1.
Course Code: EEE 409
Course Title: Optical Fiber Communication
Department of Electrical and Electronic Engineering
Hajee Mohammad Danesh
Science and Technology University, Dinajpur-5200
Course Teacher
Md. Sazedur Rahman
Lecturer
Dept. of Electrical and Electronic Engineering (EEE)
Hajee Mohammad Danesh Science and Technology University (HSTU)

2.
Course Contents:
Introduction to optical communication. Guided and unguided optical
communication system, Light propagation through guided medium, Optical
Fibers: SMF and MMF, SI fibers and GI fibers. Fiber modes, mode theory for
light propagation through fibers, single mode condition and multimode
condition. Transmission impairments: fiber loss, chromatic dispersion in a fiber,
polarization mode dispersion (PMD). Different types of fibers: DSF, DCF,
Dispersion compensation schemes. Fiber cabling process, Fiber joints/connectors
and couplers, Optical transmitter: LED and laser, Operating principles,
Characteristics and driver circuits. Optical receivers: PN, PIN and APD
detectors, Noise at the receiver, SNR and BER calculation, Receiver sensitivity
calculation. IM/DD and Coherent communication systems. Nonlinear effects in
optical fibers. Optical amplifiers, Optical modulators, Multichannel optical
systems: Optical FDM, OTDM and WDM. Optical Access Network, Optical link
design and Free space optical communication.
Course Code: EEE 409 Credit Hour: 3.00
Course Title: Optical Fiber Communication

3.
TEXT BOOKS:
1.Optical Fiber Communications – Gerd Keiser, Tata Mc Graw-Hill
International edition.
2. Optical Fiber Communications – John M. Senior, PHI.
REFERENCE BOOKS:
1. Fiber Optic Communications – D.K. Mynbaev , S.C. Gupta and Lowell L.
Scheiner, Pearson Education.
2. Text Book on Optical Fibre Communication and its Applications –
S.C.Gupta, PHI.
3. Fiber Optic Communication Systems – Govind P. Agarwal , John Wiley.
4. Fiber Optic Communications – Joseph C. Palais, Pearson Education.

4.
Contents
 Introduction to OFC
 Brief history
 Optical fiber communication system
 Advantage and Limitation of OFC

5.
Communication may be broadly defined as the transfer of
information from one point to another. When the
information is to be conveyed over any distance a
communication system is usually required. Sophisticated
techniques have been developed for this process using
electromagnetic carrier waves operating at radio
frequencies as well as microwave and millimeter wave
frequencies. However, ‘communication’ may also be
achieved using an electromagnetic carrier which is
selected from the optical range of frequencies.
Communication

6.
Communication System

7.
Block Diagram of Communication System

8.
Background of Optical Communication
Brief history of Optical Communication Technology:
 Before 1792, fire beacons or smoke was used to send information
 In 1792, Claude Chappe was invented optical telegraphy
 He was succeeded to transmit information between Paris and Lille
 By 1830, the network was extended In Europe (Bit/s < 1)
Optical telegraphy system and its inventor Claude Chappe
Relay
Station

9.
 The advent of electrical telegraphy in the 1830 replaced the use of
optical telegraphy and began the era of electrical communication
 The bit rate of electrical telegraphy was increased to ~ 10 bit/s by
using Morse Code (dots and dashes)
 The invention of telephone in 1876 enables to transmit electrical
signals in analog form, which dominate comm. for a century or so.
 The development of worldwide telephone networks led to many
advances in the design of Electrical communication systems
 Use of coaxial cable instead of wire pairs increased system capacity
considerable
Brief history of Optical Communication Technology

10.
 The first coaxial cable put into service in 1940 with 3 MHz system
capacity (300 voice channels or a single television channel)
 The bandwidth was limited by frequency dependent cable losses(~10MHz)
 This limitation was led to develop Micro-Wave communication (1~10GHz)
 The first Microwave system was operated at 4 GHz
 Most advanced coaxial cable put into service in 1975 (274 Mb/s, ~1Km)
 Microwave communications generally allow larger repeater spacing, but bit
rate is limited by the carrier frequency of such waves
Bit rate-
distance
product, BL
BL product 100
Mb/s-Km was
achieved by
1970 and limited
due to carrier
frequency
1970
108
Brief history of Optical Communication Technology

11.
 During 1950 it was realized that BL product can be further increased
if optical waves were used as the carrier
 During 1950 there was no coherent optical source nor a suitable
transmission medium
 In 1960 first LASER was developed (coherent light source)
 After 1960 first idea was developed to use glass material as a
transmission medium
 In 1966 first optical fiber was made by Kao and Hockham but loss
was 1000 dB/km
 By reducing concentration of transition-metal ions and water ions
(Fe, Cu, Cr, Ni, Mn, Cobalt and HO)
 In 1970 Kapron et al. at Corning made a fiber with α < 20 dB/km in
the wavelength region near 1 m
 In the same time GaAs semiconductor lasers operating continuously
at room temperature at 1 m were developed
 Simultaneous availability of compact light source and low loss fiber
led to a worldwide effort for developing FO Comm. systems
Brief history of Optical Communication Technology

12.
Electromagnetic Spectrum
106

13.
 Purpose: Eliminate repeaters used in inter-office trunk
lines
 Technology: 0.8 µm GaAs semiconductor lasers,
Multimode silica fibers
 Repeater Spacing: 10 km
 Limitations: Fiber attenuation 3 dB/km, Intermodal
dispersion, bit rate 45 Mb/s
 Deployed since 1974
First-generation Fiber optic Systems

14.
 Opportunity:
Development of low-attenuation fiber (removal of H2O and other
impurities), eliminate repeaters in long-distance lines
 Technology:
1.3 µm semiconductor lasers, Muti-mode fiber, low-
attenuation silica fibers, bit rate: < 100 Mb/s due to dispersion
1.3 µm semiconductor lasers, Single-mode fiber, low-
attenuation silica fibers, bit rate: 1.7 Gb/s
 Limitation:
Fiber attenuation 0.5 dB/km, repeater spacing ≈ 50 km
 Deployed since 1978
Second-generation Fiber optic Systems

15.
 Opportunity:
Long-distance Communication
 Technology:
1.55 µm single-mode semiconductor lasers, Single-mode
fiber, low- attenuation silica fibers, bit rate: 2.5 Gb/s
 Limitations:
Fiber attenuation 0.2 dB/km, repeater spacing ≈ 60~70
km, Fiber dispersion, electronic repeaters
 Deployed since 1982
Third-generation Fiber Optic Systems

16.
 Opportunity:
Development of erbium-doped fiber amplifiers and WDM technology
Technology (deployment began in 1994):
1.55 µm single-mode, narrow-band semiconductor lasers, Single-mode,
low-attenuation dispersion-shifted silica fibers, Wavelength-division
multiplexing, with bit rate 2.5 Gb/s over 21000 km and 5 Gb/s over
14300 km by 1996. Using WDM technology bit rate was possible to
increase 2.56 Tb/s by 2002
Nonlinear effects limit the following system parameters:
Signal launch power, Propagation distance without regeneration,
WDM channel separation, Maximum number of WDM channels per fiber
Fourth-generation Fiber optic Systems

