Optical amplifiers and networking

Prof.Amogha B,
Dept. of EC&E,
JIT, Davangere.

OpticalAmplifiers
 Optical amplifiers,
 Basic applications and types,
 Semiconductor optical amplifiers, EDFA.
OPTICAL NETWORKS: Introduction,
 SONET / SDH, Optical Interfaces,
 SONET/SDH rings, High – speed light
waveguides
1/1/2018 Dept. of EC&E, JIT, Davangere. 2

 Traditional Power budget were considered while
making a optical link. Repeaters are used when
power margin loss is more.
 To amplify: photon to electron generation working
on electrical reshaping and then convert to photon.
 A same process is applied for modern single
wavelength operated devices but its costly and
complex for multi-wavelength operated devices.
1/1/2018 3Dept. of EC&E, JIT, Davangere.

 Optical amplifiers applications vary from ultra-long
underground links to short range communication.
 Longer wavelength posses spectral response range
and highly reliable.
 Shorter wavelength are carried on metro and access
network links but wavelength can be altered based on
customer’s demands.
 Designed to recover quickly from rapid input power
variations as sudden channels changes.
1/1/2018 4Dept. of EC&E, JIT, Davangere.

 Three classes of the amplifiers applications
are there namely
 In-line optical amplifiers.
 Pre-amplifiers
 Power amplifiers

 In a single mode link, the effects of fiber
dispersion may be small so that limitation to
repeater spacing is fiber attenuation.
 There is no need of complete regeneration of
signal, a simple amplification of optical signal is
sufficient.
 An inline amplifiers used to decrease the
transmission loss and increase the distance
between repeaters by enhancing power levels.

 Used as front-end amplifiers for an optical
receiver where weak signals are amplified
before photo-detection.
 Decrease in SNR by thermal noise at the
receiver side can be minimized considerably.
 Provides a higher gain factor compared to
other amplifiers.

 To boost the transmitted signal at the
transmitters the power amplifiers are used.
 Increases the transmission distance from 10
to 100km based on value of amplifier gain
and fiber losses.
 Combination of pre-amplifiers and boosting
technique can achieve undersea
communication without repeaters(300km).

 A passive star coupler is used to multiplex a
single input port to several output ports.
 It take cares about the power splitter loss and
coupler insertion loss.

 Three main types of amplifiers are
 Semiconductor OpticalAmplifiers(SOA)
 Doped Fiber Amplifier.(DFA)
 Raman Amplifiers.
 All amplifiers increases power level of incident light through
stimulated emission or optical power transfer.
 Optical amplifiers are nothing but a laser without feedback.
 These just boosts the signals but unable to produce coherent
output. ( Ex: Reference source in DAC a 5V for reference but if
we connect load it unable to reproduce the same)

 The device absorbs the energy from external source
which is called as pump to active medium.
 Due to pumping action the population inversion
occur(higher orbit) in active medium.
 An incoming signal photon makes the stimulated
emission process.
 The falling electrons emits the electron hence the
optical amplifications happens.

 Posses high gain response.
 Can be used both for switching and signal
processing functions in optical network.
 Limitations is the gain changes at particular
wavelength due to rapid change in carrier response.
 Rapid gain response leads to crosstalk while
amplification.

 The active medium is made up of lightly doped SiO2
with erbium during the fabrication of core of the fiber.
 The advantage is pumping of the devices at several
different wavelength and has low coupling loss.
 Posses high bit rate and less dispersion for broad
spectrum from 1530 to 1560 nm.
 Posses less cross talk between the electronic
components.

 Stimulated Raman scattering is the principle used in these kinds of
amplifiers.
 The gain for Raman amplifiers are through lump amplifiers or by
distributed amplifier.
 Lump amplifiers with a pump lasers are fitted to the small core
radius of 80m
 Distributed amplifiers any wavelength of sources can be used as
last 20 to 40km of fiber is converted to pre-amplifiers.
 Types are
 Erbium doped fiber amplifiers and
 Travelling wave amplifiers

 These are optical networks which are used for
signal processing and switching functions.
 The signal rate is fluctuating due to rapid
change in carrier response and also effects
the signal gain for all wavelength.
 The crosstalk effects during the amplification
of broad spectrum due to rapid change in
carrier frequency.

 The active medium prepared by doping the
rare earth metals with 10-30m length of optical
fiber.
 The fiber material can be silica, fluoride based
glass.
 The popular rare earth material is erbium.
 It is operated in the range 1530 to 1560 nm

 Optical pumping : an external current (photon) is
used to push the electrons from lower to higher
energy level.
 The three energy levels are used in optical
pumping.
 Stimulated emission.
 Top level: More than a desired laser level(population
inversion)
 Middle level: Meta-stable state
 Normal state: all the energy are fed back in the form
of new photon

 From pump source to ground energy level
1.27ev band-gap separation.
 0.841 separation between ground and top
layer of metastable
 Bottom layer of metable and ground is of
0.814ev
 Bottom layer of metable and top of bottom
layer is 0.775ev.

 Process1
 Process2
 Process3
 Process 4
 Process5
 Process6
 Process7

 Pumping to active medium based on several
different wavelength.
 Low coupling loss
 Has a constant gain.
 Absorption and emission depends on glass
composition and dopants.

