Lasers & Fiber Optics
Engr. Hyder Bux Mangrio
Engr. Fayaz Hassan Mangrio
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Introduction
L&FO labs
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Lab #01: Introduction to Fiber optics Communication System
Lab #02: Optical Sources
Lab #03: Optical Detectors
Lab #04: Optical fiber attenuation losses
Lab #05: Analog voice transmission
Lab #06: Understanding basic function of S122A splicer
Lab #07: Perform Fusion/Mechanical Splicing
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Introduction
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Lab #08: Understanding the basic function of OTDR
Lab #09: Perform fiber measurement on OTDR
Lab #10: Fiber attenuation measurement using Cut-Back method
Lab #11: Optical Field Spectrum Analyzer
Lab #12: Overview of Power meter & Light Source
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Information
• Email:
hyder.bux@faculty.muet.edu.pk
• Webpage:
https://sites.google.com/a/faculty.muet.edu.pk/hyder
mangrio/
• Optical Communication Laboratory
• Consultation Timings:
Monday(8am to 3pm) & Friday (8am to 1pm)
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Laboratory
• There will be at least 13 labs covering in
13 weeks course. Each lab will be
approximately 2 hours long. The lab
report / Handout is due to the lab
assistant before next lab.
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What is lightwave technology?
• Lightwave technology uses light as the primary
medium to
carry information.
• The light often is guided through optical fibers
(fiberoptic technology).
• Most applications use invisible (infrared)
light.
(HP)
Why lightwave technology?
• Most cost-effective way to move huge
amounts of information (voice, data) quickly
and reliably.
• Light is insensitive to
electrical interference.
• Fiber optic cables have less weight and
consume less space than equivalent
electrical links.
(HP)
Use Of Lightwave Technology
• Majority applications:
– Telephone networks
– Data communication systems
– Cable TV distribution
• Niche applications:
– Optical sensors
– Medical equipment
LW Transmission Bands
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Frequency
Wavelength
(vacuum)
Longhaul Telecom
Regional Telecom
Local Area Networks
229
353
461
THz
Near Infrared
1.8
1.6
1.4
1.2
UV
1.0
0.8
0.6
0.4
HeNe Lasers
633 nm
1550 nm
CD Players
780 nm
1310 nm
850 nm
0.2
µm
Introduction to Fiber Optics
• Fiber optics is a medium for carrying information
from one point to another in the form of light. Unlike
the copper form of transmission, fiber optics is not
electrical in nature.
• A basic fiber optic system consists of a transmitting
device that converts an electrical signal into a light
signal, an optical fiber cable that carries the light,
and a receiver that accepts the light signal and
converts it back into an electrical signal.
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Introduction to fiber Optics
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Optical Sources
• Two main types of optical sources
– Light emitting diode (LED)
• Large wavelength content
• Incoherent
• Limited directionality
– Laser diode (LD)
• Small wavelength content
• Highly coherent
• Directional
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Light Emitting Diodes (LED)
• Spontaneous emission dominates
– Random photon emission
• Spatial implications of random
emission
– Broad far field emission pattern
– Dome used to extract more of
the light
• Spectral implications of random
emission
– Broad spectrum
Laser Diode
•
Stimulated emission dominates
– Narrower spectrum
– More directional
•
Requires high optical power density in the gain region
– Optical Feedback: Part of the optical power is reflected back into the cavity
– End mirrors
• Lasing requires net positive gain
– Cavity gain
• Depends on external pumping
• Applying current to a semiconductor pn junction
– Cavity loss
• Material absorption
• Scatter
• End face reflectivity
Optical Detectors
• Inverse device with semiconductor lasers
– Source: convert electric current to optical power
– Detector: convert optical power to electrical current
• Use pin structures similar to lasers
• Electrical power is proportional to i2
– Electrical power is proportional to optical power squared
– Called square law device
• Important characteristics
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Modulation bandwidth (response speed)
Optical conversion efficiency
Noise
Area
HOW DOES FIBRE OPTIC WORK ?

Carries Signals as Light Pulses
 signals
converted from electrical to light (and visaversa) by special equipment
e.g. fibre-optic “transceiver” (transmitter / receiver)
FIBRE CONSTRUCTION
125 Cladding Glass
8, 50, 62.5
Core Glass
PRIMARY BUFFER
Primary Buffer 250
Cladding 125
Core (62.5)
SECONDARY BUFFER
Secondary Buffer 900
Primary Buffer 250
Cladding 125
Core (62.5)
FIBRE MATERIAL
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Silica Glass
 used for high-speed data applications
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Plastics
 used for low-speed data / voice applications
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Composite Constructions
 used for low-speed and specialized applications
FIBRE TRANSMISSION
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Multi-Mode
 graded-index
used for short / medium distance applications
 step-index
early fibre type - no longer used

Single-Mode
 a.k.a Mono-Mode
used for long-distance / very high-speed applications
e.g. cross-country and transatlantic communications
LIGHT TRANSMISSION
MultiMode
Step Index
MultiMode
Graded Index
SingleMode
COMMON FIBRE SIZES
50 µm
62.5 µm
125 µm
125 µm
MultiMode Graded Index
100 µm
140 µm
8 µm
125 µm
SingleMode
Advantages/Disadvantages of Fiber Optics
• Advantages
a)
b)
c)
d)
e)
f)
Enormous potential bandwidth
Small size and weight
Electrical Isolation
Signal security
Low transmission loss
Potential low cost
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Advantages/Disadvantages of Fiber Optics
• Disadvantages
a)
b)
c)
High cost for connector and interfacing
Requires specialized and sophisticated tools for
maintenance and repairing
Higher initial cost in installation
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Light-Source
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What is light?
Properties of Light.
Refractive Index
Law of Refraction
Law of Reflection
Total Internal Reflection
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Refractive Index
• The guidance of the light beam which acts as a
transmission channel for information (through the
optical fiber) takes place because of the
phenomenon of total internal reflection (TIR),
which is dependent on the refractive index of the
medium.
The refractive index (n) of a medium can be written
as:
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Total Internal Reflection
• A ray of light incident on a denser medium i.e. n1<n2
According to Snell’s Law and the law of reflection we have
n1 sin θ1 =n2 sin θ2 and θ1=θ3
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Total Internal Reflection
• The angle of incidence, for which the angle of
refraction is 90º, is known as the critical angle and
is denoted by θc .Thus, when
θ1=θc =sin-1(n2/n1)
θ2=90. When the angle of incidence exceeds the
angle of critical (i.e.,θ1>θc), there is no refracted ray
and we have total internal reflection.
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Total Internal Reflection
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