COMPONENT IN FIBER OPTIC

COMMUNICATION

CHAPTER 3

Fiber Optic Sources

Two basic light sources are used for fiber optics:

light-emitting diodes (LED) laser diodes (LD)

Light-Emitting Diodes

An LED is form of junction diode that is operated with forward bias

Instead of generating heat at the PN junction, light is generated and passes through an opening or lens

LEDs can be visible spectrum or infrared

light-emitting diodes (LED)

Fiber optic sources must operate in the low-loss transmission windows of glass fiber.

LEDs are typically used at the 850-nm and 1310-nm transmission wavelengths

LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power.

They are used in applications in which data rates are in the hundreds of megahertz.

Two basic structures for LEDs are used in fiber optic systems: surface emitting and edge emitting

The output spectrum of a typical LED is about 40 nm, which limits its performance because of severe chromatic dispersion.

LEDs operate in a more linear fashion than do laser diodes. This makes them more suitable for analog modulation

Surface-emitting LEDs

Light emerges from top

Directed by device structure and packaging

Common for illumination LEDs

+ +- -

Edge-emitting LEDs

Light in junction plane

Emerges from side facet

Smaller emitting area

+ +- -

LED

Figure 8-22 shows a graph of typical output power versus drive current for LEDs and laser diodes.

Notice that the LED has a more linear output power that makes it more suitable for analog modulation

Typical applications are local area networks, closed-circuit TV, and transmitting information in areas where EMI may be a problem.

Laser Diodes

Laser diodes generate coherent, intense light of a very narrow bandwidth

A laser diode has an emission linewidth of about 2 nm, compared to 50 nm for a common LED

Laser diodes are constructed much like LEDs but operate at higher current levels

Laser Diode Construction

Laser Diode

Laser diodes (LD) are used in applications in which longer distances and higher data rates are required.

Because an LD has a much higher output power than an LED, it is capable of transmitting information over longer distances.

Consequently, and given the fact that the LD has a much narrower spectral width, it can provide high bandwidth communication over long distances.

The LD has smaller N.A. also allows it to be more effectively coupled with single-mode fiber.

The difficulty with LDs is that they are inherently nonlinear, which makes analog transmission more difficult

Laser Diode

They are also very sensitive to fluctuations in temperature and drive current, which causes their output wavelength to drift.

In applications such as wavelength-division multiplexing, in which several wavelengths are being transmitted down the same fiber, the stability of the source becomes critical.

This usually requires complex circuitry and feedback mechanisms to detect and correct for drifts in wavelength.

The benefits, however, of high-speed transmission using LDs typically outweigh the drawbacks and added expense.

Laser Diode

Laser diodes can be divided into two generic typesGain-guided laser diodes work by controlling the

width of the drive-current distribution; this limits the area in which lasing action can occur.

Index-guided laser diodes use refractive index steps to confine the lasing mode in both the transverse and vertical directions.

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; they would not show on same scale

LED versus Laser

Characteristic

LED Laser

Output power

Lower Higher

Spectral width

Wider Narrower

Numerical aperture

Larger Smaller

Speed Slower FasterCost Less More

Ease of operation

Easier More difficult

Light Detector

Optical detection occurs at the light wave receiver’s circuitry.

The photo detector is the device that receives the optical fiber signal and converts it back into an electrical signal.

The most common types of photo detectors are a) positive intrinsic negative photodiode ( PIN )

b) the avalanche photodiode (APD )

Light detector characteristic

The important characteristics of light detectors are :1. Responsitivity: Responsitivity is a measure of the

conversion efficiency of a photodetector.2. Dark current: Dark current is the leakage current

that flows through a photodiode with no light input.3. Transit time: Transit time is the time it takes a

light-induced carrier to travel across the depletion region.

4. Spectral response: Spectral response is the range of wavelength values that can be used for a given photodiode.

5. Light sensitivity: Light sensitivity is the minimum optical power a light detector can receive and still produce a usable electrical output signal.

PIN diode

The most common optical detector used with fiber-optic systems is the PIN diode

The PIN diode is operated in the reverse-bias mode As a photodetector, the PIN diode takes advantage of its wide

depletion region, in which electrons can create electron-hole pairs The low junction capacitance of the PIN diode allows for very fast

switching PIN photodiodes are inexpensive, but they require a higher optical

signal power to generate an electrical signal. They are more common in short distance communication

applications.

Avalanche Photodiode

APD photodiodes are more sensitive to lower optical signal levels and can be used in longer distance transmissions.

