Advanced DWDM Training
Advanced DWDM Training
Advanced DWDM Training
Generics
Transmitters & Receivers
Amplifiers
Non-Linear Effects
Dispersion
Generics
100/140 or 200/280 m
0.1 - 3 mm , core 80 to 99%
Cladding
Core
Refractive
Index (n)
1.480
1.460
100 m
lass
G
iO 2
Primary coating
(e.g., soft plastic)
140 m
Diameter (r)
NA
= sin
n2cladding
n2core -
Attenuation (dB/km)
OH
Absorption
Optical
Windows
1.0
0.5
900
1100
1300
1500
Wavelength (nm)
1700
~ 20 Mb/s km
~ 5 Mb/s km
1.475
1.460
1.465
1.460
Key characteristics
peak
-3 dB
BW
What is a LED?
A light-emitting diode is a p-n junction that emits light when
forward-biased:
one side of the diode is a semiconductor of type p
(majority carriers are holes)
the other side is of type n (majority carriers are
electrons)
Characteristics of LED's
Forward Bias
Laser Diodes
Laser diodes are semiconducting devices that emit
coherent light or laser light
Laser Diodes
There are two important groups of words in this
acronym:
light amplification
stimulated emission
laser has typically three components:
the lasing material: gas, semiconductor, etc.
the energy pump source
two mirrors
Emission in a Preferential
Direction
The two mirrors are placed at the ends of the tube
to force the emission in one direction.
The photons traveling along the axis of the two
mirrors bounce between the two mirrors: the number
of photons increases greatly along this direction.
Population Inversion
Atoms are lazy: they don't like to stay in an excited (or
high energy) state.
It is tiring. They prefer to relax into a lower energy) state
and enjoy life!
If we don't have a lot of excited atoms to start with, the
chance of having a lot of stimulated emission is not
great!
To get a laser to work, we need to have a population
inversion: we need to get most of the electrons to the
high energy states.
This is done by providing an initial energy to the atoms:
passing electrical current, illuminating with a bright
pulse, ...
Stimulated Emission
Application of Stimulated
Emission
Stimulated emission and electroluminescence
convert information and energy from an
electronic form to an optical form.
However in the case of stimulated emission, we
have a coherent light emitting device.
Laser Basics
Semiconductor material - electrons negatively charged particles.
Electrons can go to excited state - more energy than regular electrons.
An electron in an excited state can just spontaneously fall down to the regular
ground state.
The ground state has less energy, and so the excited-state electron must give
out its extra energy before it can enter the ground state.
It gives this energy out in the form of a photon a single particle of light.
Laser Basics
Laser Basics
More and more spontaneous emission of photons caused by electrons decaying from
the excited state to the ground state.
Need lots of these electrons to decay at the same time to give lots of light out,
2.
Need this to be happening all the time so that we have a steady stream of light.
Want them to travel back and forth through the laser time and time again :
1.
Photons can encourage other excited electrons to fall to the ground state and give
out more photons.
2.
ll the time an electric current is putting more electrons into the excited state where they
wait to fall to the ground state and give out light
peak
P
Threshold
Fabry-Perot
Two specially designed slabs of semiconductor material on top of
each other
Another material in between them forming what is known as the
active layer or laser cavity.
Electric current through the device from the top slab to the bottom
Fabry-Perot
Fabry-Perot
Light builds up enough within the active layer
Highly intense beam of light is emitted from ends of laser
Specific value of electrical current applied to the laser - spontaneous
emission to stimulated emission
Threshold current
Photons from the laser roughly the same wavelength
One color or wavelength
Fabry-Perot
Key Points
Tiny salt-grain sized devices made of semiconductor material
Electrical current puts lots of electrons into excited state: population
inversion
Excited electrons spontaneously decay to ground state and emit
photons
Photons can stimulated emission of further photons
Fabry-Perot cavity confining light and reflecting it back and forth
Most of light at one specific wavelength, but others are produced in
Fabry-Perot
SMSR
Digital Modulation
Digital Modulation:
Extinction ratio = P1 / P0
Time-division multiplexing (TDM)
~1.5 Mb/s to 10 Gb/s
P1
P0
0
Channel
2
3
4
Modulation Principles
Direct (laser current)
Inexpensive
Can cause chirp up to 1 nm
(wavelength variation caused
by variation in electron
densities in the lasing area)
External
2.5 to 40 gb/s
AM sidebands (caused by
modulation spectrum) dominate
linewidth of optical signal
DC
RF
DC
MOD
RF
External Modulators
Mach-Zehnder Principle
Laser
section
Modulation
section
Photodiode
Photodiodes are semiconducting devices that
convert light into electrical signals.
