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Börje Josefsson
SUNET
[email protected]
Börje Josefsson
SUNET
[email protected]
Lumos
Börje Josefsson
<[email protected]>
2017-05-24
Page # 4
Lumos !?!

lüm\O\s

It’s a wizard-spell that generates LIGHT

Wish optical networking companies could do
that ;-)

(After this presentation you’ll see what I mean…)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 5
Terminology
Transmission Technology

The simplest transmission system is a fiber jumper
from one piece of client equipment to another



Data goes in, data comes out
No complex operations, no settings
Just a piece of glass
10GE
“Transmission System”
Fiber pair < 80 km
(Transmit and Receive)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 7
Technical Design Elements:
Terminology







Decibels (dB) – used for power gain or loss
Decibels-milliwatt (dBm) – used for output power and receive
sensitivity
Attenuation – loss of power in dB/km
Chromatic dispersion – spreading of the light pulse in ps/nm*km
Bit Error Rate (BER) – typical acceptable rate is 10-12
Optical Signal to Noise Ratio (OSNR) – ratio of optical signal
power to noise power for the receiver
ITU Grid Wavelength – standard for the lasers in DWDM systems
Börje Josefsson
<[email protected]>
2017-05-24
Page # 8
dB versus dBm

dBm used for output power and receive
sensitivity (Absolute Value)

dB used for power gain or loss (Relative Value)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 9
Optical Attenuation



Pulse amplitude reduction limits “how far”
Attenuation in dB (or dB/km)
Power is measured in dBm:
Pi
Examples
10 dBm
10 mW
0 dBM
1 mW
-3 dBm
500 μW
-10 dBm
100 μW
-30 dBm
1 μW
P0
T
T
Börje Josefsson
<[email protected]>
2017-05-24
Page # 10
Caution…
Börje Josefsson
<[email protected]>
2017-05-24
Page # 11
Optical Budget
Basic Optical Budget = Output Power – Input Sensitivity
Pout = +6 dBm
R = -30 dBm
Budget = 36 dB
Optical Budget is affected by:






Börje Josefsson
<[email protected]>
Fiber attenuation
Splices
Patch Panels/Connectors
Optical components (filters, amplifiers, etc)
Bends in fiber
Contamination (dirt/oil on connectors)
2017-05-24
Page # 12
Bit Error Rate (BER)

BER is a key objective of the optical system design

Goal is to get from Tx to Rx with a BER < BER
threshold of the Rx

BER thresholds are on data sheets

Typical minimum acceptable rate is 10 -12
Börje Josefsson
<[email protected]>
2017-05-24
Page # 13
Fiber fundamentals
Fiber Fundamentals
Attenuation
Dispersion
Nonlinearity
Distortion
It may be a digital signal, but It’s analog transmission
Transmitted data waveform
Börje Josefsson
<[email protected]>
Waveform after xxx km
2017-05-24
Page # 15
Fiber Geometry
Core

Cladding
An optical fiber is made of
three sections:



The core carries the light signals
The cladding keeps the light in the core
The coating protects the glass
Coating
Börje Josefsson
<[email protected]>
2017-05-24
Page # 16
Propagation in Fiber
n2
q0
n1
Cladding
q1
Core
Intensity Profile



Light propagates by total internal reflections at the corecladding interface
Total internal reflections are lossless
Each allowed ray is a mode
Börje Josefsson
<[email protected]>
2017-05-24
Page # 17
Different Types of Fiber

Multimode fiber

Core diameter varies
50 μm for step index
 62.5 μm for graded index
n2
Cladding

Bit rate-distance product
>500 MHz*km


n1
Core
Single-mode fiber
Core diameter is about 9 μm
 Bit rate-distance product
>100 THz*km

n2
Cladding
n1
Börje Josefsson
<[email protected]>
2017-05-24
Core
Page # 18
Optical Spectrum
UV
S-Band: 1460–1530nm
C-Band: 1530–1565nm
L-Band:
1565–1625nm
IR
125 GHz/nm
l
Visible



Light

Ultraviolet (UV)

Visible

Infrared (IR)
850 nm
980 nm
1310 nm
1480 nm
1550 nm
1625 nm
Communication wavelengths

