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1
Outline:
Fibre and fibercharacteristics
Transmitters
Modulation
Receivers
Passive couplers
Filters
Transmission systems and optical networks
2
Optical fibre, characteristic
• Large bandwidth (theoretical 50 THZ)
• Low attenuation (0,2 dB/km at 1550nm).
• Physical size beneficial, light and thin, simplifies
installation
• Splicing and mounting connectors more complex
• Immune to electromagnetic interference
• Environmentally friendly material (sand!).
3
Propagation through fibre
• Lightpulses are reflected in the core when hitting the
cladding => approximately zero loss
Andreas Kimsås, Optiske Nett
4
Snells law
• Snells law:
nkjerne  sin  kjerne  nkappe  sin  kappe
– θkappe= 90° (for total refraction)
• Refractive index:
nmateriale 
cvakum
 nluft  1
cmateriale
• Critical angle for total reflection:
 nkappe 


 nkjerne 
 kritisk  sin 1 
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Multi-Mode vs. Single Mode Fibre
Multi mode
•Core > 50 um.
•Light being reflected
with different angle
travels different
distances
•Pulse spreading
Single Mode
• Core < 10 um =>
single mode
• Less pulse spreading
Andreas Kimsås, Optiske Nett
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Fibermodes
•
Multimode:
–
–
–
Core diameter typical 50-100μm.
NA = Numerical Aperture
Number of modes (m) depends on normalized frequency (V), a = core-diameter, NA:
V 
•
2

 a  NA
V2
V  10  m 
2
Singlemode
–
–
–
Core-diameter typically 10μm.
Criteria for single-mode is V < 2.4048
No mode-dispersion gives better transmission properties than multimode more
difficult to couple to the lightsource.
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Coupling light into the fibre
• Single modus
–
–
–
–
Coupling into the tiny 10 micrometer core is demanding
Lining up the light-source is a significant part of the production cost
Laser is preferred light-source
LED has too large beam
• Multimode
– Larger core diameter simplifies coupling
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Attenuation in the fibre
• Rayleigh-scattering:
–
–
–
–
Dominant
Inhomogenities in the fibre and the structure of the glass.
Occurs when the lightbeam hits the inhomogenities in the glass
Sets the theoretical lower limit of fibre attenuation L≈1/λ^4
• Absorption:
– Metal-ions, especially hydroksyliones (OH¯) at approx. 1400nm.
– Pollution from production, or doped material for achieving the
optical properties desired.
• Radiation loss:
– E.g variations in core-diameter and inhomogenities between the
core and the cladding, e.g. Microbends or airbubbles.
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Attenuation in the fibre
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Transmission window and applied
wavelength bands
Figur fra “Fiber Optic Communication Systems”, G. Agrawal, Wiley
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Dispersion
• Pulse spreading when propagating through the fibre.
• To much spreading results in intersymbolinterference
• Limits the maximum transmissionrate through the
fibre.
• Three types of dispersion:
– Modi-dispersion: Light travelling in different modi undergoes
different delays through the fibre. Not present in SM!
– Material-dispersion (chromatic): Refractive index is function of
wavelength
– Waveguide-dispersion: Propagation of different wavelengths
depends on the characteristic of the waveguide, e.g. Index,
geometry of core and cladding.
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Zero dispersion
• At 1300 nm in standard fibre
– Material (chromatic) dispersion is close to zero at 1300 nm
– Not minimum loss
• ~ 1500 nm in dispersion shifted fibre
– Manufactured for zero dispersion in 1500 nm region
– Design core and cladding to give negative waveguide dispersion
– At a specific wavelength, material and waveguide dispersion will
result in zero total dispersion.
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Chromatic dispersion
Figure: S. Bigo, Alcatel: Talk at Norwegian electro-optics meeting
2004
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Point to point fibre-optical system
Transmitter
(Laser+
modulator)
Receiver
(fotodiode +
amplifier)
Fibre
Important limitation:
Attenuation:
Dispersion:
Some light being absorbed in fibre Speed of light depends on wavelength
Pulse Spreading
Time
Time
Illustration: Lucent Technologies
15
Optical transmitters - LASER
Constructive interference
1. Active laser medium
2. Laser pumping energy
3. Mirror (100%)
4. Mirror (99%)
5. Laser beam
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Semiconductor laser
• Most common transmitter in optical communication
– Compact design
• Material give frequency ranges (Fermi-Dirac distr.)
•
– Population inversion: Electrons in n-region and holes in P-region
– Electrons in n-region (conduction band) combine with holes
(valence band) in p-region
1) Forward biasing create
Cavity length decides frequency
population Inversion
2) Electrons combine
with holes, releases photons
3) Stimulated emission
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Stimulated emission
Ei
Ef
18
Stimulated emission
A chain reaction!
