Optical Fibres Short notes | Electromagnetics - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


Optical Fibres Notes
Introduction
Optical fibres are thin, flexible str ands of glass or plastic that tr ansmit light sig-
nals over long distances with minimal loss. They are critical in telecommunica-
tions, internet connectivity , and medical imaging due to their high bandwidth
and immunity to electromagnetic interference.
K ey Concepts
• Optical Fibre : A waveguide that guides light via total internal reflection,
consisting o f a core and cladding.
• T otal Internal Reflection : Light remains confined in the core due to re-
flection at the c ore-cladding interface.
• Applications : High-speed internet, telephon y , cable TV , and endoscopic
imaging.
Structure and Oper ation
• Core : Centr al re gion with higher refr active index (n
1
), where light propa-
gates.
• Cladding : Surroundin g la yer with lower refr active index (n
2
), enabling to-
tal internal re flection.
• Protective Coating : Outer la yer for mechanical protection.
• Condition for T otal Internal Reflection : Occurs when the angle of inci-
dence exceeds the critical angle:
?
c
= sin
-1
(
n
2
n
1
)
• Numerical A perture (NA) : Measures the light-gathering ability:
NA =
v
n
2
1
-n
2
2
Types of Optical Fibres
• Single-Mode F ibre (SMF) :
– Small core (~ 8-10µ m), supports one mode of light.
– Low dispersion, used for long-distance communication.
• Multi-Mode Fibr e (MMF) :
– Larger core (~ 50-62.5µ m), supports multiple light modes.
1
Page 2


Optical Fibres Notes
Introduction
Optical fibres are thin, flexible str ands of glass or plastic that tr ansmit light sig-
nals over long distances with minimal loss. They are critical in telecommunica-
tions, internet connectivity , and medical imaging due to their high bandwidth
and immunity to electromagnetic interference.
K ey Concepts
• Optical Fibre : A waveguide that guides light via total internal reflection,
consisting o f a core and cladding.
• T otal Internal Reflection : Light remains confined in the core due to re-
flection at the c ore-cladding interface.
• Applications : High-speed internet, telephon y , cable TV , and endoscopic
imaging.
Structure and Oper ation
• Core : Centr al re gion with higher refr active index (n
1
), where light propa-
gates.
• Cladding : Surroundin g la yer with lower refr active index (n
2
), enabling to-
tal internal re flection.
• Protective Coating : Outer la yer for mechanical protection.
• Condition for T otal Internal Reflection : Occurs when the angle of inci-
dence exceeds the critical angle:
?
c
= sin
-1
(
n
2
n
1
)
• Numerical A perture (NA) : Measures the light-gathering ability:
NA =
v
n
2
1
-n
2
2
Types of Optical Fibres
• Single-Mode F ibre (SMF) :
– Small core (~ 8-10µ m), supports one mode of light.
– Low dispersion, used for long-distance communication.
• Multi-Mode Fibr e (MMF) :
– Larger core (~ 50-62.5µ m), supports multiple light modes.
1
– Higher dispersion, used for short-distance applications.
• Step-Index F ibre : Constant refr active index in core and cladding.
• Gr aded-Index Fibre : Refr active index decreases gr adually in the core, re-
ducing modal dispersion.
Propagation Char acteristics
• Attenuation : Loss of signal power , measured in dB/km:
Attenuation = 10 log
10
(
P
in
P
out
)
Causes incl ude absorption, scattering, and bending losses.
• Dispersion : Spreading of light pulses, limiting bandwidth:
– Modal Dispersion : In MMF , due to different path lengths of modes.
– Chr omatic Dispersion : Due to wavelength-dependent refr active index.
• Propagation Constant (ß ) : Phase change per unit length, related to the
wave number .
K ey Equations
• V-Number (Normalized Frequency) : Determines the number of modes:
V =
2pa
?
v
n
2
1
-n
2
2
wherea is the core r adius,? is the wavelength. Single-mode ifV < 2.405 .
• Propagation Time : F or a fibre of length L :
t =
Ln
1
c
wherec = 3×10
8
m/s is the speed of light.
Pr a ctical Consider ations
• Loss Minimization : Use of low-loss wavelengths (e.g., 1310 nm, 1550 nm)
to reduce attenuation .
• Dispersion Management : Gr aded-index fibres or dispersion-compensating
fibres for high- speed systems.
• Splicing and C onnectors : Minimize signal loss at fibre joints.
• Bandwidth : MMF limited b y modal dispersion; SMF supports higher band-
width over longer distances.
• T ools : Optical time-domain reflectometry (O TDR) for fault detection and
fibre char acterization.
2