Presented By Shuchi A. ParkhaniSneha P. Sawarkar

Swati R. JambhulkarSnehal K. Gaikwad

M. Tech. III sem. (Communication Engineering)

Department Of Electronics & Telecommunication EngineeringG. H. Raisoni College of Engineering, Nagpur

(An autonomous Institute under UGC act 1956 & Affiliated to Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur)

Session 2015-2016

A Seminar On

Optical Fiber Communication

CONTENTSIntroduction to optical fiber

networkOptical fibersOptical sourcesOptical detectorsOptical amplifiers

• High Speed Data Transmission (Tb/sec)

• Low Loss of Signal due to lack of Radiation And Conduction

• Low Dispersion and Attenuation Properties

• Better Electrical Isolation

• Small size and light weight

• Huge bandwidth

Advantages of Optical Networks

Major Elements of an optical fiber link

Introduction to Optical Fiber

An optical fiber is essentially a waveguide for light It consists of a core and cladding that surrounds the coreThe index of refraction of the cladding is less than that of the

core, causing rays of light leaving the core to be refracted back into the core

A light-emitting diode (LED) or laser diode (LD) can be used for the source

Advantages of optical fiber include:◦ Greater bandwidth than copper◦ Lower loss◦ Immunity to crosstalk◦ No electrical hazard◦Flexibility

Areas of application of Optical Fiber

TelecommunicationsLocal Area NetworksCable TVCCTVOptical Fiber Sensors

Optical Fiber & Communications System

Optical Fiber

Optical fiber is made from thin strands of either glass or plastic

It has little mechanical strength, so it must be enclosed in a protective jacket

Often, two or more fibers are enclosed in the same cable for increased bandwidth and redundancy in case one of the fibers breaks

It is also easier to build a full-duplex system using two fibers, one for transmission in each direction

Total Internal Reflection (TIR)

Optical fibers work on the principle of total internal reflection

With light, the refractive index is listedThe angle of refraction at the interface between

two media is governed by Snell’s law:

n1 sin1 n2 sin2

Reflection law: angle of incidence=angle of reflection

Snell’s law of refraction:

2211 sinsin nn

Total internal reflection, Critical angle

1

2sinn

nc

1

1

2sinn

nc

n 2

n 1 > n2

Incidentlight

Transmitted(refracted) light

Reflectedlight

kt

TIR

Evanescent wave

ki

kr

(a) (b) (c)

Light wave travelling in a more dense medium strikes a less dense medium. Depending onthe incidence angle with respect to , which is determined by the ratio of the refractiveindices, the wave may be transmitted (refracted) or reflected. (a) (b)

(c)

and total internal reflection (TIR).

2

1 c

902

c 1

cc 1 c 1

c 1

Critical angle

Launching optical rays to slab waveguide

1

21

12

22

1max0 2sinNA

n

nn

nnnn

Numerical aperture:

Different Structures of Optical Fiber

According to the Fiber Material◦ Glass◦ Plastic

According to the number of modes◦ Single mode◦ Multimode

According to the refractive index profile◦ Step index ◦ Graded index

Types of Optical Fibers

Transmission Windows in Optical Fiber

1st window is at 850nm◦Low loss◦Silica fibers◦10 Km repeater spacing◦Multimode fibers were

used◦140 Mbps◦Higher attenuation◦Short distance

Transmission Windows in Optical Fiber

2nd window is at 1310nm◦Low loss◦Silica fibers◦40 Km repeater spacing◦Single mode as well as Multimode fibers were used◦2.5Gbps◦Comparatively less attenuation◦ long distance

Transmission Windows in Optical Fiber

3rd window is at 1550nm◦Low loss◦Silica fibers◦90 Km repeater spacing◦Single mode and Multimode fibers were used◦Up to 10 Gbps◦lowest attenuation◦long distance

Attenuation Characteristics of an Optical Fiber

Types of Optical Sources

Monochromatic incoherent source:◦ Light emitting diode (LED)

