Introduction to Fiber
Optics
Optical fiber vs Copper
What are optical fibers
Thin strands of pure glass
Carry data over long distances
At very high speeds
Fiber can be bent or twisted
Background on Optical
Communications
Fiber-optic communication
Trapping light inside an optical fiber
Can carry any form of information
Fiber is an optical medium, which means it is capable
of transmitting light
Based on total internal reflection (TIR)
Information Technology in Theory 4
Tyndall’s Experiment
Information Technology in Theory 5
Fiber optic technology
Sources
Transmission medium
Detectors
Transceiver Fiber Optic Cable
Transceiver
Electrical
Connector Electrical
Optical Optical Optical Connector
Optical
Port Connector Connector Port
Sources of light
Light emitting diodes
Lasers
Sources
Modulate electrical signals into
optical signals
Mostly modulate at 850nm, 1300nm
and 1550 nm
Lasers give high intensity, high
frequency light
LEDs are economical
Transmission medium
Optical fiber is replacing copper
Light is used as the carrier of
information
Much higher data rate
The optical fiber
Structure of Fiber-Optic Cables –
Cladding
Cylindrical material made of glass or specialized
plastic
Central portion of the fiber
Light signal carrying the information travels
through the core
The diameter of the core can range from a couple
of micrometers (µm-one millionth of a meter) to a
couple of millimeters (mm-one thousandth of a
meter)
Information Technology in Theory 10
Structure of Fiber-Optic Cables –
Jacket
Surrounds the cladding
Insulates and protects the fiber from physical damage
and environmental effects, such as moisture, that
might interfere with the inner workings of the cable
Usually made of opaque plastic or another type of
material
Information Technology in Theory 11
How Light Travels Through Fiber
TIR is the basis of fiber-optic communication
TIR may be considered to be an extreme case of
refraction
When a light ray strikes a boundary of two materials
with different RIs, it bends, or in other terms, refracts
to an extent that depends on the ratio of the RIs of the
two materials
Information Technology in Theory 12
Total Internal Reflection
Information Technology in Theory 13
Total Internal Reflection
(continued)
Information Technology in Theory 14
Total internal reflection
Trapping light in the fiber
Fibers can be bent!!
Fig: Illustration of total internal reflection
Types of optical fibers
Single mode
only one signal can be transmitted
use of single frequency
Multi mode
Several signals can be transmitted
Several frequencies used to modulate
the signal
Losses in optical fibers
Attenuation loss
Dispersion loss
Waveguide loss
Advantages of optical fibers
Can carry much more information
Much higher data rates
Much longer distances than co-axial
cables
Immune to electromagnetic noise
Light in weight
Unaffected by atmospheric agents
Reflection in Optical Fiber
The critical angle is the angle of incidence that will produce a
900 angle of refraction.
3 specific conditions are shown here. The angle of incidence, A1
and the angle of refraction, A2.
Material 1 is more dense than material 2, so n1 is greater than n2.
n=1
n=1.5
Total Internal Reflection in Fiber
• Straight hallway • Bent hallway
• Bent hallway with a mirror
Acceptance Angle
Snell's law states that the ratio
of the sines of the angles of
incidence and refraction is
1
equivalent to the ratio of
phase velocities in the two n1 v1
media, or equivalent to the
opposite ratio of the indices n2 v2
of refraction:
1
Acceptance Angle
An Optical Fiber will only propagate light that enters the fiber within a
certain cone, known as the acceptance cone of the fiber. The half angle
of this cone is called Acceptance angle a
a n1 c
p Core
core
in
n2 Cladding
n1 c
p Core
core
a
in
n2 Cladding
To propagate the light beam down the optical fiber, the light beam at
the core and cladding interface must taken an angle less than the
critical c, From Snell’s law,
2
n sin a n1 sin c n2
sin a n1 1 sin c2
sin a n1 1 2
n1
sin a n1 sin( 90 c ) from core to cladding
sin a n1 cos c n1 sin c n2 sin 90 sin a n1
2
n2
2
a – Acceptance angle
Numerical Aperture
NA sin a
Numerical Aperture
NA n1 n2
2 2
NA describes the ability of an optical fiber to gather light signals
from the sources and to preserve them within the fiber
Relative index ,
n1 n2 n1 n2 n1 n2 n1 n2
2 2
n nn1 n2 nn1 n2
Where ‘n’ average index
1 n2
2 2
n
1
2 2 2
2
n1 n2 2 n
2n1
n1 n2 n1 2
NA n1 2
2 2
Types of Fibers
Classification based on Materials
1. Glass fiber: Made by fusing mixtures of metal oxides and silica glasses.
Ex: GeO2-SiO2 core, SiO2 cladding
SiO2 core, P2O3-SiO2 cladding
2. Plastic fiber: Made up of plastic polymers and is of low cost and flexible. Can be
handled without any special care due to its toughness and durability.
