Lecture 1 Intro - Optical Components - At - 2011

ELEN90034

Optical Networking and Design

ELEN90034 Optical Networking and Design – An Tran

Lecturers:

• Dr. An Tran

Email: [email protected]

• Mr. Trevor Anderson

Email: [email protected]

Location: NICTA, Level 2, EEE Building 193

Contact Hours

• Recommended Prerequisites:

– Not required

• Time & Place:

– Monday 2:15 pm – 5:15 pm ICT Theatre 1

• Consultation:

– To be determined

– We do not like “pop-in”. If urgent, can arrange appointment.


Course Information

• References:

– Optical Networks: A Practical Perspective by Rajiv Ramaswami and Kumar Sivarajan

– Ethernet Passive Optical Networks by Glen Kramer

• Lecture notes

– Available before the lecture. – Available before the lecture.

– Students encouraged to read reference texts before lecture.

• Additional notes

– Will provide online links to other sources of helpful learning information.


Assessment

• 60% Final exam

– Hurdle: need to pass exam to pass the subject

• 30% Mid-semester test

– Tentative date: 5 Sep 2011– Tentative date: 5 Sep 2011

• 10% Assignments


Subject Objectives

Develop skills and knowledge in:

• Fundamental optical network elements

• Optical network architectures ranging from optical access

networks to backbone optical transport networks

• Approaches and methodologies of optical network design and • Approaches and methodologies of optical network design and

optimization

• Techniques of optical network survivability

• Problem-solving skills and critical thinking in the discipline of

optical networks


Syllabus

• Introductions to optical communications and optical

components

• SDH/SONET and Gigabit Ethernet

• Optical access networks (including EPON, GPON, WDM

PON)

• Next-generation optical networks

• Optical performance monitoring

• Optical network control, management and survivability

• Energy efficiency issues in networks


ELEN90034: Optical Networking

and Design

Lecture 1: Introduction to Optical

Communications and Optical Components


Introduction

• Recently, dramatic growth in demand for communication capacity– Internet growing at about 50% annually

– Huge bandwidth demand for new applications:

• Video on demand

• Peer to peer traffic

• Interactive services

• Speed of electronics is not increasing fast enough

• Only optical systems can provide the capacity for the future

• Optical communication systems are now the preferred technology for:– Long distance networks (undersea network, national networks)

– High capacity networks (optical LAN, fibre-to-the-home)


Electromagnetic Spectrum

431-694 Optical Networking – An Tran

What is an Optical System?

• Optical fibre communication system uses optical

frequencies (1014 Hz) as carrier frequency to carry

information

– Such high carrier frequency allows modulation rates of up to 1013

bit/s

– Today rates are 2.5 Gb/s and 10 Gb/s. 40 Gb/s and 100 Gb/s are being developedare being developed

• Optical signal is confined inside an optical fibre and

isolated from surrounding environment.

• First-generation optical system: point-to-point, now

evolving into optical network.


Elements of Optical System

• Laser diode

– Drive circuitry

• Optical fibre

• Optical components:

– Coupler,

multiplexer/demultiplexermultiplexer/demultiplexer

– Filter, isolator, circulator

– Optical switches

• Optical amplifier

• Detector

– Receiver circuitry


Evolution of Optical Networks

• LED (light-emitting diode) over

multimode fibre in 0.8 µm and 1.3

µm band

• Fabry-Perot laser (multiple

modes) over single-mode fibre in

1.3 µm band1.3 µm band

• DFB laser (single mode - reduce dispersion) over single-mode fibre in 1.5 µm band to reduce loss

• Current system: WDM with

optical amplifier in 1.5 µm band


Example of a Wavelength-Routing Mesh Network

Optical

crossconnect

(OXC)

Wavelength Wavelength

conversion


Coupler

• A directional coupler is used to combine and split optical signals

• Couplers are made by fusing 2 fibers together in the middle, called fused fiber couplers. Can also be made from waveguides.

• Design parameters:

– Wavelength selective or wavelength independent

– Coupling ratio

– Excess loss– Excess loss


Star Coupler


Isolators

• An isolator is a passive nonreciprocal device.

– Lightpath can be transmitted in one direction, but not

in the opposite direction

– Example of application: anti-reflection

Isolator


Circulators

• A circulator is similar to an isolator, except that it

has multiple ports, typically three or four.