850, 1310, 1550 nm

Low-loss wavelengths
Specialty wavelengths

980, 1480, 1625 nm
Börje Josefsson
<[email protected]>
C = x l
l (nanometers)
Frequency:  (terahertz)
Wavelength:
2017-05-24
Page # 19
Fiber attenuation characteristics
Attenuation vs. Wavelength
S-Band: 1460–1530nm
L-Band: 1565–1625nm
2.0 dB/Km
Fibre Attenuation Curve
0.5 dB/Km
0.2 dB/Km
800
900
1000
1100
1200
1300
1400
Wavelength in Nanometers (nm)
Börje Josefsson
<[email protected]>
2017-05-24
1500
1600
C-Band: 1530–1565nm
Page # 20
Glass Purity
Fiber optics requires
very high purity glass
Window glass
~ 3 cm
Optical quality glass
~3m
Fiber optics
~ 14 km
Propagation distance needed to reduce the
transmitted light power by 50% (3 dB)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 21
Fraction of Power
Remaining
Fiber loss
1
0.8
0.6
80 km  100x loss
0.4
0.2
0
0
20
40
60
80
100
Distance (km)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 22
Optical amplifiers compensate loss
Input
After Loss
After Amplifier
Added Noise
Power
Wavelength
Need to maintain adequate signal-to-noise ratio,
so more distance means more signal power
Börje Josefsson
<[email protected]>
2017-05-24
Page # 23
DWDM
To get more information on a single fiber
Use more wavelengths (DWDM)
Power (dBm)
0
-10
-20
-30
1570
1580
1590
1600
W a v el e n gt h ( n m )
Börje Josefsson
<[email protected]>
2017-05-24
Page # 25
Increasing Network Capacity Options
Same bit rate, more fibers
Slow Time to Market
Expensive Engineering
Limited Rights of Way
Duct Exhaust
More Fibers
Same fiber & bit rate, more ls
Fiber Compatibility
Fiber Capacity Release
Fast Time to Market
Lower Cost of Ownership
Utilizes existing TDM Equipment
W
D
M
Faster Electronics
(TDM)
Börje Josefsson
<[email protected]>
Higher bit rate, same fiber
Electronics more expensive
2017-05-24
Page # 26
DWDM History

Early WDM (late 80s)
Two widely separated wavelengths (1310, 1550nm)


“Second generation” WDM (early 90s)
Two to eight channels in 1550 nm window

400+ GHz spacing


DWDM systems (mid 90s)
16 to 40 channels in 1550 nm window

100 to 200 GHz spacing


Upcoming DWDM systems
64 to 160 channels in 1550 nm window

50 and 25 GHz spacing

Börje Josefsson
<[email protected]>
2017-05-24
Page # 27
ITU Wavelength Grid
1530.33 nm
195.9 THz


0.80 nm
100 GHz
ITU-T l grid is based on 191.7 THz +
100 GHz
It is a standard for laser in DWDM
systems
Börje Josefsson
<[email protected]>
2017-05-24
1553.86 nm
l
193.0 THz

Freq (THz)
192.90
192.85
192.80
192.75
192.70
192.65
192.60
ITU Ch
29
28
27
26
Page # 28
Wave (nm) 1520
1554.13
1554.54
1554.94
1555.34
1555.75
1556.15
1556.55
DWDM Components
l1
850/1310
15xx
l1...n
l2
l3
Transponder
Optical Multiplexer
l1
l2
l1
l1...n
l2
l3
l3
Optical De-multiplexer
Optical Add/Drop Multiplexer
(OADM)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 29
More DWDM Components
Optical Amplifier
(EDFA)
Optical Attenuator
Variable Optical Attenuator
Dispersion Compensator (DCM / DCU)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 30
Typical DWDM Network Architecture
DWDM SYSTEM
DWDM SYSTEM
VOA
DCM
EDFA
EDFA
DCM
VOA
Service Mux
(Muxponder)
Börje Josefsson
<[email protected]>
Service Mux
(Muxponder)
2017-05-24
Page # 31
Sub-wavelength Multiplexing or MuxPonding
Ability to put multiple services onto a
single wavelength
Börje Josefsson
<[email protected]>
2017-05-24
Page # 32
Optical Signal-to Noise Ratio (OSNR)
Signal Level
X dB
Noise Level
• Depends on :
Optical Amplifier Noise Figure:
(OSNR)in = (OSNR)outNF
EDFA Schematic
(OSNR)out
(OSNR)in
Pin
• Target : Large Value for X
NF
• Don’t confuse this with attenuation etc!
Börje Josefsson
<[email protected]>
2017-05-24
Page # 33
Optical Signal to Noise Ratio
OSNR = 58 – 10 * log(N) – 10 * log(M) – NF -10log(L) +
Pout - 
The optical signal to noise ratio at the end of the link depends on the
number of channels, the number of spans, the noise figure, the OA
output power and some other factors, like gain tilt, flatness, etc.
The OSNR needed is determined by the receiver BER versus OSNR
performance characteristics. This is a measured parameter.
M Channels, N Amplifiers, L Loss Per Span, NF: Noise Figure
Pout :OA Output Power,  : Other Factors
Börje Josefsson
<[email protected]>
2017-05-24
Page # 34
Loss Management: Limitations