There are several kinds of semiconductor
photodiodes. They all work on the same principle which
is based on photoconductivity.
Photodiodes have also invaded our homes: they can
be found in appliances that can be controlled by remote
control.
Photoconductivity
Application of Photoconductivity
What is a Photodiode?
A photodiode is a p-n junction that is reversebiased.
Reverse Bias
Photodiode Types
APD Gain
Bias Voltage
Material Aspects
Silicon (Si)
Least expensive
Germanium (Ge)
Classic detector
Responsivity (A/W)
1.0
Quantum
Efficiency = 1
0.5
InGaAs
Germanium
Silicon
500
1500
1000
Wavelength nm
AGC
-g
Temperature
Control
Clock
Recovery
Decision
Circuit
Monitors
& Alarms
Remote
Control
0110
Receiver Sensitivity
Bit error ratio (BER)
versus input power (pi)
BER
Passive Components
DWDM Spectrum
RL +0.00 dBm
5.0 dB/DIV
Channels: 16
Spacing: 0.8 nm
Amplified
Spontaneous
Emission (ASE)
1545 nm
1565 nm
Blue band
Channel
Central
Central
number frequency (GHz) wavelength (nm)
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
192.200
192.300
192.400
192.500
192.600
192.700
192.800
192.900
193.000
193.100
193.200
193.300
193.400
193.500
193.600
193.700
1559.79
1558.98
1558.17
1557.36
1556.55
1555.75
1554.94
1554.13
1553.33
1552.52
1551.72
1550.92
1550.12
1549.32
1548.51
1547.72
Channel
Central
Central
number frequency (GHz) wavelength (nm)
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
194.300
194.400
194.500
194.600
194.700
194.800
194.900
195.000
195.100
195.200
195.300
195.400
195.500
195.600
195.700
195.800
1542.94
1542.14
1541.35
1540.56
1539.77
1538.98
1538.19
1537.40
1536.61
1535.82
1535.04
1534.25
1533.47
1532.68
1531.90
1531.12
Filter Characteristics
Passband
Insertion loss
Ripple
Wavelengths
(peak, center, edges)
Bandwidths
(0.5 dB, 3 dB, ..)
Polarization dependence
Stopband
Crosstalk rejection
Bandwidths
(20 dB, 40 dB, ..)
i-1
Crosstalk
Passband
i+1
Crosstalk
Isolators
Main application:
To protect lasers and optical amplifiers from light coming back (which
otherwise can cause instabilities)
Insertion loss:
Low loss (0.2 to 2 dB) in forward direction
High loss in reverse direction :
20 to 40 dB single stage, 40 to 80 dB dual stage)
Return loss:
More than 60 dB without connectors
Dielectric Filters
Thin-film cavities
Alternating dielectric thin-film layers with different refractive index
Multiple reflections cause constructive & destructive interference
Variety of filter shapes and bandwidths (0.1 to 10 nm)
Insertion loss 0.2 to 2 dB, stopband rejection 30 to 50 dB
0 dB
Incoming
Spectrum
Transmitte
d
Spectrum
Reflected
Spectrum
30 dB
Layers
Substrate
1535 nm
1555 nm
Multiplexers (MUX) /
Demultiplexers (DEMUX)
Key component of wavelength-division multiplexing technology
(DWDM)
Variety of technologies
Cascaded dielectric filters
Cascaded FBGs
Phased arrays (see later)
High crosstalk suppression essential for demultiplexing
Common
Add / Drop
Add i
Passband
Filter reflects i
Optical Amplifiers
Causes of Attenuation
The attenuation or losses ocurring in
glass fibre are due to:
absorption
scattering
geometrical effects
All these phenomena contribute to the
degradation of the fibre transmission
Light Absorption
Light is absorbed in glass by:
the material itself
the impurities present in glass
the atomic defects present in
glass
Intrinsic Absorption
Erbium Doped-Fiber
Amplifiers (EDFAs)
Few meters of optical fiber doped with a few parts per
million of the rare earth element erbium
Optical signal is injected into this fiber
With the light from a special pump laser that is
designed to excite the erbium ions
Erbium Properties
Erbium: rare element with phosphorescent properties
Photons at 1480 or 980 nm activate
electrons into a metastable state
Electrons falling back emit light in
the 1550 nm range
540
Spontaneous emission
670
Occurs randomly (time constant ~1 ms)
820
