TCOM 513 Optical Communications Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems

TCOM 513Optical Communications

Networks

Spring, 2007

Thomas B. Fowler, Sc.D.

Senior Principal Engineer

Mitretek Systems

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Topics for TCOM 513

Week 1: Wave Division Multiplexing Week 2: Opto-electronic networks Week 3: Fiber optic system design Week 4: MPLS Week 5: Optical control planes Week 6: The business of optical networking: economics

and finance Week 7: Future directions in optical networking

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Where we are

In TCOM 503 we discussed background and components– Physical basis for optical fiber– Types of optical fiber– Physics behind fiber optic devices– Light sources– Major classes of fiber optic devices

In TCOM 513 we will use this knowledge to build fiber optic networks– Some higher-level technologies– How to design networks– New trends– Economics and finance

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Topics to think about…

If you want to sell fiber optic technology, you can– Sell fiber– Sell lasers and other components– Sell custom-built networks– Sell standard services based on fiber optic technology

If you want to buy fiber optic technology, you can– Buy fiber and components, lay the fiber, and make your

own network– Buy a custom-made network– Buy standard services from telecom providers and

hardware from standard suppliers and rely on plug-and-play

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WDM

Overview of WDM Types of WDM How WDM works Light sources Transmission problems Amplifier issues Optical switches

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Virtual Session

End-to-End Messages

Physical

Presentation Presentation

Session Session

Network Network

Data Link Control

Data Link Control

PhysicalPhysical

Physical Link, e.g. electrical signals

Physical portion of code

Logical portion of

code

Virtual Network ServiceApplicationApplication

End-to-End PacketsTransport Transport

DLC DLC DLC DLC

NetworkNetwork

Bits

Packets

Frames

Physical Physical Physical

Originating site

Terminating site

Subnet node

Subnet node

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Wave Division Multiplexing (WDM) Background

Methods available to carriers to increase capacity– Lay more fiber– Increase bit rate– Increase fiber carrying capacity of existing fiber plant

Need or desire to offer new services

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Wave Division Multiplexing Overview

Method devised to increase data carrying capacity of fiber– Takes advantage of enormous data potential of fiber

while recognizing constraints• Current electrical technology

– At 40 Gbps, time division multiplexing probably at its limit

– Increasing bit rate not feasible• Way signals originate and must be switched

Essentially a frequency division multiplexing (FDM) technology– Each data stream has its own wavelength (or frequency)

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Wave Division Multiplexing Overview (continued)

Important for several reasons– Overcomes limitations of Opto-electronic systems,

where data rates limited to 10 Gbps or 40 Gbps• Allows fiber to carry far higher data rates by

simultaneously carrying multiple 10 or 40 Gbps streams

– Allows easy segregation of data traffic– Improves security through less time division

multiplexing and demultiplexing Allows carriers to offer new, secure services

– Sell s to customers

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Structure of today’s networks

end user services

end userservices

SONET

SONET

DWDM

DWDM

SONET

SONET

end user services

end user services

1

n

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Types of WDM Simple or Sparse

– Sometimes called “Coarse Wave Division Multiplexing” or CWDM

Dense– Denoted as DWDM

History

Source: Cisco

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Sparse WDM or CWDM Use of 2 or small number of different s on same fiber Has been employed for many years Key characteristic: use of separate bands (widely spaced s,

rather than closely spaced s in same band Can be built with commonly available components

– Wide separation means that wavelength selective couplers can be used to multiplex and demux signal

– Typically run at low data rates • Not due to technological limitations• Each can handle high data rates if desired

Full duplex system using sparse WDM

Source: Dutton

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Sample CDWM component

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Dense Wave Division Multiplexing

Uses closely spaced wavelengths– Each carries high data rate (up to 40 Gbps)– Current state of the art is ~320

• Yields 3 terabits per second at OC192• Yields 12 terabits per second at OC768

Physical layer technology– Transparently supports SONET, Ethernet, Fibre channel

and other protocols

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History of optical bandwidth