19
Optical transmitters - LED
20
Light Emitting Diodes (LEDs)
• Not sufficient in long distance fibre transmission
– Wideband source => dispersion
– Power is lower than for a laser
• Employed at shorter distances
– Maximum a few hundred meters, depends on bitrate
21
Optical receivers
Photodiode:
Avalanche diode = Higher sensitivity
22
Modulation
• OOK modulation (on-off-keying)
– NRZ (No Return Zero) most often used
– RZ (Return Zero), some use
– More advanced modulation formats being launched for 40 and 100
Gb/s pr. Channel systems.
• Employ phase and/or polarisation
– Phase and polarisation modulation not employed in systems for < =
10 Gb/s bitrate.
• External modulation, e.g. Employing external
modulator: MZ interferometer
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Modulation II
• Direct modulation of laser
– Switch laser on and off
– Difficult to fabric laser that can be switched at high speed,
simultaneously having proper transmission characteristics.
– Undesirable frequency variations (chirp) and Limited extinction ratio
• External modulation
–
–
–
–
Mach-Zehnder interferometer
External component being fed electrically
May be Integrated with laser
High extinction ratio prolongs transmission distance
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fibre-optical transmission at longer distances
Transmitter
(Laser+
modulator)
Receiver
(photodiode +
aplifier
Fibre
Must be compensated:
Attenuation:
Some light being absorbed
Dispersion:
Light of speed wavelength dependent
Pulse Spreading
Time
Time
Illustration: Lucent Technologies
25
What is a long distance?
• 100 m?
• 10 Km?
• 1000 Km?
26
What is a long distance?
• 100 m?
– LAN
• 10 Km?
– Access network
• 1000 Km?
– Transport network
27
Long distance optical system
• Attenuation must be compensated
– Regeneration
– Attenuation
• Dispersion must be compensated
– Dispersion compensation employing fibre
– Electronic compensation
28
Regeneration
• 1R regeneration = Amplification (Reamplification)
– Usually an optical amplifier
– Amplifies the signal without conversion to electrical
– Typically transparent for signal (shape, format and modulation)
• 2R Reamplification & Reshaping:
– Reshapes the flanks of the pulse as well as the floor and roof of the
pulse, removes noise.
– Usually electronic
– Optical solutions still subject to research
• 3R Reamplification & Reshaping & Retiming:
– Synchronisation to original bit-timing. (regeneration of clock)
– Usually involves electro-optic conversion
– Optical techniques in the research lab.
29
Optical amplifier characteristics
• Amplifier parameters:
–
–
–
–
–
Gain
Bandwidth of gain
Saturation level
Polarisation sensitivity
Amplifier noise
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Optical fibre amplifier
• Doped-fiber amplifier:
– Doping = Inserting small amounts of one material into a second
material
– An Erbium doped silica fibre is fed with a pump-signal together with
the original signal.
– Doped atoms are being excited to a higher energy level
– The pumping signal is a high power signal with a wavelength lower
than the wavelength to be amplified (typically 980 nm or 1480 nm
fore EDFA).
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Erbium Doped Fiber Amplifier
(EDFA)
• Widely deployed in optical networks
32
Optical amplifiers overview
• Semiconductor-laser amplifier:
– Signal is sendt through the active region of the semiconductor
– Stimulated emission results in a stronger signal
– May be integrated with other components (e.g. Output of a switch
or a transmitting laser)
– Widely employed in research projects on all-optical switches.
