Optical Fiber Communication.ppt

OPTICAL FIBER
COMMUNICATION
M.NARESH
M.E.,(Ph. D)
ASSISTANT PROFESSOR,ECE Dept.,
MATRUSRI ENGINEERING COLLEGE
Elective-I,EC-412

UNIT-I
• Evolution of fiber optic system
• Elements of Optical Fiber Transmission link
• Optical Fiber Modes and Configurations
• Ray Optics
• Mode theory of Circular Waveguides:
-Overview of Modes and Key concepts
- Linearly Polarized Modes
• Single Mode Fibers and Graded Index fiber structure

Evolution of Fiber optic System(1/2):
Circa 2500 B.C. Earliest known glass
Roman times-glass drawn into fibers
Venice Decorative Flowers made of glass fibers
1609-Galileo uses optical telescope
1626-Snell formulates law of refraction
1668-Newton invents reflection telescope
1840-Samuel Morse Invents Telegraph
1841-Daniel Colladon-Light guiding demonstrated
in water jet
1870-Tyndall observes light guiding in a thin water jet
1873-Maxwell electromagnetic waves
1876-Elisha Gray and Alexander Bell Invent
Telephone
1877-First Telephone Exchange
1880-Bell invents Photophone
1888-Hertz Confirms EM waves and relation to light
1880-1920 Glass rods used for illumination
1897-Rayleigh analyzes waveguide
1899-Marconi Radio Communication
1902-Marconi invention of radio detector
1910-1940 Vacuum Tubes invented and developed
1930-Lamb experiments with silica fiber
1931-Owens-Fiberglass
1936-1940 Communication using a waveguide
1876-Alexander
Graham Bell
1876 First commercial Telephone
1970 I. Hayashi
Semiconductor Laser

Evolution of Fiber optic System (2/2):
1951-Heel, Hopkins, Kapany image transmission using fiber
bundles
1957-First Endoscope used in patient
1958-Goubau et. al. Experiments with the lens guide
1958-59 Kapany creates optical fiber with cladding
1960-Ted Maiman demonstrates first laser in Ruby
1960-Javan et. al. invents HeNe laser
1962-4 Groups simultaneously make first semiconductor lasers
1961-66 Kao, Snitzer et al conceive of low loss single mode fiber
communications and develop theory
1970-First room temp. CW semiconductor laser-Hayashi &
Panish
April 1977-First fiber link with live telephone traffic-
GTE Long Beach 6 Mb/s
May 1977-First Bell system 45 mb/s links
GaAs lasers 850nm Multimode -2dB/km loss
Early 1980s-InGaAsP 1.3 µm Lasers
- 0.5 dB/km, lower dispersion-Single mode
Late 1980s-Single mode transmission at 1.55 µm -0.2 dB/km
1989-Erbium doped fiber amplifier
1 Q 1996-8 Channel WDM
4th Q 1996-16 Channel WDM
1Q 1998-40 Channel WDM

Bells Photophone
1880 - Photophone Transmitter
1880 - Photophone Receiver
“The ordinary man…will find a little difficulty in comprehending how sunbeams are to be used. Does Prof.
Bell intend to connect Boston and Cambridge…with a line of sunbeams hung on telegraph posts, and, if so,
what diameter are the sunbeams to be…?…will it be necessary to insulate them against the
weather…?…until (the public) sees a man going through the streets with a coil of No. 12 sunbeams on his
shoulder, and suspending them from pole to pole, there will be a general feeling that there is something
about Prof. Bell’s photophone which places a tremendous strain on human credulity.”
New York Times Editorial, 30 August 1880

Approaches to Optical Communication

Operating ranges of components

Elements of Optical Fiber Transmission link

Digital transmission hierarchy

Optical fiber cable installations

Spherical and plane wave fronts
Nature of Light

Field distributions in plane E&M waves
Adding two linearly polarized waves

Optical Fiber Modes and Configurations

Types: 1.step index
2. Graded index
Mode: The propagation of light a long a wave guide can be described in terms
of set of guided electromagnetic waves called “ Modes of the Waveguide”
1.The Refractive index of the Core is Uniform throughout and undergoes an abrupt
change (step) at the cladding boundary is called “STEP-INDEX”
2. The Core Refractive index is made to vary as a function of the radial distance from
the center of the fiber is called “ GRADED-INDEX”
Both the step and graded-index fibers can be further dived into single and multimode
Classes.

Comparison of single mode and multi-mode step index
and graded index optical fiber

Ray optics
Ray optics representation of skew rays travelling in a step –index fiber optical core
Two types of rays: 1. Meridional rays 2. Skew rays
Meridioanl rays confined to the meridian plan of the fiber, ray lines in a single plane, its path is
Easy to track it travels along the fiber. It is two types 1.Bound rays 2. Trapped rays.
Skew rays are not confined to a single plane, but instead tend to follow a helical –type path
Along the fiber as. It is difficult to track. It constitute a major portion of the total no. of guided
Rays.

Meridional ray representation
Meridional ray optics for step –index fiber wave guide
From the Snells law the min. critical angle SinØc= n2/n1

Light wave propagation
Light wave propagation along a fiber waveguide. Phase changes occur both as the wave traveles
Through the fiber medium an at the reflection points.

Mode theory of Circular Waveguides:
-Overview of Modes and Key concepts
- Linearly Polarized Modes
To attain a more detailed understanding of the optical power propagation mechanism in
Fiber, it is necessary to solve MAXWELL’s equations subject to the cylindrical boundary
conditions at the interface between the core and cladding of the fiber.
Key model concepts:

Single Mode Fibers and Graded Index fiber structure

Propagation modes in Single –Mode Fibers
Two polarizations of the fundamental HE11 mode in a single- mode fiber

UNIT-II
ATTENUATION & DISPERSION
• Attenuation
- Absorption losses
- Scattering losses,
- Bending Losses,
- Core and Cladding losses
• Signal Distortion in Optical Waveguides-Information Capacity determination
- Group Delay
- Material Dispersion
- Waveguide Dispersion
- Signal distortion in SM fibers Polarization Mode dispersion
- Intermodal dispersion,
• Pulse Broadening in Guided Index fibers, Mode Coupling
• Design Optimization of Single Mode fibers-Refractive Index profile and cut-off
wavelength

ATTENUATION
• Signal Attenuation also known as ‘FIBER LOSS ‘ or ‘SIGNAL LOSS’.
• Attenuation of a light signal as it propagation along a fiber is an important
consideration in the design of optical communication system.
• The Degree of Attenuation plays major role in determining the maximum
transmission distance between a Transmitter and receiver.
• The basic mechanism in a fiber are Absorption,
Scattering,
Radiative losses.
• Absorption is related to FIBER MATERIAL.
• Where as Scattering is associated both with the fiber material and with structural
imperfections in the optical waveguide.