0

0.5

1

1.5

2

2.5

3

3.5

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Fib

er

Cap

acit

y (T

bp

s)

1.7 Gbps135 Mbps565 Mbps OC-48

OC-192, 32

OC-192, 80

OC-192, 160

OC-192, 160

SONET ERA WDM ERA

320

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Dense WDM (DWDM)

Functions required for DWDM– Transmitters– Signal combiners (optical multiplexer)

• Transponder– Suitable fiber– Signal separators (optical demultiplexer)– Receivers

Source: Cisco

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Point-to-point DWDM system-detailed view

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Transponder

OEO device Converts optical signal back to electrical form Does 3Rs

– Reamplification– Reshaping– Reclocking

Converts to appropriate ITU frequency for multiplexing

Source: Cisco

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Operation of transponder-based DWDM system

transmitter

Source: Cisco

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Operation of transponder-based DWDM system (continued)

Transponder accepts laser (modulated light) input from transmitters

Wavelength of each input signal mapped to DWDM wavelength

All wavelengths multiplexed together and launched onto fiber– Amplified before launch

Amplifiers, as necessary, en route to destination Amplified at receiving end Wavelength demux at receiving end

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DWDM overview (continued)

Each optical channel allocated its own wavelength– ITU separation 0.8 nm

• Closer separation means more channels but more difficulty in construction

– Actually a wavelength range within which modulated carrier must stay

– Width of channel depends on several key factors• Modulation (always 2x modulation frequency)• Stability of signal• Tolerances of all components in system

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Bandwidth calculation– At 40 Gbps, signal bw ~ 80 Gbps– ITU spacing is 100 GHz ~ 0.8 nm– 1 nm spacing ~ 120 GHz– Indicates that 40 Gbps is limit for ITU spacing

• Requires extremely tight tolerances on all components

– Higher modulation rates => wider spacing => less aggregate bandwidth

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ITU grid for DWDM spacing

Source: Cisco

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Fitting optical channels into allocated wavelengths

Necessary to fit all channels into allocated wavelengths with no spillover

Source: Dutton

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Fitting optical channels into allocated wavelengths

Actually need “guard bands” to ensure separability of wavelengths on demux end

Source: Dutton

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Multiplexer– Must be able to combine many signals– Y-junctions inadequate because they can only combine

2 signals at a time• Requires large number to combine ~50 or 100

signals• 3 db loss with each junction too high• Commonly done with gratings

Source: Dutton

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Transmitter– Always a laser– Linewidth must fit within channel, near center

• Cannot go outside, so behavior such as chirp, drift must be small enough that it stays inside

Linewidth

Source: Dutton

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Transmission and amplification– Need to control crosstalk and other problems leading to

signal degeneration– Variables that can be adjusted

• Channel separation• Channel width• Power levels

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Demultiplexer– More difficult than multiplexing– Standard methods

• Reflective (Littrow) gratings• Waveguide grating routers• Circulators with in-fiber Bragg gratings• Splitters with Fabry-Perot filters

Receivers– Not so difficult as wavelengths already split– Operate up to 40 Gbps

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Add/drop multiplexer– Covered in TCOM 503

– Main types• Array waveguide gratings (AWGs)• Circulators and Fiber Bragg Gratings (FBGs)• Mach-Zehnder Interferometers

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Sparse, DWDM on same fiber

Source: Dutton

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Source: Tektronix

Typical OSA display for DWDM

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Real-world hardware: OC-48 (2.5 Gbps) module

Source: Cisco

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Challenges in photonic and all-photonic networks

Stabilizing wavelengths– Temperature– Material ageing– Carrier density fluctuations– Chirp and other transient effects

Wavelength conversion– All-optical networks will require wavelength conversion

in some switches• Because same wavelength may be on two incoming

signals to be switched to a single output– Not yet well developed

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Challenges in photonic and all-photonic networks (continued)