17.
Fifth-Generation !!!!!!
 Opportunity:
Development of Raman amplifiers and WDM technology, dry fiber
Technology (deployment began in 1994):
Dry fiber with low loss over the wavelength region 1.3 to 1.65 µm lead to
lightwave systems having 1000 WDM channels, Each channel 40 Gb/s,
which can be extended to 160 Gb/s in future

18.
BL product in several generations
of lightwave systems

19.
Optical fiber communication system
Optical
Transmitter
Comm. Channel
(Optical fiber)
Optical
Receiver
Input Output
Attenuation, Dispersion,
crosstalk & noise

20.
Greater bandwidth
Low attenuation
Electrical immunity (no RFI, EMI)
Greater security
Flexibility
Falling cost
Long repeater spacing
Smaller size and weight than copper cables
Advantages of OFC

21.
Disadvantages of OFC
 Disadvantages include the cost of interfacing
equipment necessary to convert electrical
signals to optical signals. (optical transmitters,
receivers) Splicing fiber optic cable is also
more difficult.
 Expensive over short distance
 Requires highly skilled installers
 Adding additional nodes is difficult

22.
Disadvantages of OFC
 Stimulated Raman Scattering (SRS):
An interaction between light and vibrations of silica molecules, causes
attenuation of short wavelength channels in WDM system
 Stimulated Brillouin Scattering (SBS):
An interaction between light and sound waves in the fiber, causes frequency
conversion and reversal of propagation direction of light
 Four Wave Mixing (FWM):
Two or more optical waves at different wavelengths mix to produce new
waves at other wavelengths
 Self Phase Modulation (SPM):
Change in signal phase due to change in intensity of the signal due to group
velocity dispersion
 Cross Phase Modulation (XPM):
It is an interaction via the non-linear refractive index between the intensity of
one light wave and optical phase of other light waves
 Some other limitations:
Dispersion, laser phase noise, relative intensity noise etc.

23.
Application of OFC
Fiber optic cables find many uses in a wide variety of industries and
applications. Some uses of fiber optic cables include:
• Medical: Used as light guides, imaging tools and also as lasers for surgeries
• Defense/Government: Used as hydrophones for seismic waves and
SONAR , as wiring in aircraft, submarines and other vehicles and also for
field networking
• Data Storage: Used for data transmission
• Telecommunications: Fiber is laid and used for transmitting and receiving
purposes
• Networking: Used to connect users and servers in a variety of network
settings and help increase the speed and accuracy of data transmission
• Industrial/Commercial: Used for imaging in hard to reach areas, as wiring
where EMI is an issue, as sensory devices to make temperature, pressure
and other measurements, and as wiring in automobiles and in industrial
settings.
• Broadcast/CATV: Broadcast/cable companies are using fiber optic cables
for wiring CATV, HDTV, internet, video ondemand and other applications.

24.
Types of Optical Communication
 Guided Optical Communication
 In guided channel communication, optical fiber is used to transmit the
light signal from transmitter to receiver. In this, the transmission of
modulated signal is through glass fibers. Single Mode fiber is used for
this transmission.
 Unguided Optical Communication
 Unguided channel communication means, that the transmission of
modulated light signal through the atmosphere or vaccum to
obtain optical communications. It can be termed as wireless optical
communication (WOC).

25.
Optical Fiber
 An optical fiber (or fibre) is a glass or
plastic fiber that carries light along its
length.
 Light is kept in the "core" of the optical fiber
by total internal reflection.

26.
Fiber optic waveguide
 An optical wave guide is a structure that "guides" a light
wave by constraining it to travel along a certain desired
path. If the transverse dimensions of the guide are
much larger than the wavelength of the guided light,
then we can explain how the optical waveguide works
using geometrical optics and total internal reflection.

27.
Construction of an Optical Fiber
An optical fiber is a waveguide for light Consists of :
Core: inner part where wave propagates
Cladding: outer part used to keep wave in core
Buffer: protective coating
Jacket: outer protective shield
SiO2 doped
with GeO2
High n
Low n

28.
Types of Optical Fiber
 According to the refractive index profile
 Step index fiber
 Graded index fiber
 According to the mode of propagation
 Single mode fiber (SM)
 Multimode fiber (MM)

29.
Single-mode fibers – used to transmit one signal
per fiber (used in telephone and cable TV). They
have small cores(9 microns in diameter) and
transmit infra-red light from laser.
Single-mode fiber’s smaller core (<10
micrometres) necessitates more expensive
components and interconnection methods, but
allows much longer, higher-performance links.
Single Mode Fiber

30.
Multi-mode fibers – used to transmit many signals per
fiber (used in computer networks). They have larger
cores(62.5 microns in diameter) and transmit infra-red
light from LED.
Multimode fiber has a larger core (≥ 50 micrometre),
allowing less precise, cheaper transmitters and receivers
to connect to it as well as cheaper connectors.
Multi Mode Fiber

31.
Step index fiber is a fiber type of cylindrical
waveguide core with inner core has a uniform
refractive index of n1 and the core is surrounded by an
outer cladding with a uniform refractive index of n2..
The refractive index of the core in graded-index fibers
is not constant but decreases gradually from its
maximum value n1 at the core center to its minimum
value n2 at the core–cladding interface.
Step index and Graded index fiber

32.
Step index and Graded index fiber

33.
Step index and Graded index fiber

34.
34
fiber optic multimode
step-index
fiber optic multimode
graded-index
fiber optic single mode

35.
35

36.
The General Principle
The classical understanding of fiber optics
comes from Snell’s Law!
• Step index fibers: by total internal reflection
• GRIN (Graded Refractive Index) fibers: by layered
changes in refractive index

37.
Total Internal Reflection
t
i n
n 
 sin
sin 2
1 
i
Exit rays
low index, n2
high index,
n1
c
i
Incident rays
t = 900
2
1 sin n
n c 

1
2
sin
n
n
c 

According to Snell’s Law

38.
Step Index Fiber
n2
n1
i
i
i
i  c for total internal reflection
Cladding
Core
Escapes core
(freedom!)
Stuck in core
Escapes from core
c = Critical Angle,
c
n1 > n2
1
2
sin
n
n
c 


39.
Graded Index Fiber
n2
n2
n1
n varies
Gradually

40.
Propagation of Light Through
Optical Fiber
40

41.
Critical angle, θc
 The minimum angle of incidence at which
a light ray ay strike the interface of two
media and result in an angle of refraction
of 90° or greater.

