PHY110 UNIT III: Fiber optics
LECTURE 2
Prof. Reji Thomas DRD-DRC March 3, 2023
Revision Lecture 1
Fiber- Thin strand of dielectric material (transmission of light)
Wire: The stand of metal (transmission of electricity)
Laser diode: Forward biased
n1 > n2
Photo diode: Reverse biased
AIR
Laser diode/LED Photo diode
n1
AIR
“Fiber optics is a technology in which electrical
signal is converted to optical signals and
OPTICAL FIBER SYSTEM transmitted through fibers and reconverted back
into electrical signals”
Prof. Reji Thomas DRC-DRD March 3, 2023
Human hair thickness ~ 100µm
Optical Fiber: Structure
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 3, 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
Quick Quiz Response on 1/03/2023 Lecture
No Question Attempts Right Wrong
1 Which type of laser is the simplest to modulate
directly by changing its excitation?
2 Which laser emits light in the visible range 400 to
700 nm?
3 Which is the proper measurement of average
power emitted by a pulsed laser?
4 Which of the following contributes to the
broadening of laser emission bandwidth?
5 A dielectric waveguide for the propagation of
electromagnetic energy at light frequencies
6 A Fiber optic telephone transmission can handle
more than thousands of voice channels. True or
false?
Prof. Reji Thomas DRC-DRD March 3, 2023
6
Which type of laser is the simplest to modulate directly by
changing its excitation?
a. Semiconductor
b. Ruby
c. Helium-neon
Ans: A
d. Neodymium-YAG
Prof. Reji Thomas DRC-DRD March 3, 2023
7
Which laser emits light in the visible range 400 to 700 nm?
a. Argon-ion
b. Nitrogen
c. Carbon-dioxide
d. Neodymium-YAG Ans: A
Prof. Reji Thomas DRC-DRD March 3, 2023
8
Which is the proper measurement of average power
emitted by a pulsed laser?
a. Energy x time
b. Pulse energy x repetition rate
c. Pulse energy / repetition rate
d. Peak power x pulse length
Ans: C
Prof. Reji Thomas DRC-DRD March 3, 2023
9
Which of the following contributes to the broadening of laser
emission bandwidth?
a. Doppler shift of moving atoms and molecules
b. Amplification within the laser medium
c. Coherence of the laser light
d. Optical pumping of the laser transition
Ans: A
Prof. Reji Thomas DRC-DRD March 3, 2023
10
A dielectric waveguide for the propagation of electromagnetic
energy at light frequencies
a. Stripline
b. Microstrip
c. Laser beam Ans: D
d. Optical fiber
Prof. Reji Thomas DRC-DRD March 3, 2023
11
A Fiber optic telephone transmission can handle more
than thousands of voice channels. True or false?
(a) True
(b) False
Ans: A
Prof. Reji Thomas DRC-DRD March 3, 2023
12
The core of an optical fiber has a
a. Lower refracted index than air
b. Lower refractive index than the cladding
c. Higher refractive index than the cladding
d. Similar refractive index with the cladding
Ans: C
Prof. Reji Thomas DRC-DRD March 3, 2023
13
Which of the following is used as an optical transmitter
on the Fiber Optical Communications?
a. Avalanche Photo Diode (APD)
b. Limited Space Accumulation diode (LSA diode)
c. P type-Intrinsic-N type diode(PIN diode)
d. Light emitting diode (LED)
Ans: D
Prof. Reji Thomas DRC-DRD March 3, 2023
14
Which of the following is used as an optical receiver in
fiber optics communications?
a. Avalanche Photo Diode (APD)
b. Tunnel diode
c. Laser diode
d. LED
Ans: A
Prof. Reji Thomas DRC-DRD March 3, 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 Lecture 5: Application of optical fibers in computer networking
and broadcasting
Prof. Reji Thomas DRC-DRD March 3, 2023
Total Internal Reflection
The phenomena in which light is totally reflected back to denser medium at the
denser-rarer boundary is known as TOTAL INTERNAL REFLECTION.
