lecture 1 intro - optical components - at - 2011
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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.
ELEN90034 Optical Networking and Design – An Tran
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.
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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 – An Tran
ELEN90034: Optical Networking
and Design
Lecture 1: Introduction to Optical
Communications and Optical Components
ELEN90034 Optical Networking and Design – An Tran
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)
ELEN90034 Optical Networking and Design – An Tran
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.
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Example of a Wavelength-Routing Mesh Network
Optical
crossconnect
(OXC)
Wavelength Wavelength
conversion
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Star Coupler
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Circulators
• A circulator is similar to an isolator, except that it
has multiple ports, typically three or four.
– Example of application: OADM
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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)
ELEN90034 Optical Networking and Design – An Tran
Static Wavelength Crossconnect
• Static wavelength crossconnect: crossconnect pattern is fixed and cannot be changed dynamically
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Diffraction Gratings
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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.
ELEN90034 Optical Networking and Design – An Tran
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.
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
AOTF (2)
ELEN90034 Optical Networking and Design – An Tran
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
• Multistage banding:
– Divide wavelengths into bands
– Demultiplexing done in 2 stages
High Channel Count Multiplexer Architectures (3)
stages
– Need a guard wavelength space between bands
– More scalable
ELEN90034 Optical Networking and Design – An Tran
High Channel Count Multiplexer Architectures (4)
• 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
ELEN90034 Optical Networking and Design – An Tran
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
431-694 Optical Networking – An Tran
– 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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Clos Switch
• Strict-sense
nonblocking
• Individual switch in
each stage uses
crossbar switchcrossbar switch
• Use smaller no. of
2x2 switches and
better loss uniformity
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Benes Switch
• Rearrangeably
nonblocking
• Use smallest no. of
2x2 switches
• Uniform loss• Uniform loss
• Require waveguide
crossover, hard to
fabricate
ELEN90034 Optical Networking and Design – An Tran
Spanke-Benes Switch
• Rearrangeably
nonblocking
• Requires n stages to
realise nxn switchrealise nxn switch
• No waveguide
crossover
• Non-uniform loss
ELEN90034 Optical Networking and Design – An Tran
Comparison of Different Switch Architectures
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
MEMS Switches (2D or Digital)
• MEMS consists of tiny movable mirrors
• Mirrors are deflected using electromagnetic, electrostatic, or piezoelectric methods
MEMS switch
ELEN90034 Optical Networking and Design – An Tran
MEMS Switches (3D or Analog)
ELEN90034 Optical Networking and Design – An Tran
MEMS Switches (3D or Analog)
ELEN90034 Optical Networking and Design – An Tran
Bubble-based Waveguide Switch
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Comparison of Different Types of Switches
ELEN90034 Optical Networking and Design – An Tran
ELEN90034 Optical Networking and Design – An Tran
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
431-694 Optical Networking – An Tran
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.
431-694 Optical Networking – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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.
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
431-694 Optical Networking – An Tran
– 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.
ELEN90034 Optical Networking and Design – An Tran
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.
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
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
ELEN90034 Optical Networking and Design – An Tran
Front-End Amplifiers
• Two types:
– High-impedance front-end amplifier
– Transimpendance front-end amplifier: higher dynamic range and better noise performance
ELEN90034 Optical Networking and Design – An Tran