42.
Acceptance angle
max


is the maximum acceptance angle to the axis of the fiber
at which light may enter into the fiber in order to propagate
max

A
B
  c for total
internal reflection
Lost by
radiation
Acceptance
cone
42

43.
Numerical aperture (NA)
• Used to describe the light-gathering or light-
collecting ability of an optical fiber.
• In optics, the numerical aperture (NA) of an
optical system is a dimensionless number
that characterizes the range of angles over
which the system can accept or emit light

44.
Numerical aperture (NA)
The NA defines a cone of acceptance for light that will be
guided by the fiber
1
Air n0
n2
n1
A 2
B
C

=90-2
> c
At the air core interface 2
1
1 sin
sin 
 n
no 
From the triangle ABC
2
2


 


45.
Numerical aperture (NA)

 cos
sin 1
1 n
no 
2
1
2
1
1 )
sin
1
(
sin 
 
 n
no
Using trigonometric relationship
For total internal reflection, 1 = a, and   c
2
1
2
1
2
2
1 1
sin 









n
n
n
n a
o 
900
n2
n1
2
2
1
1 sin
sin 
 n
n 
1
2
sin
n
n
c 

  2
1
2
2
2
1
sin n
n
n a
o 


  2
1
2
2
2
1
sin n
n
n
NA a
o 

 
1
2
1
2
2
2
1
2n
n
n 


1
2
1
n
n
n 


2
1
1 )
2
( 
 n
NA
Exmp. 2.1 and 2.2

46.
 Class Test -01

47.
Mode Theory for Light Propagation
Through Optical Fiber

48.
The mode theory uses electromagnetic wave
behaviors to describe the propagation of light along a
fiber. A set of guided electromagnetic waves is called
the modes of the fiber. The mode theory is used to
describe the properties of light that ray theory is
unable to explain. An optical mode refers to a specific
solution of the wave equation that satisfies the
appropriate boundary conditions and has the property
that its spatial distribution does not change with
propagation. The fiber modes can be classified as
guided modes, leaky modes, and radiation modes
Fiber Modes

49.
A set of guided electromagnetic waves is called the modes of an optical fiber.
Maxwell's equations describe electromagnetic waves or modes as having two
components. The two components are the electric field, E(x, y, z), and the magnetic
field, H(x, y, z). The electric field, E, and the magnetic field, H, are at right angles to
each other. Modes traveling in an optical fiber are said to be transverse. The
transverse modes, shown in figure 2-17, propagate along the axis of the fiber.
Fiber Modes

50.
The mode field patterns shown in figure 2-17 are said to be transverse electric
(TE). In TE modes, the electric field is perpendicular to the direction of
propagation. The magnetic field is in the direction of propagation. Another type
of transverse mode is the transverse magnetic (TM) mode. TM modes are
opposite to TE modes. In TM modes, the magnetic field is perpendicular to the
direction of propagation. The electric field is in the direction of propagation.
Figure 2-17 shows only TE modes. he TE mode field patterns shown in figure 2-
17 indicate the order of each mode. The order of each mode is indicated by the
number of field maxima within the core of the fiber. For example, TE0 has one
field maxima. The electric field is a maximum at the center of the waveguide and
decays toward the corecladding boundary. TE0 is considered the fundamental
mode or the lowest order standing wave. As the number of field maxima
increases, the order of the mode is higher.
Fiber Modes

51.
Like all electromagnetic phenomena, propagation of optical
fields in fibers is governed by Maxwell’s equations. For a
non-conducting medium without free charges, these
equations take the form (in SI units)
Faraday’s law : ∇×E = −
∂B
∂t
(1)
Ampere’s law: ∇×H =
∂D
∂t
(2)
Gauss’s law for static electric field / Coulomb’s law :∇·D = 0 (3)
Gauss’s law for static magnetic field: ∇·B = 0 (4)
Where, electric field E, magnetic field H, electric flux
density D, magnetic flux density B and ∇ is a vector
operator.
Maxwell’s Equation

52.
The flux densities are related to the field vectors by the
constitutive relations
D = 𝜀 E (5)
B = µ H (6)
Where, ε is the dielectric permittivity and μ is the
magnetic permeability of the medium.
Substituting for D and B and taking the curl of Eqs (1)
and (2) gives:
∇ × (∇ × E) = − με
𝜕2𝑬
𝜕𝑡2 (7)
∇ × (∇ × H) = − με
𝜕2𝑯
𝜕𝑡2 (8)
Maxwell’s Equation

53.
Then using the divergence conditions of Eqs (4) and (5)
with the vector identity:
𝛻 × (𝛻 × 𝑌) = 𝛻(𝛻 ⋅ 𝑌) − 𝛻2(𝑌)
we obtain the nondispersive wave equations:
𝛻2𝑬 = με
𝜕2𝑬
𝜕𝑡2 (9)
𝛻2𝑯 = με
𝜕2𝑯
𝜕𝑡2 (10)
where 𝛻2 is the Laplacian operator.
And velocity of light is, c =
1
με
1
2
Maxwell’s Equation

54.
If planar waveguides, described by rectangular Cartesian
coordinates (x, y, z), or circular fibers, described by cylindrical
polar coordinates (r, φ, z), are considered, then the
Laplacian operator takes the form:
𝛻2𝜓 =
𝜕2𝜓
𝜕𝑥2 +
𝜕2𝜓
𝜕𝑦2 +
𝜕2𝜓
𝜕𝑡2 (11)
or:
𝛻2𝜓 =
𝜕2𝜓
𝜕𝑟2 +
1
𝑟
𝜕𝜓
𝜕𝑟
+
1
𝑟2
𝜕2𝜓
𝜕∅2 +
𝜕2𝜓
𝜕𝑧2 (12)
respectively.
𝜓 is a field (E or H)
It is necessary to consider both these forms for a complete treatment
of optical propagation in the fiber, although many of the properties
of interest may be dealt with using Cartesian coordinates.
Solution of Maxwell’s Equation

55.
Basic solution of wave equation is,
𝜓 = 𝜓0 e𝑗 𝜔𝑡 − 𝒌 ⋅ 𝒓
where ω is the angular frequency of the field, t is the time, k is the
propagation vector which gives the direction of propagation and the
rate of change of phase with distance while the components of r
specify the coordinate point at which the field is observed. When λ
is the optical wavelength in a vacuum, the magnitude of the
propagation vector or the vacuum phase propagation constant k,
𝑘 =
2𝜋
𝜆
k is also referred to as the free space wave number.
Solution of Maxwell’s Equation

56.
Assignment:
Derive the equation of normalized frequency, V and normalized
propagation constant, b for a cylindrical fiber. Also, show the LP
modes of order l = 0, 1 against normalized frequency (V) for a
circular optical waveguide for step index fiber.
Ref: article 2.4.1, figure 2.15, 2.17.
Ref. Book: Optical Fiber Communications – John M. Senior, PHI,
3rd Edition
Solution of Maxwell’s Equation

57.
Normalized Frequency, V:
𝑉 =
2𝜋
𝜆
𝑎 𝑁𝐴 =
2𝜋
𝜆
𝑎𝑛1 2∆
1
2
Normalized propagation constant, b:
b =
𝛽
𝑘
2
−𝑛2
2
2𝑛1
2∆
Normalized Frequency

58.
Index profile for step index fiber:
n(r) = n1 r < a (core)
n2 r ≥ a (cladding)
The total number of guided modes or mode volume 𝑀𝑠 for
a step index fiber:
𝑀𝑠 =
𝑉2
2
Condition for step index fiber

59.
Index profile for graded index fiber:
n(r) =
n1
√(1− 2∆ (
𝑟
𝑎
)
𝛼
) r < a (core)
n1
√(1− 2∆) = 𝑛2
r ≥ a (cladding)
The total number of guided modes or mode volume 𝑀𝑠 for a graded
index fiber:
𝑀𝑔 = (
𝛼
𝛼+2
)
𝑉2
2
For a parabolic refractive index profile core fiber (𝛼 = 2),
𝑀𝑔 =
𝑉2
4
Parabolic refractive index profile 𝛼 = 2
Trigonometric refractive index profile 𝛼 = 1
Step index refractive index profile 𝛼 = ∞
Condition for graded index fiber

60.
Single-mode propagation of the LP01 mode in step index
fibers is possible over the range:
0 ≤ V < 2.405
So, 𝑉𝑚𝑎𝑥 = 2.405
Normalized Frequency,
𝑉𝑚𝑎𝑥 =
2𝜋
𝜆𝑐𝑢𝑡 𝑜𝑓𝑓
𝑎 𝑁𝐴 =
2𝜋
𝜆
𝑎𝑛1 2∆
1
2
Condition for single mode step index fiber

61.
Example 2.1, 2.3 to 2.8
Ref. Book: Optical Fiber Communications – John
M. Senior, PHI, 3rd Edition.
Mathematical analysis

62.
 Class Test - 02

63.
Transmission impairments: fiber
loss, chromatic dispersion in a fiber,
polarization mode dispersion (PMD).

64.
Source of Losses in Silica OF
Losses in silica fiber are mainly occur due to two
mechanisms:
 Intrinsic absorption mechanism (due to
characteristic of glass fiber)
 Extrinsic absorption mechanism (due to impurities:
such as OH bonds and transition metal ions (iron,
cobalt, copper etc.))