Eq.1
Critical angle can be obtained from Eq.1
1 = c ; 2 =90°
Eq.1a
Later we use for
If the rarer medium is air n2=1
or Eq.2
Prof. Reji Thomas DRC-DRD March 3, 2023
Acceptance Angle
Case 1: Refraction at A
Incident ray from launching medium having the refractive index
n0→ refracted into the core having refractive index n1 ,
according to the Snell’s law…
Eq.3
Prof. Reji Thomas DRC-DRD March 3, 2023
Acceptance Angle
Case 2: Refracted ray incident on interface at B
Refracted ray in the core (n1) now incident on the core-cladding
interface at an angle (),
From the triangle ABC
r= 90 - -------- Eq.4
substitute Eq.4 in Eq.3
-------- Eq.5
Prof. Reji Thomas DRC-DRD March 3, 2023
Acceptance Angle
Case3: Total internal
reflection at B
When =c total internal reflection occurs at B, that set the
maximum allowable launching angle, imax , equation 5 changes to
-------- Eq.6
But from Eq.1a -------- Eq.7
Prof. Reji Thomas DRC-DRD March 3, 2023
Acceptance Angle
cos2c+ sin2c=1 substitute for sin2c from Eq.7
-------- Eq.8
Or
substituteEq.8
Substitute Eq.8inin Eq.6
For launching from air n0=1
-------- Eq.9
imax is the acceptance angle of the fiber. Also called waveguide
acceptance angle Prof. Reji Thomas DRC-DRD March 3, 2023
Acceptance Cone
We know in 2D, imax is the acceptance angle- and is with axis of the
fiber
Substitute Eq.8 in
If you consider 3Dimension, instead of angle it is the cone (solid
angle), Light rays within the cone having full 2imax are accepted and
hence called acceptance cone (2imax ).
Larger the imax easier to launch light into the fiber
Incident at an angle more than imax refract through the cladding
and lost
Prof. Reji Thomas DRC-DRD March 3, 2023
22
What is the other name for maximum external incident angle?
a) Optical angle
b) Total internal reflection angle
c) Refraction angle
d) Wave guide acceptance angle
Ans: D
Prof. Reji Thomas DRC-DRD March 3, 2023
Relative Refractive Index
The fractional difference between the refractive indices of the core
and the cladding is relative refractive index or the fractional
refractive index difference
-------- Eq.10
is always positive because n1>n2.
Typically value of is the order of 0.01
For effective light transmission through the fiber, <<1.
Prof. Reji Thomas DRC-DRD March 3, 2023
Numerical aperture
Light gathering ability of the fiber depends on the numerical
aperture , NA and is defined as the sine of the acceptance angle
imax
-------- Eq.11
NA is also related to the relative refractive index
How? We will see now!
Prof. Reji Thomas DRC-DRD March 3, 2023
Numerical aperture
Numerical aperture can also be expressed in terms of relative refractive ,
Multiply and divide with 2n1
Approximating (n1+n2)/2~n1 and eq.10
That is
• Depends only on the refractive indices of the core and
cladding.
• Independent of the dimension of the fiber
Prof. Reji Thomas DRC-DRD March 3, 2023
Numerical aperture
• 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
High NA means fiber accept large amount of light from the source and
allows it to propagate through the fiber
Prof. Reji Thomas DRC-DRD March 3, 2023
27
The numerical aperture of a fiber if the angle of acceptance
is 15°, is
a. 0.17
b. 0.26
c. 0.50
d. 0.75
Prof. Reji Thomas DRC-DRD March 3, 2023
TYPES OF OPTICAL FIBER
Optical Fibers are classified into three major categories based on
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
Cont’d
Prof. Reji Thomas DRC-DRD March 3, 2023
TYPES OF OPTICAL FIBER
29
An additional classifications is also made by considering 2 and 3
together.. So we have the 4th category
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
We would expect GISM why not given above? It is there but..
As far as single mode fibers are concerned, advantages for GISM
fibers compared to SISM fibers are relatively small.
TYPES OF OPTICAL FIBER
ALL GLASS optical fibers
Glass 2 with n2
Glass 1 with n1
but n1 > n2 for Total Internal Reflection
Glass SiO2.. So abundant on earth crust.. so cheap.. easily available..