ATTENUATION UNITS:
• As light travels along a fiber, its decreases exponentially with distance. If
P(0) is the optical power in a fiber at the origin(at z=0),then the power P(z)
at a distance z farther down the fiber is
P(z) = P(0) e-αpz
where







)
(
)
0
(
ln
1
z
P
P
z
p

)
(
343
.
4
)
(
)
0
(
log
10
)
/
( 1








 km
z
P
P
z
km
dB p


Attenuation coefficient in units decibels per kilometer

Absorption
• Absorption is caused by 3 different mechanisms:
1. Absorption by atomic defects in the glass composition
2. Extrinsic absorption by impurity atoms in the glass material
3. Intrinsic absorption by the basic constituent atoms of the fiber material.
1 rad(Si) = 100 erg/g = 0.01 J/Kg

Effects of ionizing radiation:
a. Loss increase during steady irradiation
to a total dose of 104 rad.
b. Subsequent recovery as a function
of time after radiation has
stopped.

Optical fiber attenuation as a function of wavelength yields nominal values of 0.40db/km
At 1310nm for standard single-mode fiber absorption by water molecules.








 


63
.
4
exp
10
60
6
.
46
2
.
154 2
X
x
x
uv





 



48
.
48
exp
10
81
.
7 11
X
X
IR

Optical fiber attenuation characteristics and their limiting mechanisms for aGeO2 doped
low loss low water content silica fiber.

Scattering Loss (1/3)
 Scattering losses in glass arise from microscopic variations in the material in the
Material density, fro compositional fluctuations, and from structural inhomogenetics
Or defects occurring during fiber manufacture.
 Glass is composed of a randomly connected network of molecules. Such a structure
naturally contains regions in which the molecular density is either higher or lower
than average density in the glass. In addition , since glass is made of several oxides,
such SiO2,Geo2, and P2O5, compositional fluctuations can occur.
These 2 effects give rise to refractive index variations that occur within the glass over
distances that are small compared with the wavelength.
These index variations cause a Rayleigh-type scattering of the light.
 Rayleigh scattering in glass is the same phenomenon that scatters light from the sun n
the atmosphere, there by giving rise to a blue sky.
The expressions for scattering- induced attenuation are fairly complex owing to the
random molecular nature and the various oxide constituents of glass. For single-
component glass the scattering lost at wavelength ʎ resulting from density fluctuations
can be approximately:

T
f
B
scat T
k
n 


 2
2
4
3
)
1
(
3
8


Alternatively
T
f
B
scat T
k
p
n 


 2
2
4
3
3
8

P is the photo elastic coefficient

Fundamental mode filed in a curved optical waveguide
Bending losses:
Radiative losses occur whenever an optical fiber undergoes a bend of finite radius
Of curvature.
Two types of curvatures:
1. Macroscopic bends having radii that are large compared with the fiber diameter
such as those that occur when a fiber cable turns a corner.
2. Random Microscopic bens of the fiber axis that can arise when the fibers are
incorporated into cables.

}
)
2
3
(
2
2
2
1
{ 3
/
2
2










 
kR
n
R
a
M
Meff






2
1 )
(
2
ka
n
M


2
4
2
1
)
(


















j
f
M
E
E
a
b
F 


CORE AND CLADDING LOSSES
 Core and cladding have different indices of refraction and therefore differ in
Composition, the core and cladding generally have different attenuation coefficients,
denoted α1 and α2 rspectively. If the influence of modal coupling is ignored, the loss for a
mode of order (v,m) for a step index v=waveguide is:
p
p
p
p clad
core
vm 2
1 

 

p
pcladd
vm )
( 1
2
1 


 


 The total loss of the waveguide can be found by summing over all modes weighted
By the fractional power in that mode

SIGNAL DISPERSION IN FIBERS
 An optical signal weakens from attenuation mechanism
And broadens due to Dispersion effects as it travels along a fiber.
 Eventually these two factors will cause neighboring pulses to overlap.
After a certain amount of overlap occurs, the receiver can no longer distinguish
the individual adjacent pulses and errors arise when interrupting the received signal

Overview of Dispersion Origins:
 Signal dispersion is a consequence of factors such as
-Intermodal delay/Internalmodal dispersion
- Intramodal Dispersion
- Polarization – mode Dispersion
- High –order Dispersion effects.
Intermodal Delay appears only in multimode fibers.
 Intermodal Dispersion/Chromatic Dispersion is pulse spreading that takes place
within a single mode.
 This spreading arises from the finite spectral emission width of an optical source. The
phenomenon is also known as Group velocity Dispersion.
 The spectral width is the band of wavelength over which the source emits light. This
wavelength band normally is characterized by the root-mean-square(rms) spectral widthσλ.
Depending on the device structure of a light emitting diode(LED), the Spectral width is
approximately 4 to 9 percent of a central wavelength.
 The two main causes of intermodal dispersion are as following:
1. Material dispersion
2. Waveguide dispersion

Polarization mode dispersion
 Polarization mode dispersion results from the fact that light-signal energy at a given
wavelength in a single mode fiber actually occupies two orthogonally polarization states or
modes.
 At the start of the fiber the two polarization states are aligned. However , since fiber
material is not perfectly uniform throughout its length.
 The group Velocities of two orthogonal polarization modes are Vgx and Vgy, then
the differential time delay during of the pulse over a distance L is











gy
gx
PMD
V
L
V
L


MODAL DELAY (1/2)
• Intermodal dispersion or Modal delay appears only in multi-mode fibers
• This signal-distorting mechanism is a result of each mode having a different value of the
group delay velocity at a single frequency.
• The steeper the angle of propagation of the ray congruence, the higher is the mode number
and consequently the slower the axial group velocity. This variation in the group velocities
of the different modes results in a Group Delay spread, which is the intermodal Dispersion.
• This dispersion mechanism is eliminated by single- mode operation but is important in
multimode fibers.
• The maximum pulse broadening arising from the modal delay is the difference between the
travel time Tmax of the longest ay congruence paths and the travel time Tmin of the shortest
ray congruence paths.
• This broadening is simply obtained from ray tracing and for a fiber of length L is given by;
c
Ln
cn
Ln
L
L
c
n
T
T
T
c
