They are more expensive than the PIN photodiodes and are sensitive to temperature variations

The avalanche photodiode (APD) is also operated in the reverse- bias mode

The creation of electron-hole pairs due to the absorption of a photon of incoming light may set off avalanche breakdown, creating up to 100 more pairs

This multiplying effect gives an APD very high sensitivity

Splices and Connectors

In fiber-optic systems, the losses from splices and connections can be more than in the cable itself

Losses result from: Axial or angular misalignment Air gaps between the fibers Rough surfaces at the ends of the fibers

Fiber-Optic Connectors

Coupling the fiber to sources and detectors creates losses as well, especially when it involves mismatches in numerical aperture or in the size of optical fibers

Good connections are more critical with single-mode fiber, due to its smaller diameter and numerical aperture

A splice is a permanent connection and a connector is removable

Type of connector

Type Feature Application

Ferrule Connector (FC)

-Was designed for use in high vibration environment- provide non-optical disconnect performance-Designed with a threaded coupling for durable connection

- datacom-Telecommunication- measurement equipment

Straight Tip (ST) -Maintains the perfect alignment of the ends of the connected fibers required for efficient light transmission.- Mate with an interconnection adapter and is latched into place by twisting to engage a spring-loaded bayonet socket

-multimode fiber optic LAN

Type of connector

Type Feature Application

Subscriber Connector (SC)

- a standard- duplex fiber optic connector with a sqaure molded plastic body and push-pull locking features.

-Data communication- CATV- telephony

Subminiature (SMA)

-robust fiber optic connector that is composed of a threaded coupling housing-can withstand high temperatures without experiencing hot spots- Compatible with TO-18 transmitter/emittercomponents

- Medical - Industrial - Data / Telecom - FTTx - Mining - Oil exploration

Type of connector

Type Feature Application

Lucent / Local Connector

-similar to a RJ45 connector- Optimized for point to point interconnection and multi-channel routing application

-Data / Telecom-Local Area - Network (LAN) - FTTH / FTTP -Cable TV

Optical Couplers

An optical device that combines or splits power from optical fibers

As with coaxial cable and microwave waveguides, it is possible to build power splitters and directional couplers for fiber-optic systems

It is more complex and expensive to do this with fiber than with copper wire

Optical couplers are categorized as either star couples with multiple inputs and outputs or as tees, which have one input and two outputs

Type of coupler/adapter

Type Feature Application

ST -used to link different kinds of ST optical fiber components.- Mates with interconnection adapter and is latched into place by twisting to engage a spring-loaded bayonet socket

-Premise installationTelecommunication networks (LANs)Data processing networks

SC - a snap-in (push-pull) connection design for quick patching of cables into rack or wall mounts..

-CATVTelecommunication networksLocal Area Networks (LANs)

Type of coupler/adapter

Type Feature Application

Fiber Distributed Data Interface (FDDI)

- refer to a local area network standard such as Ethernet ar Token Ring.- Contain two ferrules in large, bulky plastic housing that uses a squeeze tab retention mechanism.

-CATVTelecommunication networks-Local Area Networks (LANs)

FC -used to link the screw type FC optical fiber connections. - can be used alone or be mounted onto fiber optic patch panels.

-CATVTelecommunication networks- Local Area Networks (LANs)

Optical Switches and Relays

Optical switch is a switch that enables signals in optical fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to another.

The simplest type of optical switch moves fibers so that an input fiber can be positioned next to the appropriate output fiber

Another approach is direct the incoming light into a prism, which reflects it into the outgoing fiber. By moving the prism, the light can be switched between different output fibers

Lenses are necessary with this approach to avoid excessive loss of light

Optical Cross Connect

Optical Cross-Connects (OXC)Wavelength Routing Switches (WRS)route a channel from any I/P port to any O/P port

Natively switch s while they are still multiplexed Eliminate redundant optical-electronic-optical

conversions

DWDMFibers

in

DWDMDemux

DWDMDemux

DWDMFibers

out

DWDMMux

DWDMMux

All-optical

OXC

MEMS Optical Switches

What is MEMS Micro-Electro-Mechanical System

What is MEMS optical switches Steerable micromirror array to direct optical light

from input port to its destination port. System-in-a-chip

2D MEMS Switches

Mirrors have only 2 positions (cross or bar)Crossbar configurationN2 mirrors

3D MEMS Switches

Mirrors can be tilted to any angles

N or 2N mirrors accomplishing non-block switching

Good scalability

Repeaters, Regenerators, and Optical Amplifiers

Boost signal after it fades with distanceNeeded to span long distances

more than 50-100 km terrestrial often shorter distances in networks

Repeater: receiver-transmitter pairRegenerator: Repeater plus signal clean-upOptical amplifiers: amplify signal as light

