PHY110 UNIT III: Fiber optics
LECTURE 3
Prof. Reji Thomas DRD-DRC March 7, 2023
Revision
Fiber- Thin strand of dielectric material (transmission of light)
Wire: The strand of metal (transmission of electricity)
Laser diode: Forward biased
Photo diode: Reverse biased? n1 > n2
Laser diode/LED Photo diode
AIR
n1
AIR
OPTICAL FIBER SYSTEM
“Fiber optics is a technology in which electrical signal is
converted to optical signals and transmitted through fibers and
reconverted back into electrical signals”
Prof. Reji Thomas DRC-DRD March 7, 2023
Optical Fiber: Structure Human hair thickness ~ 100µm
1. Core - Light guiding region
2. Cladding- confine the light to the core
3. Buffer or Sheath - protect the fiber from
physical and environmental damage
Additional functions of cladding
To maintain the uniformity along the length of the fiber
To protect the outer surface of the core
To reduce the cone of the light
Prof. Reji Thomas DRC-DRD March 7, 2023
4
TOTAL INTERNAL REFLECTION happens when a ray light
pass from the denser medium to rarer medium:
n2
n1
TOTAL INTERNAL REFLECTION
Critical angle > c
Snell’s law
Principle on which Fiber optic communication rely on is TOTAL
INTERNAL REFLECTION
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imax is the acceptance angle of the fiber.
2imax = acceptance cone
Relative Refractive Index
is always positive because n1 > n2.
Typically value of is the order of 0.01
For effective light transmission through the fiber, <<1.
Numerical Aperture (NA)
• Measure of the light gathering ability of
the fiber
• Depends only on the refractive indices of
the core and cladding.
• Independent of the dimension of the fiber
• Typical values are in the range 0.13 to 0.50
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If n1, n2 and n0 are the refractive indices of core, cladding and
medium from which the light is launched into the core, the value
of numerical aperture can exceed 1 when
NA= 𝑛𝑜 sin 𝑖𝑚𝑎𝑥
a) n0 =1
b) n0 > 1
c) n0 < 1
d) n2 > n1
Ans: B
Prof. Reji Thomas DRC-DRD March 7, 2023
Classification of Optical Fibers
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1. The material from which it is made
i. Glass optical fibers
ii. Plastic Optical fibers
iii. Plastic Clad Silica (PCS) optical fibers
2. The propagation modes through it
i. Single mode optical fibers
ii. Multimode optical fibers
3. The refractive index profile of the material used
i. Step-index optical fibers
ii. Graded index optical fibers
4. the modes and refractive index profile
i. Step-index single mode(SISM) optical fiber
ii. Step index multimode (SIMM) optical fiber
iii. ??? GISM
iv. Graded-index multimode (GIMM) optical fiber
Prof. Reji Thomas DRC-DRD March 7, 2023
9
In single mode fibre the diameter of the core is nearly equal to
a) 10 m
b) 100 m
c) 50 m
d) 125 m
Ans: A
Prof. Reji Thomas DRC-DRD March 7, 2023
V-number
Maximum number of modes Nm supported by a fiber having diameter d for a
particular wavelength is
Step index fiber Graded Index fiber
For a fiber to be single mode V < 2.4.
and the wavelength with which the fiber becomes single mode is called cutoff
wavelength c
Single mode for any wavelength > c
Single mode transmission in a multimode fiber can be realized by
reducing the diameter and/or decreasing the such that V< 2.405
Prof. Reji Thomas DRC-DRD March 7, 2023
11
Carrier wave used in fibre optical communication is
a) Radio waves
b) Ordinary light waves
c) Microwaves
d) LASER light
Ans: D
Prof. Reji Thomas DRC-DRD March 7, 2023
Quick Quiz Response on the 3/03/2023 Lecture
No Question Attempts Right Wrong
1 How does the refractive index of the core
vary in Graded Index fiber?
2 How does the refractive index of the core
vary in step Index fiber?