65.
Source of Losses in Silica OF
Intrinsic absorption loss mechanisms are:
 Material absorption:
The atomic bonds associated with the core material absorb the
longer wavelength light (Si-O; 9.2 m, Ge-O; 11.0 m; P-O; 8.1;m)
 Electron absorption:
In the ultraviolet region, light is absorbed in order to excite the
electron in a core atoms to a higher energy state.
 Rayleigh scattering:
Due to small irregularities in the structure of the fiber core, which
are caused by density fluctuations into the glass material at
manufacture. This loss reduces with forth power of  (~ -4).

66.
Attenuation wavelength Ch. Of Glass fiber

67.
Attenuation wavelength Ch. Of
Glass fiber

68.
Fiber Transmission Windows
(Bands)

69.
Fiber bending and micro bending

70.
Critical bending radius for MM
and SM fiber
Bending losses may reduce:
- Large refractive index difference
- Operating at shorter wavelength
 2
3
2
2
1
1
4
3
n
n
n
Rc















c
cs
n
n
R



996
.
0
748
.
2
)
(
20
2
3
2
1
Large bending losses
occur in SM fiber at
critical bending radius
Bending radius may reduce in
SM fiber if operating wavelength
becomes shorter for a fixed c
and n
The cutoff wavelength is the shortest wavelength at which the fiber will be single-
moded. Wavelengths shorter than the cutoff will travel in multiple modes whereas
wavelengths longer than the cutoff will travel in a single mode.
For MM fiber

71.
Other scattering losses
Mie scattering:
Due to imperfections such as irregularities in core-cladding
interface, core-cladding refractive index differences along
the fiber length, diameter fluctuations, strains, and bubbles
Stimulated Brilloiun Scattering:
Shift in incident light frequency in the acoustic range due to
scattering process, which causes reversal of propagation
direction
Stimulated Raman Scattering:
Shift in incident light frequency in the optical range causes
attenuation

72.
Problems:









out
in
P
P
dB 10
log
10
Attenuation in decibels (dB)
In OFC attenuation is usually expressed in dB/Km









out
in
dB
P
P
L
10
log
10
1

Example 3.3: When the mean optical power launched into an 8 Km length of fiber is 120 W,
the mean optical power at the fiber out is 3 W.
Determine:
a) the overall signal attenuation in dB through the fiber assuming there are no connectors or
splices;
b) The signal attenuation/Km for the fiber
c) The overall signal attenuation for 10 Km optical link using the same fiber with splices at
1 Km intervals, each giving an attenuation of 1 dB;
d) The numerical input/out power ratio in (c).
Example 3.1, 3.3 Senior, self study

73.
Dispersion
Dispersion of the transmitted optical signal causes
distortion for both digital and analog transmission
along optical fibers.
Types of dispersion:
• Chromatic or intramodal dispersion
Material Dispersion
Waveguide Dispersion
• Intermodal dispersion or Modal Dispersion

74.
• Chromatic or intramodal dispersion
Chromatic or intramodal dispersion may occur in all types
of optical fiber and results from the finite spectral
linewidth of the optical source. Since optical sources do
not emit just a single frequency but a band of frequencies
then there may be propagation delay differences between
the different spectral components of the transmitted
signal. This causes broadening of each transmitted mode
and hence intramodal dispersion. The delay differences
may be caused by the dispersive properties of the
waveguide material (material dispersion) and also
guidance effects within the fiber structure (waveguide
dispersion) .

75.
Material Dispersion
f1
f2
n1
n2
Changing optical path length due to a changing refractive index n1
Material Dispersion
Velocity of electromagnetic wave in any medium = c/n
For glass material n(), i.e, n changes with  or frequency
Output
wave
t
I
Input wave

76.
Mathematical Expression of Material Dispersion




d
d

If  is the difference in propagation time between two
sidebands then
where  is the wavelength difference  = 1-2
Dmat is the material dispersion coefficient then










g
mat
v
d
d
d
d
D
1



where  =1/vg for a unit fiber length

2
1
All optical
signals
consist of a
range of
wavelengths

77.
Mathematical Expression of Material
Dispersion





































 n
d
d
c
c
n
d
d
c
d
d
c
d
d
d
d
vg
2
2
2
2
2
1
c
N
d
dn
n
c
v
g
g











1
1
2
2
1


 d
n
d
c
v
d
d
D
g
mat 














n
2

For free space
c
n
vp


 

For any medium with n,
0
2


 
Again



c
2


78.
Single mode propagation
The RI experienced by the
wave is an average of the RI of
core and cladding depending on
the relative proportion of the
wave that travels there. Thus
the wave confined within the
core, “see” a higher RI than
that of the cladding. Therefore,
wave in the core tend to travel
more slowly than that of
cladding. Thus signals are
dispersed (because every signal
consists of a range of
wavelengths)

79.
Waveguide Dispersion
 Changing optical path length due to traveling the light wave in the core and
cladding with different phase constant, which can be changed by θi
θi
θi
t
I
Input light
wave
With different θi

80.
Waveguide Dispersion
Transit time per unit length of source linewidth
  
























 i
n
d
d
c
x
d
d
c
d
x
d cos
2
2
1
2
1 2
2
 
i
i
d
d
c
n
d
dn
n 





 cos
cos 









GVD=c= mat + wg

81.
Fiber modes
Core dia. large than  of
light
Core dia. large than  of
light
Core dia. comparable than  of
light
When light travels on a multimode fiber it is limited to a
relatively small number of possible paths (called modes).