Current state of the art at 1530-1620 nmOptical regenerators: would be nice

Repeaters and regenerators

DetectorElectronicamplifier

Transmitter

DetectorElectronicamplifier

Transmitter

Thresholding & retiming

Forward error

correction

Repeater

Regenerator

Optional

Electro-optic repeaters

Receiver converts signal to electronic formElectronics amplify signal, drive transmitterBecame obsolete

Limited to one transmission format Designed for particular data rate One optical channel per repeater Erbium-doped fiber amplifiers are better

NOT obsolete for wavelength conversion

Why regenerators are still used

Optical amplifiers are analog devices Cannot remove noise or dispersion Contribute amplified spontaneous emission

Dispersion accumulates over long distancesRegeneration used at termination pointsMost terrestrial systems <1000 km

Terminate in switches or routers Signals redistributed

Regeneration is within the switch

Optical amplifiers

Directly amplify weak optical signal Stimulated emission from excited material Laser without a resonant cavity Optical signal makes single pass

Amplify all wavelengths in their range Compatible with WDM

Purely analog devices Require fine tuning to limit noise

Types of optical amplifiers

Erbium-doped fiber amplifiers: C band 1530-1565 nm-most widely used L band 1570-1620 nm

Thulium doped fibers, S band 1470-1500 nmRaman fiber amplifiers: broadbandPraseodymium-doped fiber amplifiers

1310 nm rangeSemiconductor optical amplifiersCascaded

Erbium-fiber amplifier

Erbium fiber operation

Single pass produces gainOptical isolators prevent feedback to laserNoise from amplified spontaneous emission Erbium gain is broad: 1520-1630 nm

Depends on erbium host Designs differ for different wavelength bands Long fibers for low-gain L band 1570-1620 nm Short fibers for high-gain C band 1530-1565 nm

Erbium-fiber gain

Cascaded

Amplifiers connected in series in electronics.Two amplifiers are connected together by

using coupling capacitor.Used when topological conditions do not

allow direct communication between module and gateway.

Their use can double or triple the communication distance between point within the limit of 3 repeater in cascaded.

Noise factor

a) Thermal noise Noise due to thermal agitation of electron in a

conductor. It is present in all electronic devices and

transmission media and is a function of temperature

b) Shot noise Caused by discrete nature of electrons a signal

disturbance The pulse start when the electron escapes from the

cathode and end when the electron strikes the anode.

Noise factor

c) Dark Current Noise The relatively small electric current that flow

through photosensitive devices such as a photomultiplier tube, photodiode are charge coupled device even when no photon are entering the device.

Refer as reverse leakage current.

Signal to Noise Ratio

SNR = S/NS – represents the information to be

transmittedN – integration of all noise factors over the full

system bandwidth

SNR (dB) = 10 log 10 (S/N)

Fiber Joints (Connections)

Point where two fibers are joined together To allow light signal to propagate from one

fiber into the other with as little loss as possible

Reasons for fiber joints:Fibers and cables are not endless and

therefore must eventually be joined.Fiber may also be joined to distribution

cables and splitters.At both transmit and receive termination

Fiber Joints (Connections)

Fiber optic cables terminated in 2 ways :ConnectorsSplices

SplicingPermanent connection of two optical fibers.

Fiber splices (contd.)

Need of splicing:System design may require that fiber connections

have specific optical properties (low loss)Permit repair of damaged optical fibersCables are of limited lengths – 1 to 6km. To establish long-haul optical fiber links.Splices might be required at building entrances,

couplers, wiring closets, etc.Broadly classified into two typesArc fusion splicingMechanical splicing

Splicing: Pre-requisite

End preparationStripping :Stripping away all

protectionStripping their protective

polymer coating Thermal splicers are bestCleaning:alcohol and wipes, orultrasonic cleanerCleaving:perfect fiber end face cut  

Alignment

Fiber splice alignmentPassive : relies on precision reference

surfacesActive : monitors splice loss or uses

microscope

Arc Fusion Splicing

Localized heat melts or fuses the endsSplice loss- direct function of angles and

quality of fiber-end facesArc fusion- discharge of electric current

across the gap between two electrodes

Arc Fusion Splicing

Fiber end placed between electrodes

Electric discharge melts or fuses the ends of each fiber

Initially, a small gap between fiber ends

Pre-fusion: short discharge of electric current, eliminates fiber defects from cleaving

Surface defects can cause core distortions or bubble formations

Fusion splice--ends pressed together,-actively aligned,-longer and stronger electric discharge

Surface tension of molten glass tends to realign

Arc Fusion Splicing

Protecting the fiber:Splice protection sleeve Yields vary between 25 and 75%Sophisticated fusion splices for low loss

Mechanical Splicing

Mechanical fixtures to align and connect optical fibers

Amount of splice loss stable overtimeUnaffected by changes in environmental or

mechanical conditionsIf high splicing loss results- splice reopened

and fibers realigned

Mechanical Splicing (contd.)