Cascading filters– As signal traverses optical network, it passes through

many devices with filtering characteristics– Can give rise to unexpected effects– Network needs to be compensated for these effects

• Bandwidth• Shape• Alignment (center frequency)

Tunable lasers– Required for some proposed and current applications– Tuning times now relatively long, milliseconds– Need to go down to nanosecond range for proposed

applications

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EDFA characteristics– Flattening gain curve– Modifying gain curve to meet particular demands

Equalizing signal power– Keeping power levels equal when signals pass through

many components– E.g., add/drop mux—added signal should leave with

about same power as others just passing through Dispersion compensation

– Chromatic– PMD

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Optical cross-connects and switching elements– Available, but work continues– Newer devices should be considerably better

• Faster• Higher capacity

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Light sources for DWDM applications

Spectral width and linewidth– Conventional lasers produce narrow band of

wavelengths• May be large number (~20 to 30) present• Jumps among these bands randomly

– For WDM, need laser with only one line in its spectrum• Usually means Distributed Feedback laser (DFB) or

Distributed Bragg Reflector (DBR) laser• Other ways to accomplish this

– Linewidth requirement depends on other components in system (e.g., demultiplexer)

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Measuring linewidth with OSA

Source: Tektronix

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Light sources for DWDM applications (continued)

Wavelength stability– Extremely high degree of stability required– Drift or change of 1 nm unacceptable

• Would disrupt DWDM system• Though little effect on sparse WDM system

– Physical parameters of lasers change over time, causing drift

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Tunable and multiwavelength lasers– Tunable lasers work on principle of varying RI or

parameters of cavity– Relatively slow– Allows for fairly precise setting of center wavelength

• Manufacturing process used in ordinary lasers does not allow tight control of center wavelength

– Many are produced and tested, then labeled– Multiwavelength lasers involves synthesizing several on

same substrate, with switch to allow selection of a single one to operate

• Each as different wavelength

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Example

Difficult to produce commercially

Switch Source: Dutton

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Multiline lasers– Harness “undesirable” characteristics of F-P lasers– Large number of lines of nearly equal power below

threshold• Equally spaced• Called “Amplified spontaneous emission source” (ASE)

Stabilizing one stabilizes all

Useful range

Source: Dutton

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– Can’t modulate individual lines• Requires external modulation for each channel

– Amplifier with correct gain characteristics required to boost and equalize power of each line

Source: Dutton

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– Amplifier

– Modulation of individual lines can be done by acoustic modulator

– Not yet commercially available

Source: Dutton

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At present, most DWDM systems use separate lasers for each wavelength

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Transmission problems

Amplifier problems– Noise accumulation– Nonlinearity of gain across frequency band– Polarization dependent effects– Rapid transient power fluctuations

Dispersion– Chromatic– PMD

Nonlinearities– 4-wave mixing– Stimulated Brillouin scattering– Stimulated Raman scattering– Carrier induced phase modulation (CIP)

Polarization-dependent degradations Crosstalk

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Amplifier problems: noise

Problem is Amplified Spontaneous Emission (ASE)– Trivial if only a few stages– Much more serious if large number of stages– Arises because some excited erbium atoms decay to

ground state (undergo spontaneous emission) before encountering incoming photon

• Photon emitted with random phase, direction• Small proportion in direction of fiber

– Indistinguishable from signal– Amplified further

• Noise proportional to amplifier gain Ultimately limits amplifier spacing

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Amplifier problems: noise (continued) Control

– Demultiplexing gets rid of noise at non-signal wavelengths– Run amplifier in saturation so excess pump power doesn’t

end up as ASE– Filter out unwanted wavelengths

• ASE peaks at 1533 nm• Limits to this method

– Link design• Do not let signal decay to low level before amplifying

– Note that noise levels do not decay with distance, only signal

– Large amplifier gain means large ASE• Keep spacing as small as possible• Large distances may require repeater

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Amplifier noise accumulation

