Module 5: Digital Techniques and
Electronic Instrument Systems
5.10 Fiber Optics
Fiber optics are used to create data links.
A fiber optic data link performs 3 functions:
Electric signal is converted to an optical signal.
The optimal signal is send over an optical fiber.
The optical signal is converted again to electrical.
The fiber optic data link is
consist of 3 parts:
Transmitter (e.g. LED)
Receiver (e.g. Photodiode)
Advantages and Disadvantages of Fiber
Ethernet cable: 1Gbps
Fiber optics: 250Gbps
Immunity to electrical noise
Reduced size and weight cables
Resistance to corrosion and temperature variations.
Expensive in comparison with conventional electrical
Expensive and difficult installation.
Frequency and Bandwidth
Bandwidth: Amount of information transmitted at one
time. Can be above Tbps.
Wavelength: around 700nm.
Fiber Optic Light Transmission
Light wave is a form of energy moving in wave
When studying the propagation of light (e.g. in a fiber
optic cable), we consider it as an electromagnetic wave.
When studying light interaction with semiconductors (e.g.
in the photoelectric effect), we consider it a sum of
particles (i.e. photons).
When light transmitted from a source encounters a
substance can be:
Fiber Optic Light Transmission
Almost all light
Some light rates
No light rates
Always, an amount of light rays is absorbed by
the material and the rest is reflected (which
defines its color and makes them visible) or / and
refracted, if it is not opaque.
Reflected rays: Rays that are not absorbed nor refracted.
(e.g. in a mirror).
The incident (falling on the relecting surface.
The reflected ray.
The angle of
incident and the
angle of reflection
are the equal. (Law
Refraction of light
When the light
a medium, the
direction of rays
changes. This is
Refraction of Light
Blue lines: the real light
rays path from water to
Red lines: the light path
as if there was no
In a non smooth material the reflected light is broken
into many light beams and scattered through many
The index of refraction
Index of refraction (or refractive
index) shows how light
propagates through a medium.
n: refractive index.
c: speed light in vacuum (3x108
v: speed of light in the material.
0 λ is the wavelength
of light in the
medium and λ0 in
v is the frequency of
light multiplied with λ. fv
As angle of incidence increases, angle of
refraction increases as well.
When angle of refraction exceeds 90
degrees, then no retraction happens. All
light is reflected. (Total internal reflection).
The angle of incidence at which the angle of
reflection is 90 degrees is called critical
angle of incidence (θc).
2211 sinsin nn
2. Structure of Fiber Optics and Light
Basic structure of the Fiber Optics
Determines the mechanical
robustness of the cable.
Usually plastic, PVC etc.
Buffer (or coating):
Protects the fiber optic from
physical damage. Fiber
Dielectric material, with index
of refraction less than the core
material. Made of glass or
Light propagation medium.
Dielectric material (usually
glass). Conducts no electricity.
How the light propagates in the optical fiber
2 ways to describe the propagation:
The ray theory: to get a clear picture of the propagation
of light inside the cable.
The mode theory: to understand the behavior of the light
inside the cable (comprehending of the properties of
absorption, attenuation and dispersion).
The Ray Theory
2 types of rays propagate along the optical fiber
Meridional rays: Rays that pass through the center axis
of the core.
Skew rays: Rays that travel through the fiber without
passing through the center axis of the core.
Can be classified as:
Bound: Rays that propagate through the fiber by total
Unbound: Rays that are refracted from the core.
prevent total internal
Part of the bound rays
are refracted inside the
eventually escape from
How light rays enter the cable?
making the rays
unbound, the rays
must enter the core
with incident angle
equal or greater
than the critical
The range of
angles which make
the rays bounded
is the cone of
All rays entering the
core with angle less
than the half-angle of
the core of
and finally lost.
The Numerical Aperture
Illustrates the relation between the cone of
acceptance and the indexes of refraction.
Shows the light-gathering ability of the optical fiber
High NA means that the fiber optic “admits” more light.
The acceptance angle of
skew rates is larger than
the acceptance angle of
Most rays entering the
core are skew rays.
The contribute to the
amount of light capacity
of the fiber optic
(especially in case of high
However, they contribute
also the amount of light
losses of the fiber optic.
