Optical fiber communication Part 1 Optical Fiber Fundamentals

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Transcript of Optical fiber communication Part 1 Optical Fiber Fundamentals

  • OPTICAL FIBER

    COMMUNICATION

    Wavelength of operation

    Propagation of light in fibre

    Types of fibre

    Ray theory

    Mode theory

    Attenuation and dispersions

    Fibre Manufacturing

    Fibre to fibre coupling

    Splices and connectors

    PART I:-

    OPTICAL FIBRE

  • ADVANTAGES OF OPTICAL FIBER

    COMMUNICATIONS

    Low transmission loss and wide bandwidth.

    Small size and weight.

    Immunity to interference.

    Electrical isolation.

    Signal security.

    Abundant raw material.

  • BLOCK DIAGRAM

    Drive

    r

    Circui

    t

    Light

    Source

    LED/LD

    Optical

    Receive

    r

    Detecto

    r

    Processin

    g Circuit

    Light

    Sourc

    e

    Amplifier Detector

    Input

    signal

    o/p

    Optica

    l Fiber

    Repeater

    E/O E/O O/E

    O/E

  • FREQUENCY

    OF

    OPERATION

  • FREQUENCY OF OPERATION

  • ATTENUATION OF

    SIGNAL

  • ATTENUATION OF SIGNAL OFC Transmits all wavelengths from 800nm to 2.5m.

    Attenuation offered by different wavelengths are different.

    Windows of wavelengths are used.

    Earlier minimum attenuation sensed at 800nm to 900nm.

    Concentration of hydroxyl ions and metallic ions impurities

    reduced later .

    Glass is further purified.

    1100nm to 1600nm region gave lesser loss.

    Two popular windows centered around 1300nm and

    1550nm.

  • BASIC OPTICAL LAWS

    n1sin1 = n2sin2

  • BASIC OPTICAL LAWS

    Refractive index n = c/v

    c = 3 X 108m/s speed of light in vacuum/free space.

    v = Speed of light in material.

    n(air) = 1

    n(pure glass) = 1.5

    n1 > n2

    n1sin1 = n2sin 2

  • PROPAGATION OF LIGHT THROUGH OFC

  • OPTICAL LAWS

    1

    2

  • OPTICAL LAWS

    no sino = n1sin

    no sino = n1sin(90-)

    no sino = n1cos 1

    Also n1sin1 = n2sin2

    o is gradually increased.

    increases and 1 reduces till critical angle.

    Limiting stage is when light refracts.

    o = omax

    1 = c

    1

    2

  • NUMERICAL APERTURE STEP INDEX

    FIBER

    no sinomax = n1cos c

    n1sin c = n2sin90

    sin c = n2/ n1

    cos c = (n2

    1 - n2

    2) / n1

    Hence no sinomax = (n2

    1 - n2

    2)

    For air no = 1

    sinomax = (n2

    1 - n2

    2) = NA

    sinomax = n1(2 ) = NA

    = (n1 - n2) / n1 Core cladding index difference

    2 being small, neglected

    n1

    (n21 - n22)1/2

    n2

    c

  • WAVE PROPAGATION

  • FIBER STRUCTURE

    In principle, clad is not necessary for light to propagate.

    Light can propagate through core-air interface.

    Clad is required for

    It reduces scattering losses due to dielectric

    discontinuities at core surface.

    Adds mechanical strength to fiber.

    Protects core from absorbing external light.

  • FIBER STRUCTURE

    Low loss fiber

    Made from glass core glass cladding.

    Medium loss fiber

    Glass core plastic cladding (Plastoclad)or

    plastic core plastic cladding.

    Has high loss.

    Cheaper as clad covering is elastic abrasion resistant

    plastic material.

    Gives strength and protects fiber from geometric

    irregularities, distortion and roughness of adjacent

    surface.

  • TYPES OF FIBER

  • TYPES OF FIBER STRUCTURE

    Step index Fiber

    RI is constant throughout and changes abruptly at

    interface.

    Constant in cladding.

    Graded Index Fiber

    RI reduces gradually from center to interface and

    constant in clad.

  • GRADED INDEX FIBER

  • NUMERICAL APERTURE GRADED INDEX FIBER

    More complex.

    Function of position across core face.

    Light will propagate as guided mode at r only if it is

    within local NA(r) defined as -

  • TYPES OF FIBER STRUCTURE

    Single mode Fiber

    Thin core.