Glass or Ceramic AlignmentCapillary tube Inner diameter of tube

slightly larger than outer diameter of fiber

Transparent adhesive injected into the tube bonds the two fiber together

Adhesive also provides index matching

Relies on inner diameter of tube

Inner diameter should be appropriates

Glass Or Ceramic Alignment Tube Splices  o

Capillary tubeo

Inner diameter of tube slightlylarger than outer diameter of fibero

 Transparent adhesive injectedinto the tube bonds the twofibers togethero

Adhesive also provides indexmatchingo

Relies on inner diameter of tubeo

Inner diameter should beappropriateFig. A glass or ceramic alignmenttube for mechanical splicing.

Mechanical splicing (contd.)

V-Grooved SplicesOpen grooved substrate to

perform fiber alignmentV-groove aligns the cladding

surface of each fiber end Transparent adhesive makes

the splice permanent by securing the fiber ends to the grooved substrate

Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibers. Fig. Open V-grooved splice.

Mechanical splicing (contd.)

Spring V-Grooved Mechanical Splice

 Two positioning rods  Two rods form a groove. This

is used to align the fiber endsOuter surface of each fiber

end extends above the groove formed by the rod

A flat spring presses fiber ends into the groove

 Transparent adhesive completes the process Fig. Spring V-grooved mechanical splice.

Mechanical splicing (contd.)

Rotary SpliceFibers are mounted into a glass

ferrule and secured with adhesives  The splice begins as one long glass

ferrule that is broken in half during the assembly process.

Fiber inserted into each half of the tube, epoxied using ultraviolet cure epoxy.

 The end face of the tubes are polished and placed together using the alignment sleeve.

Added mechanical stability  The rotary splice may use index

matching gel within the alignment sleeve to produce low-loss slices.

Splicing Defect

Several defects can occur during splicing leading to useless splices

Great care needs to be taken

Dense Wavelength Division Multiplexing

It transmits multiple data signals using different wavelengths of light through a single fiber.

Incoming optical signals are assigned to specific frequencies within a designated frequency band.

The capacity of fiber is increased when these signals are multiplexed onto one fiber

Transmission capabilities is 4-8 times of TDM Systems with the help of Erbium doped optical amplifier.

EDFA’s : increase the optical signal and don’t have to regenerate signal to boost it strength.

It lengthens the distances of transmission to more than 300 km before regeneration

Dense Wavelength Division Multiplexing

Multiple channels of information carried over the same fiber, each using an individual wavelength

Dense WDM is WDM utilizing closely spaced channels

Channel spacing reduced to 1.6 nm and less

Cost effective way of increasing capacity without replacing fiber

Commercial systems available with capacities of 32 channels and upwards; > 80 Gb/s per fiber

Wavelength Division

Multiplexer

Wavelength Division

Demultiplexerl1

Al2

l3B

C

l1X

l2

l3Y

Zl1 + l2 + l3

Fibre

Simple DWDM System

Multiple channels of information carried over the same fiber, each using an individual wavelength

Unlike CWDM channels are much closer together

Transmitter T1 communicates with Receiver R1 as if

connected by a dedicated fiber as does T2 and R2 and so on

Wavelength Division

Multiplexer

Wavelength Division

Demultiplexerl1

T1l2

lN

T2

TN

l1R1

l2

lN

R2

RN

l1 + l2 ... lN

Fibre

Is DWDM Flexible?

DWDM is a protocol and bit rate independent hence, data signals such as ATM, SONET and IP can be transmitted through same stream regardless their speed difference.

The signals are never terminated within the optical layer allows the independence of bit rate and protocols, allowing DWDM technology to be integrated with existing equipment in network.

Hence, there’s a flexibility to expand capacity within any portion of their networks.

Is DWDM Expandable?

“ DWDM technology gives us the ability to expand out fiber network rapidly to meet growing demands of our customer”, said Mike Flynn, group President for ALLTEL’s communications operations.

DWDM coupled with ATM simplifies the network, reduce network costs and provide new services.