 1
2
2
1
1
min
max
sin 

MODAL DELAY (2/2)
 Maximum Bit rate B can be sent over a multimode step-index fiber. Typically the fiber
capacity is specified in terms of the “Bit rate- distance product (BL).”
 In order for neighboring signal pulses to remain distinguishable at the receiver , the pulse
Spread should be less than 1/B, Which is the width of bit period.
 The bit rate – distance product:


c
n
n
BL 2
1
2
The rms impulse response s
 due to intermodal dispersion in a step-index
Multimode fiber cane be
c
n
NA
L
c
Ln
s
1
2
1
3
4
)
(
3
2





Group Delay
 As the signal propagates along the fiber, each spectral component can be assumed to travel
independently and to undergo a time delay/Group delay per unit length In the direction
of propagation given by:
dk
d
c
dk
d
c
V
L g
g 




2
1
1 2




 Group Velocity
1
1 






 











d
dk
d
c
Vg
 Dispersion






2
2
1
1 c
V
d
d
d
d
L
D
g
g













MATERIAL DISPERSION
 Material dispersion occurs because the index of refraction varies as a function of the optical
wave length.
 To calculate material-induced dispersion , consider a plane wave propagating in the infinitely
extended dielectric medium that has a refractive index n(λ) equal to the fiber core. Then the
Propagation constant β is given by:




)
(
2 n

 Group delay from material Dispersion











d
dn
n
c
L
mat
)
(
2
2








 

 mat
mat
mat D
L
d
n
d
c
L
d
d




WAVE GUIDE DISPERSION
2
2
2
1
2
2
2
2
2
1
n
n
n
k
V
ua
b












For small values of the index difference Δ=(n1-n2)/n1 can be approximated by
2
1
2
n
n
n
k
b




Where β
)
1
(
2 

 b
k
n

Waveguide dispersion is










dk
kb
d
n
n
c
L
dk
d
c
L
wg
)
(
2
2



CHARACTERISTICS OF SINGLE MODE FIBERS
 Refractive index profile configuration used to produce different fiber types
Cutoff wavelength
Dispersion calculations
Mode field Diameter
Bending loss

THREE DIMENSION AL REFRACTIVE INDEX PROFILES

CUTOFF WAVELENGTH
 The cutoff wavelength of the first higher –order mode (LP11) is an important
transmission parameter for single mode fiber because it separated the single mode
from the multi mode regions.
 Single mode operation occurs above the theoretical cutoff wavelength given by:





 2
2
)
(
2
1
2
/
1
2
2
2
1 n
V
a
n
n
V
a
c

 P1(λ) is measured as a function of wavelength in sufficiently wide range around the
expected cutoff wavelength.
 P2(λ) is measured over the same wavelength range when a loop of sufficiently small
radius is included in the test fiber to filter the LP11 mode,







)
(
)
(
log
10
)
(
2
1



p
p
R
 The logarithmic Ration:

DISPERSION CALCULATIONS
The total chromatic dispersion in single mode fibers consist mainly of material and the
wave guide dispersion.
The resultant intermodal or chromic dispersion is represented by:



d
d
L
D
1
)
( 
The dispersion is commonly expressed in ps/(nm.km). The Broadening of σ of an
optical pulse over a fiber of length L is given by:



 L
D )
(

The Dispersion behavior varies with wavelength and also with fiber type
 The EIA and ITU-T have recommended different formulas to calculate the chromatic
dispersion for specific fiber types operating in a given wavelength region.
 To calculate the dispersion for a non-dispersion shifted fiber in the 1270-to-1340nm region,
The standards recommend fitting the measured group delay per unit length to three-term
Sellmeier equation of the form:
2
2


 C
B
A 



2
2


 C
B
A 


A, B and C are the curve-fitting parameters, and equivalent expression is
2
2
0
0
0
8 















S
Whereλ0 is the relative delay min. at zero
So is the value of the dispersion slope S(λ)=Dd/dλ at λo
The Dispersion for a non-dispersion –shifted fiber is:
















4
1
4
)
(



 o
o
S
D
To calculate the dispersion for a dispersion-shifted fiber In the 1500-to-1600nm region,
The standard recommended using the quadratic expression:
 2
0
0
2
o
S



 


Which results in the dispersion expression
o
O S
D )
(
)
( 

 


Typical mode field diameter variations with wavelength for 1300 nm optimized,
Dispersion shifted and Dispersion-flattened single-mode fibers
MODE FIELD DIAMETER (MFD)
 In a single mode fibers the geometric distribution of light in the propagating mode
is what
is needed when predicting the performance characteristics of the these fibers.
Thus a
fundamental parameter of a single-mode fiber is mode field diameter.
 Since it takes into account the wavelength-dependent field penetration into the
cladding.

Bending Loss:
Macro bending and Micro bending losses are important in the design of single-
mode fibers.
These losses are principally evident in the 1550nm region and show up as a rapid
increase
attenuation when the fiber is bent smaller than a certain bend radius.
 The lower the cutoff wavelength relative to the operating wavelength, the more
susceptible
single-mode fiber are to bending.

 The bending losses are primarily a function of the mode-field diameter. Generally
the
smaller the mode field diameter, the smaller the bending loss. This is true for the
both
matched-clad and depressed-clad fibers.
Calculated increase in attenuation at 1310nm from micro bending and macro bending
effects as a function of MFD (a) Depressed-cladding single mode fiber(V=2.514)
(b) Matched-Cladding single-mode fiber(V=2.373).
Micro bending calculations assume a correlation length Lc

Calculated Bend loss as a function of bend radius at 1300nm. The Dashed line
represents the infinite- cladding case.i.e.,n2=n3
This example gives of calculated bend loss as a function of bend radius at a 1300nm
wavelength. The fiber parameters were core radius a=3.6µm,cladding radius
b=60µm,
(n1-n2)/n2=3.56 X 10-3 and (n3-n2)/n2=0.07

