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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 5 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 6 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. 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. 9 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 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. 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 meeting 2004 14 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 16 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 17 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 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 + 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 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 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