– Example of application: OADM


Multiplexers and Filters

• Filter: separate one wavelength from multiple wavelengths

• Multiplexer: aggregate multiple wavelengths in a single output port. In reverse direction becomes demultiplexer

• Used in wavelength cross-connect and optical add-drop multiplexer


Filter Characteristics

• Insertion loss: loss from input to output

• Polarisation independence

• Temperature independence• Temperature independence

• Flat passband measured by 1-dB bandwidth

• Sharp passband skirts (or slope)


Static Wavelength Crossconnect

• Static wavelength crossconnect: crossconnect pattern is fixed and cannot be changed dynamically


Gratings

• The term grating is used to describe almost any

type of device whose operation involves

interference among multiple optical signals

originating from the same source but with

different relative phase shiftsdifferent relative phase shifts

• In WDM systems, gratings used as

demultiplexer/multiplexer to separate/combine

individual wavelengths


Diffraction Gratings


Bragg Gratings

• Any periodic perturbation in the propagating

medium serves as a Bragg grating.

• This perturbation is usually a periodic variation

of the refractive index of the medium


Fibre Bragg Gratings

• Gratings written in fibre using

photosensitivity of certain fibre type

• Exposing silica fiber doped with

germanium to UV light causes

changes in fiber refractive index.

• Advantages:

• Low loss, easy of coupling, • Low loss, easy of coupling,

polarisation insensitivity, low

temperature coefficient, simple

packaging

• High wavelength accuracy, flat

tops, high crosstalk suppression

• Applications: optical add-drop

multiplexer, dispersion compensator


Fabry-Perot Filters

• A Fabry-Perot filter consists of a cavity formed by two highly reflective mirrors placed parallel to each other

– The input light beam to the filter

enters the first mirror at right

angles to its surface.

– The output of the filter is the light – The output of the filter is the light

beam leaving the second mirror

– Interference occurs within the

cavity

• Advantages: can be tuned to select different WDM wavelengths by changing cavity length or refractive index.

• Used in Fabry-Perot lasers


Multilayer Dielectric Thin-Film Filters

• Thin-film filter is a Fabry-Perot

interferometer where mirrors are

realised using multiple reflective

dielectric thin-film layers

– Act as bandpass filter, pass

through a wavelength and reflect

other wavelengthsother wavelengths

– Passthrough wavelength

determined by cavity length

• Multiple cavities: flatter

passband and sharper passband

skirts


Multilayer Dielectric Thin-Film Filters (2)

• TFF cascaded to make multiplexer/demultiplexer

• Each filter passes a different wavelength and reflects all others

• Good temperature stability, flat passband, sharp skirts, low loss, polarisation

insensitive.


Mach-Zehnder Interferometers

• MZI: interferometric device that makes use of two interfering paths of different lengths to resolve different wavelengths

• MZI consists of 2 3-dB couplers interconnected through 2 interconnected through 2 different paths

• Used as multiplexer/demultiplexer and tunable filter by changing temperature in one arm.


Arrayed Waveguide Grating

• AWG is a generalization of MZI

– Consists of 2 multiport couplers

connected by array of

waveguides

– AWG is a device where several

copies of the same signal, but

shifted in phase by different

amounts, are added togetheramounts, are added together

• Used as multiplexer/demultiplexer or static wavelength crossconnect

• Temperature coefficient not low, require active temperature control


Acousto-Optic Tunable Filter (AOTF)

• AOTF can select several wavelengths simultaneously

• Principle of operation:

– Acoustic wave used to create a Bragg grating

– Changing the frequency changes the grating

– Tunable


AOTF (2)


High Channel Count Multiplexer Architectures

• Serial:

– The demultiplexing is done one wavelength at a time

– The demultiplexer consists of W filter stages in series, one for each of the Wwavelengthswavelengths

– Allow “pay as you grow”

– High loss and not scalable

– Non-uniform loss across channels

– Eg: multilayer dielectric thin-film filters


High Channel Count Multiplexer Architectures (2)

• Single-stage:

– All the wavelengths are demultiplexed together in a single stage

– Lower loss and better loss – Lower loss and better loss uniformity

– No. of channels limited by device capability

– Eg: AWG


• Multistage banding:

– Divide wavelengths into bands

– Demultiplexing done in 2 stages


stages

– Need a guard wavelength space between bands

– More scalable



• Multistage interleaving:

– Demultiplexing done in 2 stages

– First stage separates wavelengths into odd and even-numbered group

– Second stage separates – Second stage separates individual wavelength

– Benefit: last-stage filters can have much wider bandwidth, easier to be built

– Realised by using fiber-based MZI


Switches

• Automatic provisioning of lightpath services: replacing fibre patch panels

• Protection switching in case of fiber and network failure

• Packet switching: packet-by-packet

• External modulation: in front of laser, switching time is fraction of bit duration

• Important parameters:

– Extinction ratio: output power ratio in on and off states


– Extinction ratio: output power ratio in on and off states

– Insertion loss

– Crosstalk

– Polarisation dependent loss

Large Optical Switches

• Considered issues

– Number of switch elements required: cost and complexity

– Loss uniformity

– Number of crossovers

– Blocking characteristics:

• Nonblocking: any unused input port can be connected to any • Nonblocking: any unused input port can be connected to any

unused output port

– Strict-sense nonblocking: without requiring existing connections to be

rerouted

– Wide-sense nonblocking: use particular algorithm to route without

requiring existing connections to be rerouted

– Rearrangeably nonblocking: require rerouting of connections

• Blocking: some interconnection pattern between unused input port

and unused output can no be realised


Crossbar Switch

• Use 2x2 switches

• Wide-sense

nonblocking

• nxn crossbar switch

requires n2 2x2 requires n2 2x2

switches.

• Large difference

between shortest and

longest path


Clos Switch

• Strict-sense

nonblocking

• Individual switch in

each stage uses

crossbar switchcrossbar switch

• Use smaller no. of

2x2 switches and

better loss uniformity


Spanke Switch

• Strict-sense

nonblocking

• Use n 1xn and n nx1

switches

• Use smaller no. of • Use smaller no. of

switches

• Low insertion and

uniform loss


Benes Switch

• Rearrangeably

nonblocking

• Use smallest no. of

2x2 switches

• Uniform loss• Uniform loss

• Require waveguide

crossover, hard to

fabricate


Spanke-Benes Switch

• Rearrangeably

nonblocking

• Requires n stages to

realise nxn switchrealise nxn switch

• No waveguide

crossover

• Non-uniform loss


Comparison of Different Switch Architectures


Optical Switch Technologies

• Bulk mechanical switches

• Micro-Electro-Mechanical System (MEMS)

switches

• Bubble-based waveguide switch

• Liquid crystal switch

• Electro-optical switch

• Thermo-optic switch

• Semiconductor optical amplifier switch


Bulk Mechanical Switches

• Use mechanical means to

perform switching

• Eg: moving mirror, directional

coupler

• Low insertion loss, low

crosstalk, inexpensive

• Slow switching speed and

small no. of ports

• Used in small wavelength

crossconnect for provisioning

and protection


MEMS Switches (2D or Digital)

• MEMS consists of tiny movable mirrors

• Mirrors are deflected using electromagnetic, electrostatic, or piezoelectric methods

MEMS switch


MEMS Switches (3D or Analog)


Bubble-based Waveguide Switch


Electro-Optic Switches

• Constructed using Lithium

Niobate Mach-Zehnder

Interferometer

• Applying voltage to change

refractive index in coupling regionrefractive index in coupling region

• Relatively fast switching speed

• Can integrate into large switches

• High loss and more expensive

than mechanical switches


Thermo-Optic Switches

• Constructed using Mach-

Zehnder Interferometer

• Applying temperature to

change refractive index in change refractive index in

coupling region

• Slow switching speed

• Poor crosstalk


Semiconductor Optical Amplifier Switches

• Use semiconductor optical

amplifier as on-off device by

changing bias voltage

• Large extinction ratio• Large extinction ratio

• Fast switching speed

• SOA is expensive and

polarisation dependence


Comparison of Different Types of Switches


Lasers

• Two types:

– Semiconductor lasers use semiconductors as gain medium - most

popular type of laser due to small size and low cost

– Fiber lasers use erbium-doped fiber as gain medium

• Principle of operation:

– Optical energy is reflected at the ends of the amplifying or gain medium

or cavity, which forms an oscillation if optical waves add in phase at the


or cavity, which forms an oscillation if optical waves add in phase at the

ends (resonant wavelengths of the cavity)

– The parameters of the cavity, e.g., cavity length, determines the emitting

wavelength of a laser

Lasers (2)

• Lasing threshold: beyond this, the device produces light output, even in the absence of input signal

• This is due to spontaneous emission gets amplified without input signal and appears as light output. This is called stimulated emission.

• Multiple wavelengths exist within cavity if cavity length is integral multiple of half the wavelength.

• Multiple-longitudinal mode (MLM) laser (e.g. Fabry-Perot laser): large spectral width around 10 nm with multiple modes, not suitable for high-speed communication due to chromatic dispersion and crosstalk.


speed communication due to chromatic dispersion and crosstalk.

• Single-longitudinal mode (SLM) laser: narrow spectral width using filtering

DFB and DBR Lasers

• FP laser: light feedback from

reflecting facets

• Distributed feedback laser: light

feedback due to distributed

reflectors, provided by periodic

variation of cavity width

• Reflected waves add in phase if

period of corrugation is integral period of corrugation is integral

multiple of half the wavelength.

• Strongest transmitted wavelength is

equal twice the corrugation period.

• DFB laser: corrugation occurs within

gain region

• DBR (distributed Bragg reflector)

laser: corrugation is outside gain

medium


External Cavity Laser

• External cavity to suppress

oscillation of other modes.

• Laser oscillates only at resonant wavelengths of both primary and external cavities.

• Diffraction gratings can be used in external cavity. Wavelengths in external cavity. Wavelengths reflected determined by grating characteristic and its angle.

• ECL used primarily in test instruments and not for low-cost transmission.

• ECL can’t be directly modulated due long cavity.


Vertical Cavity Surface-Emitting Lasers (VCSEL)

• Easy to make active layer by

depositing on semiconductor

substrate. This leads to vertical cavity

with mirrors formed on top and bottom

of semiconductor wafer. Hence named

VCSEL.

• Problem with high temperature

operation.operation.

• VCSEL advantages: simpler fibre

coupling, easier packaging, easy to be

integrated into array.

• 0.85 µm VCSEL used for short-

distance multimode fiber in optical

LAN

• 1.3 µm and 1.5 µm VCSEL being

developed for single-mode fiber


Other Types of Lasers

• Light-emitting diodes (LED)

– pn-junction using spontaneous emission, no reflective facets

– Broad wavelength spectrum

– Low output power, cannot be directly modulated for > 100 Mb/s

– Can use LED slicing provides cheap source with narrow spectral width

• Tunable lasers

– Important for WDM and reconfigurable network


– Important for WDM and reconfigurable network

– External cavity lasers: varying angle and distance from grating to cavity

– Tunable VCSELs: adjusting cavity length by applying voltage to upper and lower mirrors

– Two- and three-section DBR lasers: injecting current to change wavelength and power

Direct Modulation

• On-off keying (OOK): light stream is

turned on and off depending on data

bit 1 or 0.

• Drive current set well above

threshold for 1 bit and below

threshold for 0 bit.

• Direct modulation: simple and • Direct modulation: simple and

inexpensive.

• Disadvantage: chirped pulses, where

frequency varies with time, causing

broadening of transmitted spectrum.

Chirped pulses have much shorter

transmission limit than unchirped

pulses.


External Modulation

• External modulator in front of light

source, turns light on and off. Light

source is continuously operated.

• Two ways to generate RZ pulses:

– Using mode-locked laser to generate

periodic pulses then standard

modulator

– Using 2-stage modulator to impose

clock signals before data signals.clock signals before data signals.

• Two types of external modulators:

– Lithium niobate modulators

– Semiconductor electro-absorption

(EA) modulators: using electric field

to make material to absorb incident

photons. Easy to be integrated with

DFB lasers for compact, low-cost

solution. Chirp performance not as

good as lithium niobate modulators.


Lithium Niobate Modulators

• Use electro-optic effect: applied

voltage induces change in refractive

index of material.

• Directional coupler configuration:

– Apply voltage to coupling region to

change its refractive index,

– Then determining how much power

coupled from input to output

waveguidewaveguide

• Mach-Zehnder interferometer (MZI):

– Applying voltage so that signals in 2

arms of MZI are in phase and interfere

constructively and appear at output

– When signals are out of phase, they

interfere destructively and do not

appear at output

– Have higher modulation speed and

extinction ratio than directional coupler


Photodetectors

• Principle of operation: incident photon

absorbed by electrons in valence

band, then electrons excited to

conduction band leaving a hole. When

voltage applied, electron-hole pairs

give rise to electrical current.

• In practice, use semiconductor pn • In practice, use semiconductor pn

junction to improve efficiency

• Two types:

– PIN photodiode: use intrinsic

semiconductor between pn junction

– Avalanche photodiode: have higher

gain by applying higher electric field


Front-End Amplifiers

• Two types:

– High-impedance front-end amplifier

– Transimpendance front-end amplifier: higher dynamic range and better noise performance


Lecture 1 Intro - Optical Components - At - 2011

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