UNIT-III
OPTICAL SOURCES POWER LAUNCHING AND COUPLING
 Direct and indirect Band gap materials
 LED structures
 Light source materials, Quantum efficiency
 LED power, Modulation of LED, laser Diodes
 Modes and Threshold condition
 Rate equations, External Quantum efficiency
 Resonant frequencies, Laser Diodes, Temperature effects
 Introduction to Quantum laser, Fiber amplifiers
 Power Launching and coupling, Lensing schemes
 Fiber-to-Fiber joints, Fiber splicing

TOPICS FROM SEMI CONDUCTOR PHYSICS
Pure-crystal energy-band diagram

Reverse bias condition Forward bias condition

Direct and Indirect Band gaps:

Double-heterostructure configuration
LIGHT –EMITTING DIODES (LEDs):
1. LED Structures:

2.Light Source materials
Semi conductor material Band gap energy (ev)
Si 1.12
GaAs 1.43
Ge 0.67
Inp 1.35
Ga0.03Al0.03As 1.51
Band gap energies of some common semiconductor materials
 The semiconductor material that is used for the active layer of an optical sourc
have a Direct band gap.
 In direct band gap semiconductor, electrons and holes can recombine directly
the band gap without needing a 3rd particle to conserve momentum.
 The most important of these compounds are made from III-IV materials.
III-Group: Al, Ga or In
V-Group: P,As or Sb

 The alloys GaAlAs and InGaAsP are chosen to make semiconductor
light sources because it is possible to match the lattice parameters of the
hetro-structure interfaces by using a proper combination of binary,
ternary and quaternary materials.
---A very close match between the crystal lattice parameters of the two
adjoining hetro-junctions is required to reduce interfacial detects and to
minimize strain in the device as the temperature varies.
---These factors directly affect the radiative efficiently and life time of the
light source.
--Using the fundamental quantum-mechanical relationship between
energy E and frequency v.

hc
hv
E 

)
(
240
.
1
)
(
ev
E
m
g



The peak emission wavelength λ in micrometers can be expressed as a
function of the band gap energy Eg in electron volts by the equation

The relationships between the band gap energy Eg and the crystal lattice spacing
ao for various III-V Compounds are plotted.

Typical spectral patterns for edge-emitting and surface-emitting LEDS at 1310nm.
LED type Material Wavelength
(nm)
Operating
current (mA)
Fiber coupled
power (µm)
FWHM
(nm)
SLED GaAlAs 850 110 40 35
ELED InGaAsP 1310 100 15 80
SLED InGaAsP 1310 110 30 150
Typical characteristics of surface-and edge-emitting LEDs

Quantum Efficiency and LED power(1/4):

t
e
n
n

 0
 An excess of electrons and holes in p-type and n-type material created in a
semiconductor light source by carrier injection at the device contacts.
 The excess carrier density decays exponentially with time according to the
relation:
The excess carriers can recombine either radiatively or non-radiativley.
In radiative recombination a photon energy hv is emitted.
In Non-radiative recombination effects optical obsorption in active region.
 when there is a constant current flow into a LED, an equilibrium condition is
established. i.e., the excess density of e and holes is equal since the injected c
are created and recombined in pairs such that charge neutrility is maintained i
the device.
 The total rate at which carriers are generated is the sum of the externally supp
The thermally generated rates.


n
qd
J
dt
dn


qd
J
n


nr
t
t
R
R
R


int

 The Externally supplied rate is given by J/qd, where d is thickness of recombination
 The thermal generation rate is given by n/г,Hence the rate equation for ca
recombination in an LED can be written as:
The equilibrium condition is found by setting above equation equal to zero:
 This relationship gives the steady-state electron density in the active region wh
a constant is flowing though it.
 The internal quantum efficiency in the active region is the fraction of the
electron-hole pairs that recombine radiatively:
Rτ-radiative recombination rate, Rnτ-non radiative recobination rate

T
nr
r 



 


/
1
1
int
q
I
R
R nr /






q
hcl
hv
q
I
P out


int
int
 For exponential decay of excess carriers, the radiative recombination is 
 R
n
r /

Non-radiative recombination 
 n
nr R
n /

then the internal quantum efficiency can be expressed as:
Where the bulk recombination life time τ is
nr
r 


1
1
1


If the current injected into the LED is I, then the total number of recombination per
second
 Substitute above equation in internal quantum efficiency then
q
I
Rr int


 Noting Rr is the total no. of photons generated per second and that each photo
energy hv, then the optical power generated internally to the LED is:

2
)
1
(
1


n
n
ext

2
int
int
)
1
( 


n
n
p
p
P ext

2
2
1
2
1
)
(
4
)
0
(
n
n
n
n
T




c
d
T
ext







0
)
sin
2
)(
(
4
1
External quantum efficiency
The external quantum efficiency can be calculated from the expression
Where T(Φ) is the Fresnel transmission coefficient or Fresnel transmissivity this factor dep
on there incidence angle Φ but for simplicity we can use the expression for the normal in
Assuming the out side medium is air and letting n1=n2 then T(0)=4n/(n+1)2 ,the
external quantum efficiency is
Optical power emitted from the LED is
Quantum Efficiency and LED power(4/4
Only light falling within a core
defined by the critical angle Φc w
be emitted from an optical source

Modulation of an LED(1/3):
 The response time or frequency response of an optical source dictates
how fast an electrical input drive signal can vary the light output level.
Following 3 factors largely determine the response time:
1. Doping level in the active region
2. Injected carrier life time гi in the recombination region
3. parasitic capacitance of the LED.
If the drive current is modulated at a frequency w, the optical output power of
the device ill vary as
 2
1
2
0 )
(
1
)
( i
P
P 
 

Where Po is the power emitted at zero modulation frequency.
The modulation bandwidth of LED can be defined in either in electrical or optical t


Frequency response of an optical source showing the electrical and optical 3-dB bandwidth
points
 An optical source exhibits a linear relationship between light power and
current, so currents rather than voltages are compared in optical systems.