82.
Modal Dispersion
Time taken by the ray1 (axial) to travel fiber length L will be
Tmin
Ray2 will take maximum time to travel fiber length L,
Tmax


cos
cos
tan 1
1
max
c
LN
N
c
L
velocity
ce
dis
T
g
g



Using Snell’s law
c
LN
N
c
L
velocity
ce
dis
T
g
g
1
1
min
tan




 cos
sin
1
2


n
n
c
………
….(i)
…………
.(ii)
…………
.(iii)
t
I
Input light
wave
n1
n2
L
θi =c
c
Ray2
θ
θa
Ray1(axial
)
θi=900

83.
Modal Dispersion
Substituting Eq.(ii) into Eq.(iii) we have
2
2
1
max
g
g
cN
LN
T 
The difference in arrival time between the Ray1 and Ray2 is
 
2
1
2
1
1
2
2
1
min
max g
g
g
g
g
g
g
s N
N
cN
LN
c
LN
cN
LN
T
T
T 






If we take Ng1/n1≈Ng2/n2 then dispersion per unit length
n
cn
N
n
n
cn
N
N
N
cN
N
L
T g
g
g
g
g
g
s


2
1
2
1
2
1
2
1
2
1
)
(
)
( 




Modal dispersion per unit length n
cn
Ng


2
1
mod 
Modal dispersion is very sensitive for MM step index fiber

84.
Effect of Dispersion on OFC
n1
n2
1 0 1 1
L2
Long length fiber
Input pulse
n1
n2
1 0 1 1
L1
1 0 1 1
Short length fiber
Output pulse
Input pulse
t
t
Output pulse
No zero level Indistinguishable pulse
Intersymbol
Interference
t

85.
Wavelength dependence of Ng and n

86.
Polarization mode dispersion (PMD) is a source of
pulse broadening which results from fiber
birefringence and it can become a limiting factor for
optical fiber communications at high transmission
rates. It is a random effect due to both intrinsic
(caused by noncircular fiber core geometry and
residual stresses in the glass material near the core
region) and extrinsic (caused by stress from
mechanical loading, bending or twisting of the fiber)
factors which in actual manufactured fibers result in
group velocity variation with polarization state.
Polarization mode dispersion

87.
Different types of fibers: DSF, DCF,
Dispersion compensation schemes

88.
Dispersion-shifted fiber (DSF) is a type of optical fiber made to optimize both low
dispersion and low attenuation.
Dispersion Shifted Fiber is a type of single-mode optical fiber with a core-clad index
profile tailored to shift the zero-dispersion wavelength from the natural 1300 nm in silica-
glass fibers to the minimum-loss window at 1550 nm. The group velocity or intramodal
dispersion which dominates in single-mode fibers includes both material and waveguide
dispersion. Waveguide dispersion can be made more negative by changing the index
profile and thus be used to offset the fixed material dispersion, shifting or flattening the
overall intramodal dispersion. This is advantageous because it allows a communication
system to possess both low dispersion and low attenuation. However, when used in
wavelength division multiplexing systems, dispersion-shifted fibers can suffer from four-
wave mixing which causes intermodulation of the independent signals. As a result,
nonzero dispersion shifted fiber is often used.
DSF

89.
A Dispersion Compensation Module (also called a
Dispersion Compensation Fiber (DCF)) provides
fixed chromatic dispersion compensation for diverse
and disaster recovery DWDM networks.
This can support medium to long distance fiber optic
systems ranging from 30 km to 600 km.
DCF

90.
Double-clad fiber (DCF) is a class of optical fiber with a
structure consisting of three layers of optical material
instead of the usual two. The inner-most layer is called the
core. It is surrounded by the inner cladding, which is
surrounded by the outer cladding. The three layers are
made of materials with different refractive indices.
DCF

91.
Self Study:
Dispersion compensation schemes

92.
Fiber cabling process

93.
 Liquid-phase technique
 Vapour-phase deposition technique
 Modified chemical vapour deposition (MCVD)
 Plasma-activated chemical vapour deposition (PCVD)
Preparation method of Optical Fiber

94.
Fabrication of telecommunication-grade silica fibers involves
two stages. In the first stage a vapor-deposition method is used
to make a cylindrical preform with the desired refractive-index
profile. The preform is typically 1 m long and 2 cm in diameter
and contains core and cladding layers with correct relative
dimensions. In the second stage, the by using a precision-feed
mechanism that preform is drawn into a fiber feeds the preform
into a furnace at the proper speed. To make the preform
commonly used methods are modified chemical-vapor
deposition (MCVD), outside-vapor deposition (OVD), and
vapor-axial deposition (VAD). [Govind P. Agrawal]
Fabrication Method

95.
Modified chemical vapour deposition (MCVD)
Figure 2.21: MCVD process for fiber fabrication [Govind P. Agrawal]

96.
Modified chemical vapour deposition (MCVD)
Figure 2.21 shows a schematic diagram of the MCVD process. In this process,
successive layers of SiO2 are deposited on the inside of a fused silica tube by mixing
the vapors of SiCl4 and O2 at a temperature of about 1800◦C. To ensure uniformity, a
multiburner torch is moved back and forth across the tube length using an automatic
translation stage. The refractive index of the cladding layers is controlled by adding
fluorine to the tube. When a sufficient cladding thickness has been deposited, the core
is formed by adding the vapors of GeCl4 or POCl3 (Phosphoryl Trichloride). These
vapors react with oxygen to form the dopants GeO2 and P2O5:
GeCl4 +O2 → GeO2 +2Cl2,
4POCl3 +3O2 → 2P2O5 +6Cl2.
The flow rate of GeCl4 or POCl3 determines the amount of dopant and the
corresponding increase in the refractive index of the core. A triangular-index core can
be fabricated simply by varying the flow rate from layer to layer. When all layers
forming the core have been deposited, the torch temperature is raised to collapse the
tube into a solid rod of preform. The MCVD process is also known as the inner-vapor-
deposition method, as the core and cladding layers are deposited inside a silica tube.

97.
Fiber drawing
Figure 2.22: Apparatus used for fiber drawing. [Govind P. Agrawal]

98.
Fiber drawing
The fiber drawing step is essentially the same irrespective of the process used to
make the preform. Figure 2.22 shows the drawing apparatus schematically. The
preform is fed into a furnace in a controlled manner where it is heated to a
temperature of about 2000◦C. The melted preform is drawn into a fiber by using a
precision-feed mechanism. The fiber diameter is monitored optically by
diffracting light emitted by a laser from the fiber. A change in the diameter
changes the diffraction pattern, which in turn changes the photodiode current.
This current change acts as a signal for a servocontrol mechanism that adjusts the
winding rate of the fiber. The fiber diameter can be kept constant to within 0.1%
by this technique. A polymer coating is applied to the fiber during the drawing
step. It serves a dual purpose, as it provides mechanical protection and preserves
the transmission properties of the fiber. The diameter of the coated fiber is
typically 250 µm, although it can be as large as 900 µm when multiple coatings
are used. The tensile strength of the fiber is monitored during its winding on the
drum. The winding rate is typically 0.2–0.5 m/s. Several hours are required to
convert a single preform into a fiber of about 5 km length.

99.
Fiber joints/connectors and
couplers

100.
Fiber Connectors
Fiber connectors should have the following properties:
 The fiber termination, which protects and locates the fiber
ends
 The fiber end alignment to provide optimum optical
coupling
 Protect the fiber ends from the environments and provide
adequate strength at the joint

101.
Why we need joints
 Fiber can only be installed in lengths upto 2Km, for
longer spans a joint is needed.
 For the repair of damaged fiber.
 For test purpose at terminal equipment.
 All of the fiber cable in a building cannot be installed
as one continues cable run.
 joints are needed to complete network cabling.
 Temporary access is needed for test purposes.

