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Outline:
Fibre and fibercharacteristics Transmitters Modulation ReceiversPassive couplersFiltersTransmission 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:
– θkappe= 90° (for total refraction)
• Refractive index:
• Critical angle for total reflection:
kappekappekjernekjerne nn sinsin
kjerne
kappekritisk n
n1sin
1 luftmateriale
vakummateriale n
c
cn
5
Andreas Kimsås, Optiske Nett
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
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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:
• 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.
NAaV 2
210
2VmV
7
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
8
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
10
Transmission window and applied wavelength bands
Figur fra “Fiber Optic Communication Systems”, G. Agrawal, Wiley
11
Dispersion• Pulse spreading when propagating through the fibre.• To much spreading results in intersymbol- interference• 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.
12
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.
13
Chromatic dispersion
Figure: S. Bigo, Alcatel: Talk at Norwegian electro-optics meeting2004
14
Point to point fibre-optical system
Transmitter(Laser+
modulator)
Receiver(fotodiode +
amplifier)Fibre
Important limitation:
Attenuation:Some light being absorbed in fibre
Dispersion:
Time Time
Pulse Spreading
Illustration: Lucent Technologies
Speed of light depends on wavelength
15
Optical transmitters - LASER
1. Active laser medium2. Laser pumping energy3. Mirror (100%)4. Mirror (99%)5. Laser beam
Constructive interference
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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
• Cavity length decides frequency1) Forward biasing create population Inversion2) Electrons combine with holes, releases photons3) Stimulated emission
17
Stimulated emission
Ei
Ef
18
Stimulated emission
A chain reaction!
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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 receiversPhotodiode:
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
23
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
24
fibre-optical transmission at longer distances
Transmitter(Laser+
modulator)
Receiver(photodiode +
aplifierFibre
Must be compensated:
Attenuation:Some light being absorbed
Dispersion:
Time Time
Pulse Spreading
Illustration: Lucent Technologies
Light of speed wavelength dependent
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
30
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).
31
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
33
Available wavelength range depends on amplifier technology
0
0,1
0,2
0,3
0,4
0,5
1200 1300 1400 1500 1600
Wavelength (nm)
Lo
ss (
dB
/km
)
EDFAEDFAC - bandC - band
1530-15621530-1562
EDFAEDFAL - bandL - band
1570-16001570-1600
ALTERNATIVE AMPLIFIER TECHNLOGIES: RAMAN AND SOA
PDFAPDFA1300 nm1300 nm
Commercially available Still subject to research
34
Long distance fibre-optical transmission
Transmitter(Laser+
modulator)
Receiver(photodiode +
amplifier)Fibre
To be compensated:
Dispersion:
Time Time
Pulse Spreading
Illustration: Lucent Technologies
Speed of light is wavelength dependent
EDFA
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)
Receiver(fotodiode +
amplifier)Long Fibre
Compensation of amplitude and dispersion
EDFADCF
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
52
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
0
0,1
0,2
0,3
0,4
0,5
1200 1300 1400 1500 1600
Wavelength (nm)
Lo
ss (
dB
/km
)
25 THz
Optical fibre loss spectre
53
Fibre optical transmission system
ReceiverOptical fibreSingle modus
Electric input data
Laser & modulator
Amplifieror
regeneratorOptical fibreSingle modus
Electric output data
Wavelength Division Multiplexing (WDM) transmission system:Add lasers & modulators + receivers
Time Division Multiplexing = TDM
54
Fibre optical transmission system
ReceiverOptical fibreSingle modus
Electric input data
Laser & modulator
Amplifieror
regeneratorOptical fibreSingle modus
Electric output data
Polarisation multiplexing:Doubles capacity
PBS
PBSCombine! At Telenor 32 WDM X 2.5 Gb/s TDM
55
From regenerator to optical amplifier
11 11
11 1111 1144 44
11
22
33
44
11
22
33
44
Tidligere utbygging
RegeneratorTerminalFiber
Før: 1 kanal pr fiber
Optiskforsterker
MultiplekserDemultiplekser
2,5 Gb/s =30000
Opptil20 000 000
WDM: 4-128 kanaler
pr fiberNåværende utbygging
Wavelength Division Multiplexing(WDM), mangedobler kapasitet i fiber
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)
– = 10 ps for 10 Gb/s, = 2.5 ps for 40 Gb/s
• PMD new fibre <= 0.1 ps/ (Alcatel standard fibre)• Distance limits of new fibre
TDM Bitrate
2.5 Gb/s 10 Gb/s 40 Gb/s 160 Gb/s 640 Gb/s
PMD Max length
1.6*105 Km 10,000 Km 625 Km 40 Km 2.5 Km
= DPMD/
km
L
- Can be compensated, currently expensive- PMD on installed fibre can be as high as e.g. 2 ps/- Distance limit: 1.5 km at 40 Gb/s
km
58
The good, The bad, The Ugly
Non-linear effects:
Impact & Applications
Superhero! Enables signal processing components
Ugly pulses!Nightmare! For system designers
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
Frequencyff11 ff22-- ff22 ff2222 -- ff1122 ff11
Original Wavelengths
New Wavelengths
Distortion by non-linear effects• Four-Wave Mixing (FWM)
– Intermodulation products
– In WDM and as intrachannel products for high TDM rates
Number of new wavelengths = N2(N-1)/2where N = number of original wavelengths
N
248
FWM Products
224
224
Especially a problem in WDM!
Lucent-97
61
Distortion by non-linear effects• Suppress by
– Moderate channel powers
– Avoid zero dispersion fibre
– Polarisation interleaving of channels
Lucent-97
0
Dispersion-Shifted Fiber (25 km)
Wavelength (1 nm/div.)
1546.55
Signals
Especially a problem when D = 0!
N
248
FWM Products
224
224
Mixing products
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
Output spectrum
Input spectrum
150 nm10 THz 20
Ram
an G
ain
Frequency shift
Power is shifted to upper from lower channels
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