– Recently employed in commercially available compact tunable
laser-modules
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Available wavelength range depends on
amplifier technology
Loss (dB/km)
0,5
0,4
PDFA
1300 nm
EDFA
C - band
1530-1562
0,3
EDFA
L - band
1570-1600
0,2
0,1
ALTERNATIVE AMPLIFIER TECHNLOGIES: RAMAN AND SOA
0
1200
1300
1400
1500
1600
Wavelength (nm)
Commercially available
Still subject to research
34
Long distance fibre-optical transmission
Transmitter
(Laser+
modulator)
Receiver
(photodiode +
amplifier)
EDFA
Fibre
To be compensated:
Dispersion:
Speed of light is wavelength dependent
Pulse Spreading
Time
Time
Illustration: Lucent Technologies
35
Dispersion in transmission fibre
•
•
•
•
Dispersion depends on fibretype
G652, “Standard fibre” -17 Ps/nm*km @ 1550 nm
Dispersion shifted fibre: 0 dispersion @ 1550 nm
Non – Zero (NZ) dispersion shifted fibre: -3 to -6
Ps/nm*km
36
Dispersion Compensating Fibre
(DCF)
• Negative dispersion compared to transmission fibre
• Much higher dispersion/km => Shorter fibre than
transmission fibre required for achieving zero
dispersion
37
Long distance fibre-optical transmission
Transmitter
(Laser+
modulator)
EDFA
Long Fibre
Compensation of amplitude and dispersion
DCF
Receiver
(fotodiode +
amplifier)
38
Noise from optical amplifiers
• Amplified Spontaneous Emission (ASE)
– Photons are being emitted without stimulation
• Noise distributed through the entire amplification
band
• May be limited through filtering out the wavelengths
where amplification is desirable
• Optical filter needed
39
Interference between two light
sources
• Constructive
– Light in phase results in addition and
increased intensity
• Destructive
– Light out of phase (180 degrees) results in
extinguished pulse
40
Mach-Zehnder interferometer
• At given frequencies the delay equals duration of a
wavelength => constructive interference
• At given frequencies the delay equals duration of half
a wavelength => destructive interference
41
Mach-Zehnder based modulator
• Modulates phase of one or both paths
– E.g voltage on => phase being changed => extinguished pulse
Electronic modulation
42
Series of Mach-Zehnder
• Applicable as an optical filter
• Adjustable delay enables adjustable frequency
– A chain of filters helps sharpening up the filter characteristic
– Very fast adjustment-time: As low as 100 ns
– High attenuation (multiple stages)
43
Etalon based adjustable filter
• Cavity with parallell mirrors in each end
• Free spectral range (FSR)
– Periode between repetition of pass-band
• Finesse
– FSR/width of channel
• Fabry-Perot
– Mechanical, large range adjustable, slow adjustment - 10 ms.
Adjustable to n wavelengths
44
Acusto-optical filter
• RF waves converted to sound-waves in a piezo
electrical crystal (transducer)
• Soundwaves results in mechanical movements
• Mechanical movements in crystal alters refractive
index
• The crystal then works as a grating
• Adjustment within 10 Micro-seconds
• Possible to filter out several frequencies
simultaneously by sending several RF waves with
different frequency to a transducer
45
Filters with fixed wavelength
• Gratingbased filters e.g. Diffraction gratings
– Flat layer of transparent material, constructive interference in
bumps for a given wavelength, destructive for other wavelengths
• Arrayed Waveguide Grating (explained later)
46
Optical couplers
• One or more fibers in, several fibres out
– Divides the optical signal on several fibres.
• Signal power is divided on the output-fibres
• Splitting ratio is varying
– 50/50, 50 % on each of two fibres
– 10/90, 10 % in one, 90 % in a second.
• Attenuation from input to output depends on splitting
ratio
– 50/50 splitter results in 3 dB attenuation (halving the power)
Combiner
Splitter
47
Optical couplers
• Coupler employed as splitter:
– One input divided on two or more outputs
– Splitting ratio (α) indicates share of power to each output
– 1x2 splitter is typical 50:50, however some power is being reflected (40-50
dB weaker than payload signal). This is called return-loss.
– Connection-loss between fibre and coupler also attenuates the signal
• Coupler employed as combiner:
– Opposite use as a splitter; several inputs, single output.
– Returnloss and connection-loss as for the splitter
48
Arrayed waveguide Grating
• 1 X N or N X N coupler divides the light on N
waveguides of different length
• Waveguides is then coupled together, resulting in
interference
• On each of the N outputs, constructive interference is
achieved for a specific wavelength and destructive
interference for the other wavelengths
49
Multiplexing/Demultiplexing
• Optical multiplexing: Couple several waveguides
together into a fibre.
• Optical demultiplexing: Separate wavelengths from
an input fibre into several output fibres with a single
wavelength in each.
• Is this useful?
50
Transmission systems and
aspects for optical networks
By: Steinar Bjørnstad
As part of the training course ”optical networks”
51
Overview
transmission, transmission
effects and limitations
• Wavelength Division Multiplexed (WDM) systems
• Give a brief introduction to limiting effects in optical
transmission systems
– Polarization Mode Dispersion
– Non-linear effects
– Limiting factors
WDM and bandwidth utilization
• Optical fibre has unique transmission properties
– 25 THz bandwidth available in low loss region
– Another 75 THz available (higher attenuation)
• How can we utilize the bandwidth?