Roughly speaking,

– SNRoutput = SNRinput – Amplifier noise figure

• SNR = signal-to-noise ratio– Indicates that with enough amplifiers, SNR will go to 0– Modern EDFAs have noise figure ~ 3 db– If SNR starts at 30 db, can only use 10 amplifiers before

repeater necessary Modern amplifier spacing is ~ 40-80 km

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Amplifier problems: nonlinear gain

Typical gain profile

3.5 db

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Amplifier gain compensation device

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Amplifier problems: nonlinear gain (continued)

Less of a problem now– Gain flatness typically on the order of 1 db or less over

amplified range– Regeneration still required for long hauls

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Amplifier problems: polarization dependent effects

PMD Review: in SMF, light traverses fiber with two polarizations orthogonal to each other– Energy shifts between them randomly– If wave hits device which does not respond equally to the

polarizations, energy is lost• SNR goes down• Also, pulse smeared out over time, eye diagram closes

Source: Yafo Networks

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Amplifier problems: polarization dependent effects (continued)

In amplifier, gain slightly higher in orthogonal polarization (PDG)– Causes reduction in SNR of about 0.1 db– In long distance applications, can reduce SNR by 5 db

In addition, ASE is unpolarized and experiences fixed gain (PDL)

• Further reduction in SNR which varies over time• Exacerbates PMD problem

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Amplifier problems: Rapid Transient Power Fluctuations

Rapid change in system load can trigger amplifier gain changes– Amplifier stores only small amount of energy– If one channel stops, gain of other channels goes up– Transient propagated down line to other amplifiers

• Causes saturation and a period of errors which can last a few milliseconds

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Amplifier typical spec sheet

Source: Nortel

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Dispersion PMD

– Origin already discussed– Spec quoted in ps/nm/km– Example: 1500 nm wavelength, 17 ps/nm/km dispersion– Assume spectral width of 6 nm, distance 10 km

• Dispersion = 17 ps/nm/km x 6nm x 10 km = 1020 ps• At 1 Gbps, pulse is 1 ns• This would yield smearing of 102%, system would fail• 20% is usually max allowable = 200 ps• Requires dispersion on order of 2.2 ps/nm/km or

narrower spectral width

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Dispersion (continued)

– Worse with higher speeds• At 10 Gbps, pulse is 100 ps• If distance is to be 40 km, spectral width 0.2 nm, max

dispersion is 2.5 ps/nm• Yields dispersion of 20 ps

– General formula:

dispersion spec x spectral width x distance < 0.2 x 1/data rate

Modern fibers have dispersion on order of 0.5 ps/nm/km

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Chromatic dispersion

Index of refraction not constant Since index of refraction is determined by speed of light in

the medium, follows that speed of light in medium is function of – Will lead to dispersion of information bearing light

waves over distance– Called “material dispersion”

Waveguide dispersion– Light travels in both core and inner cladding at slightly

different speeds (faster in cladding) Material and waveguide dispersion opposite effects

– Can be balanced to allow for zero dispersion at a particular wavelength between 1310nm and 1650 nm

Total effect called “chromatic dispersion”

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Chromatic dispersion (continued)

Source: Corning

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Chromatic dispersion (continued)

Source: Nortel

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Nonlinearities

4-wave mixing– Problem arises when two or more waves propagate in

same direction on SMF– Signals mix to produce new signals at linear

combinations of original frequencies– Example: 2 frequencies (wavelengths): 1 and 2

• New frequencies appear at 2 1 - 2 and 22 - 1 • In WDM systems, new frequencies coincide with

frequencies already in use, appearing as noise– Effect greater with reduced channel spacing, grows

exponentially with increased signal power– Chromatic dispersion mitigates it– Can be reduced by using uneven spacing

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Nonlinearities (continued)

Source: Dutton

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Stimulated Brillouin Scattering (SBS): scattering of light backwards to transmitter

– Caused by mechanical (actually acoustical) vibrations in fiber inducing changes in RI