Ex: Polysterene core, Methyl methacrylate cladding
Polymethyl methacrylate core and co-polymer cladding
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Classification based on number of modes
Number of modes
Optical fiber is a dielectric waveguide.
Energy in the fiber is propagated by electric and magnetic field
vectors of electromagnetic wave; which can be analysed by
Maxwell’s field equations.
Maxwell’s equations have discrete sets of solutions called the modes.
Number of modes propagating in an optical fiber can be determined
by a factor known as “horizontal wave number” (V).
2 a
V NA a radius of the core
2
Maximum number of modes supported by a step index fiber is:
1 2
Nm V
2
Maximum number of modes supported by a graded index fiber is:
1 2
Nm V
4
For V < 2.405: Only one mode is supported (single mode fiber)
For V > 2.405: Can support more than one mode (Multimode fiber)
The wavelength corresponding to V = 2.405 is known as the cut-off
wavelength of the fiber.
V
c
2.405 3
Single mode fiber
These fibers have very narrow core (~ 10 m in diameter).
Hence allow only one mode (TE, TM or TEM) to pass through it.
NA and acceptance angles are small for these fibers which allows
only the transmission of fundamental modes.
Amount of dispersion is very less.
Used for very high speed, large bandwidth and long distance
transmission.
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Multi mode fiber
These fibers have relatively wide core (~ 50 m in diameter).
Allow different modes to pass through it together.
There are two rays travelling in the core: axial ray (along the axis)
and marginal ray (near the fiber surface).
Marginal ray travels longer distance than the axial ray.
This time delay causes distortion in the pulse leading to dispersion.
Results in broadening of light pulses reducing the transmission speed
and transmission bandwidth.
Best designed for short transmission
distances and is suited for use in
LAN systems and video surveillance
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Classification based on refractive index profile
Step-index fiber
Refractive index of the core is uniform throughout and undergoes an
abrupt change (step) at the cladding boundary.
Multimode step-index fiber
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Graded-index fiber
Core refractive index is made to vary as a function of the radial
distance from the center of the fiber. Also known as inhomogeneous
core fibers.
1/ 2
r
n(r ) n1 1 2 For r < a, Core
a
n(r ) n1 (1 2)1/ 2 n2 For r >= a, Cladding
Relative refractive index difference
Profile parameter (Gives the characteristic refractive index of core)
= 1 (triangular profile), 2 (parabolic), (step-index)
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Graded index profiles, giving best result for multimode optical
propagation have nearly parabolic refractive index profile.
In this case, the pulse dispersion is less than that in step-index fiber.
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Fiber optic communication system suffers from the following three major impediments
Dispersion
Attenuation
Nonlinear effects
Dispersion
Spreading of light pulse as it travels down the length of an optical fiber
Dispersion causes temporal pulse spreading
Pulse overlap results in indistinguishable data
Inter symbol interference (ISI)
Dispersion is related to the velocity of the pulse
• There are two major types of dispersion in fiber-optics
– Intermodal
– Intramodal
• Intermodal
– When an optical pulse is launched into the fiber, the optical pulse is distributed over all
modes of fiber
– Here we consider the propagation of light within the fiber in terms of guided
electromagnetic waves called “modes”.
– Different modes will travel with different propagation angles, hence these modes takes
different routes but travel with the same velocity, but at the end of fiber they come at
different timings.
– This causes pulse widening
– This is called intermodal dispersion or modal dispersion.
• Measuring intermodal Dispersion
• To ascertain this let us go for some mathematical calculations
• A zero order mode travelling along the fiber axis needs some time to reach the receiver it is
given by
t0 = L / v
L – length of the link
v = c / n1 – velocity of light within the core
• The highest order mode propagating at critical angle needs time of
tc = L / (v cos ac)
Therefore, pulse widening due to intermodal dispersion is
DtSI = tc – t0
= [L / (v cos ac)] – [L / v]
= L / v [ (1/cos ac) – 1]
= L / v [ (n1 / n2) – 1] [since cos ac = n2 / n1
= L / v [ (n1 - n2) / n 2]
= Ln1 / c [ (n1 - n2) / n 2] since v = c / n1
Since n2 ≈ n we can write the equation
DtSI = Ln1 / c [ (n1 - n2) / n]
DtSI = [Ln1 / c] (D)
Where D is the relative refractive index
• Solution to this intermodal dispersion was done with graded index fiber.