Stimulated emission
980
By electromagnetic wave
Metastable
Emitted wavelength & phase are
1480
state
identical to incident one
Ground state
Amplified
spontaneous
emission (ASE)
Erbium Doped-Fiber
Amplifiers (EDFAs)
Erbium Doped-Fiber
Amplifiers (EDFAs)
Erbium Doped-Fiber
Amplifiers (EDFAs)
Erbium has several energy levels, but its ions are usually in the ground state
(unexcited)
The ions can be excited with a 1480-nanometer pump laser into the first excited
state
Left there for long enough, they will fall back down to the ground state
Falling back to the ground state, the ions have some extra energy to get rid of,
which they each give out as a photon (a single particle of light)
Ions fall back to the ground state and give out photons without any aid
whatsoever
Spontaneous emission because
Such spontaneous emission can build up in the amplifier and is known as
Amplified spontaneous emission or ASE
ASE is an undesirable effect and adds noise to the amplifier system
Erbium Doped-Fiber
Amplifiers (EDFAs)
Incoming Optical signal at around 1550nm :
Causes some of those excited ions to fall down to the
ground state and give out a photon each
Stimulated emission because the signal is directly
causing the photons to be emitted
Emitted photons are at the exact same wavelength as
the signal and so are now a part of the signal.
Signal now has more photons representing it than
before, so it has been Amplified
Process continues down the few meters of this fiber,
until lots of photons have joined the signal photons and
the signal has been greatly amplified
Erbium Doped-Fiber
Amplifiers (EDFAs)
Amplification :
At several wavelengths around 1550nm
Amplification can be achieved via C-Band EDFA designs
(Conventional-band 1530nm and 1580nm)
Amplification can be achieved via L-Band EDFA designs
(Long-band 1580nm and 1610nm)
The amount of amplification varies at different wavelengths:
Much effort put into EDFA designs to achieve similar
levels of amplification at all wavelengths
Gain flattening
Erbium Doped-Fiber
Amplifiers (EDFAs)
Erbium Doped-Fiber
Amplifiers (EDFAs)
The 980nm pump excites the erbium ions into a much higher
state than the 1480nm pump :
Ions only stay in that higher state for a very short period of time
(maybe nanoseconds)
Moving down to the next state.
Stick around there for several milliseconds
Erbium Doped-Fiber
Amplifiers (EDFAs)
Key Points
Few meters of regular fiber doped with a tiny amount of erbium
Signal passes through this fiber along with light from pump laser
Pump laser excites erbium ions, which give extra energy to signal
Amplification possible at many wavelengths around 1550nm
Pumping with 980nm laser is more effective than 1480nm pumping
Commonly used in submarine systems, and increasingly on land
Optically transparent
Unlimited RF bandwidth
Wavelength transparent
Input
1480 or
980 nm
Pump
Laser
Coupler
Isolator
Output
C-band
Gain [dB]
S(blue) L(Red)
1500
1520
1540
1560
1580
1600
Wavelength
Blue: 480 nm
Red: 633 nm
Output Spectra
+10 dBm
Amplified signal
spectrum
(input signal saturates
the optical amplifier)
ASE spectrum when
no input signal is
present
-40 dBm
1525 nm
1575 nm
Time-Domain Properties
Input Signal
on
Turn-On
Overshoot
of
on
~ 10 .. 50 s
Gain x Signal
ASE level
(signal absent)
ASE level
(signal present)
~ 0.2 .. 0.8 ms
of
on
Gain (dB)
40
30
Input:
-30 dBm
-20 dBm
-10 dBm
20
10
1520
-5 dBm
1540
1560
1580
Wavelength (nm)
7.5
5.0
1520
1540
1560
Wavelength (nm)
1580
Gain Compression
Total output power:
Amplified signal + ASE
EDFA is in saturation if almost
all Erbium ions are consumed
for amplification
Total output power remains
almost constant
Lowest noise figure
Preferred operating point
Power levels in link stabilize
automatically
Total P
Max
-3 dB
out
Gain
-30
-20
-10
in (dBm)
Compensation techniques
Signal pre-emphasis
Gain flattening filters