P(w)=I2 (w)/R the ratio of the output electrical power at the frequency w to the
power at zero modulation is














)
0
(
)
(
log
10
)
0
(
)
(
log
10 2
2
I
I
p
p
Ratioelec


Where I(w) is the electrical current in the detection circuitry. the electrical 3-dB
points occurs at that frequency point where the detected electrical power P(w)=
That happens when
2
1
)
0
(
)
(
2
2

I
I 
Or I(w)/I(0)=1/√2=0.707
Some times, the modulation BW of an LED is given in terms of the 3-dB BW of the
Modulated optical power P(w),at that frequency p(w)=Po/2.The 3-dB BW is determined
From the ratio of the optical power at frequency w to the un-modulated value of the optic
Power. Since the detected current is directly proportional to the optical power, the ratio is














)
0
(
)
(
log
10
)
0
(
)
(
log
10
I
I
p
p
Ratiooptical



LASER DIODES
 Lasers come in many forms with dimensions ranging from the size of
A grain of salt to one that will occupy an entire room.
 The lasing medium can be Gas,
liquid,
an insulating crystal(solid state),
a semiconductor.
 Despite their differences, the basic principle of operation is the same for each type of
 Laser action is the result of three key processes.
Photon absorption,
Spontaneous emission,
and Stimulated emission.
These three processes are represented by the simple two-energy diagrams.

Laser transition processes
E1 is the ground state energy and E2 is the excited-state energy.
 According to the planck’s law: a transition between these two states involves
the
absorption or emission of photon of energy hv12=E2-E1.
 E2 is the unstable state, the electron will shortly return to the ground state,
there by
emitting a photon of energy hv12, this occurs without and external
stimulation and
is called spontaneous emission.
 The electron can also be induced to make a down ward transition from the

LASER diode Modes and Threshold conditions
Fabry-Perot resonator cavity

havior of the resonant wavelength in a Fabry- perot cavity for 3 values of the
mirror reflectivity

Optical output vs. drive current
 At low diode currents, only spontaneous radiation is emitted. Both the
spectral range and the lateral beam width of this emission are broad like
that of an LED.
 A Dramatic and sharply defined increase in the power o/p occurs at the
lasing threshold.
The threshold current Ith is conventionally defined by extrapolation of the
lasing region of the power-versus-current curve.
At high power o/p the slope of the curve decreases because of

Laser Diode Rate Equations:
 The relationship between optical output power and the diode drive current
can be determined by examining the rate equations that govern the interaction
of photon and electrons in the active region.
 The total carrier population is determined by carrier injection, spontaneous
recombination and stimulated emission.
For a pn junction with a carrier – confinement region of depth d , the rate
equation
ph
sp
R
Cn
dt
d







=stimulated emission +spontaneous emission + Photon loss


Cn
n
qd
J
dt
dn
sp



Which governs the no. of photons ‘Ø’
=injection + spontaneous recombination + stimulated
emission
Which governs the no. of electrons
‘n’
C-coefficient describing the strength of the optical absorption and emission interactions
Rsp- rate spontaneous emission into the lasing mode
τph- photon life time, τsp- spontaneous recombination life time
J-is the injection-current density

0
1


ph
n
C

qd
J
t
n th
sp
th

ph
s
sp
s
th R
Cn


 


0
s
th
sp
th
Cn
n
qd
J





0
sp
ph
th
ph
s R
J
J
qd


 

 )
(
 The steady state is characterized by the left-hand sides of above equation being equa
First ,assuming Rsp is negligible and noting that dφ/dt must be positive when φis small,t
 This shows that n must exceed a threshold value nth in order for φ to increase.
This threshold value can be expressed in terms of the threshold current Jth need to main
An inversion level n=nth in the steady state when no.of photos φ=0
This expression defines the current required to sustain an excess electron density in the la
When spontaneous emission is the only decay mechanism. Consider the photon and elect
equations in the steady –state condition at the lasing threshold.
and
Φs-steady state photon density
Adding above two equations using for the term nth/τsp, and solving for Φs Yields the no. of photons
per unit volume:

External Quantum Efficiency:
th
th
i
ext
g
g



)
( 


)
(
)
(
)
(
8065
.
0
mA
dI
mW
dP
m
dI
dP
E
q
g
ext 

 

External differential quantum efficiency is defined as the no. of photons
emitted per radiative electron-hole pair recombination above threshold.
Gain co-efficient remains fixed at gth Where , ηi-internal quantum efficiency
This is not a well defined quantity in laser diodes, but most measurements Show that
ηi≈0.6-0.7 at room temperature.
Experimentally ext. Quantum efficiency is calculated from the straight, line portion of
the curve for the emitted optical power P versus drive current I.
Where
Eg-band gap energy
dP-incremental change in emitted optical pow
dI-Drive current
λ-emisssion wavelength

Resonant Frequencies:
m
L 
 2
2 


 /
2 n

v
c
Ln
n
L
m
2
2
/








 

 2
2
0
2
)
(
exp
)
0
(
)
(



 g
g
Let us examine the resonant frequencies of the laser
m is an integer,
using for the propagation constant
Where c=vλ
This states that the cavity resonates when an integer number m of half-wavelengths spans the region
between the mirrors
Some lasers are single-mode and some are multi-mode. The relation ship between
gain and frequency can be assumed to have the Gaussian form:
Where λo-wavelength at the center of the spectrum
σ - spectral width of the gain
g(o)-max. gain proportional to the population inversion

1
2
1 

 m
v
c
Ln
m
m
v
c
Ln
m
2

v
c
Ln
v
v
c
Ln
m
m 


 
2
)
(
2
1 1
Ln
c
v
2


Ln
2
2

 

 This can be related to the wavelength spacing Δλ through the relationship Δv/v=Δλ/λ
From which we have the frequency spacing
Subtracting these two equations yields
and
 Here, we consider only the longitudinal modes. Note ,however, that for each longitudina
there may be several transverse modes that arise from one or more reflections of the
propagating wave at the sides of the resonator cavity.
 consider two successive modes of frequencies vm-1 and vm represented by the integer
m-1 and m.

Fabry-Perot GaAlAs/GaAs laser diode spectrum
The output spectrum laser follows the typical gain-versus-frequency plot
Where the exact no. of modes , their heights ,and their spacing's depend on the laser con

Laser Diode Structures and Radiation Patterns:
 A basic requirement for efficient operation of laser diodes is that, in addition to
transverse optical confinement and carrier confinement between hetro-junction layers,
the current flow must be restricted laterally to a narrow stripe along the length of the
laser.
 Numerous novel methods of achieving this, with varying degrees of success, have
been proposed, but all strive for the same goals of limiting the number of lateral modes
so that lasing is confined to a single filament, stabilizing the lateral gain, and ensuring a
relatively low threshold current.
 Dielectric waveguide materials are fabricated in the lateral direction.
The variations in the real refractive index of the various materials in these structures
control
the lateral modes in the laser. These devices are called index-guided lasers.
 If a particular index-guided laser supports only the fundamental transverse
mode and the fundamental longitudinal mode, is known as a single-mode laser.
Index-guided lasers; (a) Positive –index waveguide
(b) Negative-index waveguide
In positive index- waveguide the central region has a higher refractive index than the
outer regions. These lasers are more popular.

