Optical Components for WDM Light-wave Networks

Borella et al,Proceedings of the IEEE,

August, 1997.

2

Contents• I. Introduction• II. Optical Fiber• III. Optical Transmitters• IV. Optical Receivers and Filters• V. Optical Amplifiers• VI. Switching Elements• VII. Wavelength Conversion• VIII. Designing WDM Networks: Systems

Considerations• IX. Experimental WDM Lightwave Networks• X. Conclusion

3

I. Introduction

• The network medium may be– a simple fiber link,– a passive star coupler (PSC) (for boradcast and sel

ect network)– a network of optical or electronic switch and fiber

links

4

• The transmitter block– consists of one or more optical transmitters

• The transmitter– fixed to a single wavelength

or tunable across a range of wavelengths

– consists of a laser, a laser modulator, or an optical filter for tuning purpose.

– For multiple optical transmitters, a multiplexer or coupler is needed to combine the signals onto a single fiber.

5

• The receiver block may consist of– tunable filter + photodetector – demultiplexer + photodetector array

• Amplifiers are needed in various locations to maintain the strength of optical signals.

6

• Transmitter block– fixed wavelength + m

ultiplexer – tunable across a rang

e of wavelengths

• Receiver block– demultiplexer + phot

odetector array– tunable filter + photo

diode

7

0.5dB

0.2dB

II. Optical Fiber• Rayleigh sc

attering

• Peak loss in 1400 nm due to OH- impurities in the fiber

0.5dB

0.2dB

8

Advantages– low attenuation

long distance (X00 KM)low bit error rate (BER=10-11)

– small size, flexible,

– difficult to break, reliable in corrosive environments, deployable at short notice,

– immune to electomagnetic intererence and does not cause interference,

– made from sand (cheap, readily available and environmentally sound)

9

A. Optical Transmission in Fiber

• Fiber is a thin filament of glass that acts a a waveguide.– A waveguide is a physical medium or path that allows the

propagation of electromagnetic waves, such as light.

• Two types of fiber: multimode and single mode

core = 纖核cladding = 纖覆

Multimode Singlemode

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• Total internal reflection (完全內部反射 ) little loss

11

– Speed of Light• in vacuum: cvac = 3 x 108 m/s

in other material: cmat cvac

– Refraction Index (n) 折射率nmat = cvac / cmat 1

– Snell‘s Law• na sina = nb sinb

• nb / na = sina / sinb

• when a , then b until b = 90o, then sinb = 1 sina = nb / na 1 (i.e. nb na)

• Critical incident angle crit sin-1(nb / na)then No Refraction, i.e. Total Internal Reflection

na

nb

a

b

12

Numeric Aperture (N.A.)

• nair sinair = ncore sin(90 - crit) = ncore (1 - sin2 crit )1/2

• sin crit = nclad / ncore nair sinair = (n2

core - n2clad)1/2

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• Two types of core-cladding implementations– step index– graded index

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B. Multi-mode versus Single-mode

• Normalized Frequency: V = k0 a (n2core - n2

clad)1/2 where k0 = 2 /

a = radius of the core = wavelength in vacuum

• The number of modes: m 0.5 V2

core = 纖核cladding = 纖覆

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Multi-mode• Advantages

– core diameter is relatively large

– injection of light into the fiber with low coupling loss can be accomplished by using inexpensive, large-area light source, such as LED‘s.

• Disadvantages– each mode

• due to different incident angles propagates at different velocity

and arrives at the destination at different times a pulse spread out in the time domain

intermodel dispersion distance of propagation• reduced through the use of graded-index fiber

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Single-mode• To reduce the intermodel dispersion

– to reduce the number of modes

– to reduce the core diameter

– to reduce the numerical aperture

– to increase the wavelength of the light

single-mode fiber

• Advantages– eliminating intermodel dispersion

– transmission over longer distance

• Disadvantages– high concentration of light energy is needed, such as laser

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C. Attenuation in Fiber

• P(L) = 10-AL/10 P(0)– P(L) the power of the optical pulse

at distance L km from the transmitterA the attenuation constant (in dB/km)P(0) optical power at the transmitter

• Lmax = (10/A) log10 (P(0) / Pr)

– Lmax the maximum distance between the transmitter and the receiver

– Pr the receiver sensitivity

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D. Dispersion in Fiber• The widening of a pulse duration as it travels t

hrough a fiber

to interfere with neighboring pulses (bits)

• Three types of dispersions:– Intermodel Dispersion

• multiple modes propagates with differnet velocites

– Material or Chromatic Dispersion• refraction index wavelength

– Waveguide Dispersion• propagation of different wavelength waveguide chara

cteristics

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E. Nonlinearities in Fiber• 1) Nonlinear Refraction

– Index of refraction optical intensity of signals propagating through the fiberThe phase of the light at the receiver

• the phase of the light sent by the transmitter,

• the length of the fiber, and

• the optical intensity.

– Two types of Nonlinear Refraction• Self-Phase Modulation (SPM)

Cross-Phase Modulation (XPM)

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– Self-Phase Modulation (SPM)• caused by variations in the power of an optical signal

• results in variations in the phase of the optical signal

• The amount of phase shift NL = n2k0L|E|2

where n2 = nonlinear coefficient for the index of refractionk0 = 2/L = length of the fiber|E|2 = optical intensity

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– Cross-Phase Modulation (XPM)• caused by the change in intensity of an optical signal pr

opagating at a different wavelength

• results in a shift in the phase of the optical signal

• Advantage: to modulate a pump signal at one wavelength from a modulated signal on a different wavelength. Such technique is used in Wavelength Conversion devices.

22

• 2) Stimulated Raman Scattering (SRS)– Light incident with molecules creates scattered lig

ht at a longer wavelength than that of the incident light.

– A portion of the light traveling at each frequency in a Raman-active fiber is downshifted across a region of lower frequencies: the Stokes wave.

– The range of the frequencies occupied by the Stokes wave is determined by the Raman gain spectrum, which covers the range of around 40THz below the freq. of the input light. In silica fiber, max. gain at 13THz below input light.

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• 3) Stimulated Brillouin Scattering (SBS)– similar to SRS except that the frequency shift is ca

used by sound waves rather than moleculer vibration.

– Stokes wave propagates in the opposite direction of the input light,SBS occurs at relatively low input powers for wide pulses (greater than 1s) but has negligible effect for short pulses (less than 1s)

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• 4) Four-Wave Mixing (FWM)– Two wavelengths, operated at frequencies f1 and f2,

mix to cause signals at 2f1-f2 and 2f2-f1: sidebands

– Sidebands can cause interference if they overlap with frequencies used for data transmission

– can be reduced by using unequally spaced channels

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F. Couplers• All devices that combine light into or split out

of a fiber.

• SplitterCombinerCoupler

• Splitting Ratio – the amount of power that goes to each output

– Ex. 50:50 for a 1 x 2 splitter

• Return Loss (reflected in the opposite direction): 40-50 dBInsertion Loss (when directing the light into the fiber):

2 x 1 combiner1 x 2 splitter 2 x 2 coupler

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PSC (Passive Star Coupler)• Light coming from any input port is broadcasted to every outpu

t port.

• Pout = Pin / N (excess loss is ignored)

• Example: – Using combiners, splitters, and couplers :

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III. Optical Transmitters• A. How a Laser Works

– Laser = Light Amplication by Stimulated Emission of Radiation

– Stable atom : whose electrons are in the lowest possible energy levels (ground state)

– When an atom absorts energy, it becomes excited and moves to a higher states (unstable atom). It moves quickly back to the ground state by releasing a photon.

– Quasi(準 )-stable: electrons staying in the excited states for a longer periods to time without constant excitation.By applying enough energy, population inversion occurs, i.e. more electrons are in the excited state than in the ground state. It emit more light than it sbsorts.

28• The general structure of a laser– (1) The excitation device applies current to the lasing medium, whi

ch is made up of a quasi-stable substance.

– (2) The lasing medium is excited and emits a photon.

– (3) The photon reflects off the mirrors and passes back through the medium again.

– (4) When a photon passes very close to an excited electron, the electron releases its energy and return to the ground state and releases another photon, which will have the same direction and coherency (frequency) (stimulating photon)

29– (5) Photons for which the frequency is an integral fraction of the cavity length will coherently combine to build up light at the given frequency in the cavity until the energy is removed as rapidly as it is being applied.

– (6) The mirrors feed the photons back and forth, so further stimulated emission can occur and higher intensity of light is produced.

– (7) Some of the photons will escape through the partially transmitting mirror in the form of narrowly focused beam of light. The frequency can be adjusted by changing the length of the cavity.

– f = (Ei - Ef) / h where Ei = initial (quasi-stable) state of the electronEf = final (ground) state of the electron

h = Planck‘s constantEi - Ef = Boltzmann distribution (temperature)

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• Semiconductor Diode Laser– 1) Bulk Laser Diode:

• a p-n junction with mirrored edges

• Quasi-stable: over-doped impurities to provide extra electrons in an n-type semiconductor andextra holes in a p-type semiconductor.

• Forward bias the p-n junction combine electrons in “n” with holes in “p“ emitting a beam of light (frequency ?)

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– 2) Multiple Quantum Well (MQW) Laser• Thin alternating layers

confining the position of the electrons and holes to a smaller number of energy states

• The quantum wells are placed in the region of p-n junction

• By confining the possible states of electrons and holeshigher resolutionlow-linewidth (very narrow frequency range)

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• B. Tunable and Fixed Lasers– Laser Characteristics:

• Laser Linewidth = the spectral width of the light gerneated by the laser

– affects the spacing of channels

– affects the amount of dispersion, thus, the maximum bit rate

• Frequency Instability = variations in the laser frequency– Mode Hopping : a sudden jump in the laser frequency cuased by

a change in the injection current above the a given threshold

– Mode Shift : changes in frequency due to temperature changes

– Wavelength Chirp : variations in the frequency due to variations in injection current

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• Number of Longitudinal Modes = the number of wavelengths that are amplified by the laser

– wavelength = 2L/n will be amplified,where L is the length of the cavity

n is an integer

– the unwanted logitudianl modes may results in significant dispersion; thus, it is desirable to have only a single logitudinal mode

• For tunable lasers:– Tuning Range = the range of wavelengths over which the laser

may be operated

– Tuning Time = the time required for the laser to tune from one wavelength to another

– Continuously tunableDiscretely tunable

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– Tunable Lasers• 1) Mechanically Tuned Lasers:

– Fabry-Perot cavity that is adjacent to the lasing medium (external cavity) to filter out unwanted wavelengths

– physically adjusting the distance between two mirrors

• 2) Acousto-optically Tuned Lasers:– the index of refraction in the external cavity is changed by usi

ng sound waves

• 3) Electro-optically Tuned Lasers:– the index of refraction in the external cavity is changed by usi

ng electrical current

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• 4) Injection-Current Tuned Lasers:– a diffraction grating (柵 ) inside or outside of the lasing medium, w

hich consists of a waveguide in which the index of refraction alternates periodiclly between two values

– only wavelengths that match the period and indexes of the grating will be amplified

– tuned by injecting a currnet that changes the index of the grating– DFB (Distributed Feedback) Laser:

grating is placed in the lasing medium, DBR(Distributed Bragg Reflector) Laser:

grating is moved to the outside of the lasing medium

• 5) Laser Arrays:– a set of fixed-tuned lasers, each with a different wavelength

36

• C. Optical Modulation– Analog: AM, FM, PM

Digital: ASK, FSK, PSK– Binary ASK (on-off keying, OOK):

• Simple Preferred

• Direct Modulation:– turning the laser on and off,

• External Modulation:– modulates the light coming out of the laser

• Mach-Zehnder (MZ) Interferometer– a drive voltage is applied to one of the two waveguides,

creating an electric field that cuases the signals in the two waveguides to be either in phase or 180o out of phases, resulting in the light being either passed or blocked.

– Up to 18 GHz

– Intergrated laser and modulation cost effective

37

• A. Photodetectors– Direct detection

• converts a stream of light into a stream of electrons, the stream of electrons is then amplified and passed to a threshold device to determine a stream of digital 0’s or 1’s

• PN photodiode (a p-n junction)PIN photodiode (an intrinsic material between p- and n-type )

• light incident on the junction will create electron-hole pairs in both “n” and “p” regions, resulting a current flow.

– Coherent detection• Phase information is used in the encoding and decoding

• the incident light is added to the local oscillator (a monochromatic laser), then is detected by a photodetector.

• more elaborate, difficult to maintain phase information

IV. Optical Receivers and Filters

38• B. Tunable Optical Filters– Filter Characteristics

• Tuning range • Tuning time • Free Spectral Range (FSR)

– the transfer function, or the shape of the filter passband, repeats itself after a certain period

• Finesse– the ratio of the FSR to (3-dB)channel bandwidth (f)

• Finesse f number of channels

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• To increase the number of channels: Cascading filters with different FSR‘s

• Example:Cascading a high-resolution filter with a low-resolution filter, each with four channels within FSR,results in 16 unique channes.

40– The Etalon• consists of a single cavity formed by two parallel mirrors

• light from an input fiber enters the cavity and reflects a number of times between the mirrors

• by adjusting the distance of the mirrors (mechanically), a single wavelength can be chosen to propagate through the cavity

• Modifications– Multipass: light passes the same cavity multiple times

– Multicavity: multiple etalons with different FSR‘s are cascaded to increase finesse effectively

– Fabry-Perot filterSingle-cavity Fabry-Perot Etlon : max. number of channels 0.65F ( F = the finesse) Two-pass Fabry-Perot Etlon : max. number of channels 1.4FTwo-cavity Fabry-Perot Etlon : max. number of channels 0.44F

– Tuning range : virtually entire low-attenuation rangeTuning time : order of milliseconds

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– MZ Chain• A splitter + a combiner + a delay

• The adjustable delay controls one of the the path length,resulting in a phase difference when combined.

• Wavelengths with 180o phase difference are filtered out.

• By constructing a chain of these elements,a single desired optical wavelength can be selected.

42– Acousto-optic Filters -not easy to filter out cross talk

• RF waves are passed through a tranducer.

• The sound waves change the transducer‘s refraction index.

• Light incident upon the transducer will refract at an different angle.

– Electro-optic Filters• Current changes the crystal‘s refraction index

– Liquid-Crystal Fabry-Perot Filters + low power + inexpensive

• The cavity consists of an LC.

• The refractive index of the LC is changed by an current

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• C. Fixed Filtersmay be used to implement optical multiplexers and demultiplexers or receiver devices

– Grating Filters• a flat layer of transparent material (glass or plastic) with a ro

w of parallel grooves (凹槽 )

• reflecting light at all angles

• at an angle, only a certain wavelength adds constructively

• place a filter at the proper angle to select a certain wavelength

– Fiber Bragg Gratings - low insertion loss, varied with temp.

• inducing a grating directly into the core of a fiber

– Thin-Film Interference Filters - poor thermal stability, high insertion loss, poor spectral frofile

• similar to fiber Bragg grating, except that the low index and high index materials onto a substrate layer?

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V. Optical Amplifiers• All-optical amplification

Opto-electronic amplification• 1R (regeneration) amplification

+ total data transparency (independent of modulation format)- noise is amplified as well. Used by all-optical networks of tomorrow

2R (regeneration, reshaping*) amplification* reproduce the original pulse shape of each bi

t3R (regeneration, reshaping, reclocking*) amplification

* applied only to digitally modulated signals. Used by SONET, SDH

45

• A. Optical Amplifier Characteristics– Gain (output power / input power)– Gain Bandwidth– Gain Saturation (3-dB gain)– Polarization Sensitivity– Amplifier Noise

• dominant source is ASE (amplified spontaneous emission), which arises from the spontaneous emission of photons in the active region of the amplifier.

46

• EDFA = Erbium(鉺 )-Doped Fiber Amplifier

• PEFFA = Praseodymium(鐠 )-Doped Fluoride(氟化物 ) Fiber Amplifier

47

• B. Semiconductor-Laser Amplifier + integratable

– A modified semiconductor laser.A weak signal is sent through the active region, which, via stimulated esission, results in a stronger signal.

– Fabry-Perot Amplifier• reflectivity = 30 %

– Traveling -Wave Amplifier (TWA)• reflectivity = 0.01 %

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• C. Doped-Fiber Amplifier– Length of fiber is doped with an element (rare eart

h, 稀土 ) that can amplify light• EDFA = Erbium(鉺 )-Doped Fiber Amplifier

• PEFFA = Praseodymium(鐠 )-Doped Fluoride(氟化物 ) Fiber Amplifier

Pump wavelength980 nm (10 dB/nW gain)1480 nm (5 dB/nW gain)

49

• EDFA– the 3-dB gain bandwidth = 35 nm– the gain saturation power = 10dB

10 dB

35 nm

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VI. Switching Elements• Electronic Switches

- data switching with electro-optic conversion- electronic control of switching - flexibility, slow, extra delay.

Optical Switches- data switching without electro-optic conversion- electronic control of switching- transparent switching

51

• Two classes of switches:

Relational Switches- relation between inputs and ouputs is a function of control signals applied to the device and is independent of the contents of the signal and data inputs (data transparency).- loss of flexibility- used for circuit switching

Logic Switches- the data signals control the state of the device- used for packet switching

52

• A. Fiber Crossconnect Elements– Wavelength insensitive

(incapable of demultiplexing different wavelength signals on a given input fiber)

– Example: 2 2 cross-connect element• cross state• bar state

Cross state Bar state

53

• Two types of crossconnect technologies:

(How to connect the input to the output?)

– 1) Directive Switch• physically directed

– 2) Gate Switch• using amplifier gates

54

•1) Directive Switches

– (a) Directive Switches

– (b) Reversed Data-Beta Coupler

– (c) Balanced Bridge Interferometric Switch

– (d) Intersecting Waveguide Switch

×Electrode 電極

55• 2) Gate Switches– N N gate switch

• 1 N splitters + N2 optical amplifiers + N 1 couplers• controlling optical amplifier on or off to pass only selected signa

ls to the outputs• Example: 2 2 amplifier gate switch (Fig. 20)• Semiconductor optical amplifer: 8 8 switch

1300-nm, low polarization dependence (1 dB)fairly low cross talk ( < - 40 dB), bulky, expensiv

e Integrated optical amplifer: 4 4 switch

1550-nm, high polarization dependence (6-12 dB) fairly low cross talk ( < - 40 dB),

56

• Wavelength Routing Devices– Routing is based on the wavelengths of the signals– demultiplexing

+ switching each wavelenght (optional) + multiplexing

• Two types of wavelength routing:– Non-reconfigurable

no switching stage between MUX and DEMUX

– Reconfigurableswitching stage in between, controlled electronically

57

• B. Non-reconfigurable Wavelength Router

– a stage of multiplexers + a tage of demultiplexers

– hardwired– Example: 4 4 (Fig. 21)

• Fixed routing matrix

Multiplexers Demultiplexers

DCBA

HGFE

LKJI

PONM

LOBE

PKFA

HCNI

DGJM

58

– WGR (Waveguide Grating Router)AWG (Arrayed Waveguide Grating)

• Max. of N2 connections v.s. N for passve-star coupler Integrated device low cost fixed routing

• Two passive-star coupler– N N‘ & N’ N,

(N <<N’)

– Seperated by angles & ‘

• One grating array– N‘ waveguides

– Lengthsl1 < l2 < … < lN‘

• Different phase shift =…

Input transmitted to output with = 2n

59• C. Reconfigurable Wavelength-Routing Switch (WRS) Wavelength-Selective Cross Connect (WSXC)– P P Reconfigurable WRS

• Photonic Switches inside,built from 2 2 optical crosspoint elements (tunable/reconfigarable) arranged in a banyan-based fabric

P Demultiplexers P MultiplexersM Switches

60

• D. Photonic Packet Switches– Composed of logic devices– The switch configuration is a function of data on the inpu

t signal.– Problem: Resource Contention

• Multiple packets contend for a common resource in the switch.

• Contention is resolved through:– buffering for an electronic switch

– delay lines for an optical switch(a long length of fiber that introduces propagation delays that are on the order of packet transmission times)

– Examples• Staggering Switch

• Contention Resolution by Delay Lines (CORD)

• HLAN Architecture

61– Staggering Switch• “almost-all-optical” - fully optical data + electronic control Transparent - the payload may be encoded in an

arbitrary format or at an arbitrary data rate Lack of random-access optical memory ?

(unequal)

• Yet to be implemented

• suffer from cross talk and power limitations

62

– Contnesion Resolution by Delay Lines (CORD)• A number of 2 2 cross-connect elements and delay lines

• One packet may be switched to the delay line while the other packet is switched to the proper output.

63– HLAN Architecture• A helical unidirectional bus is divided into three separated

“segments”.

• A headend periodically generates equal-sized empty frame.

• Guaranteed-bandwidth traffic GBW segmentBandwidth-on-demand traffic BOD segmentData is received RCV segment

• 100 Gb/s or more

64VII. Wavelength Conversion• Fig. 27:

– two WDM cross connects (S1 and S2) and five access stations (A-E)three linepaths has been set up (C-A on 1, C-B on 2, and D-E on 1 ?)

– Wavelength-continuity constraint(the same wavelength is allocated on all the links in the path)

2

1 ?

1

65

• Fig. 28 (a)

– Wavelength-continuity

– Node 1 - Node 2 : 1 Node 2 - Node 3 : 2

Node 1 - Node 3 ?

• Fig. 28 (b)

– Wavelength-conversion

– Node 1 - Node 2 : 1 Node 2 - Node 3 : 2

Node 1 - Node 3 2 1

– Improve the efficiency

Wavelength-continuity constraint

Wavelength conversion

• Wavelength Converter– s c

66

• A. Wavelength Conversion TechnologiesTwo types:

- opto-electronic - all-optical

- that employ coherent effects- that use cross modulation

– 1) Opto-electronic Wavelength Conversion• up to 10 Gb/s

• more complex and more power consumption

• O/E affects data transparency (data format & data rate)

Photodetector Tunable Laser

O/E E/O

67

– 2a) Wavelength Conversion Using Coherent Effects• based on wave-mixing effects (Fig. 31)

• preserve data transparency (phase & amplitude)

• the only approach that allows simultaneous conversion of a set of multiple input wavelengths to another set of multiple output wavelengths

• 100+ Gb/s

• n = 3 : Four-Wave Mixing (FWM)4th wave is generated : fijk = fi +fj -fk

n = 2 : Difference Frequency Generation (DFG)

(pump wavelength)(pump wavelength)(Continuous Wave)

68

– 2b) Wavelength Conversion Using Cross Modulation• Using optical-gating wavelength conversion techniques

on active semiconductor optical devices such as a) SOA (semiconductor optical amplifier) in

XGM (Cross-Gain Modulation) modeXPM (Cross-Phase Modulation) mod

eb) Semiconductor Laser

69– SOA in XGM (Cross-Gain Modulation) mode simple, 10 Gb/s invertion

– SOA in XPM (Cross-Phase Modulation) mode power efficient

(inverted)

(inverted | noninverted)

70

– Semiconductor Laser• inverted output

• 10 Gb/s

• Bandwidth 1 GHz

71

• B. Wavelength Conversion in Switches not very cost effective

since not all the WC‘s may be required all the time

Wavelength Converter

M inputs M outputsN wavelengths M N

72

–Fig. 36 (a)• Share-Per-Node• Converter Bank

– a collection of a few wavelength conveters

– accessed by any wavelength on any incoming fiber

–Fig. 36 (b)• Share-Per-Link

• Converter Bank– accessed by

particular links

Share-per-node

Share-per-link

73– Share-with-local switch• Opto-electronic wavelength conversion is used either in