Each amplifier adds noise, thus the
optical SNR decreases gradually
along the chain; we can have only
have a finite number of amplifiers
and spans and eventually electrical
regeneration will be necessary

Gain flatness is another key
parameter mainly for long amplifier
chains

Noise Figure > 3 dB
Typically between 4 and 6
section
span
Rule of thumb: Distance of spans
are dependant on the number of
spans in a section
Börje Josefsson
<[email protected]>
2017-05-24
Page # 35
Designing for Distance
L = Fiber Loss in a Span
Pin
Pout
S
Amplifier Spacing

D = Link Distance
Link distance (D) is limited by the minimum acceptable
electrical SNR at the receiver


Pnoise
G = Gain of Amplifier
Dispersion, Jitter, or optical SNR can be limit
Amplifier spacing (S) is set by span loss (L)
Closer spacing maximizes link distance (D)
 Economics dictates maximum hut spacing

Börje Josefsson
<[email protected]>
2017-05-24
Page # 36
Wavelength Capacity (Gb/s)
Link Distance vs. OA Spacing
Amp Spacing
20
60 km
10
80 km
100 km
5
120 km
140 km
2.5
0

2000
4000
6000
8000
Total System
Length (km)
System cost and and link distance both depend strongly
on OA spacing
Börje Josefsson
<[email protected]>
2017-05-24
Page # 37
Error Correction
Information
Transmitter



Transmission
Channel
Information
Receiver
“Detect is good, correct is better”.
Error correcting codes both detect errors and correct
them
Forward Error Correction (FEC) is a system that:



“Noise”
adds additional information to the data stream
corrects eventual errors that are caused by the transmission
system.
Low BER achievable on noisy medium
Börje Josefsson
<[email protected]>
2017-05-24
Page # 38
FEC performance, theoretical
FEC gain  6.3 dB @ 10-15 BER
Bit Error Rate
1
BER without FEC
10 -10
Coding Gain
BER floor
10 -20
BER with FEC
10 -30
-46
Börje Josefsson
<[email protected]>
-44
-42
-40
-38
-36
2017-05-24
-34
-32
Received Optical
power (dBm)
Page # 39
FEC in DWDM systems
9.58 G
10.66 G
9.58 G
10.66 G
IP
FEC
FEC
IP
SDH
FEC
FEC
SDH
.
.
.
.
FEC
FEC
ATM
2.48 G


2.66 G
2.66 G
2.48 G
FEC implemented on transponders (TX, RX, 3R)
No change on the rest of the system
Börje Josefsson
<[email protected]>
2017-05-24
ATM
Page # 40
Uni- versus bi-directional DWDM
DWDM systems can be implemented in two different ways
• Uni-directional:
l1
l3
l5
l7
wavelengths for one direction
travel within one fiber
l2
l4
l6
l8
Fiber
l1
l3
l5
l7
l2
l4
l6
l8
Fiber
two fibers needed for
Uni -directional
full-duplex system
• Bi-directional:
a group of wavelengths for each
direction
single fiber operation for fullduplex system
Börje Josefsson
<[email protected]>
2017-05-24
Fiber
l5
l6
l7
l8
l1
l2
l3
l4
Bi -directional
Page # 41
Fiber Anomalies
Types of dispersion
• Chromatic Dispersion
Different wavelengths travel at different speeds
Causes spreading of the light pulse
• Polarization Mode Dispersion (PMD)
Single-mode fiber supports two polarization states
Fast and slow axes have different group velocities
Causes spreading of the light pulse
Börje Josefsson
<[email protected]>
2017-05-24
Page # 43
Data distortion from dispersion
10101
0110
010
Propagation Length
160 km
80 km
1’s
0 km
0’s
NRZ distortion very pattern dependent
Börje Josefsson
<[email protected]>
2017-05-24
Page # 44
A Snapshot on Chromatic Dispersion
Interference




Affects single channel and DWDM systems
A pulse spreads as it travels down the fiber
Inter-Symbol Interference (ISI) leads to performance
impairments
Degradation depends on:



laser used (spectral width)
bit-rate (temporal pulse separation)
different fiber types
Börje Josefsson
<[email protected]>
2017-05-24
Page # 45
Solution for chromatic dispersion
Dispersion
Saw tooth compensation
DCU
Dispersion
+D
DCU
-D
Length
Total dispersion averages to ~ zero
The precision required in the DCU devices scales as the square of the bit rate
Börje Josefsson
<[email protected]>
2017-05-24
Page # 46
Dispersion limits (without compensation)

DL = 16,000 ps/nm at 2.5 Gb/s NRZ
16,000 / 17ps/nm*km
~ 1000 km in SMF
16

DL = 1,000 ps/nm at 10 Gb/s NRZ
1000 / 17
~ 60 km in SMF
16

DL = 62.5 ps/nm at 40 Gb/s NRZ
62.5 / 17
~ 3.5 km in SMF
(Gb/s)2  Dispersion  Distance < 100,000
Börje Josefsson
<[email protected]>
2017-05-24
Page # 47
Not all fiber is created equal
Fiber
Cross
Section
Polarization mode
dispersion is caused
by slight fiber
asymmetry
Elliptical Fiber Core
PMD also changes randomly with environmental effects
(e.g. temperature), requiring adaptive compensation.