3 Possible number of allowed paths of light in
the optical fiber is called modes of
propagation. State true or false
Prof. Reji Thomas DRC-DRD March 7, 2023
13
How does the refractive index of the core vary in Graded
Index fiber?
a) Tangentially
b) Radially
c) Longitudinally
d) Transversely
Ans: B
Prof. Reji Thomas DRC-DRD March 7, 2023
14
How does the refractive index of the core vary in step
Index fiber?
a) Tangentially
b) Radially
c) Longitudinally
d) No variation
Ans: D
Prof. Reji Thomas DRC-DRD March 7, 2023
15
Possible number of allowed paths of light in the optical fiber
is called modes of propagation. State true or false
a) True
b) False
Ans: A
Prof. Reji Thomas DRC-DRD March 7, 2023
Fiber optics
Lecture 1: 1/03/23 Fiber optics introduction, optical fibers, optical fiber as a
dielectric wave guide, Total internal reflection
Lecture 2: 3/03/23, Acceptance angle, relative refractive index, numerical
aperture, Classification of fibers, Step index and graded index fibers,
V-number, and modes of propagation
Learned the fundamentals of optical fiber, the fundamental parameters of optical
fibers and propagation of light through optical fiber, learned about
the types of fibers
Lecture 3: 7/03/23; Losses associated with optical fibers; learn the reason for
data loss
Lecture 4: 10/03/23 Application of optical fibers in computer networking and
broadcasting
Prof. Reji Thomas DRC-DRD March 7, 2023
Losses and Dispersion in optical fibers
When the optical signal is made to propagate through the optical
fiber, signal strength reduces and shape of the wave changes due to
1a. Attenuation Intrinsic
a) Absorption loss/attenuation
b) Scattering
1b. Radiative loss Extrinsic
Bending of optical fiber loss/attenuation
2. Dispersion
a) Intermode
b) Intramode
i. Material dispersion
ii. Waveguide dispersion
Loss of amplitude of the signal: attenuation
Change in shape of the signal: Distortion/dispersion
Prof. Reji Thomas DRC-DRD March 7, 2023
LOSSES IN OPTICAL FIBERS
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ATTENUATION
The loss of signal amplitude is known as attenuation
The loss of optical power as light travels down the fiber exponentially
depends on the distance
Po is the power at distance L, Pi input power and is the fiber
attenuation constant expressed in /km
In unit of dB/km, is defined as,
In ideal case, Pi = Po and attenuation is zero
LOSSES IN OPTICAL FIBERS
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1. Losses due to Attenuation
a) Material absorption
Imperfection and impurities in the fiber account for 3-5 % loss
OH- common impurity-
Due to water trapped during the manufacturing process or Humidity from
the atmosphere
Cu, Ni,Cr,V, Mn impurities in glass absorb visible wavelength
Electronic absorption at UV and vibrational absorption at IR wavelengths are
un avoidable
Absorption found to be minimum around the wavelength 1.3 µm (1300 nm)
Propagation of light with wavelength above 1.7 µm or 1700nm is not possible
due to infrared (IR) absorption
LOSSES IN OPTICAL FIBERS
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1. Losses due to Attenuation
b) Rayleigh scattering
Account for the 96% of attenuation in the fiber
Microscopic density variation causes changes in
refractive index locally in the fiber
These obstructions act as scattering centers and
scatter light in all direction- Rayleigh scattering
It varies as 1/4 – high at lower wavelength
Lower wavelength limit is 800 nm (0.8 µm)
Material absorption set the upper wavelength limit at 1700 nm
and Rayleigh scattering set the lower wavelength limit at 800
nm. So light having wavelength in the range 800-1700 nm is
used in optical communication!
LOSSES IN OPTICAL FIBERS
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800-1700nm is used in optical communication
1. Losses due to Attenuation
nm loss dB/km
820-880 2.2
1220-1320 0.6
1550-1610 0.2
The band of wavelength at which the attenuation is a minimum is
called optical window or transmission window or low-loss window
The range 1550-1610 is most preferable
1300 nm is suitable as the dispersion is minimum
LOSSES IN OPTICAL FIBERS
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2. Losses due to bending of Optical fiber
a) Macrobend
Fiber is bend in noticeable way and strain
induced ‘n’ change TIR conditions and light
escape
b) Microbend
Small scale local bend on the fiber
Not clearly visible
Indicative of pressure on the fiber
Light refracted into the cladding as TIR
condition get changed
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What causes microscopic bend?
a) Uniform pressure
b) Non-uniform volume
c) Uniform volume
d) Non-uniform pressure
Ans: D
Prof. Reji Thomas DRC-DRD March 7, 2023
24
The loss in signal power as light travels down a fiber is called
a. Dispersion
b. Scattering
c. Absorption
d. Attenuation
Ans: D
Prof. Reji Thomas DRC-DRD March 7, 2023
25
Which range of wavelength is most preferable for the
transmission light through optical fiber with minimum loss??
