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Les Ondes Optiques Riad Haïdar ONERA Département d’Optique Théorique et Appliquée

Les Ondes Optiques

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Les Ondes Optiques. Riad Haïdar. ONERA Département d’Optique Théorique et Appliquée. l = 100 mm f = 3 GHz. l = 1 mm f = 300 GHz. RADIO FREQUENCIES. MICRO WAVES. OPTICAL FREQUENCIES. Propagation : optical fibers. The ElectroMagnetic Spectrum. Fiber propagation. n 1 > n 2. n 2. - PowerPoint PPT Presentation

Text of Les Ondes Optiques

  • Les Ondes OptiquesRiad HadarONERA

    Dpartement dOptique Thorique et Applique

  • The ElectroMagnetic Spectrum

  • Fiber propagationn1 > n2n2

  • Reflection & Refraction

  • Un fil de verre seul peut aussi conduire la lumire...>> MAIS il n'y a pas confinement au voisinage du centre.>> Ce sera une fibre multimode.>> Le milieu extrieur peut influencer la propagation.

    Pour les communications sur de longues distances, onutilise une fibre monomode pour minimiser les problmes de dispersion.Remarque

  • Fiber performance

  • Optical attenuation in glass

  • Fiber attenuation (SiO2)

  • Other Fiber losses (1)pissure : fusion bout bout de deux fibres.>> Pertes typiques de 0,05 dB sur les fibres standard tlcoms.

  • Other Fiber losses (1)

  • Other Fiber losses (2)Courbures :

    >> Il y a modification des conditions de rflexion : un rayon totalement rflchi dans un guide droit, peut s'chapper par rfraction lorsque le guide est courb.

    >> Les fibres monomodes tolrent un rayon de courbure de l'ordre de 10 cm sans perte notable

    >> les pertes croissent exponentiellement avec la courbure.

  • Thorie du GuidageDeux approches sont possibles : la thorie gomtrique : (optique des rayons), valable pour des curs de dimensions beaucoup plus grandes que la longueur d'onde. la thorie ondulatoire : elle utilise les quations de Maxwell avec les conditions aux limites. Elle conduit la notion de mode, valable pour toute dimension de coeur.

    Pour des diamtres beaucoup plus grands que la longueur d'onde les deux thories se rejoignent.

  • Les rayons se divisent en deux types : Les rayons hlicodaux, qui ne coupent jamais laxe Les rayons mridionauxA chaque inclinaison qm correspond un groupe de rayons >> on parle de mode.Chaque mode est caractris par sa vitesse de phase VP lie l'angle qm par :>> Il y a autant d'inclinaisons que de modes.Thorie du Guidage

  • Modes & Rays

  • Mode intensity profilesOptical modes:Planar:

    Single-mode if V Fiber:

    Single-mode if V 2.405V number>> determines how many modes a fiber supports

  • Number of modesNumber of modes in step-index fiber if V > 2.405 Optical power in the cladding (gaine optique)for large values of V

  • Numerical Aperture

  • Numerical Aperture

  • DispersionLes diffrentes composantes du signal se propagent selon des temps diffrents dans la fibre optique.

    Deux causes essentielles : Diffrence de trajet (dispersion modale) Diffrence de vitesse (dispersion chromatique)

  • Dispersion (intermodal)

  • Deux longueurs donde l1 et l2 voyagent des vitesses diffrentesDispersion (chromatic)Bonne nouvelle : La dispersion s'annule vers 1300nm.

  • Dispersion in numbers

    Type de fibre

    rco / rgo

    Multimode

    100 / 140 m

    Monomode

    9 / 125 m

    Monomode

    9 / 125 m

    Longueur d'onde (nm)

    1300

    1300

    1550

    Dispersion M

    (ps/ nm/ km)

    22.000

    3,5

    20

  • Dispersion and frequency1010011000freq (MHz)Attenuation (dB/km)Coaxial

  • Les Solutions : Emploi de source monochromatique. Fibre dispersion dcale : dcaler le zro de dispersion vers 1550nm (car attnuation min 1550nm) aplatir la dispersion dans le domaine 1300 -1550 nm.

    Pour cela, on doit raliser des profils dindice spciaux cur segment (de type W par exemple) ou/et triangulaire.

    >> Une transmission sur 100 km sans rpteur est alors ralisable. Dispersion (chromatic)

  • Dbit = BP * Efficacit(Bits / s) = (Hz) * (Bits / s / Hz)

    1 BP disponible autour de 1550nm :~ 15 THz2 Meilleure isolation en l => Meilleure EfficacitAujourdhui :0,2 Bits/s/HzDici 24 mois :0,5 Bits/s/HzRadio :10 Bits/s/Hz

    Objectif long terme : 150 Tbits/sDispersion (enjeux)

  • Fiber typesrefractiveindex

  • Fiber classification (1)Core diameter (coeur)50 - 400 mCladding (gaine) 125 (500) m2nd coating (2nde gaine) 250 - 1000 mNA (ouverture numrique)0.16 - 0.5Attenuation1 - 4 dB/kmBandwidth6 - 25 MHz.kmApplicationShort distance, low costlimited bandwidthMM-SI: Multi Mode - Step Index fiber

  • Fiber classification (2)Core diameter50 m standardCladding125 m2nd coating200-1000 mNA0.2 - 0.3Attenuation1 dB/km (1300 nm)Bandwidth150 MHz.km - 2 GHz.kmApplicationMedium distance communicationLED/Laser sourcesMM-GI: Multi Mode - Graded Index fiber

  • Fiber classification (3)Core diameter3-10 mCladding50-125 m2nd coating200-1000 mNA~0.1 (not used)[email protected] - [email protected] dB/kmBandwidth>> 500 MHz.kmApplicationLong distance communicationLasers, standard fiberSM-SI: Single Mode - Step Index fiber

  • G.652 : fibre monomode standard (SMF)Dispersion 17ps/nm/km 1550nm>> Faible dbit

    G.653 : fibre dispersion dcale (Shifted Dispersion Fibre)Dispersion = 0 1550nm mais sensible aux effets non linaires>> Dbits levs (> 10Gbits/s)>> Pas WDM

    G.655 : lavenir ! Compromis entre G.652 et G.653Dispersion = 8ps/nm/km 1550nm et insensible aux effets non linaires>> Dbits levs (> 10Gbits/s) >> WDM (120 canaux dmontrs en 2000) Today Fibers

  • Today Fibers0+20-2013001550wavelength (nm)Dispersion (ps/nm/km)G.652G.653SDFSMF

  • Silica fibers preform fabricationGases inO2, HeSiCl4GeCl4BBr3POCl3SilicatubeHeating ringGases outDepositpreformfurnaceDiameter controlPolymer coating solutionPolymer curingPulling driveTake-up reelModified chemical vapor deposition for preform fabricationPulling machine

  • Fiber materialsSilica glass fiberstarting material: pure silica (SiO2) in the form of fused quartz (amorphous)modification of refractive index by addition of impuritieslowering refractive index : B2O3, Fraising refractive index : P2O5, GeO2

    Polymer optical fiber (POF)large core (multimode)large refractive index difference between core and claddingeasy handlingrelatively high losses

  • Advantages of Optical communication Huge bandwidth Small and light Low loss Electrical isolation No EMI (Lightning, interference) Security (no tapping) Reliability Low cost per bit

  • Les Sources Laser (Light Amplification by Stimulated Emission of Radiation)

    Laser fibre dope lErbium Laser semi-conducteur1 - les plus utiliss pour intgration (qques m)2 - deux typesDistributed FeedBack (DFB), incluant la zone de gainDistributed Bragg Reflector (DBR), ne lincluant pasLight Sources

  • Caractristiques des lasers utiliss dans les tlcomsLes Sources AccordablesLight Sources

    Technique d'accord

    Gamme de couverture

    Vitesse d'accord

    Laser accord mcanique

    500 nm

    1-10 ms

    Laser accord lectro-optique

    7 nm

    1-10 ns

    Laser accord par injection de courant

    10 nm

    1-10 ns

  • f < 1Gbits/s (1 GHz) : OK Entre 1 GHz et 10 GHz : - La diode na plus le temps de laser- Phnomne de CHIRP : l se met fluctuer>> CHIRP + dispersion des fibres : Pb !Light ModulationLa modulation interne

  • La diode met en continu, on place un obturateur en sortie Limite lectronique : 10 Gbits/sLight ModulationLa modulation externe

  • Que module-t-on ?

    Modulation du champ rayonn Modulation en amplitude(ASK), frquence (FSK) ou phase (PSK) Source ncessairement cohrente : laser Fibre monomode indispensable

    Modulation dintensit Seule la puissance rayonne est module. Nul besoin dune source cohrente Toutes les fibres conviennentLight Modulation

  • Exemples 1 Gbit/sFibre multimode saut dindice :L = 10 mFibre multimode gradient dindice :L = 1 kmFibre monomode : DEL 1,5mL = 500 m DFB 1,5m + modulation directeL = 250 km DFB 1,5m + modulation externeL = 2500 kmLimite due la dispersion.

  • Les rseaux optiques classiques sont brids en dbit :2 Gbits/sElectronic Multiplexing

  • La solution : on combine Mux/Demux lectronique et optiquen x 2 Gbits/sn x lWavelength Division Multiplexing WDM

  • Aujourdhui

    Historiquement, le WDM consistait discriminer les voies montantes (1,5m) et descendantes (1,3m)Progrs : 2000 : Mux WDM 80 longueurs donde 2Gbits/s (160Gbits/s!) 2001 : Mux WDM 200 longueurs donde 2Gbits/s (500Gbits/s!)Espacement inter-canaux : dl ~ 50GHz (0,4nm) autour de 1550nmWavelength Division Multiplexing WDM

  • Amplificateurs OptiquesEDFA (erbium doped fiber amplifier)

  • EDFA (erbium doped fiber amplifier)

  • EDFA (erbium doped fiber amplifier)courbe de gain

  • LEDFA convient tous types de modulation : amplitude ASK : tout photon incident induit un photon stimul frquence FSK et phase PSK

    Gain jusqu 40dB dans une bande de 3 THz ([1,53 - 1,56m])

    Utilisation de canaux autour de 1,5m si espacs de 100GHz

    Bruit large spectre d lmission spontane >> filtrable

    Temps de rponse : 10msEDFA (erbium doped fiber amplifier)

  • Amplificateur semi-conducteur SCOAConversion lectro-optique

  • PDFFA : praseodynium doped fluroide fiber amplifierAmplificateurs Optiques

    Type d'amplificateur

    Zone de gain

    Largeur de bande

    Temps de rponse

    Gain

    SCOA

    Quelconque

    40 nm

    1 ns

    25 dB

    EDFA

    1525 nm - 1560 nm

    35 nm

    10 ms

    25-51 dB

    PDFFA

    1280 nm - 1330 nm

    50 nm

    ?

    20-40 dB

  • A note on dB and dBmdBoptical signals: electrical signals:

    dBmabsolute power value (with 1 mW as reference) power level in dBm: electrical dB = 2 x optical dB