102.
Type of fiber connectors
 Cylindrical ferrule connectors
 Bioconical ferrule connectors
 Double eccentric connectors
 Duplex and multiple fiber connectors
 Expanded beam connectors

103.
Cylindrical ferrule connector

104.
Bioconical ferrule connector

105.
Double eccentric connector

106.
Duplex and multiple fiber connector

107.
Expanded beam connector
Lenses
Optical Fiber Optical Fiber

108.
Graded Index (GRIN) Rod Lens
An alternative lens geometry to facilitate efficient beam
expansion and collimation within expanded beam
connectors is that of GRIN rod lens. Its diameter varies
typically from 0.5 mm to 2 mm
  10 to 50
GRIN Rod Lens

109.
Graded Index (GRIN) Rod Lens
  10 to 50
GRIN Rod Lens
The GRIN-rod lens, which arose from developments on graded index fiber waveguides
,comprises a cylindrical glass rod typically 0.2 to 2 mm in diameter
which exhibits a parabolic refractive index profile with a maximum at the axis similar to
graded index fiber. Light propagation through the lens is determined by the lens
dimensions and, because refractive index is a wavelength-dependent parameter, by
the wavelength of the light. The GRIN-rod lens can produce a collimated output beam
with a divergent angle of between 1° and 5° from a light source situated on, or near to,
the opposite lens face.

110.
The refractive index variation with radius therefore causes all the input rays to follow
a sinusoidal path through the lens medium. The traversion of one sinusoidal period is
termed one full pitch and GRIN-rod lenses are manufactured with several pitch
lengths.Three major pitch lengths are as follows:
1. The quarter pitch (0.25 pitch) lens, which produces a perfectly collimated output
beam when the input light emanates from a point source on the opposite lens face.
Conversely, the lens focuses an incoming light beam to a point at the center of the
opposite lens face.Thus the focal point of the quarter pitch GRIN-rod lens is
coincident with the lens faces, thus providing efficient direct butted connection to
optical fiber.
2. The 0.23 pitch lens is designed such that its focal point lies outside the lens when
a collimated beam is projected on the opposite lens face. It is often employed to
convert the diverging beam from a fiber or laser diode into a collimated beam, as
illustrated in.
3. The 0.29 pitch lens is designed such that both focal points lie just outside the lens
end faces. It is frequently used to convert a diverging beam from a laser diode into a
converging beam. Hence, it proves useful for coupling the output from a laser diode
into an optical fiber, or alternatively for coupling the output from an optical fiber into a
photodetector.
Graded Index (GRIN) Rod Lens

111.
Quarter pitch lens
0.25P
0.25P

112.
0.23 and 0.29 pitch lens
0.23P
0.29P
Source Fiber
Fiber
GRIN
GRIN

113.
Fiber joint loss
 In fiber-fiber connection the optical loss
encountered at interface.
 The loss in optical power through a
connection is defined as
Po is the power emitted from the source fiber
Pi is the power accepted by the connected fiber

114.
Fiber joint loss
 Intrinsic coupling losses are limited by reducing fiber mismatches
between the connected fibers.
 This is done by procuring only fibers that meet stringent geometrical
and optical specifications
 Extrinsic coupling losses are limited by proper connection
procedures.

115.
Reducing Fresnel Loss
 To reduce the amount of loss from Fresnel
reflection, the air gap can be filled with an index
matching gel.
 The refractive index of the index matching gel
should match the refractive index of the fiber core.
 Index matching gel reduces the step change in the
refractive index at the fiber interface.

116.
Misalignment losses
These losses depends upon the fiber type , core diameter and
the distribution of the optical power.

117.
Fiber Splice
A permanent joint formed between two individual optical
fibers in the field or factory is known as a FIBER SPLICE.
Used to establish long haul optical fiber links
Two types of splicing:
1. Fusion splicing
2. Mechanical splicing

118.
Fiber Coupler
An optical fiber coupler is a device that distributes light from
a main fiber into one or more branch fibers. The latter case is
more normal and such devices are known as multiport fiber
couplers. Optical fiber couplers are often passive devices in
which the power transfer takes place either:
• through the fiber core cross-section by butt jointing the
fibers or by using some form of imaging optics between
the fibers (core interaction type)
• through the fiber surface and normal to its axis by
converting the guided core modes to both cladding and
refracted modes which then enable the power-sharing
mechanism (surface interaction type)

119.
Multiport Coupler
Multiport optical fiber couplers can also be subdivided into
the following:
1. Three- and four-port* couplers, which are used for signal
splitting, distribution and combining.
2. Star couplers, which are generally used for distributing a
single input signal to multiple outputs.
3. Wavelength division multiplexing (WDM) devices, which
are a specialized form of coupler designed to permit a number
of different peak wavelength optical signals to be transmitted
in parallel on a single fiber. In this context WDM couplers
either combine the different wavelength optical signal onto
the fiber (i.e. multiplex) or separate the different wavelength
optical signals output from the fiber (i.e. demultiplex).

120.
Fiber Coupler

121.
Mid Term Syllabus ( First slide to Previous slide)

122.
Optical transmitter: LED and laser, Operating
principles, Characteristics and driver circuits

123.
Types of optical sources
There are three main types of optical light source
are available:
 Wide band sources (incandescent lamps)
 Incoherent sources (LEDs)
 Coherent sources (LASERs)

124.
Energy state basic
The interaction of light with matter takes place in discrete packets of energy or
quanta, called photons. Furthermore, the quantum theory suggests that atoms exist
only in certain discrete energy states such that absorption and emission of light
causes them to make a transition from one discrete energy state to another. The
frequency of the absorbed or emitted radiation f is related to the difference in energy
E between the higher energy state E2 and the lower energy state E1 by the
expression: E = E2 - E1 = hf
where h = 6.626 × 10-34 J s is Planck’s constant.

125.
Light Emission
Absorption and Emission of Radiation

126.
If the photon energy hν of the incident light of frequency ν is about
the same as the energy difference Eg = E2 -E1, the photon is absorbed
by the atom, which ends up in the excited state. Incident light is
attenuated as a result of many such absorption events occurring inside
the medium. The excited atoms eventually return to their normal
“ground” state and emit light in the process.
In the case of spontaneous emission, photons are emitted in random
directions with no phase relationship among them. LEDs emit light
through the incoherent process of spontaneous emission.
Stimulated emission, by contrast, is initiated by an existing photon.
The remarkable feature of stimulated emission is that the emitted
photon matches the original photon not only in energy (or in
frequency), but also in its other characteristics, such as the direction of
propagation. All lasers, including semiconductor lasers, emit light
through the process of stimulated emission and are said to emit
coherent light.
Absorption and Emission of Radiation

127.
Absorption and Emission of Radiation
Absorption
Spontaneous
emission
Stimulated
emission
Initial state Final state

128.
Population inversion
Under the conditions of thermal equilibrium given by the Boltzmann
distribution (Eq. (6.2)) the lower energy level E1 of the two-level atomic
system contains more atoms than the upper energy level E2. This situation,
which is normal for structures at room temperature, is illustrated in Figure
6.2(a). However, to achieve optical amplification it is necessary to create a
nonequilibrium distribution of atoms such that the population of the upper
energy level is greater than that of the lower energy level (i.e. N2 > N1). This
condition, which is known as population inversion, is illustrated in Figure
6.2(b).
In order to achieve population inversion it is necessary to excite atoms into the
upper energy level E2 and hence obtain a nonequilibrium distribution. This
process is achieved using an external energy source and is referred to as
‘pumping’. A common method used for pumping involves the application of
intense radiation (e.g. from an optical flash tube or high-frequency radio field).
In the former case atoms are excited into the higher energy state through
stimulated absorption.