– Electronic components can not process signals beyond ~100 GHz
Optical fibre loss spectre
0,5
Loss (dB/km)
52
0,4
0,3
25 THz
0,2
0,1
0
1200
1300
1400
1500
Wavelength (nm)
1600
53
Fibre optical transmission
system
Amplifier
Laser &
modulator
Electric
input data
or
Optical fibre
Optical fibre
Single modus regenerator Single modus Receiver
Time Division Multiplexing = TDM
Wavelength Division Multiplexing
(WDM) transmission system:
Add lasers & modulators + receivers
Electric
output data
54
Fibre optical transmission
system
Amplifier
Laser &
modulator
Electric
input data
or
Optical fibre
Optical fibre
Single modus regenerator Single modus Receiver
Combine! At Telenor
PBS 32 WDM X 2.5 Gb/s TDM
Polarisation
multiplexing:
Doubles capacity
PBS
Electric
output data
55
Wavelength Division Multiplexing
Frommangedobler
regeneratorkapasitet
to optical
(WDM),
i fiber
amplifier
2,5 Gb/s =
30000
Før: 1 kanal pr fiber
Terminal
11
11
11
11
44
Opptil
20 000 000
Regenerator
Fiber
11
11
44
WDM:
4-128 kanaler
pr fiber
11
Tidligere utbygging
Nåværende
utbygging
11
22
22
33
33
44
44
Optisk forsterker
Demultiplekser
Multiplekser
56
Polarisation Mode Dispersion
(PMD)
• Fibre has two principle states of polarization
– Light travels with different velocity in the two states
– Difference in arrival time: Differential Group Delay (DGD)
• Caused by elliptic fibre core
– Bad fabrication process
– Optical components may also cause PMD
• PMD is frequency dependent
• It varies with time
– Statistical process, Maxwellian probability density
– PMD: Mean value over time of DGD (expressed in picoseconds)
57
PMD impact
• Maximum tolerable PMD
– 10 % - 20 % of bit period (Depends on modulation format)
– Dt = 10 ps for 10 Gb/s, Dt = 2.5 ps for 40 Gb/s
km
• PMD new fibre <= 0.1 ps/
• Distance limits of new fibre
(Alcatel standard fibre)
Dt = DPMD/ L
TDM
Bitrate
2.5 Gb/s
10 Gb/s
40 Gb/s
PMD Max
length
1.6*105 Km 10,000 Km 625 Km
160 Gb/s
640 Gb/s
40 Km
2.5 Km
- Can be compensated, currently expensive
- PMD on installed fibre can be as high as e.g. 2 ps/ km
- Distance limit: 1.5 km at 40 Gb/s
58
The good, The bad,
The Ugly
Non-linear effects:
Impact & Applications
Superhero! Enables signal Nightmare! For
processing components
system designers
Ugly pulses!
59
Non-linear effects
• Scattering effects in fibre medium
– Stimulated Brillouin Scattering (SBS): Backward scattering from
acoustic waves
– Stimulated Raman Scattering (SRS): Interaction of light waves with
phonons (molecular vibrations)
• Fibre refractive index dependence on optical power
– Four Wave Mixing (FWM)
– Self-Phase Modulation (SPM)
– Cross-Phase Modulation (XPM)
60
Distortion by non-linear effects
• Four-Wave Mixing (FWM)
– Intermodulation products
– In WDM and as intrachannel products for high TDM rates
Original Wavelengths
2 f1 - f2
2 f 2 - f1
f2
f1
Frequency
New Wavelengths
Number of new wavelengths = N2(N-1)/2
where N = number of original wavelengths
Especially a
problem in WDM!
FWM
N Products
2
4
8
2
24
224
Lucent-97
61
Distortion by non-linear effects
• Suppress by
– Moderate channel powers
– Avoid zero dispersion fibre
– Polarisation interleaving of channels
Dispersion-Shifted Fiber (25 km)
Signals
Mixing products
Especially a
problem when D = 0!
0
1546.55
Wavelength (1 nm/div.)
FWM
N Products
2
4
8
2
24
224
Lucent-97
62
Raman amplifiers principle
• Using the transmission fibre as a gain medium
• Pumping the fibre forwards and/or backwards
• Coupled in through couplers or multiplexers
Sender
(Laser+
modulator)
Mottaker
(fotodiode +
forsterker)
Fiber
Laser
Pump
forward
Laser
Pump
Backward
63
Stimulated Raman
Scattering
(SRS)
• SRS in high channel count WDM systems
– Higher wavelengths experience gain
– Lower wavelengths attenuation
Power is shifted
to upper from
lower channels
Raman Gain
150 nm
10 THz
20
Input spectrum
Output spectrum Frequency shift Stolen-79
64
Raman amplifiers
• Decreases noise
• Increases bandwidth
• High pump powers needed
– High demands for installation
• Expensive (very)
• Not widely deployed
65
Raman amplification
benefits