    Uses high power through precision LASER.

    Low distortion.

    Low intermodal dispersion.

    High Bandwidth.

  • TYPES OF FIBER STRUCTURE

    Multimode Fiber

    Large core diameter.

    Easy power launching.

    Easy connectorization.

    Cheap.

    Uses cheaper light source as LED and less complex

    circuitry.

    But power output is low.

    Intermodal dispersion high.

    Graded index fiber reduces dispersion hence has high

    BW.

  • RAYS AND MODES

    Light travels in form of ray with total internal reflection.

    During launching, infinite number of rays launch inside.

    Few discrete rays travel down the fiber.

    Propagation of light uses set of electromagnetic waves .

    Called Modes of waveguide.

    Or trapped modes of waveguide.

  • RAYS AND MODES

    Each guided mode has a pattern of E and H field lines

    repeated along the fiber in interval of wavelength.

    Certain discrete number of modes can propagate along the

    fiber.

    They are those EM waves satisfying

    homogeneous wave equations in the fiber.

    Boundary conditions at waveguide surface.

    Propagation characteristics of light in OFC can be explained

    by

    Ray optics.

    Electromagnetic field theory.

  • RAY OPTICS

    Light rays are perpendicular to phase front of the wave.

    Family of waves for one mode gives a set of light called Ray congruence.

    Each ray of a set travels at same angle relative to fiber axis.

    Discrete number of ray sets exist inside fiber due to Phase condition.

  • RAY OPTICS

    Two types of phase changes.

    One while reflection.

    Other while travelling

  • RAY OPTICS

  • RAY OPTICS

    Phase shift 1: Totally internally reflected twice.

    Depends upon whether polarization is normal or

    parallel to plane of incidence.

    With n = n1/n2 and 1 < c , Phase change at each

    reflection:

  • RAY OPTICS

    Phase shift 2: due to wave travel from A to B and B to

    C.

    2 = k1s

    K1 = Propagation constant in medium of RI n1.

    s= total distance travelled.

    Total phase change must be integral multiple of 2.

  • RAY OPTICS

    Total phase change must be integral multiple of 2.

    Other angles cancel out each other.

    Total angle of , 3 etc will cancel out completely.

    Hence..

    M = number of discrete ray sets allowed to propagate inside fiber.

  • MODE THEORY FOR CIRCULAR WAVE GUIDE

    Ray optics has limitations.

    It does not deals with coherence or interference

    phenomenon.

    It doesnt give the field distribution of individual mode.

    Doesnt show coupling of power between modes of wave

    guides.

    Hence the mode theory.

  • TYPES OF RAYS - MERIDIONAL RAYS

    Confined to the meridional planes of the fiber, i.e. planes

    containing axis of symmetry of fiber, core axis.

    A given Meridional ray propagates in a single plane

    along fiber axis, hence easy to track.

    Bound Rays Trapped in fiber core according to Snells

    law of reflection and refraction.

    Unbound Rays Rays refracted out of fiber core

    according to Snells law of refraction and can not be

    trapped in core.

  • TYPES OF RAYS - SKEW RAYS

    Propagates without passing through core of fiber.

    Not confined to single plane, but follow helical path along fiber.

    Difficult to track these rays as they do not lie in single plane.

  • SKEW RAYS.

    Direction of ray changes by angle 2 at each reflection

    where is angle between projection of ray in two

    dimensions and radius of fiber core.

    Skew rays show smoothening effect on distribution of light

    transmitted even if light launched in fiber is not uniform.

    Numerical aperture of skew rays is greater than

    meridional rays.

  • ACCEPTANCE ANGLE OF SKEW RAYS.

    cos = RB/AB = RB/BT * BT/AB

    Under limiting condition becomes c.

  • ACCEPTANCE ANGLE OF SKEW RAYS.

    sin c = n2/n1

    Also no sino = n1sin

    Under limiting condition -

    sinas = NA/cos

  • MODE THEORY FOR CIRCULAR WAVE GUIDE

    Field pattern of three modes shown.

  • MODE THEORY FOR CIRCULAR WAVE GUIDE

    Three categories of mode:

    Bound modes are those modes which are confined in

    core of waveguide.

    Refracted modes are those which are scattered out of

    clad due to roughness of surface or absorbed by coating

    of clad.

    Leaky modes are those which are partially confined to

    core region

    attenuate continuously, radiating their power out of

    core as they propagate.

    Due to tunnel effect.

  • MODE THEORY FOR CIRCULAR WAVE GUIDE

    For a particular mode to be confined , the condition is: is propagation constant.

    If < n2k, power leaks out of core into cladding region.

    Significant power loss due to leaky modes.

    Modes that sustain have very small loss throughout

    fiber propagation.

  • MODE THEORY FOR CIRCULAR WAVE GUIDE

    Assuming linear isotropic dielectric material having no

    current and free charge, Maxwell's equations are:

  • ELECTROMAGNETIC WAVE PROPAGATING ALONG

    CYLINDRICAL WAVEGUIDE

  • 1

    2

    3

    4

    5

    6

  • ELECTROMAGNETIC WAVE PROPAGATING ALONG

    CYLINDRICAL WAVEGUIDE

    c2 = 1/

  • WAVE EQUATIONS FOR CYLINDRICAL OPTICAL FIBER

    WAVEGUIDE.

  • TEM MODE

    Occurs through free space/ parallel wire/ co-axial cable.

    Ez = 0, Hz = 0.

    Et , Ht are perpendicular to each other and to direction

    of propagation.

  • TE MODE

    Occurs through metallic waveguide.

    Ez = 0, Hz = finite.

    Electric field lies entirely in transverse plane.

    Magnetic field vector has component in direction of Z as well as transverse.

    Propagation in z direction takes place with group velocity vg.

    E-H plane moves at angle normal to itself with speed of light.

  • TM MODE

    Occurs through metallic waveguide.

    Hz = 0, Ez = finite.

    Magnetic field lies entirely in transverse plane.

    Electric field vector has component in direction of Z as well as transverse.

    Propagation in z direction takes place with group velocity vg.

    E-H plane moves at angle normal to itself with speed of light.

  • HYBRID MODE

    Hz = finite, Ez = finite.

    Both Magnetic field and Electric field vectors have

    components in direction of Z as well as transverse.

    Et > Ht --- EH mode

    Ht > Et --- HE mode

    Propagation in z direction takes place with group velocity vg.

    E-H plane moves at angle normal to itself with speed of light.

  • NOTE:

    Meridional rays take place only in TE and TM mode

    which is completely guided.

    Skew rays propagate entirely in the hybrid HE and

    EH mode. May contribute to losses through leakage

    and radiation.

  • SOLUTION FOR WAVE EQUATIONS FOR CYLINDRICAL

    OPTICAL FIBER WAVEGUIDE.

    Putting value of Ez in wave equation, we get

    Similar equation for Hz.

    Bessels equation, whose Solutions are called Bessel's

    functions.

  • REQUIREMENTS FROM SOLUTION

    To sustain, field inside core must be sinusoidal.

    Field should be exponentially decaying outside the core

    i.e. in cladding.

    Depending on q, we have to chose that only that

    possibility which satisfies above two equations and

    find q to achieve this solution.

    Various possibilities for the solution are:--

  • 1. q is real

    Bessel function of order and argument qr. OR

    Newmahn function of order and argument qr.

  • 1. Q IS REAL

    Bessel Function:-

    Oscillatory behavior inside core.

    Amplitude reduces as order increases.

    For = 0, i.e., lowest order mode Jo is finite = 1.

    Favorable inside core.

    Neumann Function:-

    Oscillatory behavior inside core.

    Amplitude reduces as order increases.

    For = 0, i.e., Neumann function tends to -

    Not desirable condition as field strength along axis in

    infinite.

    HENCE:- Bessel function as solution to Bessel

    equation inside core with q = real.

  • 2. q is imaginary

    Modified Bessel function of first kind OR

    Modified Bessel function of second kind

    qr/j is real

  • 2. Q IS IMAGINARY

    Modified Bessel function of first kind:

    q is imaginary hence argument qr/j is real.

    Monotonically increasing function of argument.

    Not desirable in cladding.

    Modified Bessel function of second kind

    Monotonically decreasing function of argument.

    Desirable in cladding.

    HENCE:- Modified Bessel function of second

    kind

    as solution to Bessel equation inside cladding

    with q = imaginary.

  • ELECTROMAGNETIC WAVE PROPAGATING ALONG

    CYLINDRICAL WAVEGUIDE INSIDE CORE

    Field must be finite, hence sinusoidal inside core as r 0.

    Same as Bessel function.