A large portion of the number
of skew rays becomes leaky
Leaky rays, although they
are predicted to be totally
reflected, they are partially
refracted due the curved
shape of the fiber boundary.
The Mode Theory
Mode theory considers light to be electromagnetic
Describes light propagation in the optical fiber as a
propagation of electromagnetic waves.
A light wave can be
represented as a
Plane wave’s surfaces
are parallel plates,
normal to the direction
of the propagation.
3 characteristics of a
Planes having the same
phase are called
c: speed of light in the
f: frequency of light
n: index of refraction
Only wavefronts with angle of incident
equal or grater with the critical angle will
propagate inside the core.
All wavefronts propagating inside the
core must have phase equal to an
integer multiplied with 2π in order for
light to be transmitted.
If they total phase is not integer x
2π, then destructive interference
takes place. (one wavefront
eliminates the other).
This is the reason that a finite number of
modes can be propagate along the fiber.
A set of electromagnetic
field patterns , form a
Light inside the fiber optics
wave is composed by
an electric field and a
If the magnetic field is in
the direction of the
propagation, the mode is
If the electric field is the
direction of the
propagation, it is called
Different electromagnetic wave patterns can propagate
in a fiber optic. These patterns are called modes.
The number of maxima of each pattern is the order of
the mode (e.g. TE0 has one maximum, TE2 has 3,
Each mode can be realized with different wavelength.
Low order and High order modes
Single vs. Multi-mode Optical Fibers
Not every mode can propagate in any fiber
The number of modes than can propagate
ncore, ncladding, wavelength (λ) of the fiber optic
and core diameter (α).
Normalized frequency (V)
show the number of modes
that a fiber optic can
The larger the V, the more
modes can be supported.
Single vs. Multi-mode Optical Fibers
Each fiber optic has a
frequency, for a specific
Fiber optics with V < 2.405
can support only one
mode. They are called
single mode fibers.
Fiber optics with V > 2.405
can support more modes.
They are called multi-
Y axis shows the propagation
The higher the propagation
constant, the less the
dispersion of the wave during
The higher the mode order, the
higher the dispersion.
Single vs. Multi-mode Optical Fibers
Single mode fibers:
The core diameter is small: 8 – 10 μm.
Only the lowest mode can be propagated (around 1300nm
Normalized frequency less or equal to 2.405.
Lower signal loss in comparison with multi-mode and higher
bandwidth. Low fiber dispersion.
They use laser diodes to generate light.
Can propagate up to 4 modes (TE0, TE1, TE2, TE3. Also called OM1,
OM2, OM3, OM4).
Typical core sizes: 50 – 100 μm.
The light enters easily the fiber and the connections are easier.
They use LED to generate light (cheaper, less complex and last
However, they have higher dispersion than the single mode.
Specifically they exhibit mode dispersion: Modes arrive at the receiver at
More on Multi-mode Fibers
Multi-mode fibers are basically because they are cheaper and easier to install.
The limited bandwidth they offer in comparison with the single-mode fibers is sufficient
for more applications.
OM1 multi-fiber optics (TE0) are 62.5/125μm fibers and offer 200/500MHz x Km.
OM2 multi-fiber optics (TE0 & TE1) are 50/125μm fibers and offer 500MHz x Km.
OM3 multi-fiber optics (TE0 & TE1 & TE2) are 50/125μm and offer 2GHz x Km. Are designed
for 2Gbps transmission.
OM4 multi-fiber optics (TE0 & TE1 & TE2 & TE3) are designed for speeds from 40 to 100Gbps.
Increasing the mode used, we increase the bandwidth. However, intermodal
dispersion limits the bandwidth.
1 Gb Ethernet
OM1 (62.5/125) up to 2000 meters 275 meters] 33 meters Not supported Not supported
OM2 (50/125) up to 2000 meters 550 meters 82 meters] Not supported Not supported
OM3 (50/125) up to 2000 meters 550 meters 300 meters 100 meters[ 100 meters[
OM4 (50/125) up to 2000 meters 1000 meters 550 meters 150 meters[ 150 meters[
Performance of Fiber Optics
Performance is affected by:
Bandwidth in telecommunications is defined as the difference
between the highest and the lowest frequency of the signal.