– In effect, fiber becomes a diffraction grating

– Mainly a problem at high power levels, narrow linewidth, small core size

• Not usually a problem if power below 5 mw

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Stimulated Raman scattering (SRS): similar to SBS– Effect originates in molecular rather than acoustical

vibrations– Primarily a problem with multiple wavelength systems at

high powers– Rule of thumb: total power x total bw < 500 GHz/W

• Example: 100 channels, spacing 200 GHz (~1.6 nm) gives total bw of 20,000 GHz

• Total power must be less than 500/20,000 = 25 mw or about 0.25 mw/channel

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Carrier-induced phase modulation (CIP)– Arises from Kerr effect

• Change in refractive index due to E field of light wave– Causes change in phase of pulse as power varies over

pulse time– Generally negligible for On-Off keyed systems

• Only a problem for systems requiring coherent detection

Cross-phase modulation– Arises from same effect, but when different signals

simultaneously present– Power induced changes from one signal affect others

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Crosstalk

Arises in devices that filter and separate wavelengths– Small proportion of power that should be in one channel

ends up in others (usually adjacent)– Major problem in WDM systems– Worse with close spacing– Figure of -30 db as minimum

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Current state-of-the-art Alcatel sent 125 channels at OC768 (40 Gbps) over 1500 km

1/8/02)– Aggregate bandwidth of 5 Tbps– Hybrid Erbium/Raman amplifiers– Figure of merit: 7.5 Pb km/sec

Bell Labs sent 64 channels at OC768 (40 Gbps) over 4,000 km (3/22/02)– Aggregate bandwidth of 2.56 Tbps– Figure of merit: 10 Pb km/sec– Used 100 km spacing of amplifiers– Raman amplifiers– Differential phase shift keying (DPSK) encoding– Optimal dispersion compensation

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Theoretical limits on fiber capacity

Work at Bell Labs suggests limit of about 100 Tb/sec– Limited by noise, interference

Current systems ~ 2 Tb/sec Lab work ~ 10 Tb/sec Not much additional work done in recent years because of

collapse of industry after 2000

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Optical switching for WDM

Optical space-division switches

4x4 switch implemented with digital optical switch elements

4x4 switch implemented with cross-connects

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Optical switching for WDM (continued)

Cross-connects can be made with technologies discussed in 503– Resonant couplers– Mach-Zehnder interferometers

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Optical switching for WDM (continued)

Resonant couplers– Normal operation: coupling length set so that signals

cross– Voltage applied: RI of waveguides changes, effectively

changing coupling length, so that crossover does not occur

Source: Dutton

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Optical switching (continued)

Optical switch is “not smart”– Switches everything on input port to output – This means all wavelengths multiplexed together

What is needed is smarter switch which can switch individual wavelengths

Called “optical switching node”– Switch any input to any output port– No OEO conversion– Bit rate and protocol independent

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Optical switching node

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Optical switching node (continued) Can also be implemented with Micro Electro-mechanical

Systems (MEMS)– Extremely tiny mirrors which can pop up and down

under electronic control• “Silicon micromirror”• Can be fabricated on chips

40 Gbps

Source: Tellium

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MEMS

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MEMS (continued)

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MEMS (continued)

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MEMS (continued)

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MEMS 3D arrays

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MEMS (continued)

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Video demo from Onix Microsystems

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Optical switching node (continued)

Main problems– May have two of same wavelength switched to same

output WDM multiplexer• Results in garbage• Not a problem in small networks with only a few

nodes– Lots of s to choose from so no duplication

– Switching done by network management software, not through internal information contained in wavelengths

• Not a router

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Solution to wavelength problem: use wavelength converter to groom traffic for output WDM– Problem: wavelength converters very expensive

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Most currently available switches utilize electronic fabric– Convert wavelengths to electricity, switch, then convert

back to light at needed wavelength– Examples

• Sycamore SN3000• Sycamore SN16000• Tellium• VIP V-MAN 160• Nortel

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OEO switch benefits

Documents

TCOM 513 Optical Communications Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems