Fundamentals of bidirectional transmission over a single optical fibre
Optical fibre transmission
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Transcript of Optical fibre transmission
OPTICAL FIBER TRANSMISSION
INTRODUCTION
Optical fiber is a coaxial cylindrical arrangement of two homogeneous dielectric material.
Fiber consist of a central core of refractive index n1 and cladding of refractive index n2.
1. Step index fiber: the cross-sectional refractive index has a step function at the interface between the core and the cladding.
2. Graded index fiber: refractive index profile varies
as a function of the radial coordinate r in the core but is constant in the cladding.
NEED FOR OPTICAL FIBER TRANSMISSION
Information carrying capacity is one of the most important criteria for communication.
According to Shannon theorem C = BW log2 (1+ SNR) where C : information carrying capacity of the
channel BW : Bandwidth of the channel With increase in bandwidth channel capacity increases.
Bandwidth is approximately 10 percent of the carrier frequency.
CONTD..
TRANSMISSION CHANNEL
FREQUENCE OF OPERATIO
N
Coaxial cable
1 MHz to 100 MHz
Microwave 1 GHz to 100 GHz
Optical fibers
100 THz to 1000 THz
TRANSMISSION CHANNEL
CARRYING CAPACITY
Coaxial cable 13000 channels
Microwave terrestrial link
20000 channels
Satellite link 100,000
channels Optical fibers 300,000
channels
TOTAL INTERNAL REFLECTION
θ1 θ1 θ1
θ2
θ1
θ2
θ1=incident angle /reflected angle
θ2=refracted angle
θ2
HOW OPTICAL FIBER CONDUCTS LIGHT
n1 core
n2 cladding
n2 cladding
n1 core
αC
θ1C
BLOCK DIAGRAM
OPTICAL SOURCES
1. LED <a> HOMOSTRUCTURED LED-
HERE BOTH p-TYPE AND n-TYPE SEMICONDUCTOR
HAVE SAME ENERGY GAP
<b> HETEROSTRUCTURED LED- IT CONSISTS OF TWO ADJOINING
SEMICONDUCTOR MATERIALS WITH DIFFERENT BANDGAP ENERGY.
2. LASER
LIGHT EMMITING DIODE
VFB
LED
R
ELECTRONIC CIRCUIT OF A LED
At the p and n junction of a semiconductor material a depletion region is created because of the electron and hole recombination.A depletion voltage is developed at the junction which prevents further recombination.So an external voltage called forward bias voltage (VFB>VD ) is applied.
BASIC MECHANISM IN OPTICAL SOURCES
When the pn junction of both LED and laser diode is forward biased, electrons and holes are injected into the p and n regions, respectively.
These injected minority carriers can recombine either radiatively,
causing a photon of energy hv to be emitted, or nonradiatively,
where recombination energy is released in the form of heat.
The nonradiative recombinations take excited electrons from useful, radiative recombinations and decrease the efficiency of the process.
INTERNAL QUANTUM EFFICIENCY:
It is the fraction of electron-hole pairs that
combine radiatively. ηint :INTERNAL QUANTUM EFFICIENCY ηint= Rr/ (Rr + Rnr) where Rr : Radiative recombination Rnr : Nonradiative recombination
LASERS Laser is a device that amplifies light by stimulated
emission of radiation FEATURES OF STIMULATED RADIATION 1. Narrow spectral width 2. High intensity 3. High degree of directivity 4. Coherence E2
E1
ABSORPTION SPONTANEOUS EMISSION
STIMULATED EMISSION
hν12
hν12
hν12
hν12(in phase)
LIGHT AMPLIFICATION AND POSITIVE FEEDBACK
MIR
RO
R
MIR
RO
R 2
MIR
RO
R 1
EN
ER
GY
EN
ER
GY
POPULATION INVERSION
VALENCE BAND
CONDUCTION BANDEN
ER
GY
EX
TER
NA
L EN
ER
GY
PHOTODETECTOR The photodetector senses the luminescent power falling
upon it and converts the variation of this optical power into a correspondingly varying electric current.
Photodiode is a type of semiconductor based photodetector used exclusively because of its small size, suitable material, high sensitivity, and fast response time.
2 types of photodiodes used are— <a> pin photodiode <b> avalanche photodiode(APD)
PIN PHOTODETECTOR
The device consists of a p and n region separated by a very lightly n-doped intrinsic (i) region.
A reverse bias voltage is applied across the device so that the intrinsic region is fully depleted of carriers.
p iHole electron
RLLOAD REGISTER
BIAS VOLTAGE
n
n
PHOTODIODE
hνphoton
I
PRINCIPLE OF PHOTODETECTOR
When light having photon energies greater than or equal to the band-gap energy of the semiconductor material is incident on a photodetector, the photons can give up their energy and excite electrons from the valence band to the conduction band.
This process generates electron-hole pairs, known as photocarriers.
These carriers are generated in the depletion region where most of the incident light is absorbed.
CONTD.. The high electric field present in the depletion region
causes the carrier to separate and be collected across the reverse-bias region.