Large wavelength content Incoherent Limited directionality

Monochromatic coherent source:◦ Laser diode (LD)

Small wavelength content Highly coherent Directional

Source CharacteristicsImportant Parameters

◦Electrical-optical conversion efficiency◦Optical power◦Wavelength◦Wavelength distribution (called linewidth)◦Cost◦Compact

Basic Light Emission Processes

Pumping (creating more electron-hole pairs)◦ Electrically create electron-hole pairs◦ Optically create electron-hole pairs

Emission (recombination of electron-hole pairs)◦ Spontaneous emission ◦ Simulated emission

Semiconductor Light Sources

A PN junction (that consists of direct band gap semiconductor materials) acts as the active or recombination region.

When the PN junction is forward biased, electrons and holes recombine either radiatively (emitting photons) or non-radiatively (emitting heat). This is simple LED operation.

In a LASER, the photon is further processed in a resonance cavity to achieve a coherent, highly directional optical beam with narrow linewidth.

LED vs. laser spectral width

Single-frequency laser (<0.04 nm)

Standard laser (1-3 nm wide)

LED (30-50 nm wide)

Wavelength

Laser output is many timeshigher than LED output

Types of LED

The basic LED types used for fiber optic communication systems are

Surface-emitting LED (SLED) Edge-emitting LED (ELED)

LED performance LED performance differences help link designers decide

which device is appropriate for the intended application.

For short-distance (0 to 3 km), low-data-rate fiber optic systems, SLEDs and ELEDs are the preferred optical source.

Typically, SLEDs operate efficiently for bit rates up to 250 megabits per second (Mb/s). Because SLEDs emit light over a wide area (wide far-field angle), they are almost exclusively used in multimode systems.

LED performance For medium-distance, medium-data-rate systems,

ELEDs are preferred. ELEDs may be modulated at rates up to 400 Mb/s.

ELEDs may be used for both single mode and multimode fiber systems.

Both SLDs and ELEDs are used in long-distance, high-data-rate systems. SLDs are ELED-based diodes designed to operate in the superluminescence mode.

SLDs may be modulated at bit rates of over 400 Mb/s.

April 18, 2023 Optical Fiber Communication 27

Surface-emitting LED

April 18, 2023 Optical Fiber Communication 28

Edge-emitting LED

Quantum Efficiency

Internal quantum efficiency is the ratio between the radiative recombination rate and the sum of radiative and non-radiative recombination rates

For exponential decay of excess carriers, the radiative recombination lifetime is n/Rr

and the nonradiative recombination lifetime is n/Rnr

)/(int nrrr RRR

Drawbacks

Large line width (30-40 nm)Large beam width (Low coupling to the fiber)Low output power Low E/O conversion efficiency

AdvantagesRobust Linear

The LASER

Light Amplification by ‘Stimulated Emission’ and Radiation (L A S E R)

Coherent light (stimulated emission)Narrow beam width (very focused beam)High output power (amplification)Narrow line width because only few

wavelength will experience a positive feedback and get amplified (optical filtering)

Fundamental Lasing OperationAbsorption: An atom in the ground state might

absorb a photon emitted by another atom, thus making a transition to an excited state.

Spontaneous Emission: Random emission of a photon, which enables the atom to relax to the ground state.

Stimulated Emission: An atom in an excited state might be stimulated to emit a photon by another incident photon.

Optical light sources convert electrical signals into optical signals and launch them.

Commonly used light sources include LEDs, ELEDs, SLEDs, and LDs.

LEDs produce incoherent light whereas a Laser Diode produces coherent light.

Incoherent light sources used in multimode systems and Laser Diodes/Tunable Lasers are used in single mode systems

Laser diodes must operate above their threshold region to produce coherent light, otherwise operating as ELED.

