Transcript Ingen lysbildetittel
1 Outline:
Fibre and fibercharacteristics Transmitters Modulation Receivers Passive couplers Filters Transmission systems and optical networks
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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!).
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Propagation through fibre
• Lightpulses are reflected in the core when hitting the cladding => approximately zero loss
Andreas Kimsås, Optiske Nett
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Snells law
n kjerne
sin • Snells law:
kjerne
– θ kappe = 90 ° (for total refraction)
n kappe
sin
kappe
• Refractive index:
n materiale
c vakum c materiale
n luft
1 • Critical angle for total reflection:
kritisk
sin 1
n kappe n kjerne
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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 V
10
m
V
2 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 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.
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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) Fibre Receiver (fotodiode + amplifier) Important limitation: Attenuation: Some light being absorbed in fibre Dispersion: Speed of light depends on wavelength
Pulse Spreading Time Time
Illustration: Lucent Technologies
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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 • Cavity length decides frequency 1) Forward biasing create population Inversion 2) Electrons combine with holes, releases photons 3) Stimulated emission
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Stimulated emission
E i E f
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Stimulated emission
A chain reaction!
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Optical transmitters - LED
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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
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Optical receivers
Photodiode: Avalanche diode = Higher sensitivity
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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) Fibre Receiver (photodiode + aplifier Must be compensated: Attenuation: Some light being absorbed Dispersion: Light of speed wavelength dependent
Pulse Spreading Time Time
Illustration: Lucent Technologies
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What is a long distance?
• 100 m?
• 10 Km?
• 1000 Km?
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What is a long distance?
• 100 m?
– LAN • 10 Km?
– Access network • 1000 Km?
– Transport network
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Long distance optical system
• Attenuation must be compensated – Regeneration – Attenuation • Dispersion must be compensated – Dispersion compensation employing fibre – Electronic compensation
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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.
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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
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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
0,5 0,4 0,3 0,2 0,1 0 1200 PDFA 1300 nm EDFA C - band 1530-1562 EDFA L - band 1570-1600 ALTERNATIVE AMPLIFIER TECHNLOGIES: RAMAN AND SOA 1300 Commercially available 1400 1500 Wavelength (nm) Still subject to research 1600
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Long distance fibre-optical transmission
Transmitter (Laser+ modulator) EDFA Fibre To be compensated: Dispersion: Speed of light is wavelength dependent
Pulse Spreading
Receiver (photodiode + amplifier)
Time Time
Illustration: Lucent Technologies
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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
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Dispersion Compensating Fibre (DCF)
• Negative dispersion compared to transmission fibre • Much higher dispersion/km => Shorter fibre than transmission fibre required for achieving zero dispersion
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Long distance fibre-optical transmission
Transmitter (Laser+ modulator) Long Fibre EDFA Compensation of amplitude and dispersion DCF Receiver (fotodiode + amplifier)
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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
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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
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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
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Mach-Zehnder based modulator
• Modulates phase of one or both paths – E.g voltage on => phase being changed => extinguished pulse Electronic modulation
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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)
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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
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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
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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)
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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
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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
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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
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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?
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Transmission systems and aspects for optical networks
By: Steinar Bjørnstad As part of the training course ”optical networks”
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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
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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?
0,5
– Electronic components can not process signals beyond ~100 GHz
Optical fibre loss spectre 0,4 0,3 25 THz 0,2 0,1 0 1200 1300 1400 1500 Wavelength (nm) 1600
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Fibre optical transmission system
Laser & modulator Optical fibre Single modus Amplifier or regenerator Optical fibre Single modus Receiver Electric input data Time Division Multiplexing = TDM Electric output data Wavelength Division Multiplexing (WDM) transmission system: Add lasers & modulators + receivers
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Fibre optical transmission system
Laser & modulator Optical fibre Single modus Amplifier or regenerator Optical fibre Single modus Receiver Electric input data PBS Combine! At Telenor 32 WDM X 2.5 Gb/s TDM Polarisation multiplexing: Doubles capacity PBS Electric output data
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Wavelength Division Multiplexing From regenerator to optical amplifier
2,5 Gb/s = 30000 Terminal Fiber Før: 1 kanal pr fiber Regenerator Tidligere utbygging 1 1 1 4 4 WDM: 4-128 kanaler pr fiber Opptil 20 000 000 1 1 Nåværende utbygging 2 2 3 3 Optisk forsterker Multiplekser Demultiplekser
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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)
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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 (Alcatel standard fibre) • Distance limits of new fibre Dt = D PMD /
L
TDM Bitrate 2.5 Gb/s 10 Gb/s 40 Gb/s 160 Gb/s 640 Gb/s PMD Max length 1.6*10 5 Km 10,000 Km 625 Km 40 Km
- 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
2.5 Km
km
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The good, The bad, The Ugly
Non-linear effects: Impact & Applications
Superhero! Enables signal processing components Nightmare! For system designers Ugly pulses!
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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)
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Distortion by non-linear effects
• Four-Wave Mixing (FWM) – Intermodulation products – In WDM and as intrachannel products for high TDM rates
Original Wavelengths 2 f 1 - f 2 f 1 f 2 Frequency New Wavelengths 2 f 2 - f 1 Number of new wavelengths = N 2 (N-1)/2 where N = number of original wavelengths Especially a problem in WDM!
N FWM Products 2 4 8 2 24 224
Lucent-97
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Distortion by non-linear effects
• Suppress by – Moderate channel powers – Avoid zero dispersion fibre – Polarisation interleaving of channels
Dispersion-Shifted Fiber (25 km)
Signals
0 1546.55
Wavelength (1 nm/div.)
Mixing products
Especially a problem when D = 0! N FWM Products 2 4 8 2 24 224
Lucent-97
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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) Laser Pump forward Fiber Laser Pump Backward Mottaker (fotodiode + forsterker)
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Stimulated Raman Scattering (SRS)
• SRS in high channel count WDM systems – Higher wavelengths experience gain – Lower wavelengths attenuation 150 nm 10 THz 20
Power is shifted to upper from lower channels
Input spectrum Output spectrum Frequency shift Stolen-79
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Raman amplifiers
• Decreases noise • Increases bandwidth • High pump powers needed – High demands for installation • Expensive (very) • Not widely deployed
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