    For r < a, solutions are Bessel function of first kind of order .

    F1(r) = J (ur)

  • ELECTROMAGNETIC WAVE PROPAGATING ALONG

    CYLINDRICAL WAVEGUIDE INSIDE CLADDING

    Field must decay exponentially outside core as r .

    Same as Modified Bessel function of second kind

    Hence For r > a, solutions are Modified Bessel function

    of second kind

    F1(r) = K(wr)

  • CONDITION ON

    From definition of Modified Bessel Function of Second kind, K(wr) = e

    -wr .

    e-wr 0 when r

    Hence w2 = 2 k22 must be >0.

    Hence k2 and defines cutoff condition.

    Cutoff condition is the condition when mode is no longer bound to the core region.

    Condition on J (ur) can be deduced from the fact that u must be real inside core for F1 to be real.

    Hence k1

    Therefore -

  • MODAL EQUATION

    Solution for will depend upon boundary conditions.

    Tangential component E and Ez of E inside and outside of interface at r = a must be same.

    Similarly Tangential component H and Hz of E inside and outside of interface at r = a must be same.

    Let Ez = Ez1 inside core and Ez = Ez2 outside core-clad boundary.

    Inside core q2 is given

    by-- Outside core q2 is given

    by--

    ----1

    Hence In cladding, q2 = - w2.

  • MODAL EQUATION

    Hence condition on E1 and E2 at r = a is

    ---2

    Similarly

    ---3

    ---4

  • MODAL EQUATION

    Four unknown coefficients A, B, C, D.

    Solution will exist if the determinants of these

    coefficients is zero.

  • MODAL EQUATION

    Evaluating this determinant gives eigenvalue equation for --

  • MODAL EQUATION

    Solving eigenvalue equation for indicates---

    Only discrete values is allowed within the range

    k2 k1

    Equation for some lower order modes can be given as -

  • BESSEL FUNCTION OF ORDER

  • ROOTS OF BESSEL FUNCTION OF ORDER

  • MODES IN STEP INDEX FIBER

    J-Type Bessel function similar to harmonic function.

    Oscillatory behavior for real k.

    Hence m roots for each .

    Roots given as m

    Corresponding modes are either TEm, TMm, HEm, or EHm.

    For dielectric fiber waveguide, all modes are hybrid except =0.

    For = 0 -- 0

  • MODES IN STEP INDEX FIBER

  • CUT-OFF CONDITION-NORMALIZED FREQUENCY OR V

    NUMBER Cutoff condition for a mode:

    at which mode is no longer confined to core / guided region.

    Field no longer decays outside core region.

    Related to a parameter called V number or Normalized

    frequency.

    and w2a2

    Dimensionless number V determines how many modes the fiber can support .

  • V can also be expressed as Normalized Propagation

    Constant b as

    b = 1 a2u2/V2 V2 = a2(u2 +

    w2)

    NORMALIZED PROPAGATION CONSTANT B

    Also

  • CUT-OFF

    CONDITION

    Each mode can exist only for the value of V that exceeds the

    limiting value.

    Mode is cut-off when /k = n2.

    HE11 has no cut-off. It ceases to exists when core dia of fiber

    is zero.

  • DESIGN OF SINGLE MODE FIBER

    From /k Vs V graph, there is only one mode HE11 till

    V=2.405.

    PROB A step index fiber has normalized

    frequency of 26.6 at wavelength 1300nm.If the

    core radius is 25m, find numerical aperture.

  • NUMBER OF MODES M IN MM FIBER

    A ray will be accepted by the fiber if it lies

    within angle defined by NA .

    The solid acceptance angle of fiber -

  • NUMBER OF MODES M IN MM FIBER

    For electromagnetic radiation of wavelength from a laser

    or fiber, number of modes per unit solid angle is 2A/ 2.

    A = a2

    2 is because plane wave can have 2 polarization

    orientations.

  • POWER FLOW IN STEP INDEX FIBER Power flowing in core and cladding can be obtained by

    integrating poynting vector in axial direction.

    M = V2/2

  • SIGNAL DEGRADATION IN OPTICAL FIBER

    Signal attenuation.

    Determines maximum repeater less distance

    between Transmitter and Receiver.

    Signal Distortion due to Dispersion (Pulse

    broadening).

    Determines information carrying capacity.

    Bandwidth.

  • ATTENUATION

    Expressed as dB/Km.

    L = fiber length.

    Caused by

    Absorption

    Scattering

    Bending

  • ATTENUATION- ABSORPTION

    Absorption by atomic defects or imperfections

    as Missing molecules.

    High density cluster of atom groups.

    Oxygen defects in glass.

    Negligible w.r.t. other causes.

  • ATTENUATION- ABSORPTION

    Extrinsic absorption of photons by impurity

    atoms in glass as Iron, chromium, cobalt, copper

    OH ions from hydro-oxygen flames.

    Absorption results in energy level transition of electrons.

    Charge exchange between OH ions.

    Less than 0.5dB/Km in range of operation

    with better methods.

  • VAD SILICA FIBER WITH VERY LOW OH ION

    CONTENT

  • ATTENUATION - ABSORPTION

    Intrinsic absorption by glass materials itself. Due to absorption bands in ultraviolet region (Energy

    level transition).

    Tail of the curves enter the operation region.

    Small as compared to IR absorption.

    E and loss inversely proportional to wavelength.

    Typically 0.1dB/Km at 1200nm.

    Follows empirical relation as: Urbachs rule (E-Photon Energy)

  • SIGNAL DEGRADATION - ABSORPTION

    Intrinsic absorption by glass materials itself. Crystal lattice vibration in Infra red region

    If frequency lies within resonant frequency of vibration.

    Tail of the curves enter the operation region.

    Typically 0.1dB/Km at 1500nm.

  • SIGNAL DEGRADATION - SCATTERING

    Microscopic variations in material density.

    Glass is randomly connected network of molecules having

    higher or lower than average density.

    Compositional fluctuations of SiO2, GeO2, and P2O5.

    Give refractive index fluctuations.

    If fluctuation distance very small w.r.t wavelength,

    cause Rayleigh-type scattering of light.

    i.e. photons moving in all directions.

    Effective signal strength gradually reduces.

    Proportional to -4.

    Reduces with increase in wavelength.

  • SIGNAL DEGRADATION - SCATTERING

    MIE scattering

    When RI fluctuation distance comparable to wavelength.

    Can be reduced by-

    Reducing imperfections during manufacturing.

    Carefully controlled extrusion and coating.

    Increasing fiber guidance by increasing .

  • OPTICAL

    FIBER

    ATTENUATION

    CHARACTERIS

    TICS OF LOW

    LOSS, LOW OH

    SILICA FIBER

  • SIGNAL DEGRADATION - BENDING

    1.MACROSCOPIC BENDING

    Bending radius larger than fiber diameter.

    Coiling, corner turns.

    No longer supports Total internal reflection for few rays.

    Light refracts and power is lost.

    Can be explained by mode theory.

  • SIGNAL DEGRADATION - BENDING

    1.MACROSCOPIC BENDING

  • 1.MACROSCOPIC BENDING

    The radiation loss is present in every bent fiber no matter

    how gentle the bend is.

    Radiation loss depends upon how much is the energy

    beyond xc.

    For a given modal field distribution if xc reduces, the

    radiation loss increases.

    The xc reduces as the radius of curvature of the bent fiber

    reduces, that is the fiber is sharply bent.

  • 1.MACROSCOPIC BENDING

    Lower order modes - fields decay rapidly in the cladding,

    more confined in core.

    Higher order modes - more slowly decaying energy in the

    cladding .

    The higher order modes hence are more susceptible to the

    radiation loss compared to the lower order modes.

    The number of modes therefore reduces in a multimode

    fiber in presence of bends.

    Energy on outer part of cladding has to travel faster than

    light to keep pace with energy in core.

    Not possible, hence gets lost.

  • 2.MICROSCOPIC BENDING

    Small scale fluctuations in radius of curvature of fiber axis.

    Or non uniform lateral pressure during cabling.

    Can not maintain Total Internal Reflection if ray hits bends.

    Due to bends, Power couples from guided modes to leaky modes

    0.1 to 0.2dB/Km

  • CORE CLADDING LOSSES

    Core and clad have different composition and RI.

    Hence different attenuation coefficients 1 and 2.

    Loss for mode (vm) - P is total power.

  • SIGNAL DISTORTION IN OPTICAL FIBER

  • GROUP DELAY

    Group delay is time required for a mode to travel along fiber length L.

    Assume modulated optical signal excites all modes equally at input.

    Each mode carries equal amount of energy through fiber.

    Each mode contains all spectral components in the wavelength band of source.

    Phase velocity at which phase of a particular frequency travels in space.

    vp = / = c/n1 Group velocity at which overall wave (group of

    frequencies) travels in space.

    Vg = / = c k/

    Each spectral component travels independently.

  • GROUP DELAY

    Each spectral component undergoes time delay or group

    delay per unit length in direction of propagation given as-

    Group velocity depends on wavelength.

    Each spectral component of a mode take different

    amount of time to travel.

    Pulse spreads.

  • GROUP DELAY

    Assuming spectral width is not too wide

    Delay difference per unit wavelength = dg/ d

    For spectral components which are apart and lie /2 above and below central wavelength 0, total delay difference over distance L is -

    If spectral width of an optical source is characterized

    by rms value , then pulse spreading can be

    approximated by rms pulse width.

  • GROUP DELAY

    Dispersion = pulse spread as function of wavelength.

    Measured in picoseconds/km/nm.

    = pulse broadening per unit distance per unit spectral width

    D

    1

  • INTRAMODAL DISPERSION MATERIAL DISPERSION

    Also called chrominance dispersion or spectral dispersion.

    RI varies for wavelength and phase velocity.

    vp = / = /T, n= c/ vp

    Source has finite spectral width.

    Different wavelength travels with different phase velocities.

    Delay. Shorter wavelength more delay.

    LASER better than LED

  • VARIATION OF RI

    WITH WAVELENGTH

    FOR SILICA.

    NOTE THE FLATTER

    REGION OF LEAST

    VARIATION AROUND

    WAVELENGTH OF

    OPERATION.

  • MATERIAL DISPERSION

    Assuming dispersion is due to only material dispersion.

    g = mat

    Time delay per unit length = mat /L = 1/ Vg = d/d

    = 2c/

    d = -2 c/ 2 d

    mat /L = -2/2 c d /d

  • MATERIAL DISPERSION

    Total pulse spread mat is fractional material dispersion per

    unit spectral width taken over entire spectral width .

    D is material dispersion per unit length per unit

    spectral width.

    Dmat = ?

    Material dispersion can be reduced either by

    Choosing source with narrower spectral width ,

    Or by operating at longer wavelength.

    Proportional to curvature of RI profile.

  • DISPERSION VS

    WAVELENGTH

    If D is less than zero, the medium is said to have

    positive dispersion.

    Light pulse is propagated through a normally

    dispersive medium, the result is the lower wavelength

    components travel slower than the higher wavelength

    components.

    RI increases with reduction in wavelength.

    If D is greater than zero, the medium has negative

    dispersion.

    Pulse travels through an anomalously dispersive

    medium, lower wavelength components travel faster

    than the higher ones.

    RI increases with increase in wavelength.

    Pulse spreads in both case.

    n= c/ vp

  • MATERIAL DISPERSION REDUCES WITH WAVELENGTH

  • INTRAMODAL DISPERSION WAVEGUIDE

    DISPERSION Assuming RI of material independent of wavelength.

    80% power in Core.

    20% power in clad.

    RI of clad is smaller.

    Clad power travels faster than core power.

    Cause pulse spreading.

  • INTRAMODAL DISPERSION WAVEGUIDE

    DISPERSION Need to make group delay independent of fiber configuration.

    Group delay expressed in terms of normalized propagation

    constant b.

    Solving for ---

    (n1-n2)/n2

    Group delay due to Wave guide dispersion =

  • WAVEGUIDE DISPERSION

    is obtained by eigenvalue equations and expressed in

    terms of Normalized Frequency V.

    In multimode fiber, waveguide dispersion is very small

    w.r.t. material dispersion, hence ignored.

  • DISPERSION IN SINGLE MODE FIBER

    Waveguide dispersion and material dispersions are

    of same order.

    Pulse spread wg occurring over wavelength derived

    from derivative of group delay w.r.t. wavelength -

  • DISPERSION IN SINGLE MODE FIBER

    It is 0.2 to 0.1 for V from 2 to 2.4.

    Find waveguide dispersion at V=2.4, =0.001, n2=1.5

    Material dispersion at 900nm =

  • DISPERSION VS WAVELENGTH

  • DISPERSION IN SINGLE MODE FIBER

    Material dispersion dominates at 900nm.

    At longer wavelength as 1.310m, total dispersion is

    almost zero.

    It is operating wavelength for single mode.