They can add current and new TDM systems to their existing technology to create a system with virtually endless capacity expansion

DWDM System Characteristics

Well-engineered DWDM systems offer component reliability, system availability, and system margin. Although filters were often susceptible to humidity, this is no longer the case.

An optical amplifier has two key elements: the optical fiber that is doped with the element erbium and the amplifier.

Automatic adjustment of the optical amplifiers when channels are added or removed achieves optimal system performance.

In the 1530- to 1565-nm range, silica-based optical amplifiers with filters and fluoride-based optical amplifiers perform equally well.

The system wavelength and bit rate can be upgraded but planning for this is critical.

DWDM Components

Transmitter : Laser with precise stable wavelength.Link: Optical fiber that exhibits low loss and

transmission performance in relevant wavelength spectra.

Receiver: Photo detectors and Optical demultiplexers using thin film filters or diffractive elements.

Optical add/drop multiplexers and optical cross connect components.

DWDM component –Mux/demux

DWDM terminal demultiplexerThe terminal demultiplexer breaks the multi-

wavelength signal back into individual signals and outputs them on separate fibres for client-layer systems (such as SONET/SDH) to detect.

DWDM component -OADM

Optical Add/drop multiplexer (OADM)Between multiplexing and demultiplexing points

in a DWDM system, there is an area in which multiple wavelengths exist.

DWDM component -OSC

Optical Supervisory Channel (OSC)The OSC carries information about the multi-wavelength

optical signal as well as remote conditions at the optical terminal or EDFA site.

The “out–of–band” Optical Supervisory Channel (OSC) allows the supervision of all the NEs along the WDM path; moreover it gives some order–wires (data channel and voice channel) to the users.

Out-of-band, means the OSC is using a different band than the DWDM system is normally running in, which normally would be the U-band.

ITU standards suggest that thee OSC should utilize an OC-3 signal structure, though some vendors have opted to use 100 Megabit Ethernet or another signal format.

DWDM System with optical amplifier

Transmitters

DWDM Multiplexer

Power Amp

Line Amp

Line Amp

Receive Preamp

200 km

DWDM DeMultiplexer

Each wavelength behaves as if it has it own "virtual fibre"

Optical amplifiers needed to overcome losses in mux/demux and long fiber spans

Receivers

Optical fibre

DWDM System with Add-Drop

Transmitters

DWDM Multiplexer

Power Amp

Line Amp

Receive Preamp

200 km

DWDM DeMultiplexer

•OADM can drop a number of incoming wavelengths and insert new optical signals on these wavelengths. The remaining wavelengths of the WDM link are allowed to pass through. •The wavelengths that it adds/drops can be either statically or dynamically configured.

ReceiversAdd/Drop Mux/Demux

Optical fibre

DWDM Coupler

8 wavelengths used (4 in each direction). 200 Ghz frequency spacing Incorporates a Dispersion Compensation Module (DCM)Expansion ports available to allow denser multiplexing

DWDM versus TDM

DWDM can give increases in capacity which TDM cannot match

Higher speed TDM systems are very expensive

DWDM Standards

ITU Recommendation is G.692 "Optical interfaces for multichannel systems with optical amplifiers"

G.692 includes a number of DWDM channel plans

Channel separation set at:

50, 100 and 200 GHz

equivalent to approximate wavelength spacings of 0.4, 0.8 and 1.6 nm

Channels lie in the range 1530.3 nm to 1567.1 nm (so-called C-Band)

Newer "L-Band" exists from about 1570 nm to 1620 nm

Supervisory channel also specified at 1510 nm to handle alarms and monitoring

Optical Spectral Bands

Wavelength in nm1200 17001300 1400 16001500

S BandC Band

L Band

5th WindowE Band

2nd WindowO Band

Applications of DWDM

DWDM is ready made for long-distance telecommunications operators that use either point-to-point or ring topologies.

Building or expanding networks Network wholesalers can lease capacity, rather

than entire fibers. The transparency of DWDM systems to various bit

rates and protocols.Utilize the existing thin fiber DWDM improves signal transmission

Advantages

Robust and simple designWorks entirely in the Optical domainMultiplies the capacity of the network many foldCheap ComponentsHandles the present BW demand cost effectivelyMaximum utilization of untapped resourcesBest suited for long-haul networks

Disadvantages

Dispersion Chromatic dispersion Polarization mode dispersion

Attenuation Intrinsic: Scattering, Absorption, etc. Extrinsic: Manufacturing Stress, Environment, etc.

Four wave mixing Non-linear nature of refractive index of optical fiber Limits channel capacity of the DWDM System

PREPARED BY: MAIZATUL ZALELA BINTI MOHAMED SAIL