a) 820-880 nm
b) 1220-1320 nm
c) 1550-1610nm
d) None of the above
Ans: C
Prof. Reji Thomas DRC-DRD March 7, 2023
26
Which range of wavelength used in optical communication?
a) 800-1700 nm
b) 1700-2500 nm
c) 200-800 nm
d) None of the above
Ans: A
Prof. Reji Thomas DRC-DRD March 7, 2023
https://www.youtube.com/watch?v=zx07PShjJmk
DISPERSION
27Rainbow and Splitting of white light to spectrum of colours are the most
familiar example of dispersion
Due to the wavelength () dependence of refractive index (n)
𝑑𝑛
= 0 No dispersion
𝑑
𝑑𝑛
< 0 Normal dispersion
𝑑
𝑑𝑛
> 0 Anomalous dispersion
𝑑
However, dispersion in optical fibres is due to the group velocity (Vg)
dispersion and that causes pulses to spread and consequently this distortion
degrades the signals over long distances. 𝑐
𝑉𝑝 =
𝑛
𝑉𝑔 = 𝑉𝑝 No dispersion
𝑉𝑔 < 𝑉𝑝 Normal dispersion
𝑉𝑔 > 𝑉𝑝 Anomalous dispersion
DISPERSION
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Distortion of the signal in an
optical fiber due to DISPERSION
a) Intermodal Dispersion
b) Intramodel Dispersion
i. Material dispersion
ii. Waveguide dispersion
DISPERSION
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a) Intermodal Dispersion
• Input power divided into various modes and propagate
through the fiber
• Lower order modes take less time compared to higher
modes
• Pulse broadens and hence reduces transmission rate
• Can be reduced in Graded index fiber
DISPERSION
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b) Intramodal Dispersion
b.1 Material dispersion
b.2 Waveguide dispersion
Material dispersion is a
wavelength dependence of
the fiber material refractive index.
Light pulse is a wave packet, having components of several wavelengths..
Glass dispersive medium.. Different wavelength component travel at
different speed..
Pulse broadening occurs as a result
Material dispersion can be reduced by reducing the spectral width or by
increasing the wavelength
DISPERSION
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b) Intramodal Dispersion
b.1 Material dispersion
b.2 Waveguide dispersion
• Refractive index variation along the waveguide means different
speed for different wavelength
• Also results is pulse broadening
SMF- Only material and waveguide dispersions (only
intramodal dispersion) exist
MMF- intermodel, waveguide and material dispersion ( both
intramodal and intermodal dispersion) exist
32
A single mode step index (SMSI) fiber has low intermodal
dispersion than the corresponding multimode. True or false?
a) True
b) False
Ans: A
Prof. Reji Thomas DRC-DRD March 7, 2023
33
Which of the following has more distortion?
a) Single mode step-index fiber
b) Graded index fiber
c) Multimode step-index fiber
d) Glass fiber
Ans: C
Prof. Reji Thomas DRC-DRD March 7, 2023
34
In which of the following there is no distortion?
a) Graded index fiber
b) Multimode step-index fiber
c) Single step-index fiber
d) Glass fiber
Ans: A
Prof. Reji Thomas DRC-DRD March 7, 2023
35
When more than one mode is propagating, how is it dispersed?
a) Intra-modal dispersion
b) Inter-modal dispersion
c) Material dispersion
d) Waveguide dispersion
Ans: B
Prof. Reji Thomas DRC-DRD March 7, 2023
Fiber optics
Lecture 1: 1/03/23 Fiber optics introduction, optical fibers, optical fiber as
a dielectric wave guide, Total internal reflection
Lecture 2: 3/03/23, Acceptance angle, relative refractive index,
numerical aperture, Classification of fibers, Step index and
graded index fibers, V-number, and modes of propagation
Learned the fundamentals of optical fiber, the fundamental parameters of
optical fibers and propagation of light through optical fiber,
learned about the types of fibers
Lecture 3: 7/03/23; Losses associated with optical fibers; learn the reason
for data loss
Lecture 4: 10/03/23 Application of optical fibers
Prof. Reji Thomas DRC-DRD March 7, 2023
37
Prof. Reji Thomas DRC-DRD March 7, 2023