129.
The LASERs
Light Amplification by Stimulated
Emission of Radiation (LASER)
Type of lasers
 Solid state lasers
 Semiconductor lasers
 Gas lasers
 Dye lasers

130.
Basic construction of Laser
Mirror with
100%
reflective
Mirror with
partially
reflective
Gain medium
Laser pump
Energy to create non
equilibrium state
Photon
multiplication
Initial
state
Amplified
light !!

131.
The light emitting diodes
A forward-biased p–n junction emits light through spontaneous emission, a
phenomenon referred to as electroluminescence. In its simplest form, an LED
is a forward biased p–n homojunction. Radiative recombination of electron
hole pairs in the depletion region generates light; some of it escapes from the
device and can be coupled into an optical fiber. The emitted light is incoherent
with a relatively wide spectral width (30–60 nm) and a relatively large angular
spread.

132.
The light emitting diodes
Drawbacks of LEDs:
• Lower optical power (microwatts)
• Lower modulation bandwidth
• Harmonic distortion

133.
The light emitting diodes
Merits of LEDS:
• Simpler fabrication
• Low cost
• Reliability
• Generally less temperature dependence
• Simpler drive circuitry
• Linearity

134.
LED structures
Ohmic
contacts
Light output
n-type substrate
p-type
epitaxial
layer
Planar LED
Ohmic contact
p-type
n-type
Dome LED

135.
Double Heterojunction LED

136.
Surface emitting LED

137.
The LED structures can be classified as surface-emitting or edge-emitting, depending on
whether the LED emits light from a surface that is parallel to the junction plane or from
the edge of the junction region. Both types can be made using either a p–n homojunction
or a heterostructure design in which the active region is surrounded by p- and n-type
cladding layers. The heterostructure design leads to superior performance, as it provides a
control over the emissive area and eliminates internal absorption because of the
transparent cladding layers. Figure 3.8 shows schematically a surface emitting LED
design referred to as the urrus-type LED [22]. The emissive area of the device is limited
to a small region whose lateral dimension is comparable to the fiber-core diameter. The
use of a gold stud avoids power loss from the back surface. The coupling efficiency is
improved by etching a well and bringing the fiber close to the emissive area. The power
coupled into the fiber depends on many parameters, such as the numerical aperture of the
fiber and the distance between fiber and LED. The addition of epoxy in the etched well
tends to increase the external quantum efficiency as it reduces the refractive-index
mismatch. Several variations of the basic design exist in the literature. In one variation, a
truncated spherical microlens fabricated inside the etched well is used to couple light into
the fiber [23]. In another variation, the fiber end is itself formed in the form of a spherical
lens [24]. With a proper design, surface-emitting LEDs can couple up to 1% of the
internally generated power into an optical fiber.
Surface emitting LED

138.
Edge emitting LED

139.
LASER or LED as light source?
Semiconductor lasers emit light through stimulated emission. As a result of
the fundamental differences between spontaneous and stimulated emission,
they are not only capable of emitting high powers (∼ 100 mW), but also
have other advantages related to the coherent nature of emitted light. A
relatively narrow angular spread of the output beam compared with LEDs
permits high coupling efficiency (∼ 50%) into single-mode fibers. A
relatively narrow spectral width of emitted light allows operation at high bit
rates (∼ 10 Gb/s), since fiber dispersion becomes less critical for such an
optical source. Furthermore, semiconductor lasers can be modulated
directly at high frequencies (up to 25 GHz) because of a short
recombination time associated with stimulated emission. Most fiber-optic
communication systems use semiconductor lasers as an optical source
because of their superior performance compared with LEDs.

140.
Electrical bandwidth
)
(
)
(det
log
10 10
source
input
power
electrical
ector
output
power
electrical
REdB 
Electrical bandwidth: The ratio of the electrical power output (at the
detector) to the electrical input power
in
in
out
out
dB
R
I
R
I
RE 2
2
10
log
10

2
10
log
10 






in
out
dB
I
I
RE
Electrical 3 dB point occur when
2
1
,
2
1
2














in
out
in
out
I
I
or
I
I

141.
Optical bandwidth
)
(
)
det
(
log
10 10
source
at
d
transmitte
in
power
optical
ector
at
received
out
power
optical
ROdB 
Optical bandwidth: The ratio of the optical power output (received
at the detector) to the electrical input power (transmitted at the source)







in
out
dB
I
I
RO 10
log
10
Optical 3 dB point occur when
2
1







in
out
I
I
Example: 7.7 (self study)

142.
Modulation bandwidth
Electrical 3 dB point
Optical 3 dB point
Frequency
Electrical BW
Optical BW
0.50
0.707
Iout/Iin

143.
Although an optical source is a major component of optical transmitters, it is not the only
component. Other components include a modulator for converting electrical data into
optical form (if direct modulation is not used) and an electrical driving circuit for
supplying current to the optical source. An external modulator is often used in practice at
bit rates of 10 Gb/s or more for avoiding the chirp that is invariably imposed on the
directly modulated signal.
Transmitter

144.
Driving Circuit

145.
The purpose of driving circuitry is to provide electrical power to the optical source and to
modulate the light output in accordance with the signal that is to be transmitted. Driving
circuits are relatively simple for LED transmitters but become increasingly complicated for
high-bit-rate optical transmitters employing semiconductor lasers as an optical source
Figure 3.26 shows a simple driving circuit that controls the average optical power through
a feedback mechanism. A photodiode monitors the laser output and generates the control
signal that is used to adjust the laser bias level. The rear facet of the laser is generally used
for the monitoring purpose (see Fig. 3.25). In some transmitters a front-end tap is used to
divert a small fraction of the output power to the detector. The bias-level control is
essential, since the laser threshold is sensitive to the operating temperature. The threshold
current also increases with aging of the transmitter because of gradual degradation of the
semiconductor laser. The driving circuit shown in Fig. 3.26 adjusts the bias level
dynamically but leaves the modulation current unchanged. Such an approach is acceptable
if the slope efficiency of the laser does not change with aging. As discussed in Section
3.5.1 and seen in Fig. 3.20, the slope efficiency of the laser generally decreases with an
increase in temperature. A thermoelectric cooler is often used to stabilize the laser
temperature. An alternative approach consists of designing driving circuits that use dual-
loop feedback circuits and adjust both the bias current and the modulation current
automatically.
Driving Circuit

146.
Optical receivers: PN, PIN and APD
detectors, Noise at the receiver, SNR and
BER calculation, Receiver sensitivity
calculation.

147.
What is photodetector
The detector is an essential component of an
optical fiber communication system and is one of
the crucial elements which dictate the overall
system performance. Its function is to
Photodetector is an important elements in OFC,
which converts optical signal into electrical form.