Bandwidth in data transmission is defined as bits transmitted
Attenuation & Dispersion
Fiber optics properties that affect
Attenuation is a result of:
If the signal strength is reduced
below a specific point, the
receiver is unable to detect it.
Dispersion is the spreading of the signal. The spreading limits how fast data
can be transmitted along the fiber.
The receiver is unable to distinguish between input pulses caused by the
spreading of each pulse.
Attenuation is the loss of power as the light travels
inside the fiber. It is caused by:
L: Fiber’s length in km.
Pi: Signal’s power in the input.
Po: Signal’s power in the output.
attenuation units: dB/Km
Signal’s input power is 400W and
the power at the receiver is 100W.
The length of the fiber is 1Km.
attenuation = 10/1 * log(400/100) =
6 dB / Km
Explaining Decibel scale
dB is a logarithmic scale,
If P2 is twice the P1, the ratio is
10*log(2) = 3dB.
If P2 is half the P1, then the ratio is
10*log(1/2) = -3dB
If P1 is 1,000,000 the P1, then the
ratio is 10*log(106) = 60dB.
log(10n) = n*log(10) = n
log (1) = 0
Absorption is the conversion of optical power into
another energy form, such as heat.
Factors contributing to light absorption:
Imperfections in the structure of the fiber material.
The intrinsic material properties.
Photons are absorbed by electrons and excites them to higher
The extrinsic material properties.
Impurities in the atomic structure of the silicon (e.g. iron, nickel,
chromium) results in the absorption of photons by the electrons
of these metal and electrons excitation to higher energy levels.
Scattering is the result of density fluctuations in the
During the fabrication process of the fiber, there are areas
where the density of the molecular structure is higher or
Macrobends: Too sharp bend.
A part of the mode is converted to a higher order mode, which is
lost inside the cladding.
Microbends: Imperfections in the fiber or external force
which deforms the cable.
2 types of dispersion:
Intramodal dispersion: Occurs in all kinds of optical fibers,
where more than one wavelenghts are used (e.g in WDM).
Different wavelengths of signals of the same mode travel at
different speeds inside the fiber, so exit the fiber at different times.
Intermodal dispersion: Occurs only in multi-mode fibers.
Each mode travels at different speed inside the fiber, so, they do
not exit from the fiber at the same time.
Wave Division Multiplexing
Each signal is transmitted in the fiber using a different
e.g. the light of LED is split to separate channels, where each
channel has a different wavelength.
Can be used in single or multi-mode fibers.
Can increase the bandwidth of the fiber up to 40 times.
Step-index and Graded-index fiber cables
refractive index of the
core is uniform.
refractive index of the
core is varies gradually,
as a function of radial
distance from the fiber
Single mode fibers are
fibers performance is
superior to step-index
Why Graded-index fibers exhibit high
Light travels faster in materials with low
Therefore, rays that travel the longer
distance, propagate through lower
refractive indexes. So, they travel faster.
So, the modal dispersion of the fiber
Graded-index fibers specifications
The relative refractive index difference shows the
difference between the core and the cladding
nn Most common values of graded-index multimode
Δ = 0.02
They are the most common fiber optic cables
used in aircrafts.
A basic advantage is the low sensitivity to
microbend and macrobend. This allows relatively
sharp bending during the installation procedure.
Tight buffer cables are used for fiber
installed inside buildings, aircrafts, etc.
Loose-tube buffer cables are typically
1: Jacket (3mm)
2: Tight buffer (900μm)
3: Fiber optic
Normally used in patch boards
patch boards: panels in telecommunication
centers where temporary connections of circuits
Tight buffer (900μm)
Low cost cable, suitable for
Fibers are not individually
Several zip-core cables
Each fiber is individually
Large and more
However, the easy
eventually make then
Gel-filled loose-tube cable
Several fibers inside
each gel-filled loose
Gel is used to repel
Designed to endure high
Can be burried into the
Dry loose-tube cable
High durability and
easier installation than
Steel or aluminum
Extremely high durability
in harsh environments.
Submarine communication cables
In 2010 submarine
optical cables linked all
3: steel bars
Fiber Type Diameter (µm) Buffer Color
Multimode 50/125 Orange
Multimode 62.5/125 Slate
Multimode 85/125 Blue
Multimode 100/140 Green
Singlemode - Yellow
Fiber Optic Data Links
Fiber optic data links perform 3 basic functions:
Conversion of the input electrical to optical signal.
Transmission of the optical signal through the fiber.
Conversion of the optical signal to output electrical signal.
Fiber optic connections permit the transfer of optical
power from one component to another. A connection
a fiber optic splice (permanent join) or
a connector (allows reconfiguration) or
a coupler (combination of optical signals)
Poor connections degrade the system performance
Optical Fiber Coupling Losses
Coupling losses are due to:
Poor fiber end preparation
Mismatches between the fiber characteristics
When optical fibers are connected a part of the
source power will be reflected back, to the source
It is caused by a small air gap that may exists in the
interface, causing a discontinuity of the refracting index.
Index matching gel.
3 main coupling errors:
Angular misalignment (θ
must be less than 1
Matching gel reduces
coupling loss from fiber
However, it does not
misalignment losses and
Poor Fiber End Preparation
First we remove the
buffer (usually with
Then, using a diamond
blade and constant
tension between the two
ends is used to break
Finally the polishing
using and abrasive
paper and a microscope.
e.g. difference in core diameter.
different n1, n2.
Relative refracting index difference
Fiber Optic Splices
A device that connects a fiber optic with the other
2 kinds of splices:
Aligning takes place mechanically.
The end of the two fibers is melted by heat to fuse them
A tube, with a diameter
slightly larger than the fiber
The fiber optics are placed
inside the tube and they are
A groove where the two
fibers are placed.
A transparent adhesive is
placed after the alignment,
to make the alignment
permanent and to reduce
the coupling losses.
The fiber is inserted into
each half of the splice.
It’s epoxied with an
ultraviolet cure epoxy.
Using the alignment
sleeve the two halves are
Mechanical stability is
added due to the two
They are used extensively
in aircraft applications due
to their durability in
and easy installation.
The two ends of the fiber
optics are melted to form
Takes longer time than
training by the operators
and the final result
depends greatly on their
Fiber Connector characteristics
Connectors allow system
Every connector is defined
by its ferrule diameter.
Ferrules (made of ceramic)
hold the end of the fibers
and keeps them aligned.
There are different
connectors for single mode
and multi-mode fibers.
There are screw and push-
Most connector types are
made in two types:
Very fast installation in
comparison with splices.
Installation procedure is
almost the same, in all
Plug of the fiber optic in
the connector and cramp.
Polishing and cleaning of
the fiber end.
Fiber Connector evaluation
2 parameters define the quality of the connector:
Insertion loss: The power difference between the source
and the destination fibers (around 0.5dB).
Return loss: The amount of reflected power (around 40 –
A high quality connector has low insertion loss and
high absolute return loss value.
UPC vs. APC Connectors
In all fiber optic connections (using
splices or connectors) a part of the
incoming light ray is reflected back to
the source fiber.
It is preferable the reflected light to be
lost in the cladding, than be reflected
back in the core of the source fiber.
Angled polished connectors (APC)
are cut in 8 degrees angle to increase
the possibility of the reflected light to
be lost in the cladding.
Depending on the application
requirements, a proper selection
between UPC and APC can be made.
APC are usually single mode.
UPC vs. APC comparison
UPC has lower insertion loss, but APC has superior
return loss, due to its diameter.
Types of Connectors
Most common types:
SC connectors (square
connector or subscriber
connector or standard
A push-pull connector
2.5mm ferrule diameter
Extremely common in
datacom and telecom.
LC connectors (lucent
Half the size of SC.
1.25 mm ferrule diameter
Boeing 787 and Airbus 380 use a new ARINC
664 is a full duplex bus, using a twisted pair of optical
Transmitting and receiving for each LRU connected in the
network is independent.
It is considered rather a network, than a bus. LRUs are
connected to the bus through switches.
All connections are 10Mbps or 100Mbps.
The network is called AFDX (Avionics Full-Duplex
Fiber Optic Couplers
Some fiber optic links require
more than simple point-to-
Such connections are more
These configurations require
devices that can distribute an
optical signal from one fiber
among two or more fibers.
These devices are called
Example 1x2 couplers
distributes equally the input
signal among two outputs.
Each output signal has less
than half power of the input
signal. (losses around 3dB).
Types of couplers
Optical splitters: splits
the optical power of a
signal optical fiber in two
or more fibers.
combines the signal of
more than one optical
fibers to one fiber.
6. Fiber Optic Measurement Techniques
Cut-off Wavelength (in single mode only).
Pi and Po measurement
in a known length L.
Cut-back to 2m and
with the same Pi.
measurement in a
short length fiber is
necessary to ensure
Cut-off Wavelength Measurement
The wavelength a which a mode seizes to propagate.
It depends on the fiber length and the bending conditions.
Attenuation is measured with
specific bending radius for
Cut-off wavelength is the
longest λ for which attenuation
is equal to 0.1dB.
The fiber bandwidth is
defined as the lowest
frequency at which the
magnitude of the fiber
frequency response has
decreased to one-half
its zero frequency value.
Intramodal (Chromatic) Dispersion
Differential group delay τ(λ) is the difference
between the arrival time of different wavelengths.
We use a reference fiber and the fiber under testing.
The pulse delay for the reference fiber of length Lref and
the delay is tin(λ). The pulse delay for the test fiber of Ls is
The differential group delay for a specific λ per unit of
Fiber Geometry Measurement
Specific fiber geometry characteristics, such as the
cladding diameter are obtained by video camera
images, analyzed by a computer.
Core diameter is measured by the range of the
intensity of light at the end of the fiber.
Numerical Aperture Measurement
It is measured with a similar way with the core
Insertion and Return Loss Measurement
Insertion loss is similar to attenuation, but is used to
indicate the attenuation in an optical fiber connector.
P0 is power measured without the
P1 is the power measured with the
Return loss is measured using a
2x2 (or 2x1) coupler. The
returning power is measured in
one of input ports of the coupler.
Pi is power in the source fiber.
Pr is the power reflected.
return loss may be defined as
10logPr/Pi. Then, it will be negative.
Optical Time-Domain Reflectometer
Electronic instrument used to characterize a fiber optic.
Injects a series of optical pulses into the fiber under test.
It estimates attenuation, reflected losses, scattered light etc.
It is used for field measurements (i.e. installed fiber optics, not in laboratory).
Fiber Optic Transmitter
Converts electrical signals to optical signals and
lunches the optical signal into the fiber.
The part of the transmitter that converts electrical energy
(current) to optical energy (light) is the optical source.
Optical sources must:
Launch sufficient light power to the fiber to overcome
attenuation and coupling losses.
Emit light at proper wavelengths.
Optical sources are LEDs.
Emit infrared light normally at 850, 1300 or 1550nm.
Today, common LEDs have been replaced with VCSELs.
(Vertical Cavity Surface Emitting Laser)
High speed, low
Can used to
Multi-mode SC type.
Can transmit up to 1.65 or 3.5Gbps.
Electrical InputOptical Output
4 digital data inputs are multiplexed
with slightly different wavelengths.
VCSELs produce light of slightly
Light is forwarded to an SC type
fiber optic cable.
Fiber Optic Receivers
that convert light into
Fiber Optic Transceiver
Interfaces between fiber optics and
Electrical Input /
10 Gb Ethernet Transceiver
SR - 850 nm, for a maximum of 300 m
LR - 1310 nm, for distances up to 10 km
ER - 1550 nm, for distances up to 40 km
ZR - 1550 nm, for distances up to 80 km
Formula Units Explanation
A number that shows the size of the
L: fiber optic length
Power Frequency Response:
Po(f): Power at the output
Pi(F): Power at the input
Differential group delay:
tin: the group delay of the reference fiber with
Tout: the group delay of the test fiber with
Pi: power at the source fiber.
P0: power at the destination fiber.
Pi: power at the source fiber.
Pr: power reflected.