3 Fundamental structures for confining optical waves in the lateral direction
(a) In the gain – induced guide, electrons injected via a metallic stripe contact alter the
Index of refraction of the active later
(b) The positive index waveguide has a higher refractive index in the central portion of
the active
region
(c ) The negative – index waveguide has a lower refractive index in the central portion of

Short-wavelength (800-900nm)
GaAlAs
Long-wavelength (1300-1600nm) InGaAsP
 To make the buried hetrostructure (BH) LASER, one etches a narrow mesa stripe (1-
2μm wide)
in double- hetro structure material.
 The mesa is then embedded in high-resistivity lattice- matched n-type material with an
appropriate band gap and low refractive index.
 A no. of variations of this fundamental structure have been used to fabricate high-
performing
laser diodes.

(a) Selectively diffused (b) Varying –thickness (c ) bent- la
Positive –index Optical –wave-confining structure
 Index- guided lasers can be made using any one f four fundamental
structures.
 These are the buried hetro-structure, a selectively diffused construction, a
varying
Thickness structure and a bent-layer configuration.

 4 Basic methods for achieving current confinement in laser diodes:
Inner-stripe confinement Regrowth of back-biased pn
junctions
In addition to confining the optical wave to a narrow lateral stripe to achieve
continuous high optical output power, one also needs to restrict the drive current
tightly to the active layer so more than 60% of the current contributes to lasing.

Energy-band diagram for a quantum layer in a multiple quantum-well laser

Single-Mode Lasers:
Basic Architecture of a vertical- cavity surface-emitting laser
 high-speed long-distance communications one needs single-mode lasers, which must
contain only a single longitudinal mode and a single transverse mode. Consequently, the
width of the optical emission is very narrow.

3 types of Laser structures using built-in frequency selective resonator grat
1.Distributed- feed back
(DFB) Laser
2. Distributed –Bragg
reflector(DBR) Laser
3. Distributed- reflector
(DR) Laser

Output spectrum symmetrically distributed around λB in an idealized distributed feedbac

Temperature Effects:
An important factor to consider in the application of laser diodes is the temperature depen
of The threshold current Ith(T).
 This parameter increases with temperature in all types of semiconductor lasers because
various Temperature-dependent factors.
 The complexity of these factors prevents the formulation of a single equation that holds
For all devices and temperature ranges.
0
)
( T
T
z
th e
I
T
I 
To-threshold temp.
Iz - constant
For
GaAlAs laser diode
To is 120-1650 C
For
GaAlAs quantum –well
laser diode
To is 4370 C

 In the example given above , the threshold current increases by a factor of about
1.4
between 20 and 60 degree centigrade.
 In addition , the lasing threshold can change as the laser ages.
Consequently , if a constant optical output power level is to be maintained as the
temp. of
the laser changes or as the laser ages, it is necessary to adjust the dc-bias current
level.
One possible method for achieving this automatically is an optical feedback
scheme.
Optical feed back can be carried out by using a photo detector either to sense the
variation
In optical power emitted from the rear facet of the laser or to tap off and monitor a
small
portion of the fiber coupled power emitted from the facet.
 photo detector compares the optical power output with a reference level and
adjusts the dc-
bias current level automatically to maintain a constant peak light output relative to

 Standard method of stabilizing the optical output of a laser diode is to use a miniature
thermoelectric cooler.
 This device maintains the laser at a constant temperature and thus stabilizes the output
Normally a thermoelectric cooler is used in conjunction with a rear-facer detector feed ba
loop.

PART-II
 Power Launching and coupling
 Lensing schemes
 Fiber-to-Fiber joints
 Fiber splicing
In implementing an optical fiber link, two of the major system operations
are
1.how to launch optical power into a particular fiber form some type of
luminescent source and
2. how to couple optical from one fiber into another.

 Launching optical power form a source into a fiber entails considerations
Such as:
 Numerical apertarture,
 Core size
 Refractive –index profiles
 Core –cladding index difference of the fiber
 Size
 Radiance
 Angular power distribution of optical source
 A measure of the amount of optical power from a source that can be
coupled into
A fiber is usually given by the coupling efficiency η defined as
S
F
P
P

 F
P Power coupled into the fiber
Ps- power emitted from the light source
 The launching or coupling efficiency depends on the type of fiber that
Is attached to the source and on the coupling process;

SOURCE TO FIBER POWER LAUNCHING
 A convenient and useful measure of the optical output of a luminescent source is its rad
(or brightness) B at a given diode drive current.
Radiance is the optical power radiated into a unit solid angle per unit emitting surface ar
Is generally specified interims of watts per square centimeter per steradian;
 Since power that can be coupled into a fiber depends on the raidance, the Raidance of
optical source than the total output is the important parameter When considering
source-to-fiber coupling efficiencies.
Spherical coordinate system
For characterizing the
Emission pattern
From
An optical source
R,Ө,and Ø spherical coordinat
system

 Surface-emitting LEDs are characterized by their lambertian output pattern.
The power delivered at an angle Ө,measured relative to a normal to the emitting surface, v
cos Ө because the projected area of the emitting surface varies as cos Ө with viewing dir
The emission pattern for a lambertian source thus follows the relationship:
Radiance patterns for a lambertian Source and the lateral output of
a highly
Directional laser diode, Both sources have Bo normalized to unity
B(Ө,Ø) = Bo cos Ө
Bo-radiance along the
Normal to the radiating surface

Power coupling calculation
Schematic diagram of a light source coupled to an optical fiber. Light
outside of the acceptance angle is lost
)
,
( s
s
s
A
s
s
f
A
B
d
dA
p 

 
 
The coupled power can be found using the relationship
s
sand
A  are the area and solid emission angle of the
source
a
forr
NA
P
P s
s
Step
LED 
 2
)
(
,

















2
)
0
(
2
2 2
2
2
2
1
2
2
,





 NA
B
a
n
B
a
P o
o
Graded
LED
For step-index fiber:
For graded-index fiber:
0
2
2
B
r
P s
s 

Where

POWER LAUNCHING VERSUS WAVELENGTH
 Optical power launched into a fiber does not depend on the wavelength
Of the source but only on its brightness: i.e., Radiance.
2
0
B
M
P
S

The no. of modes that can propagate in a multimode graded-index fiber Of core size
a and index profile α is:









2
1
2
2 


 an
M

LENSING SCHEMES
 If source-emitting area is larger than the fiber –core area, then the resulting Optical
power
coupled in to the fiber is the maximum that can be achieved . This is the result of
fundamental
energy and radiance conservation principle(also know as law of brightness).the
emitting area
 However, if the emitting area of the source is smaller than the core , a miniature
lens may
be placed between the source and the fiber to improve the power-coupling
efficiency.
The function of the micro-lens is to magnify the emitting area of the source to match
the core
area of the fiber end face exactly. If the emitting area is increased by a magnification
factor
M, The solid angle within which optical power is coupled to the fiber form the source
is
increased by the same factor.
 These techniques can improve the efficiency, they also create additional
complexities.

Examples of possible lensing schemes used to improve optical source-to-
Fiber coupling efficiency

Nonimaging Microshere: One of the most efficient lensing method
The focal point can be found from the Gaussian lens formula
r
n
n
q
n
s
n 


'
'
2
2
2










s
L
s
L
r
R
r
R
M


Placing the LED close to the lens surface thus results in a magnification M of the
emitting area. This is given by the ratio of the cross-sectional area of the lens to That
of the emitting area:

Theoretical coupling efficiency in units
Of (NA)2 for a surface – emitting LED as a function
Of the light emitting diameter, coupling is to
A fiber with a core radius a =25µm
The optical power PL that can be coupled into a full aparture angle 2Ө is given by:

2
2
sin









s
L
S
L
r
R
P
P
The theoretical coupling efficiency
1
)
(
1
)
(
{ 2
2
2
max 










a
r
for
NA
a
r
for
NA
r
a
s
s
s


FIBER-TO-FIBER JOINTS (1/4)
A significant factor in any fiber optic system installation is the requirement to
Interconnect
fibers in a low loss manner.
 These interconnects occur at the optical source,
at the photo-detector
at the intermediate points with in a cable where two fibers
are joined.
 The particular technique selected for joining the fibers depends on whether a
permanent bond
or an easily demountable connection is desired.
 A permanent bond is generally referred to as a splice, where as a demountable
joint is known
as “a connector”.
 Every joining technique is subject to certain conditions that can cause various
amounts of
Optical power loss at the joint. The loss at a particular junction or thought a
component is
Called the insertion loss.

Different modal distributions of the optical beam emerging from a fiber result in different
degrees of coupling loss.
(a) When all modes are equally excited, the output beam fills the entire output NA
(b) for a steady-state modal distribution, only the equilibrium NA is filled by the output
beam

Mechanical Misalignment:
Three types of mechanical misalignments that can occur between two joints fi
 Mechanical alignment is a major problem when joining two fibers, owing to their
Microscopic
size.
 A standard multimode graded-index fiber core is 50-100µm in diameter, which is roughly
the
Thickness of the human hair.

Axial offset reduces the shaded common core area of the two fiber end
faces
Axial displacement results when the of the two fibers are separated by a Distance d.
Longitudinal separation occurs when the fibers have the same Axis but have a gap s
between
their end faces.

Output power loss effect when fiber ends are separated longitudinally by

Experimental comparison of loss(in dB) as a function of mechanical misalignm

Fiber End-Face Preparation:
Two examples of
Improperly cleaved
Fiber ends:
Controlled-fracture procedure for fiber end
preparation
 One of the firs steps that must be followed before fibers are connected or Spliced to
each other
is to prepare the fiber end faces properly.
 In order not to have light deflected or scatted at the joint, the fiber ends must be flat,
perpendicular to the fiber axis, and smooth.
 End preparation techniques that have been extensively used include sawing,
grinding and
polishing, controlled fracture, and laser cleaving.

FIBER SPLICING:
 A Fiber splicing is a permanent or semi-permanant joint between two fibers
 These are typically used to create long optical links or in situations where frequent conn
and Disconnections are not needed.
 In making and evaluting such splices, one must take into account the geometrical differe
In the two fibers
1. Fiber mis-alignments at the joint
2. mechanical strength of the splice
 Fiber splicing techniques:
1. Fusion spice
2. V-groove mechanical splice
3. Elastic- tube splice

Fusion splicing of optical fibers
V-groove of optical fibers splicing
Elastic –tube splice

Splicing single-mode fibers:
In single-mode fibers the lateral (axial) offset misalignment present
the Most serous loss.
This loss depends on the shape of the propagating mode. For
Gaussian Shaped beams the loss between identical fibers is:
}
log{exp
10
2
.

















w
d
L lat
SM
Where w-spot size mode-field radius, d-lateral displacem
 For a gap s with a material of index n3, and letting G=s/kw2 ,the gap
loss for identical single-mode fiber splices is:
)
4
(
)
(
64
log
10 2
4
2
1
2
3
2
1
,




G
n
n
n
n
L gap
SM

Connector Return loss
Model of an index-matched connection with perpendicular fiber end faces

Connection with angled end faces having a small gap of width a separating the finer end

OPTICAL FIBER COMMUNICATION
UNIT-IV
PHOTO DETECTORS , OPTICAL RECIEVER OPERATION

CONTENTS
• PIN and APD diodes,
• Photo detector noise
• SNR
• Detector Response time
• Avalanche Multiplication Noise
• Comparison of Photo detectors
• Fundamental Receiver Operation, preamplifiers
• Error Sources, Receiver Configuration
• Probability of Error
• Quantum Limit

Quality Factors
• Responsivity
– Ratio of photons incident to current produced
• Quantum Efficiency
– Ratio of photons incident to EHP produced
• Capacitance
• Gain-Bandwidth Product
– FWHM, rise time, fall time
• Noise
– Signal-to-noise ratio