Discovered in the 90s (Fibers installed before 1993 may have
significant PMD)
Most older fiber not characterized for PMD
Börje Josefsson
<[email protected]>
2017-05-24
Page # 48
Polarization Mode Dispersion (PMD)
Fast
Side View
Different polarizations
travel at different speeds
Slow
PMD = Time Delay
The optical pulse tends to broaden as it travels down the fiber;
this is a much weaker phenomenon than chromatic dispersion
and it is of little relevance at bit rates of 10Gb/s or less. The
order of magnitude is ~1 ps/√km
• 0.5 ps/√km of PMD at 10 Gb/s over 100 km leads to a pulse broadening of
5 ps (5% of “eye opening”.)
Börje Josefsson
<[email protected]>
2017-05-24
Page # 49
Polarization Mode Dispersion (PMD)
Linear
i z   im ( z ) t   rhs
0
Ψ ( z )  Wˆ ( z ) Ψ (0)
z


Wˆ ( z )  exp  i  dz ' mˆ ( z ' )
0


pulse splitting
broadening
jitter
 
Jˆ ( z; )  Wˆ ( z )Wˆ ( z )  iˆ
1
Poole, Wagner ‘86
Poole ’90;’91
Statistics of PMD
vector is Gaussian.
Börje Josefsson
<[email protected]>
Polarization (PMD) vector
(of first order)

 2 
4 3
P( |  |)  3
exp   2 
 
  
  2 Dm z
2017-05-24
Differential group delay
(DGD)
Page # 50
Four Wave Mixing (FWM)
Beating between two signals
generates a signal at the difference frequency:
1
2
Into the fiber…

1
Out of the fiber…
212
2
221

n(t) = no + n2[E12+E22+2E1*E2cos(w1-w2)t]
Börje Josefsson
<[email protected]>
2017-05-24
Page # 51
The 3 “R”s of optical networking
The options to recover the signal from degradation are:
Pulse as it enters the fiber
Pulse as it exits the fiber
Re-amplify
Re-shape
DCU
Phase Re-Alignment
Phase Variation
O-E-O
Re-time
(Re-generate) t Optimum t
s
Sampling Time
Börje Josefsson
<[email protected]>
t
t
ts Optimum
Sampling Time
2017-05-24
ts Optimum
Sampling Time
Page # 52
Different fiber types etc.
Conventional Single-Mode Fiber (SMF)
30
S C L
Dispersion (ps/nm)
20
D(1530-1565nm)
= 16 - 19 ps/nm*km
10
0
D = 0.065 ps/nm2km
-10
Aeff = 85 μm2
-20
-30
1250
1350
1450
1550
1650
Wavelength (nm)
First single-channel systems operated at 1310 nm (good laser
materials) WDM systems moved to 1550 nm: wider loss-window, but
higher dispersion Dispersion limit = 1000 km at 2.5Gb/s in SMF.
Börje Josefsson
<[email protected]>
2017-05-24
Page # 54
Dispersion-Shifted Fiber – Oops!
30
S C L
Dispersion (ps/nm)
20
10
0
-10
-20
-30
1250
1350
1450
1550
1650
Wavelength (nm)
DSF: Zero dispersion at 1550nm, so no compensation required.
However, FWM severely limits optical power levels.
Small effective core area, very nonlinear
Börje Josefsson
<[email protected]>
2017-05-24
Page # 55
Some types of Single-Mode Fiber

SMF-28(e) (standard, 1310 nm optimized, G.652)


DSF (Dispersion Shifted, G.653)


Most widely deployed so far, introduced in 1986, cheapest
Intended for single channel operation at 1550 nm
NZDSF (Non-Zero Dispersion Shifted, G.655)
For WDM operation, optimized for 1550 nm region
 TrueWave, FreeLight, LEAF, TeraLight…

Latest generation fibers developed in mid 90’s
 For better performance with high capacity DWDM systems

MetroCor, WideLight…
 Low PMD ULH fibers

Fibers before 1993 may have significant PMD
Börje Josefsson
<[email protected]>
2017-05-24
Page # 56
Now, don’t you wish it was this easy…
Questions?
Börje Josefsson
<[email protected]>