You had computer just because of Si and SiO2 did good with each other
up to 2007
SiO2 core : B2O3 -SiO2 cladding
GeO2-SiO2 core : SiO2 cladding
Prof. Reji Thomas DRC-DRD March 3, 2023
TYPES OF OPTICAL FIBER
ALL PLASTIC optical fibers
Fluro-carbon polymer or
silicone
PMMA or POLYSTERENE
but n1 > n2 for Total Internal Reflection
High NA 0.6 and large acceptance angle of 77°
Low cost
High mechanical stability
March 3, 2023
Prof. Reji Thomas DRC-DRD
TYPES OF OPTICAL FIBER
PLASTIC CLADD SILICA (PCS) optical fibers
Polymer/Teflon
Quartz
but n1 > n2 for Total Internal Function
High NA 0.6 and large acceptance angle of 77°
Low cost
High mechanical stability
But High loss
Used for low distance
Prof. Reji Thomas DRC-DRD March 3, 2023
TYPES OF OPTICAL FIBER
SINGLE MODE OPTICAL FIBER
Only one path for light to pass
Very small core diameter ( ~10 µm)
Low band width 40 GHz
Mostly used in long distance and low cost circuits, like T.V. cable
March 3, 2023
Prof. Reji Thomas DRC-DRD
TYPES OF OPTICAL FIBER
Multimode optical fiber
Prof. Reji Thomas DRC-DRD March 3, 2023
TYPES OF OPTICAL FIBER
Step-index Optical fiber
Uniform/constant refractive index of the core
Core have higher refractive index than cladding
Abrupt change in ‘n’ at the interface
Graph of radial distance vs. refractive index seems like a step
Prof. Reji Thomas DRC-DRD March 3, 2023
TYPES OF OPTICAL FIBER
Graded-index Optical fiber
Refractive index is highest at the center and decrease towards core-cladding
interface- n varies gradually with radial distance
Symmetric distribution along the diameter- concentric circles
Same ‘n’ at the interface- core material and cladding material have nearly same n
Coaxial tube of material with different n
Prof. Reji Thomas DRC-DRD March 3, 2023
37
How does the refractive index of the core vary in step
Index fiber?
a) Tangentially
b) Radially
c) Longitudinally
d) No variation
Prof. Reji Thomas DRC-DRD March 3, 2023
38
How does the refractive index of the core vary in Graded
Index fiber?
a) Tangentially
b) Radially
c) Longitudinally
d) Transversely
Prof. Reji Thomas DRC-DRD March 3, 2023
V-number
V-number sets the upper limit of the number of modes (Nm) that
can be transmitted in a multi mode optical fiber, given by the
relation
n2
n1
d - the diameter of the core and - wavelength of the light
Prof. Reji Thomas DRC-DRD March 3, 2023
Maximum number of modes Nm supported by
Step index fiber Graded Index fiber
For a fiber to be single mode V ≤ 2.4 for graded index fiber. and
the wavelength with which the fiber becomes single mode is
called cutoff wavelength c
Single mode transmission in a multimode fiber can be realized
by reducing the diameter or decreasing the
Prof. Reji Thomas DRC-DRD March 3, 2023
41
Possible number of allowed paths of the light of wavelength
in the optical fiber is called modes of propagation. State
true or false
a) True
b) False
Prof. Reji Thomas DRC-DRD March 3, 2023
Optical fiber as a dielectric wave guide
Number of Modes: Possible number of allowed paths of light in the
optical fiber.. So in a simple sense …mode = path????
Launching angle
0 ≤ ≤ imax
Accepted by the
fiber to propagate
Light rays path along which waves (E field or B field) are in phase
inside the fiber are knows as mode in real sense.
Prof. Reji Thomas DRC-DRD March 3, 2023
Optical fiber as a dielectric wave guide
Light rays launched at an angle = imax are higher order modes
Light rays launched at an angle << imax are lower order modes
In the lower order modes, fields are concentrated near the center of
the waveguide (core).. Less chance to loose light
In the higher order modes concentrated towards the edge of the
wave guide ( interface) and more chance of loosing light to the
cladding
Higher order modes travels more distance compared to lower order
modes
Prof. Reji Thomas DRC-DRD March 3, 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 3, 2023
45
Prof. Reji Thomas DRC-DRD March 3, 2023