– A graded index fiber has the center of the core having highest refractive index and
gradually decreasing towards the end of the core.
• It is estimated that the modal dispersion of graded index fiber is D/8
times less than in the case of step index fiber
Transmission Characteristics
The transmission through an optical fiber is limited by attenuation (or loss)
and dispersion.
In 1970s, it was realized that the attenuation was largely due to absorption
in the glass caused by impurities such as iron, copper, manganese etc.
Hence, research was stimulated towards a new generation of “pure” glasses
for use in optical fiber communication. It lead to silica based glass fibers
with losses less than 0.2 dB/km.
The other characteristic is bandwidth which is mostly limited by signal
dispersion within the fiber. It determines the number of bits of transmission
transmitted in a given time period.
1
attenuation
Attenuation determines the maximum transmission distance prior to signal
restoration. OFC became especially attractive when the transmission losses
of fibers were reduced below those of the competing metallic conductors.
(< 5 db/km)
Signal attenuation in optical fibers (or that of metallic cable) is usually
expressed in the units of decibel. Decibel is used for comparing two power
levels.
Pi For a particular optical wavelength,
dB 10 log10
Po Pi input (transmitted) optical power
Po output (received) optical power
In OFC, attenuation is usually expressed in dB per unit length (dB/km)
Pi dB signal attenuation/length
dB L 10 log10
Po
L Fiber length 2
Material absorption losses in silica glass fibers
This loss mechanism is related to material composition and the
fabrication process for the fiber. Absorption of light may be :
Intrinsic: caused by the interaction with one or more of the
major components of the glass
Extrinsic: caused by impurities within the glass
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4
5
6
7
8
9
10
(Mie Scattering)
11
Bending loss
12
Attenuation spectra for
fused silica based glass
13
dispersion
Dispersion of the transmitted optical signal causes distortion for analog as
Hence, the number of optical signal pulses which may be transmitted in a
well as digital transmission along optical fibers.
given period and therefore the information carrying capacity of the fiber, is
Dispersion mechanisms cause broadening of the transmitted light pulses as
restricted by the amount of pulse dispersion per unit length. The pulse
they travel along the channel.
broadening increases linearly with fiber length and thus the bandwidth is
inversely proportional to distance.
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Intramodal (Chromatic) dispersion
Results from the finite spectral linewidth of the optical source.
Optical light sources do not emit just a single frequency but a band of
frequencies. Hence, there may be propagation delay differences between the
different spectral components of the transmitted signal. This causes
broadening of each transmitted mode and hence intramodal dispersion.
15
The delay differences may be caused by:
Dispersive properties of the waveguide material (material dispersion)
Guidance factors within the fiber structure (waveguide dispersion)
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Material Dispersion
Results when different spectral components of a pulse travel at different
group velocities.
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A material is said to exhibit material dispersion when the 2nd order
refractive index of core with respect to wavelength is not equal to zero.
d 2n
d 2n 0
Material dispersion D() is given by: D ( ) d 2
c d 2
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19
Waveguide Dispersion
Results from the variation in group velocity with wavelength for a
particular mode.
Angle between the ray and the fiber axis varies with wavelength which
subsequently leads to a variation in the transmission times for the rays,
hence dispersion.
More prominent in case of single mode fibers than in the multimode fibers.
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Intermodal dispersion
Sometimes referred to as Modal (or mode) dispersion.
When numerous waveguide modes are propagating, they all travel with
different group velocities.
Parts of the wave arrive at the output before other parts, spreading out the
waveform. Hence, it is also known as multimode dispersion.
It is independent of the source linewidth.
It does not occur in a single mode fiber.
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22
Multimode step index fiber
Paths taken by the axial and an extreme meridional ray in a perfect multimode
step index fiber is shown here.
23
TMin Minimum delay time (time taken for the axial ray to travel along a fiber
of length L)
TMax Maximum delay time (time taken for the meridional ray to travel along a
fiber of length L)
Delay
difference for
<< 1
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• Optical Sources
o Optical source is often considered to be the active component in an optical
fiber communication system
o Fundamental function is to convert electrical energy into optical energy
(light)
• Three main types of optical sources
o Wide band continuous spectra source (incandescent lamp)
o Monochromatic incoherent sources (Light Emitting Diodes LED)
o Monochromatic coherent sources (Laser)
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Characteristics of optical sources for OFC
Light output should be highly directional.
Most accurately track the electrical input signal to minimize distortion and
noise. Ideally, the source should be linear.
Should emit light at wavelengths where the fiber has low losses and low
dispersion and where the detectors are efficient.
Should have a very narrow spectral linewidth in order to minimize
dispersion in the fiber.
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