Transcript component in fiber optic communication

CHAPTER 3
COMPONENT IN
FIBER OPTIC
COMMUNICATION
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
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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, closedcircuit 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
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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 types
 Gain-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)
Laser output is many times
higher than LED output; they
would not show on same scale
Standard laser
(1-3 nm wide)
LED (30-50 nm wide)
Wavelength
LED versus Laser
Characteristic
Output power
Spectral width
LED
Lower
Wider
Laser
Higher
Narrower
Numerical
aperture
Speed
Cost
Ease of
operation
Larger
Smaller
Slower
Less
Easier
Faster
More
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 lightinduced 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
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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
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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:
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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
-Telecommunicati
on
- measurement
equipment
Straight Tip (ST)
-multimode fiber
optic LAN
-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
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/emitter
components
- 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
installation
Telecommunicatio
n networks
(LANs)
Data processing
networks
SC
- a snap-in (push-pull) connection
design for quick patching of cables
into rack or wall mounts..
-CATV
Telecommunicatio
n networks
Local 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.
-CATV
Telecommunicatio
n 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.
-CATV
Telecommunicatio
n 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
DWDM
Demux
DWDM
Fibers
in
DWDM
Mux
DWDM
Fibers
out
All-optical
DWDM
Demux
OXC
DWDM
Mux
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 configuration
 N2 mirrors
3D MEMS Switches
 Mirrors can be tilted
to any angles
 N or 2N mirrors
accomplishing nonblock switching
 Good scalability
Repeaters, Regenerators, and Optical Amplifiers
 Boost signal after it fades with distance
 Needed to span long distances
 more than 50-100 km terrestrial
 often shorter distances in networks
 Repeater: receiver-transmitter pair
 Regenerator: Repeater plus signal clean-up
 Optical amplifiers: amplify signal as light
 Current state of the art at 1530-1620 nm
 Optical regenerators: would be nice
Repeaters and regenerators
Detector
Electronic
amplifier
Transmitter
Repeater
Optional
Detector
Electronic
amplifier
Regenerator
Thresholding
& retiming
Forward
error
correction
Transmitter
Electro-optic repeaters
 Receiver converts signal to electronic form
 Electronics amplify signal, drive transmitter
 Became 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 distances
 Regeneration used at termination points
 Most 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 nm
 Raman fiber amplifiers: broadband
 Praseodymium-doped fiber amplifiers
 1310 nm range
 Semiconductor optical amplifiers
 Cascaded
Erbium-fiber amplifier
Erbium fiber operation
 Single pass produces gain
 Optical isolators prevent feedback to laser
 Noise 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
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.
a)
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/N
S – represents the information to be transmitted
N – 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
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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 :
 Connectors
 Splices
Splicing
 Permanent 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 fibers
 Cables 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 types
 Arc fusion splicing
 Mechanical splicing
Splicing: Pre-requisite
End preparation
Stripping :
 Stripping away all protection
 Stripping their protective
polymer coating
 Thermal splicers are best
Cleaning:
 alcohol and wipes, or
 ultrasonic cleaner
Cleaving:
 perfect fiber end face cut
Alignment
Fiber splice alignment
 Passive : relies on precision reference surfaces
 Active : monitors splice loss or uses microscope
Arc Fusion Splicing
 Localized heat melts or fuses the ends
 Splice loss- direct function of angles and quality
of fiber-end faces
 Arc 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
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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 overtime
 Unaffected by changes in environmental or
mechanical conditions
 If high splicing loss results- splice reopened and
fibers realigned
Mechanical Splicing (contd.)
Glass or Ceramic Alignment
Capillary 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
Mechanical splicing (contd.)
V-Grooved Splices
 Open grooved substrate to
perform fiber alignment
 V-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 Vgrooved splice.
Mechanical splicing (contd.)
Spring V-Grooved Mechanical
Splice
 Two positioning rods
 Two rods form a groove. This
is used to align the fiber ends
 Outer 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 Splice
 Fibers 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
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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
A
B
C
1
Wavelength
Division
Multiplexer
Fibre
Wavelength
Division
Demultiplexer
2
3
1
2
1 + 2 + 3
3
 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
X
Y
Z
Simple DWDM System
1
T1
Wavelength
Division
Multiplexer
Fibre
Wavelength
Division
Demultiplexer
2
T2
TN
N
1
2
1 + 2 ... N
N
R1
R2
RN
 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
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
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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 demultiplexer
 The terminal demultiplexer breaks the multiwavelength 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
Receivers
DWDM
Multiplexer
Optical
fibre
Power
Amp
Line
Amp
Line
Amp
Receive
Preamp
DWDM
DeMultiplexer
Transmitters
200 km
 Each wavelength behaves as if it has it own "virtual fibre"
 Optical amplifiers needed to overcome losses in mux/demux and long
fiber spans
DWDM System with Add-Drop
Add/Drop
Mux/Demux
DWDM
Multiplexer
Power
Amp
Optical
fibre
Receivers
Line
Amp
Receive
Preamp
DWDM
DeMultiplexer
Transmitters
200 km
•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.
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
2nd Window
O Band
5th Window
E Band
S Band
C Band
L Band
1200
1300
1400
1500
Wavelength in nm
1600
1700
Applications of DWDM
 DWDM is ready made for long-distance
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telecommunications operators that use either point-topoint 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 design
 Works entirely in the Optical domain
 Multiplies the capacity of the network many fold
 Cheap Components
 Handles the present BW demand cost effectively
 Maximum utilization of untapped resources
 Best 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