 Consists of doped fiber, pump lasers,
couplers(tap and passive wavelength), optical
isolators.
 Di-chronic coupler combines the pump and
signal power into amplifiers of different
wavelength
 Tap couplers is wavelength splitting ratio
with 99:1 to 95:5(Used to compare incoming
signal with output power)

 Optical isolators avoids the back
propagation of amplified signal.
 Pumping of lasers posses two types based on
the direction of pumping towards signal path
namely
 Co-directional pumping.
 Counter pumping.
 Dual pump schemes posses two pumping
lasers which has 35 db gain

 Counter directional pumping- Provides higher
gain .
 Co-directional pumping has better noise
performance at 980nm
 Larger population inversion occurs at
1480nm.

 Output signal of any amplifiers increases which saturates
the gain of amplifiers.
 Gain of the amplifiers depends on population inversion as
shown in graph.
 The fundamental physics law: The energy extracted from
source should be less than pump power stored in device.
 The in-equality in equation tells the possibility of pump
photons are lost due to interaction with impurities or
spontaneous emission in the pump energy

 Links operating at 10Gb/s

 Optical fibers are used to connect a wide range of
applications varies in transmission capacity and
speed.
 The connection can vary from shorter distance to
connecting two countries via ocean.
 Increase in the usage of optical fiber has increased
in every field and devices(Hardware : Computer
storage devices) also supports high speed

 Basic terminology and general networks:
Four transmission implementation.( Long,
metro, access and LAN)
 Topology of the network, SONET and SDH
 Physical layer of SONET/SDH.

 Different network architecture: defines the
network terminology (Figure)

 Stations: Collection of devices that are
connected to network for communication
purpose. Ex: computers, router, printer, fax etc
 Networks: To establish the connection between
the stations, transmission paths are used to
make the interconnection between them.
 Node: Is a point where one or more
communication lines meets and used to
connect to stations.

 Trunk: is a term used to refer a transmission line which
connects nodes or network.
 Used for carrying large traffic loads.
 Topology: It is the logical manner in which all the
nodes are connected together to form a channels of
network to carry information.
 Switching and routing: Transfer the information from
source to destination through series of intermediate
node is called as switching. Selecting the specific
path for reaching destination is called as routing.

 Local Area Network(LAN)
 A network used to connect the shorter distance
customers or area.
 Ex: Office building, campus.
 In networking domain campus refers that
something which is at lesser distance.
 LANs are usually owned, used and operated
by a single person/ organization.

 Commonly called as metro networks.
 The range of coverage is usually large
compared to LAN, it connects whole building,
entire city.
 These MANs are operated by the
telecommunication organization

 Covers a large geographical area.
 Used for connecting between two cities
 Example: Undersea lines are used.
 These networks are owned and control by
private organizations or telecommunication
service providers

 Enterprise networks are owned and operated
by a private organization.
 Important thing is it can be used by members
of that organization.
 Public networks are owned by telecom
service provider which are used by public
people ex: leased lines or dedicated lines.

 Central Office
 Backbone
 Long haul Network
 Passive optical network: Optical distribution
network doesn’t posses optoelectronic
components used to provide access. Used to
get connection between fiber to network.

 We know that physical layer provides the
physical connection between nodes but optical
layer provides the light path service over the
link.
 A lightpath is an end to end optical connection
that pass through several intermediate node.
 It supports wavelength switching, multiplexing,
adding and dropping of wavelength

 SONET stands for synchronous optical network.
 Except in North America it is called as SONET in rest
parts of the world it is called as Synchronous Digital
Hierarch (SDH).
 Covers basic concepts of SONET, optical interface and
network implementations.
 Frame format of SONET/SDH.

 STS( Synchronous Transport Signal) all other
series of SONET is the integer multiple of 51.84
Mb/s i.e. STS-N.
 STS-N signal is used to modulate the optical
source.
 To avoid long 0’s and 1’s the logical STS-N is
scrambled.
 After conversion from electrical to optical the
output from physical layer is called as OC-
N(Optical Carrier) N ranges from 1-768.

 In SDH the basic rate is STS-3 (155.52Mb/s)-
STS(SynchronousTransport Module)
 STS 1 – Level 1 for general STS-M.
 SDH doesn’t makes a difference between
logical electrical signal and optical signal

 First 3 columns carry network management
information( transport overhead).
 Next 87 columns are Synchronous Payload
Envelope carries user data and 9 bytes has path
overhead (POH).
 POH monitors the signal labeling, status,
tracking function.
 This 9 bytes (POH) can be placed anywhere in
the frame.
 Individual add or drop in the SONET/SDH is
feasible feature.

 N greater than 1 is going to add the number
of columns multiplied by 3. 3*3=9.
 9 columns has transport overhead.
 90*N(3) is left for carrying data frames.

 By using the STS- N frame format the data
speed can be achieved to certain extend
 For higher speed we are moving for STM-M
 Where N=3M; M=1,4,16 and 64is as same as
SONET N but N=3M (N=3, 12, 48 and 192)

 To get the interconnection between the
equipments of different manufactures with
the SONET/ SDH.

 SONET/SDH is configured as mesh or ring
like structure.
 In order to connect equipments and
overcome the faulty devices a loop like
structures are used.
 It is also called as self- healing ring, the traffic
flow can automatically switched to alternate
or standby path due to faulty path.

 Features
1) It contains 2 or 4 rings running between nodes
(can make eight possible structure).
2) Operating signals can travel either in clockwise
direction(uni-directional ring) or anti-clockwise
direction(bi-directional ring).
3) During link failure the signals are transformed
using the protective ring.

 Out of eight combination the popular two
structures explained
1. Two fiber Uni-directional path switched ring
(UPSR)
2. Two fiber or four fiber, Bi-directional path
switched ring(BLSR).

 Primary ring- clock wise direction
 Protection ring – counter clock wise
direction(anti-clock wise direction).
 Node 1 to node 3 transfer of signals

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