  • FIN

    2. Propagation of light in fibers2.2. Wave Propagation ModelThe number of modes (including all degenerate modes) is given by the expressions shown here, which are approximately valid for a large number of modes. It is very interesting to see that this number of modes can be written as twice the solid angle x area product divided by wavelength square (this ratio is one for a diffraction limited source). This also implies that the radiance of the light emitted from a fiber end is inversely proportional to the number of modes for given intensity. In a multimode fiber with a typical core diameter of 50m and Dn of 0.01 the number of modes is of the order of 500.The entire modal discussion so far was focused on the Step Index case. In the GI-case everything is conceptually similar (but quantitatively different)In the parabolic graded index fiber (a-profile with a=2) the number of modes is smaller than for step index fiber for the same refractive index contrast. In a single mode fiber the number of modes is 2 (2 degenerate modes). Light emitted from a single mode fiber is in very good approximation a diffraction limited source. 1. Types of fiber and fabrication technologyThere are a variety of methods to produce Silica fiber. One can start from liquid materials or from gaseous materials. An important further distinction has to be made between continuous production and preform production. In the first case the fiber is directly made out of the starting materials.More important nowadays is the preform method in which a thick (e.g. 2cm diameter and 50 cm length) cylindrical rod is first made, being a longitudinally compressed version of the fiber to be made. A pulling machine then converts the rod into a fiber which can be tens of km long (for a single rod).The most widely used method to make the preform is the so-called Modified Chemical Vapour Deposition (MCVD) techniqueIn the MCVD technique one starts form a cylindrical Silica tube, which is mounted in a rotatable frame. The source gases are passed through the tube. At the same time an annular heating ring (generally a gas torch) heats part of the tube and moves slowly back and forth. A deposit will then form at the inner side of the tube by thermal oxidation. By controlling the gas flows an arbitrary index profile can be obtained. At the end of the process the tube is collapsed into a rod by passing once more the heating ring at very high temperature along the tube.The next step is to mount the preform in a pulling tower. There the preform is heated again to deform the preform into a (more than 100 times) thinner fiber. This is done with extreme precision. In the same machine a primary polymer coating is applied to protect the silica surface against microcracks and then the fiber is wound on a reel.1. Types of fiber and fabrication technologyThe most important optical fiber is based on amorphous Silica (a type of glass) which has a refractive index around 1.5. In order to produce an index difference between core and cladding, impurities are added to the material during fabrication. B2O3 and F are used for the cladding and P2O5 and GeO2 are used for the core.Silica fibers are not the only type of glass fibers. Some other glass types are also used, in particular fluoride fibers (with Fluor-bonds rather than Oxygen-bonds). They hold the promise of extremely low losses but are presently mainly important for fiber amplifiers. The most important fluoride material is ZBLAN (ZrF4 - BaF2 - LaF3 - AlF3 - NaF).In applications of very short distances fibers made of polymer can be used (Polymer Optical Fiber or POF) . They are cheap and can be handled more easily than silica fibers, when it comes to preparing a high quality fiber end. They often have a core diameter close to the cladding diameter (cladding is very thin) and are therefore always multimode. The cladding diameter is often larger than for Silica fiber and ranges from 125mm to a few millimeters. The refractive index difference between core and cladding is generally higher than in Silica fibers. The losses are much higher than in Silica fiber.2. Propagation of light in fibers2.4. AttenuationThe concept of working with dBs is often a cause for confusion. An optical power ratio (e.g. ratio between output and input power, or, ratio between powers in two systems) expressed in dB means that one takes 10 log of the ratio. For example : an optical fibre has 6 dB loss means : Pout/Pin = 0.25 (formally one should say : the fibre has - 6dB transmission).A similar definition is used for electrical signals. Depending on whether one works with ratios of voltages or currents on one hand or with ratios of electrical power on the other, one has to work with either 20 log or 10 log (to get the same number in dB).A problem arises when an optical signal is converted into an electrical signal (or vice versa). For most types of conversion devices (photodiodes, laser diodes), this conversion is a linear conversion between optical power and electrical current. Therefore, when comparing two signals, the optical power ratio in dB(opt) will translate into an electrical power ratio in dB(el) which is twice the dB(opt)-value.To express absolute power levels, one uses dBm. This is the power ratio (in dB) as compared to 1mW. Hence a power level of 1 mW = 0 dBm, 10 mW = 10 dBm, 100 mW = 20 dBm etc.Try to remember the following (approximate) conversion between dB and power ratio. 0 dB=1+1 dB=+25%;-1 dB=-20%+3 dB=+100% (or x2);-3 dB=-50% (or :2)+6 dB=x4;-6 dB=:4+10 dB=x10;-10 dB=:10+20 dB=x100;-20 dB=:100