Additional doping of amplifier with Fluorides
Gain Competition
Total output power of some EDFA remains almost
constant even if input power fluctuates significantly
If one channel fails (or is added) then the remaining
ones increase (or decrease) their output power
Output power
after channel
one failed
Equal power of
all four
channels
EDFA Categories
In-line amplifiers
Installed every 30 to 70 km along a link
Good noise figure, medium output power
Power boosters
Up to +17 dBm power, amplifies transmitter output
Also used in cable TV systems before a star coupler
Pre-amplifiers
Low noise amplifier in front of receiver
Remotely pumped
Electronic free extending links up to 200 km and more
(often found in submarine applications)
TX
Pump
RX
Pump
Security Features
Input power monitor
Turning on the input signal can cause high output power spikes
that can damage the amplifier or following systems
Control electronics turn the pump laser(s) down if the input signal
stays below a given threshold for more than about 2 to 20 s
Backreflection monitor
Open connector at the output can be a laser safety hazard
Straight connectors typically reflect 4% of the light back
Backreflection monitor shuts the amplifier down if backreflected
light exceeds certain limits
Raman Amplification
Raman Amplification
Raman Amplification
Raman Amplification
Requires no special doping in the optical fiber
Distributed amplification
happens throughout the length of the actual transmission fiber
Versus all in one place in a small box (as with an EDFA for
example).
Into the same fiber that is carrying the signal
Add a high-power pump wavelength (say of a few watts power)
Will amplify the signal along many kilometers of fiber until the pump
signal eventually fades away
Pumping at the beginning of the fiber = forward pumping or co-pumping
Pumping from the far end of the fiber known as backward pumping
or counter-pumping (usually better performance)
combination of the two (co-counter pumping)
Different pump wavelengths to achieve the required amplification at
every wavelength
Raman Amplification
Key Points
Amplifies in the actual transmission fiber, over many km
Lower wavelength (higher energy) pump light scatters
from atoms in fiber
Scattered light loses energy, then has higher
wavelength, joining signal
Several pump wavelengths needed for flat amplification
of WDM system
Co-pumping from fiber start, counter-pumping from end;
co-counter is both
Transmission Effects
Optical non-linearities:
Stimulated Brillouin Scattering (SBS)
Stimulated Raman Scattering (SRS)
Four Wave Mixing (FWM)
Self Phase Modulation (SPM)
Dispersion properties:
Chromatic Dispersion (CD)
Polarization properties:
Polarization Mode Dispersion (PMD)
Scattering
Kerr Effect
Non-Linear Effects
Nonlinear Effects
Linear system
Output is directly proportional to the input
Input increases, Output grows to same degree
Nonlinear system
No straight-line relationship, but bit of a curve
Output does not scale linearly with Input
Input signal doubles in power, Output is less than double
Nonlinear Effects
Nonlinear Effects
Nonlinear effects become noticeable at
High optical powers
WDM systems
Higher and higher bit-rates being used
Low bit-rate systems they can often be ignored completely
Nonlinear Effects
Nonlinear Effects
Key Points
Output no longer scales linearly with input
Nonlinear effects more significant with high optical powers
Intensity-dependence of refractive index the Kerr effect
Kerr effects : self-phase modulation, cross-phase
modulation, four-wave mixing
Scattering effects: stimulated Raman and stimulated
Brillouin scattering
Some useful effects e.g. Raman amplification but most
are undesirable
Kerr Effect
Optical Kerr effect: n = n0 + n2 I (intensity-dependent
refractive index)
non-linear phase shift
self-phase modulation (SPM), i.e.