Detector Technologies
MSM
(Metal Semiconductor Metal)
PIN
APD
Waveguide
Contact InP p 1x1018
Multiplication InP n 5x1016
Transition InGaAsP n 1x1016
Absorption InGaAs n 5x1014
Contact InP n 1x1018
Substrate InP Semi insulating
Semiinsulating GaAs
Contact InGaAsP p 5x1018
Absorption InGaAs n- 5x1014
Contact InP n 1x1019
Absorption Layer
Guide Layers
Simple, Planar,
Low Capacitance
Low Quantum
Efficiency
Trade-off Between
Quantum efficiency
and Speed
High efficiency
High speed
Difficult to couple int
Gain-Bandwidth:
120GHz
Low Noise
Difficult to make
Complex
Key: Absorption Layer
Contact layers
Layer Structure Features

Photo Detection Principles
Device Layer Structure
Band Diagram
showing carrier
movement in E-field
Light intensity as a
function of distance below
the surface
Carriers absorbed here must
diffuse to the intrinsic layer
before they recombine if they are
to contribute to the photocurrent.
Slow diffusion can lead to slow
“tails” in the temporal response.
Bias voltage usually needed
to fully deplete the intrinsic “I”
region for high speed
operation

PIN Photodiodes
• Large absorption area
• Simple fabrication
• Unity gain
• Speed dependent on width
• Thermally generated
carriers create noise
within region

PIN photodiodes
Energy-band diagram p-n junction
Electrical Circuit

PIN PHOTO DETECTOR
Photons with energies greater
Than or equal to the band gap
Energy Eg can generate free
Electron-hole pairs that act as
Photocurrent carries

Characteristics of Photo-detectors
 







 
 
 
 
  
    

Number of Collected electrons
1
Number of Photons *Entering* detector
/
Number of Collected electrons
1 1
Number of Photons *Incident* on detector /
Photo Current (Amps)
W
i
ph W
e p
o
e
i q
R e
P h
R  
 






 
 
 
 
 
   
 
 
   
 
1 1
Incident Optical Power (Watts)
1 1
ph o
ph W
p
o
W
p
o
i RP
i q
R e
P h
R e
P
q
h
• Internal
Quantum Efficiency
•External
Quantum efficiency
• Responsivity
•Photocurrent
Incident Photon Flux
(#/sec)
Fraction Transmitted
into Detector
Fraction absorbed in
detection region

Detector Sensitivity vs. Wavelength
Absorption coefficient vs. Wavelength
for several materials
(Bowers 1987)
Photodiode Responsivity vs. Wavelength
for various materials
(Albrecht et al 1986)

Comparison of responsivity and quantum efficiency

Avalanche Photodiodes
• High gain due to avalanche
multiplication effect
• Increased noise
• Silicon has high gain but low
noise
• Si-InGaAs APD often
used(diagram on right)
• High resistivity p-doped layer
increases electric field across
absorbing region
• High-energy electron-hole pairs
ionize other sites to multiply the
current
• Leads to greater sensitivity
n
+
p
+
p
i
Electric
field
Depletion region

APD Detectors
Signal Current


 
  
 
s
q
i M P
h
APD Structure and field distribution (Albrecht 1986)

Responsivity
M
h
e
R




2. PHOTO DETECTOR NOISE:
    L
B
L
D
p
N R
TB
k
B
qI
B
M
F
M
I
I
q
i /
4
2
2
)
( 2
2





Signal to Noise Ratio
ip= average signal photocurrent level based on modulation index m
iD=dropped current
    L
B
L
D
p
p
R
TB
k
B
qI
B
M
F
M
I
I
q
M
i
N
S
/
4
2
2 2
2
2




L
e
B
e
p
p
R
TB
k
B
M
F
M
qI
i
M
N
S
/
4
)
(
2
)
(
/ 2
2
2



Reverse-biased pin photodiode
Photo diode not fully deployed
Rise and Fall times
4. DETECTOR RESPONSE TIME:

Photo diode pulse responses under Various detector
parameters
The response time of photo detector together with its output circuit depends mainly on the
following 3 factors
1. The transit time of photo carriers in the depletion region
2. The diffusion time of the photo carriers generated outside the depletion region
3. The RC time constant of the photodiode and its associated circuit
td= w/vd

Typical Characteristics of P-I-N and Avalanche photodiodes
or comparison of Photo-detectors

Comparisons
• PIN gives higher bandwidth and bit rate
• APD gives higher sensitivity
• Si works only up to 1100 nm; InGaAs up to 1700, Ge
up to 1800
• InGaAs has higher  for PIN, but Ge has higher M
for APD
• InGaAs has lower dark current

Fundamental Receiver Operation
Signal path through an optical data link

Receiver Functional Block Diagram

Error Sources:
NOISE SOURCES
Internal:
Switch and power supply transients
External:
Electric power lines, motors, radio transmitters, lightning
Short noise arises in electronic devices because of the discrete nature of current flow in the
device.
Thermal noise arises from the random motion of electrons in a conductor

Digital Receiver performance
In digital receiver the decision-circuit output signal voltage Vout(t) would always
exceed the threshold voltage When a “1” is present and would be less than the
threshold when “o” pulse was sent.
 In actual system, deviations from the average value are caused by various noises,
interfaces from adjacent pulses and conditions where in the light source is not
completely extinguished during a zero pulse
Probability of Error
 Quantum Limit
Bt
N
N
N
BER e
t
e


Probability of Error:
A simple approach is to divide the no. of errors occurring over certain time
interval t by the no. of pulses .
Where B=1/Tb is the bit rate

Probability distribution for received
logic 0 and logic 1 Signal pulses.
The different widths Of the two
distributions are caused
By various signal distortion effect.
Gaussian noise statistics
Of a binary signal
Showing
Variances around the
ON and OFF signal
levels






2
/
2
1
)
(
Q
x
e dx
e
Q
P
BER

Q
e
Q
erf Q
/
2
1
2
1
2
1 2
/
2


















BER versus Q factor
BER vs SNR

QUANTUM LIMIT
 In designing an optical system, it is useful to know what the fundamental physical
bounds are on the system performance.
 Suppose we have a ideal photo detector that has a unity quantum efficiency and produces
No dark current, i.e., no electron – hole pairs are generated in the absence of an optical pulse.
 In the given above condition to finding the min. received optical power required for
Specific bit-error rare performance in the digital system. This min. received power level is
know as the “QUANTUM LIMIT”


 N
r e
P )
0
(
Error Probability
N- avg. no. of electron-hole pairs
E-min. optical pulse energy
Ƭ-photo detector time intervel

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