This gives rise to a current flow in the external circuit, known as photocurrent.
Optical radiation is absorbed in the semiconductor material as
P(x) = P0(1- e^(αS(λ)x)) where αS(λ)=Absorption coefficient at a wavelength
λ P0 : Incident optical power level P(x) : Optical power absorbed in a distance x
CHARACTERISTICS OF PHOTODIODE
QUANTUM EFFICIENCY (η): It is the number of electron-hole carrier pair generated per incident photon of energy hν.
η= (I /q) / (P0/hν) where I = average photocurrent generated P0 = optical power incident on the
photodetector
RESPONSIVITY : It specifies the photocurrent generated per unit optical power.
R =I / P0=(ηq) / (hν)
AVALANCHE PHOTODIODE
Avalanche photodiode (AVD) internally multiply the primary signal photocurrent.
This increases the receiver sensitivity, since the photocurrent is multiplied before encountering the thermal noise associated with the receiver circuitry.
The carrier multiplication M is a result of impact ionization.
RAPD= (ηq /hν)M = R0M where RAPD : Responsivity of AVD
DIFFICULTIES FACED BY OPTICAL FIBERS
1. ATTENUATION
2. DISPERTION
ATTENUATION
Losses in an optical fiber can be classified as 1. Intrinsic losses : These are associated with a given
fiber material. (a) Material resonance (b) Raleigh scattering 2. Extrinsic losses : These are associated with
fabrication. cabling and installation processes. (a) Absorption losses (b) Bending losses
CONTD..
λ(nm)
500 1000
1500 2000
0.1
1
10
Att
enuati
on(d
B/k
m)
RAYLEIGH SCATTERING
INFR
ARED
ABS
ORP
TION
ULTRAVIOLET ABSORPTION
CONTD..
LOSS
λ(nm)
ABSORPTION LOSS PEAKS
SCATTERING LOSS
DISPERSION
1. Chromatic dispersion (a) Material dispersion (b) Waveguide dispersion
2. Polarization-mode dispersion
CHROMATIC DISPERSIOND
ISPER
SIO
N P
AR
AM
ETER
D(λ
) Dmat (λ)
D(λ)
Dwg(λ)
λ(nm)1100 1200 1300 1400 1500 1600
CONTD.
13101500
1 1
2
3
1. CONVENTIONAL FIBER
2. DISPERSION SHIFTED
3. DISPERSION FLATTENED
COPING WITH CHROMATIC DISPERSION
There are two basis technique for dispersion compensation.
1. DISPERSION COMPENSATION FIBER(DCF) The positive dispersion of the conventional
fiber is compensated with the negative dispersion characteristic so that the total dispersion of the link will be almost zero.
2. DISPERSION COMPENSATION GRATING(DCG) Chirped fiber bragg grating (FBG) is the most
developed DCG. FBG reflects a set of wavelength. The shorter
wavelengths are reflected almost immediately and the longer wavelengths penetrate deeper into the grating before they will be reflected.
PULSE SPREADING COMPENSATION BY USING DCF
DISPERSION COMPENATING FIBER
CONVENSIONAL FIBER
LD PD
POLARISATION MODE DISPERSION Two modes travel along a singlemode fiber at different
velocities because of fiber’s birefringence. This effect results in the form of pulse spread called polarization-mode dispersion.
PMD (polarization mode dispersion) is caused by the
refractive indexes along x-axis and y-axis.
This difference in the refractive index is called birefringence (B).
B=nx – ny In order to cope with PMD, we use special fibers and other
components that allows to preserve and control the state of mode polarization.
CONTD..
Polarization maintaining fibers have very low birefringence.
Low birefringence is achieved by having very high asymmetry in the core or cladding.
Besides using PM fibers we have to use all other fiber optic component to maintain the state of polarization. This set contains PM connectors, fiber optic polarizer and PM splitters.
AREAS TO BE IMPROVED IN OPTICAL FIBER COMMUNICATI ON
1. INTEGRATION OF TRANSCEIVERS INTO ONE-SINGLE CHIP
For full duplex communications, a transmitter and receiver
are combined in one unit called a transceiver.
2. REPLACEMENT OF OPTO-ELECTRONIC COMPONENT WITH
OPTICAL COMPONENT. The replacement of opto-electronic regenarators with
optical amplifiers.
ADVANTAGES OF OPTICAL FIBER
1. LOW TRANSMISSION LOSS AND WIDE BANDWIDTH 2. SMALL SIZE AND WEIGHT 3. IMMUNITY TO INTERFERENCE
4. ELECTRICAL ISOLATION 5. SIGNAL SECURITY 6. ABUNDANT RAW MATERIAL 7. NO CROSSTALK
REFERENCES [1]Gerd Keiser, ”Optical Fiber Communication,” Tata
McGraw-Hill, Second Edition, 2000.
[2]D.Myanbaev, and L.Scheiner,” Fiber-Optic Communications Technology,’ Pearson Education, Second Edition, 2006
Thank You
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