148.
Characteristics of photodetector
 High sensitivity at the operating wavelength
 High fidelity
 Short response time to obtain a suitable
bandwidth
 Noise should be minimum
 Stability of performance characteristics
 Small size
 Low cost

149.
Photodetector types
Photo-
detectors
Photomulti-
plier
tubes
Vacuum
Photo-
diodes
pn-PD P-i-N PD APD
PD used in OFC

150.
V-I characteristics of PD
I
V
Region 2 Region 1
Region 3
Increasing
optical
power
Photovoltaic
mode
Photoconductive
mode

151.
Photodetection principles
Eg
hf >Eg
- +
p n

152.
Photon absorption in intrinsic
material
E2 - E1
hf >E2 – E1
E2
E1
To excite an electron incident
photon should have energy
E
hc
E
E
hc




1
2
0


153.
Absorption coefficient
Absorption coefficient is a measure of how good the material is
for absorbing light of a certain wavelength
d
 
)
exp(
1
)
1
(
0
d
hf
r
e
P
Ip 




The photo current Ip produce by
incident light of optical power P0
e : Electronic charge
r : Fresnel reflection coefficient

154.
Absorption coefficient of
various materials

155.
Quantum efficiency
The quantum efficiency n is defined as the fraction of incident photons
which are absorbed by the photodetector and generated electrons which
are collected at the detector terminal
p
e
r
r


n = Number of electrons collected/ Number of incident photons
rp: Incident photon rate
re: Corresponding electron rate

156.
Relationship between
responsivity and n
hf
P
r
r p
e
0

 

)
( 1
0

 AW
P
I
R
p
hf
P
rp
0

where Ip: Photocurrent, P0: Incident optical power
The incident photon rate rp in terms of optical power and
photon energy can be written as
The responsivity R of a photodetector is defined as
Electron rate can be defined as
Output photocurrent is:
hf
e
P
Ip
0

 Thus
hc
e
hf
e
R






157.
Wavelength dependence of
responsivity
Responsivity (A/W)
0.44
0.88
0.5 1.0 c
Ideal Si PD
Typical PD
Exp. 8.1, 8.2
J. Senior

158.
p-n photodiodes
E-Field
Depletion
region
Absorption
region
hf
x
p
n
Diffusion region
Load

159.
Output Ch. of a typical p-n
photodiodes
Reverse bias (V)
Current A
10 20 30 40
200
400
600
800
High light level
Low light level
Dark current (no light)

160.
p-i-n Photodiode
E-Field
Depletion region
Absorption region
hf
x
p
i
Load
n

161.
p-i-n photodiode structures
Metal contact
SiO2
Antireflection
coating
Depletion layer
P+
n+
hf
Front illuminated Si PD
i
Metal contact
n+
p+
i
Antireflection
coating
Reflection
coating
Side illuminated Si PD

162.
Speed of response of PD
There are three main factors that limit the
speed of response of a PD
 Drift time of carrier (depletion region)
 Diffusion time of carriers (outside of
depletion region)
 Transition capacitance

163.
Speed of response of PD
Drift time of carriers through the depletion region:
d
drift
v
w
t 
w : width of depletion region
vd : drift velocity
For electric field 2x104 v/cm, vd=107cm/s,
tdrift=0.1 ns when w=10 micron
Diffusion time of carriers outside the depletion region:
c
difft
D
d
t
2
2

d : carriers diffusion distance
Dc : diffusion coefficient
For 10 m diffusion distance, hole
diffusion time 40 ns whereas electron
diffusion time is only 8 ns

164.
Speed of response of PD
Time constant incurred by the capacitance of the PD with its load:
w
A
Cj


To maximize the speed of response, the transit time need to minimize by
Increasing bias voltage
Decreasing layer thickness
Increasing bias voltage resulting to increase drift speed, which lead to
reduce drift time. Further depletion layer thickness may increase with
bias voltage
Quantum efficiency will fall with decreasing layer thickness, w. It also
increase junction capacitance, which lead to rise RC time constant. Thus
device speed will slowdown

165.
PD response to a rectangular
input pulse
W W
P n n n
P P
+
- +
+
-
-
Large C
Narrow W
Low C and
W>>1/s

166.
Avalanche photodiodes
hf
Load
n
p
p+
i
Gain region
Absorption
region
E-field

167.
Silicon reach through APD
p+
p

n+
50m Absorption
region
Gain region
E-field
When reverse biased voltage is 10% less of the avalanche breakdown
limit, the depletion layer reaches through to the  region

168.
APD response time
APD response time is limited by:
 Transit time of the carrier across the
absorption region
 Time taken for avalanche multiplication
 RC time constant

169.
APD responsivity
hc
e
hf
e
R





)
( 1
0

 AW
P
I
R
p
Responsivity for p-i-n PD
Responsivity for APD PD )
( 1
0

 AW
P
MI
R
p
M: APD gain
Responsivity for p-i-n PD
hc
e
M
R



Responsivity for APD PD

170.
Basic structure of an optical
receiver
PD
Preamplifier Post-amplifier Pre-detection
filter
Electrical
signal
Optical
signal

171.
Sources of noise in an optical
receiver
Photo-
detection
Avalanche
gain
Detector
load bias
Electronic
gain
Optical
signal
Photodetector
Amplifier
Electrical
signal
Noise
• Quantum shot
• Dark current
• Surface leakage
Noise
Excess noise due
to random gain
mechanisms
Noise
Thermal
Noise
• Thermal noise
• Device (active
element)
• Surface leakage
currents

172.
Photodetector noises
DS
n
DB
n
Q
n
PD
n i
i
i
i
2
2
2
2



Q
n
i
2
DB
n
i
2
DS
n
i
2
: Due to quanta of light generating packets
of electron-hole pairs
: Due to thermally generated dark currents
occurring in the PD bulk material
: Due to surface leakage currents

173.
Signal to noise ratio of p-i-n
PD
c
n
eq
DS
eq
DB
Q
n
s
i
B
qI
B
qI
i
I
N
S
2
2
2
2
2 



S/N for shot noise
limited condition:
eq
s
s
Q
n
s
B
I
q
I
i
I
N
S
2
2
2
2
2


S/N for thermal noise
limited condition: eq
L
s
c
th
s
KTB
R
I
i
I
N
S
4
2
2
2


Beq: Noise equivalent bandwidth
IDB: Bulk leakage current
IDS: Surface leakage current

174.
Signal to noise ratio of APD PD
c
n
eq
DS
eq
DB
eq
s
s
i
B
qI
B
M
F
M
qI
B
M
F
M
I
q
M
I
N
S
2
2
2
2
2
2
)
(
2
)
(
2 



S/N for shot noise
limited condition:
eq
s
s
Q
n B
M
F
M
I
q
M
I
i
Is
N
S
)
(
2 2
2
2
2
2
2


S/N for thermal noise
limited condition:
c
th
s
i
M
I
N
S
2
2
2

M: Multiplication factor,
F(M): Excess noise factor due to random fluctuation of APD gain

175.
APD Noise
k
W
k
k
M
e 




)
)
1
(
exp(
1

x : is an empirical constant which is less than 1
F(M) can be approximated by:
K:e/h
e: Electron ionization coefficient
h: Hole ionization coefficient
x
M
M
F 
)
(

176.
APD Noise
e
e
e
e
M
M
K
KM
M
F
)
1
2
)(
1
(
)
(




h
h
h
h
M
M
K
KM
M
F
)
1
2
)(
1
1
(
)
(




F(M) depends on the value of K and type of carrier
undergoing multiplication
For Si APD with M=100 and K=0.02, Fe(M) ~ 4
For Ge APD with M=20 and K=0.5 gives Fe(M) ~ 11

177.
S/N for shot noise
limited condition:
eq
s
s
Q
n B
M
F
M
I
q
M
I
i
Is
N
S
)
(
2 2
2
2
2
2
2


S/N for thermal noise
limited condition:
c
th
s
i
M
I
N
S
2
2
2

Signal to noise ratio of APD PD
eq
n
in
L
eq
n
L
B
KTF
P
R
R
B
KTF
SR
N
S
4
4
2
2


S/N for thermal noise
limited condition: