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Elements of optical networking

Megabit and Gigabit and Terrabit and ???

by Prof. Dr. V. Brückner

1. The optical channel - DWDM

2. Limits by attenuation

3. Limits by dispersion

Megabit and Gigabit and Terrabit and ???

kB/s

MB/s

GB/s1 ms1 µs1 ns

Digital Bit pattern

transmitter

TB/s

I1

I2

I3

about 40 channels

1 000 01 1 1

Dense Wavelength Division Multiplexing(DWDM)

Bit duration

DWDM: the single optical channel:

The optical channels(wavelength consideration)

frequency

f0

Δf

100%

50%

wavelength

λ0

Δλ Line width

wavelength

frequency

f0

wavelength

λ0

The optical channels(wavelength consideration)

DWDM: the single optical channel:

Δλ≈1 pm line width

wavelength

3 optical channels:

frequency

100 GHz 100 GHz195.8 THz195.7 THz 195.9 THz

(193.1 + n.0.1) THz

ITU-T G.694.1for 100 GHz:

f

c f

cf

f

c

2

2

at λ = 1531.12 nm and Δf = 100 GHz -> Δλ = 0.75 nm

wavelength

1531.12 nm1530.37 nm 1531.87 nm

0.75 nm 0.75 nm

channel spacing

Requirements to wavelength stability of lasers: ±2.5% (±0.02 nm)!!!!!

The optical channels(wavelength consideration)

3 optical channels + modulation:

wavelength

1531.12 nm1530.37 nm 1531.87 nm

0.75 nm 0.75 nm

channel spacing

The optical channels(modulation)

Optical Time Division Multiplexing(OTDM)

Principle:Shuffling in time

(different run times in glass fibers)

3 optical channels + modulation:

wavelength

1531.12 nm1530.37 nm 1531.87 nm

0.75 nm 0.75 nm

channel spacing

The optical channels(modulation)

Equal length of time slots:Synchronous digital Hierarchie

(SDH)

Flexible length of time slots :Asynchronous Transfer Modus

(ATM)

STM level Bit rate

STM-256 39813,12 MBit/s

STM- 64 9953,28 MBit/s

STM- 16 2488,32 MBit/s

STM- 4 622,08 MBit/s

STM- 1 155,52 MBit/s

Synchron Transport Moduls:

fiber

1

n MUX DEMUX

1

n

WDM

OTDM with WDM:

The optical channels(modulation)

Principle :Shuffling in time

e.g. for STM-256 (40 GB/s)

S1 : 1 , τ1 < 10 ps

S2 : 1 , τ1 < 10 ps

Sm: 1 , τ1 < 10 ps

.

.

. t

m time slots a 25 ps = 1/40GBps

. . .

S1 S2 Sm

τ1 τ1 τ1

Principle:Generation of a frame of m time slots,

in total 40 GB/s (40 GBps)

Transmission capacity n * 40 GB/s

3 optical channels + modulation:

wavelength

1531.12 nm1530.37 nm 1531.87 nm

0.75 nm 0.75 nm

40 GB/s ~ fM = 40 GHz (0.3 nm)

Modulation with 40 GB/s (STM-256):

channel spacing

safety clearance

Safety clearance: 0.15 nm

The optical channels(modulation)

Resulting spectrum: Side bands f0 ± fM

Optical channels: scaling

wavelength

Present time: 40 channels - about 30 nm (40 GBps) = 1.6 TBps

1531.12 nm1501.87 nm

future: 80 channels - about 60 nm (40 GBps) = 3.2 TBps

160 channels - about 120 nm ?? (40 GBps) = 6.4 TBps

320 channels - about 240 nm ???? (40 GBps) = 12.8 TBps

The optical channels(wavelength + modulation)

Optical channels – conclusions:

or: 40 channels - about 30 nm (40 GBps) = 1.6 TBps

or: 80 channels - about 60 nm (20 GBps) = 1.6 TBps

or: 20 channels - about 15 nm (80 GBps) = 1.6 TBps

either: 10 channels - about 7.5 nm (160 GBps) = 1.6 TBps

Transfer of a data rate of 1.6 TBps:

Which way is better? Which problems arise?

The optical channels(wavelength + modulation)

Problem: low channel number -> higher channel distance -> technically less complicatedhigher bit rate -> shorter time slots -> technically more complicated (Dispersion)

Problem: lower bit rate -> less dispersion influence -> technically less complicatedhigher channel number -> shorter channel distance -> techncally more complicated (demultiplexing)

or

laser

modulator

MUXWDMDWDM

glass fiber

core9 µm

cladding125 µm

losses - attenuation

The optical channels(limits by attenuation)

laser

modulator

MUX

fiber The fiber:power in fibers

P0

z

P

100%

50%

physics: power engineering: level

kzePzP 0)(

W, mW dB, dBm

)1

log(10mW

Pp

mWdBm

)log(10in

outdB

P

Pa

p0

z

p

-3dB

attenuation in fibers (α): losses per kilometer (dB/km)

The optical channels(limits by attenuation)

700 800 900 1000 1100 1200 1300 1400 1500 1600 1700Wavelength [nm]

0,1

1

10

loss

es [

dB

/km

] Rayleigh scattering

OH absorption

standard SMF

AllWaveTM

Best values: 0.2 dB/km at 1500 nm

IR absorption

Si

O

OO

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

2nd 3d LS

DWDM: 40 channels, 50 GHz distance

H

H

vibrations

The optical channels(limits by attenuation)

700 800 900 1000 1100 1200 1300 1400 1500 1600 1700wavelength [nm]

0,1

1

10

loss

es [

dB

/km

]

3d LS

DWDM: 40 chanals, 50 GHz

200 nm

Max. bandwidth in SSMF: 200 nm or 25 THz

The optical channels(limits by attenuation)

laser

modulator

MUX

fiber

The fiber:amplification

Rec S T A

LD

opticaldata

opticaldata

electricaldata

ID

I0

3R regeneration: amplification (Re-amplification), clock reinstallation (Re-timing) and pulse formation (Re-shaping)Classical way:

>>

The optical channels(limits by attenuation)

3R regeneration: amplification (Re-amplification), clock reinstallation (Re-timing) and pulse formation (Re-shaping) optical way:

Optical amplification: Erbium-doped fiber amplifier EDFA

Raman amplifier ROASemiconductor optical amplifier SOA

no Re-timing, no Re-shaping!!!

The fiber:amplificationlaser

modulator

MUX

fiber

>>

The optical channels(limits by attenuation)

kBit/s

MBit/s

GBit/s

1 ns

Bit pattern

Laser

Modulator

DWDM

The optical channels(limits by dispersion)

idealrect pulse

time

t 2 1.975 5

r t( )

t

realrect pulse

, 3, 5, 7 usw.

time

t 2 1.975 5

g t( )

t

time

real, Gaussianpulse after

a short glass fiber

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

time

broadened, Gauss-shaped pulse after dispersion in a

long glass fiber

p

dis

Influence of dispersion to a single Bit

dis > p

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

t 2 1.975 5

g t( )

t

The optical channels(limits by dispersion)

L0 = 0 L1 > L0 L2 > L1 L3 > L2

2 Bits 2 Bits 2 Bits ??

1 oder 2 Bits?????

time time time time

The optical channels(limits by dispersion)

Dispersion

Modal dispersion (MMF)

Chromatic Dispersion (MMF and SMF)

Material dispersion (MMF and SMF)

Waveguide dispersion (SMF)

Polarization mode dispersion (SMF) Dep

enden

t on

The optical channels(limits by dispersion)

modal dispersionModal Dispersion in SI-Fibers (MMF)

Different runtime Δtg for all paths, because of different paths ways

Δtg

The optical channels(limits by dispersion)

modal dispersionmodal dispersion in GI-Fibers (MMF)

Nearly the same runtime for all paths, because different paths and velocities are compensated

The optical channels(limits by dispersion)

Chromatic Dispersion (CD)= Material Dispersion + Waveguide Dispersion

Different wavelengths in the LD-Spectrum have different transit times

Light sources are not monochromatic, but have a finite broad spectrum

t

t3t2t1

The optical channels(limits by dispersion)

P

Material dispersion

L

1

2

3 Selection of 3 wavelengths:3 > 2 > 1 thus n3 < n2 < n1 thus v3 > v2 > v1

3

2

1

Path difference

in Multi-Mode and Single-Mode fibers

Spectral width -> tg by different propagation speeds

n

λ

Dispersion

The optical channels(limits by dispersion)

tg by different propagation speedsin core and cladding (in SMF only)

L

d1= 9 µm

d2 = 5µm< d1

P

x

P

x

Situation: nK > nM thus vK < vM

Wave guide dispersion

The optical channels(limits by dispersion)

SMF DFF DSFindex

core core core

DChrom (ps/km*nm)

SMFDSF

DFF (µm)

chromatic dispersion

The optical channels(limits by dispersion)

bit pattern dispersion

dispersioncompensation

pulse broadening(overlap of Bits)

chromatic dispersion

The optical channels(limits by dispersion)

dispersion compensating fiber

5 km DCF 40 km SMF

Spleisses

40 km SMF5 km DCF

SMF

8 parts SMF + 1 part DCF

DCF

-300

Dps/km*nm

-200

-100

0

+100

1.1

(µm)

1.3 1.5 1.7

compensation:DCF

core

n

The optical channels(limits by dispersion)