Laser diodes are much faster in switching response than LEDsTunable laser is able to produce coherent light output with

controlled variable wavelength Tunable laser is used in multi wavelength systems by replacing a

system where many sources are coupled into a multiplexing device system

Introduction to the Photodetectors

Three steps for the operation of a photodetector:

◦(i) carrier generation by incident light absorption and generation

◦(ii) carrier transport (and/or multiplication) by current gain mechanism photocurrent

◦(iii) interaction of current with the external circuits to provide the output signal current collection

Simple point-to-point link

System Requirements1. Transmission Distance2. The data rate or channel

bandwidth3. The bit-error rate (BER)

Elements of Link/ Network Design

• Transmitter : Operating wavelength (),

Linewidth (), Rise time, Bit-rate, Line format, Power level

• Fiber : SMF/MMF, Fiber type – SMF28,

DSF, etc, Cable loss

• Rx : PSEN, PSAT, Rise time

Elements of Link/ Network Design (cont.)

• Connection: No. of splice, Splice lossNo. of connectors, Connector

Loss

• In Line Devices: Splitter, Filter, Attenuator,

Amplifier Insertion loss, Gain

Optical Amplifiers

•An optical amplifier is a device which amplifies the optical signal directly without ever changing it to electricity. The light itself is amplified.

•Reasons to use the optical amplifiers: ReliabilityFlexibilityWavelength Division Multiplexing (WDM)Low Cost

•Variety of optical amplifier types exists, including:Semiconductor Optical Amplifiers (SOAs)Erbium Doped Fibre Amplifiers (EDFAs) (most common)

Basic Concepts

Most optical amplifiers use stimulated emission

An optical amplifier is basically a laser without feedback

Optical gain is realized when the amplifier is pumped optically (or electrically) to achieve population inversion

Gain depends on wavelength, internal light intensity and amplifier medium

Two types: semiconductor optical amplifiers and fiber doped amplifiers

OFAs in the Network

Relatively simple construction

Reliable, due to the number of passive

components

Allows easy connection to external fibers

Broadband operation > 20 nm

Bit rate transparent

Ideally suited to long span systems

Integral part of DWDM systems

Undersea applications for OFAs are now

common

In-line Amplifier

PowerAmplifier

Preamplifier

Transmitter

Transmitter

Transmitter

Optical Amplifiers

Optical Amplifier

Fiber Link

Optical Amplifier

Fiber Link

Optical Receiver

Optical Receiver

Optical Receiver

Optical Amplifier Applications

Fiber Link

Type GainMaximum Output

power Noise figure

Power Amplifier

High gain High output power moderate

In-line Medium gain Medium output

powermoderate noise

figure

Preamplifier Low gain Low output powerLow value < 5 dB

essential

Selecting Amplifiers

Semiconductor Optical Amplifiers

Similar to Laser diodes but the emission is triggered by input optical signal

Work in any wavelength.Have high integration, compact and low

power consumption .Gain fluctuation with signal bit rate .Cross talk between different

wavelengths .Two types: Fabry-Perot or Traveling Wave

Amp.

Erbium Doped Fiber Amplifier

A pump optical signal is added to an input signal by a WDM coupler Within a length of doped fiber part of the pump energy is transferred to the

input signal by stimulated emission For operation 1550 nm the fiber dopant is Erbium Pump wavelength is 980 nm or 1480 nm, pump power 50 mW Gains of 30-40 dB possible

Working of an EDFA

Characteristics of EDFA

Efficient pumpingLow insertion lossHigh output power Low noiseVery high sensitivityLow distortion, minimal interchannel crosstalk

Advantages

Low crosstalkHappen to operate in most transparent region

of the spectrum for glass fiberExtremely long excited state lifetime (on the

order of 10 ms)

Disadvantages

Can only work at wavelengths where Er+3 fluoresces

Requires specially doped fiber as gain medium

Requires long path length of gain medium (tens of meters in glass)

Gain very wavelength-dependent and must be flattened

Gain limited by cooperative quenching

Thank you