spectral broadening
temporal pulse distortion (when
combined with chromatic dispersion)
Linear regime
Non-linear regime
f113
f213
f123
f112
f223
f312
f132
f221
f231
f321
f332
f331
Dispersion Properties
Dispersion
Dispersion
Dispersion
After traveling through many km of optical fiber, it is
possible for pulses to spread out in time :
Tightly defined 100 ps duration pulses at the beginning
could possibly "smear" out into 120 ps, 150 ps, or even 200
ps pulses
Increasingly difficult, if not impossible, to distinguish two
adjacent bits, as they will have smeared into each other
The problem becomes worse at higher bit rates where the
duration of the pulses (the "bit-period") becomes even
smaller
Types of Dispersion
Intermodal Dispersion
Intramodal Dispersion
There are two contributions to the intramodal
dispersion:
the material dispersion of the glass
the waveguide dispersion
Waveguide dispersion is usually smaller than
material dispersion and depends on the index
profile of the fibre
C. Dispersion
ps/(nm km)
SMF
NZDSF
DSF
20
10
0
1200 1300 1400 1500 1600 1700
-10
Chromatic Dispersion
The most well understood forms of dispersion occur
because different wavelengths of light travel at slightly
different speeds in optical fiber
Material dispersion causes different wavelengths to travel at
different speeds due to the variation of refractive index of the fiber
core with wavelength
Waveguide dispersion - proportion of the light also travels in the
cladding of the fiber, which has a different refractive index again
and therefore propagates light through it at a different speed to the
core
"chromatic dispersion"
Chromatic Dispersion
In a Wavelength Division Multiplexing (WDM) system it is not
necessarily a problem for each different signal wavelength to travel
at a different speed, as they are demultiplexed at the end and
detected separately anyway
Chromatic dispersion is still a problem :
individual signal wavelengths contains a range of different
wavelengths
Standard Fabry-Perot lasers give out a broad range of
wavelengths, actually spanning as much as 2 nm or so in total
Distributed Feedback (DFB) lasers have much purer emission, but
nevertheless still contain maybe a 0.2 nm range of wavelengths
Chromatic Dispersion
Chromatic Dispersion
A light pulse will typically have a curved shape when
looking at its intensity with respect to time
At each of these points in time the pulse could contain the
whole spread of wavelengths being emitted
With positive chromatic dispersion :
Shorter wavelengths travel faster than the longer ones
After a while, the shorter wavelengths have moved
forward in time with respect to the longer wavelengths
Beginning of the pulse in time and the end of the pulse in
time have spread further apart and the pulse has
experienced
chromatic dispersion
Chromatic Dispersion
Measured in ps/nm/km
Every km of fiber traveled through, a pulse with a 1 nm
spread of wavelengths will disperse by 1 ps for a
dispersion of 1 ps/nm/km
With a 1 ps/nm/km chromatic dispersion, a 10-Gbit/s
pulse with a 0.2nm spectral width will have spread by a
whole bit period (100 ps) after 500 km of fiber and will
then be completely indistinguishable
Chromatic Dispersion
The amount of chromatic dispersion experienced in fiber :
Dependent on the wavelength at which light is being transmitted
Chromatic Dispersion
Chromatic dispersion does not limit the distance of optical systems to a
few hundred km
Create negative chromatic dispersion
longer wavelengths travel faster
Chromatic dispersion can be corrected, or "compensated
Use of such specially designed optical fibers
Dispersion slope" can make perfect compensation at all wavelengths
quite tricky
Systems utilising the low loss 1550 nm region of optical fibre can pick
a type of fiber that reduces the amount of chromatic dispersion
experienced from the very large 15 or so ps/nm/km in the standard type
Polarization Properties
Rx
Fast
PSP
"unpolarized Light
If the electric fields could all be lined up with each other,
then the same would be true for the magnetic fields
"linearly polarized
Conclusion
A compromise has to be found between two conflicting requirements in
WDM systems:
increase channel spacing to reduce four wave mixing
between channels and to facilitate add and drop function
decrease channel spacing due to the limited optical gain
bandwidth of optical amplifiers and to reduce Raman
scattering between channels
Scattering
Scattering is a complex phenomenon arising from
the interaction between an electromagnetic
radiation such as light with small particles or
molecules.
The incident radiation is partially deflected in all
directions by the small particles.
There are several types of scattering depending on
what happens to the incident radiation. We will only
mention:
Rayleigh scattering
Mie scattering
Rayleigh Scattering
Rayleigh Scattering
Rayleigh scattering occurs when radiation "hits" a
spherical particle or a molecule whose diameter is
smaller than the wavelength of the radiation.
Some of the radiation continues in its original
direction but some of it is deflected or scattered in
other directions without change in energy: the collision
is called elastic.
The power of Rayleigh scattering is inversely
proportional to the fourth power of the wavelength.
Short wavelength radiation is scattered more than long
wavelength radiation
Mie Scattering
Mie Scattering
Commercial Designs
EDF
EDF
Output
Isolator
Input
Isolator
Pump Lasers
Input
Monitor
Telemetry &
Remote Control
Output
Monitor