Transcript CSC/ECE 775: Optical Networks Rudra Dutta, Fall 2006

CSC/ECE 778: Optical Networks
Rudra Dutta, Fall 2007
Fiber-Optical Communication and Switching
Outline
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We want/need to understand effect on
networking
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What components are possible, limitations
Quick overview of representative technology
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Optical Connection and Power Budget
– Fundamentals of Fiber Optic Transmission
– Transmission Impairments and Solutions
– Lasers and Photodetectors
– Other Optical Components (Couplers, Filters,
Multiplexers, Switches, OADMs, Amplifiers)
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Layering and Optical Services
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Generalized protocol layering can create
complicated multi-layer networks
 In this context, “optical layer” is another layer
close to physical layer, but possibly
implementing network semantics of its own
Network
Data Link
Network
Data Link
Physical
Network
Physical
Physical
Data Link
Physical
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User Apps
IP
ATM
SONET
Optical
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Why Fiber?
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Huge bandwidth: 30-50 THz
Low losses (intrinsic): 0.2 db/Km
Low bit error rates (BER): 10-11
Low power requirements: 100 photons/bit
Immunity to electromagnetic interference (EMI)
Low cross-talk
Repeater-less amplification (EDFAs)
Low cost, maintenance
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Optical Endpoint
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Optical Power Budget
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Finite power available at source (laser)
 Minimum detectable receiver power
 Must account for all losses between source and
receiver
 Optical networks are power-budget limited, not
bandwidth limited
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Optical Power Budget (cont'd)
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Wavelengths of Importance
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Optical Fiber
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Optical waveguide
 Cylindrical core surrounded by cladding (+ protective
covering)
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Single-mode vs. multimode fiber
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made of same transparent material (glass, plastic)
difference is value of refractive index n = c / v
single-mode: core diameter 8-12µm, link length > 2Km
multimode: core diameter 50µm, link length < 2Km
Step-index vs. graded-index fiber
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step-index: refractive index constant across core diameter
graded-index: refractive index varies along core diameter
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Refractive Index Profiles
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Geometric Optics: Snell's Law
n1 sin i = n2 sin t
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Geometric Optics: Total Reflection
Critical angle: c = sin-1 (n2 ÷ n1)
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Maximum Cone of Acceptance
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Transmitter-to-Fiber Coupling
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Modes: The Wave Picture
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Allowed Ray Angles
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Only allowed ray angles result in guided modes
AB = d sin m = m /2 leads to half wavelength in the
core
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m : integer, : optical wavelength in the core
Mode: one possible path that a guided ray can take
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Transmission Impairments
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Factors affecting transmission distance and bandwidth:
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attenuation
dispersion
non-linear effects
Must minimize their effects for high performance
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improvement and redesign of fiber itself
compensating for these factors
Attenuation problem solved  dispersion effects
significant
 Dispersion effects reduced  non-linear effects
dominant
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Attenuation
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Decrease in optical power along the length of
the fiber
 Varies with wavelength
 Attenuation coefficient: adB = - 10/L log10
(PR÷PT) (dB/Km)
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L : length of fiber
PT : power launched into the fiber
PR : power received at end of fiber
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Power Losses
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Material absorption: due to
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Rayleigh scattering: medium is not absolutely uniform
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resonances of silica molecules
impurities -- most serious is peak at 1390 nm due to OH ions
refractive index fluctuates  light is scattered
scattering proportional to -4  dominant at  < 800 nm
Waveguide imperfections: relatively small component
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nonideal fiber geometries
due to bending, manufacturing imperfections
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Low Loss Region of An Optical Fiber
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Erbium-Doped Fiber Amplifiers
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EDFA Principle of Operation
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Ei : energy level
 Ni : population of erbium ions at energy level Ei
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normally (no pump/signal): N1 > N2 > N3
pump/signal present: population inversion N2 > N1
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EDFA Properties
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Emission:
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stimulated  amplification
spontaneous  noise  amplified spontaneous
emission  limit on number of EDFAs along the
fiber
Energy levels are narrow bands  each
transition associated w/ a band of wavelengths
 amplify wide band around 1550nm
 Replace expensive and complicated electronic
units
 Signal remains in optical form  transparency
