diameter of cladding is about 150 to
250μm.
• Propagation of light in MMF:
• Multimode step index fibre allows finite number of
guided modes.
• The direction of polarization, alignment of electric
and magnetic fields will be different in rays of
different modes.
• Many zigzag paths of propagation are permitted in a
MMF.
• The path length along the axis of the fibre is shorter
while the other zigzag paths are longer.
• The lower order modes reach the end of the fibre
earlier while the high order modes reach after some
time delay
3) Graded index (grin) fiber
• structure
• Propagation of light in SMF
 Materials:
• Optical fibres are fabricated from glass or plastic which
are transparent to optical frequencies. Step index
fibres are produced in three forms:
1. A glass core cladded with a glass having a slightly
lower refractive index,
2. A silica glass core cladded with plastics and
3. A plastic core cladded with another plastic.
Generally the refractive index step is the smallest for all
glass fibres, a little larger for the plastic clad Silica PCS
(fibres) and the largest for all plastic construction.
• All Glass Fibres:
• The basic material of optical fibres is silica (SiO2). It
has a refractive index of 1.458 at λ=850nm.
• The materials of different refractive index are
obtained by doping silica material with various
oxides.
• If the silica is doped with Germania (GeO2) or
phosphorous pentoxide (P2O5), the refractive index
of the material increases.
• Such materials are used as core materials and pure
silica is used as cladding material in these cases.
• When pure silica is doped with boria (B2O3) or
fluorine, its refractive index decreases.
• These materials are used for cladding when
pure silica is used as core material.
• The examples for fibre compositions are
1. SiO2 core – B2O3.SiO2 cladding
2. GeO2.SiO2 core – SiO2 cladding
• All Plastic Fibres:
• In these fibres, Perspex (PMMA) and polysterene
are used for core. Their refractive indices are 1.49
and 1.59 respectively.
• A fluorocarbon polymer or a silicone resin is used
as a cladding material. A high refractive index
difference is achieved between the core and the
cladding materials.
• Therefore, plastic fibres have large NA of the
order of 0.6 and large acceptance angles up to
77o.
• The main advantages of the plastic fibres are low
cost and higher mechanical flexibility.
• The mechanical flexibility allows the plastic fibres to
have large cores, of diameters ranging from 110 to
1400μm.
• They are temperature sensitive and exhibit very high
loss.
• Therefore, they are used in low cost applications and at
ordinary temperatures (below 80oC).
• Examples of plastic fibres compositions are
1. Polysterene core n1=1.60 NA=0.60
-Methyl methacrylate cladding n2=1.49
2.Polymethyl methacrylate core n1=1.49 NA=0.50
-cladding made of its copolymer
• PCS Fibres:
• The plastic clad silica (PCS) fibres are composed of
silica cores surrounded by a low refractive index
transparent polymer as cladding.
• The core is made from high purity quartz.
• The cladding is made of a silicone resin having a
refractive index of 1.405 or perfluoronated ethylene
propylene (Teflon) having a refractive index of 1.338.
• Plastic claddings are used for step-index fibres only.
• The PCS fibres are less expensive but have high losses.
Therefore, they are mainly used in short distance
applications.
 Acceptance Angle:
• Considering a step index optical fibre into which light is
launched at one end. Let the refractive index of the core be n1
and the refractive index of the cladding be n2 (n2 < n1).
• Let n0 be the refractive index of the medium from which light
is launched into the fibre.
• The light ray enters the fibre an angle θi
• The ray refracts at an angle θr and strikes the
core-cladding interface at an angle ф.
• If ф is greater than critical angle фc, the ray
undergoes total internal reflection, since n1 > n2.
• When the angle ф is greater than ф c, the light will
stay within the fibre.
• Applying Snell’s law to the launching face of the
fibre, we get

• ---------------(1)
•
• If θi is increased beyond a limit, ф will drop below
the critical value фc and the ray escapes from the
sidewalls of the fibre.
• The largest value of θi occurs when ф = фc .
• In Δle ABC
• ----------(2)
• Using eq(2) in (1), we get
• -----------------(3)
•
• The angle θ0 is called the acceptance angle of the fibre.
• Acceptance angle is the maximum angle that a light ray can
have relative to the axis of the fibre and propagate down
the fibre.
• When angles less that θ0 will undergo repeated total
internal reflections and reach the other end of the fibre.
• Hence, larger acceptance angles make it easier to launch
light into fibre.
• In three dimensions, the light rays contained within the
cone having a full angle 2θ0 are accepted and transmitted
along the fibre
• Therefore, the cone is called the acceptance cone. Light
incident at an angle beyond θ0 refracts through the cladding
and corresponding optical energy is lost.
 Fractional Refractive Index Change:
• The fractional difference Δ between the refractive
indices of the core and the cladding is known as
the fractional refractive index change.
• It is given by
• The value of Δ is always positive because n1 must
be greater than n2 for the total internal reflection
condition.
• In order to guide light rays effectively through a
fibre, Δ<<1 and Δ is of the order of 0.01
 Numerical aperture
• The numerical aperture NA is defined as the sine of the
acceptance angle.
• Numerical aperture determines the light gathering ability
of the fibre. It is a measure amount of light that can be
accepted by a fibre
 Numerical Aperture NA

Maximum acceptance angle

αmax is that which just gives
total internal reflection at the
core-cladding interface, i.e.
when α = αmax then θ = θc.
Rays with α > αmax (e.g. ray B)
become refracted and penetrate
the cladding and are eventually
lost.

( − n22 )
1/ 2
2πa
NA = (n12 − n )
2
n NA
2 1/ 2 sin α max = 1
= V= NA
2 no no λ
2αmax = total acceptance angle
NA is an important factor in light launching designs into the optical
fiber.