– the switch (Fig. 37)

– the network access station (Fig. 38)

• Share-with-local switch architecture (Fig. 37)– O/E at RxE ESW E/O at TxE

– May be locally-added, or locally-dropped, or retransmitted on a different wavelength

O/E E/O

74

• Share-with-local network access station architecture (Fig. 38)– O/E & E/O at the network access station

75

VIII. Designing WDM Networks: Systems Consideration

• Keep in mind:– Functionality of the networks

– Capabilities and limitations of the network componets

• Issues:– A. Channels– B. Power Considerations– C. Cross talk– D. Additional Considerations– E. Elements of Local-Area WDM Network Design– F. WDM WAN Desig Issues

76

• A. Channels– Number of wavelengths to use

• Total available bandwidth or spectral range of the components

– Fiber medium: at 1300 and 1550 nm with 200 nm bandwidth each

– Amplifier: 35-40 nm bandwidth

– Injection-current laser: 10nm tuning range

– Fabry-Perot filter: entire low-attenuation region tuning range

– electro-optic filter: 16 nm tuning range

• Channel spacing– Channel bit rates

– Optical power budget

– Non-linearities in the fiber

– Resolution of transmitters and receivers

– A higher number of channels more network capacity higher network costs and more complex protocols

77

• B. Power Considerations– SNR– Signal power degrades due to losses such as

• attenuation in the fiber

• splitter losses

• coupling losses

• optical amplifier losses

– Different characteristics for the three main applications for optical amplifiers

• Transmitter power booster : immediately after the transmitter

• Receiver preamplifier : before detection at a receiver photo-detector

• In-line amplifier : used within the network

78

• C. Cross Talk– Interband cross talk

• interference from signals on different wavelengths

• affects channel spacing

• can be removed by using appropriate narrow-band filers

– Intraband cross talk• interference from signals on the same wavelengths on an

other fiber

• usually occur in switching nodes

• cannot easily be removed through filtering

• can accumulate over a number of nodes

79

• D. Additional Considerations – Dispersion

• a pulse to broaden as it propagates along the fiber

• limits the spacing between bits & the max. transmission rate

the max. fiber distance for a given rate

– Architectural (topology) considerations• choice of which transmitter-receiver pairs to operate on whi

ch wavelength

• fault tolerance and reliablity

– Standards

80

•E. Elements of Local-Area WDM Network Design

–Single-hop protocolMulti-hop protocol

–Network Medium• PSC

– passive

– fairly reliable

– additional HW for routing

– no wavelength reuse

–Network Nodes• Fixed Tx / Rx

• Fixed Tx / Rx

81

• F. WDM WAN Design Issues– Electronic/optical networks v.s. all-optical networks

–Multiple light-paths to share each fiber links

–Wave length reuse

82

IX. Experimental WDM Light-wave Networks

• A. LAN Testbeds– Bellcore‘s LAMBDNET

• each node:one fixed DBF laser transmitter + N fixed receivers

• simple, multicast, not scalable, costly

– IBM‘s Rainbow• 32 PS/2‘s + 32 200-Mbps WDM channels (1 Gb/s)

• each node:a fixed DFB laser transmitter + a tunable Fabry-Perot filter receiver

• When IDLE, scan for SETUP request

• Not scale well

83

• B. WAN Testbeds– MWTN (Multi-wavelength Transport Network)

• by RACE (Research and Development in Advanced Communications Technologies) in Europe

– MONET (Multi-wavelength Optical Networking)• by AT&T, Lucent,...

– ONTC• Bellcore, Columbia University, Hughes Research Lab,...

– AON• AT&T, DEC, MIT Lincoln Lab,...

84

– MONET : local exchange + cross connect + long distance

85

– ONTC

(ATM Switch)

(2 2)

86

– AON

LAN

MAN

Nationwide Backbone

87

X. Conclusion

• As optical device technology continues to improve, network designers need to be ready to take advantage of new device capabilities while keeping in mind the limitations of such devices.