 “Distributed” amplifiers
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Semiconductor Optical Amplifiers (SOAs)
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Similar to semiconductor laser
Consist of active medium (p-n junction)
Energy levels of electrons confined to 2 bands
 EDFA E1, E2
Mobile carriers (holes, electrons) play the role of
erbium ions
Has several disadvantages compared to EDFAs
Useful when combined with other components
into optoelectronic integrated circuits (OEICs)
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preamplifier in optical receiver
power amplifier in optical transmitter
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Dispersion
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A narrow pulse spreads out as it propagates along the
fiber
Intersymbol interference:
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pulse overlaps neighboring pulses
sharply increases the BER
Dispersion imposes a limit on the bit rate that can be
supported
Intermodal vs. chromatic dispersion
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Intermodal Dispersion
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Most serious form of dispersion
Occurs in multimode fibers
Different modes of a wavelength travel at
different speeds
Multimode fibers limited to low bitrate-distance
products
Solutions:
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use single-mode fibers for large bitrate-distance
products
(8 µm < 2a < 10 µm  only one mode is guided)
use graded-index fibers
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Graded Index Fibers
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Propagation in Graded Index Fibers
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Rays are bent as they approach the cladding
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Rays further from core travel faster (due to lower n)
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Intermodal dispersion reduced by several orders of
magnitude
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Chromatic Dispersion
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Two sources of chromatic dispersion:
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material dispersion, DM
waveguide dispersion, DW
Chromatic dispersion: D = DM + DW
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Material Dispersion
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The physical effect that allows raindrops to form
rainbow
 Refractive index of a material changes with
wavelength  different wavelengths travel at
different speeds along the fiber
 Different delays cause spreading of output
pulse, depending on:
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wavelength span of source
– length of fiber
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Waveguide Dispersion
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DW is a function of fiber geometry
 Dispersion-shifted fibers:
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DW causes zero-dispersion point to shift to 1550 nm
range
min dispersion range coincides with min loss range
Dispersion-flattened fibers: dispersion profile
close to zero for a wide spectral range
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Dispersion Profile of Single-Mode Fiber
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Non-Linear Effects
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Stimulating Raman Scattering (SRS):
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Stimulating Brillouin Scattering (SBS):
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light interacts with fiber medium  inelastic collisions
not important in single-channel systems (thresh. about 500mW)
involves transfer of power: hi freq. wave  lo freq. wave
introduces cross-talk in multiwavelength systems
no cross-talk, low threshold power (few mW for 20-Km fiber)
Four-Wave Mixing
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three signals present at neighboring freq: f1, f2, f3
new signal produced, e.g., f4 = f1 + f2 - f3
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Solitons
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Distortion, non-linearities: distort, broaden a propagating
pulse
Right combination of distortion, non-linearity:
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compensate each other
produce a narrow, stable pulse (soliton)
solitons travel over long distances without any distortion
solitons in opposite directions pass thru transparently
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Ideal situation for long-distance communication
 EDFAs needed to maintain solitons over long distances
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Lasers
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Light amplification by stimulated emission of
radiation
 Schawlow and Townes, 1958
 First solid-state laser by Maiman, 1960
 Today, lasers exist in myriad forms
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Semiconductor Energy State Diagrams
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Fabry-Perot Cavity
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Part of light leaves cavity through right facet, part is
reflected
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Resonant wavelengths: L = m /2
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Single-Wavelength Operation
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FP laser cavity supports many
modes/wavelengths of operation
Monochromatic light needed for high bitratedistance products
Geometry is modified to achieve singlewavelength operation
Distributed Bragg Reflector (DBR) lasers
Distributed Feedback (DFB) lasers
Expensive, widely used in long-distance
communication
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Tunability
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Laser tunability important in WDM network
applications:
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slow tunability (ms range): set up lightpaths in
wavelength routing networks
fast tunability (µs or ns range): multiple access (TWDMA) applications
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Tunability (cont'd)
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Mechanically tuned: change FP cavity length
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Injection current tuned: change refr. index in
DFB/DBR lasers
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(tuning range: 10-20 nm, tuning time: 100-500 ms)
(tuning range: 4 nm, tuning time: 10s of ns)
Multiwavelength laser arrays
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built in single chip
one or more lasers can be activated simultaneously
light from each laser fed to star coupler
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Optical Receivers
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Photodetectors
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Filters
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Various technologies:
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Fabry-Perot filters
Multilayer interference (MI) filters
Mach-Zehnder interferometers
Arrayed waveguide grating
Acousto-optic tunable filter
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Tunability important
 Can be used as MUX/DEMUX, wavelength
routers
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MI Filters
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Bandpass filter
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Passes thru particular wavelength, reflects all other
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Cascade multiple filters to create a MUX/DEMUX
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MI Filters as MUX/DEMUX
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MUX/DEMUX: Logical View
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Directional Couplers
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Coupling possible when waveguides placed
close together
 Coupling ratio controlled by voltage
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Couplers: Logical View
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P1’ = a11 P1 + a12 P2, P2’ = a21 P1 + a22 P2
 For ideal symmetric couplers:
a11 = a22 = a, a12 = a21 = 1-a
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Couplers
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Star Coupler:
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Power Splitter:
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P2 = 0, a = 1/2
Switches:
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a = 1/2, 2x2 star coupler (3-dB coupler)
Cascade 2x2 couplers to build NxN star coupler
a = 0,1; 2x2 switch
cascade 2x2 switches to build NxN switch
Real devices are lossy:
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a11 + a12 < 1, a21 + a22 < 1
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Internal Structure of Star Coupler
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Gratings
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Gratings: Principle of Operation
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Multiple narrow slits spaced equally apart on the
grating plane
 Light incident on one side of grating transmitted
through slits
 Diffraction: light through each slit spreads out in
all directions
 Different s interfere constructively at different
points of imaging plane  separate WDM signal
into constituent wavelengths
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Bragg Gratings
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Bragg grating: any periodic pertrubation in
propagating medium
 Perturbation is usually periodic variation of
refractive index
 Bragg gratings used in many photonic devices:
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DBR lasers: Bragg gratings written in waveguides
Fiber Bragg gratings (FBG): written in fiber
Acousto-optic tunable filters: Bragg grating formed by
propagation of an acoustic wave in the medium
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FBG as Add-Drop Multiplexers
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OADM: Logical View
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Optical Switches
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Mechanical switches
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Bubble-Based switches
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couplers, ratio modified by changing refr. index (ns range)
Thermo-Optic switches
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bubbles in optical fluid reflect beam (10s of ms range)
Electro-Optic switches
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directional couplers, ratio modified by bending (ms range)
MEMS mirrors moved in and out of path (100s of ns range)
refractive index function of temperature (ms range)
Semiconductor Optical Amplifier (SOA) switches
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SOA, change in voltage to use as on-off switch (ns range)
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MEMS Optical Switching
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MEMS = micro-electro-mechanical system
 Movable mirrors to reflect light
 2D MEMS: a 2-state pop-up MEMS mirror
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state ``0'': popped up position light reflected
state ``1'': flat (folded) position light passes through
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2D MEMS Switches
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Analog Beam-Steering Mirror
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Mirror can be freely rotated on two axes to reflect a light
beam
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3D MEMS Switch
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Static Optical Switches
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Reconfigurable Optical Switches
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Wavelength Converters
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Spectrum Partitioning
c = f, f - c/2
 100 Ghz is about .8 nm at 1,550 nm range
 10-Ghz spacing:
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100 Ghz spacing OK for optical switches
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very dense by current standards
can accommodate 1 Gbps digital bit rates
can accomodate 1 Ghz analog bandwidths
OK for receivers, but too close for wavelength routing
WDM limit today
Waveband routing alleviates throughput loss
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But better switching technology nullifies advantage
However, continue to be useful because needs “coarser” filters
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Spectrum Partitioning (cont'd)
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Waveband vs. Wavelength
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