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

- Engineers Way

www.erway.in