  • INTERMODAL DELAY

    In MM Step index fiber, each mode has different group velocity.

    Higher order mode, slower axial group velocity due to steeper angle of propagation.

    Higher order modes travels slower than lower order modes.

    Pulse spreads.

    Can be eliminated in MM Graded index fiber or single mode fiber.

  • FIBER MANUFACTURE REQUIREMENTS FROM

    MATERIAL

    Must be possible to make long thin flexible fiber.

    Transparent at particular optical wavelength.

    Able to make physically compatible material having

    slightly different refractive indices for core and cladding.

  • FIBER MANUFACTURE

    I- Glass-Glass Fiber

    Glass core and glass cladding.

    Fragile, needs heavy strengthening covering.

    Least attenuation.

    Longer distance transmission.

  • FIBER MANUFACTURE

    I- Glass-Glass Fiber

    Glass as silica (SiO2) with RI of 1.458 at 850nm

    Addition of GeO2 and P2O5 increases RI.

    Addition of B2O3 and fluorine decreases RI.

  • FIBER MANUFACTURE

    I- Glass-Glass Fiber

    Combinations can be

    GeO2 - SiO2 core, SiO2 Cladding.

    P2O5 - SiO2 core, SiO2 Cladding.

    SiO2 core, B2O3 -SiO2 Cladding.

    GeO2 - B2O3 - SiO2 core, B2O3 -SiO2 Cladding.

  • FIBER MANUFACTURE

    II- Plastic clad Glass Fiber

    Glass core and plastic cladding.

    Higher losses.

    Short distance (several hundred meters).

    Reduced cost.

    Core Silicon resin RI = 1.405 at 850nm

    Clad is Teflon FEP (Perfluoronated ethylene propylene)

    with RI = 1.338.

    Large NA with large RI difference.

    Core dia of 150 to 600m.

    LED as source.

  • FIBER MANUFACTURE

    III- Plastic Fiber

    Very short distance (100m max).

    High attenuation.

    Low cost, tough, durable, inexpensive.

    Core dia of 110 to 1400m.

    LED as source.

    Polystyrene core (1.6), methyl methacrylate clad (1.49).

    NA = 0.6.

    Polymethyle methacrylate core(1.49), its co-

    polymer(1.40), NA = 0.5

  • FIBER MANUFACTURE

    Preforms are made with core and cladding.

    By reacting pure vapours of metal halides(SiCl4 , POCl3

    and GeCl4) with oxygen.

    Vapours are collected to make a loose structure.

    Sintered at 1400C to make clear glass rod.

    10 to 25mm in diameter and 60 to 120cm long.

  • FIBER FABRICATION OVPO

    OUTSIDE VAPOUR PHASE OXIDATION

  • OUTSIDE VAPOUR PHASE OXIDATION

    Graphite rod or ceramic mandrel used to deposit soot.

    Impurity levels controlled to make core and cladding.

    Mandrel is removed and Porous tube is sintered in dry

    atmosphere .

    Equations-

    SiCl4 + O2 SiO2 + 2Cl2

    GeCl4 + O2 GeO2 + 2Cl2

    4POSiCl3 + 3O2 2P2O5 + 6Cl2

    4BBr3 + 3O2 2B2O3 + 6Br2

    POSiCl3 Phosphorous Oxychloride

    2BBr3- Boron Tribromide

  • FIBER FABRICATION OVPO

    OUTSIDE VAPOUR PHASE OXIDATION

    Sintered in dry atmosphere

    above 1400C Fiber drawing

  • FIBER

    FABRICATION

    VPAD

    VAPOUR PHASE

    AXIAL DEPOSITION

  • VAPOUR PHASE AXIAL DEPOSITION

    Soot deposited axially.

    Two separate torches for clad and core.

    Preform continuously rotated for uniform deposition.

    Torches are correspondingly fed with metal halides.

    Advantage:

    No central hole.

    Continuous process so low production cost.

    Better yield.

    No gap between torch chamber and sintering chamber.

    Clean environment.

  • VAPOUR PHASE AXIAL DEPOSITION

    Equations

    SiCl4 + 2H2O SiO2 + 2H2 + 2Cl2

    GeCl4 + 2H2O GeO2 + 2H2 + 2Cl2

    2POSiCl3 + 3H2O P2O5 + 3H2 + 3Cl2

    2BBr3 + 3H2O B2O3 + 3H2 + 3Br2

    POSiCl3 Phosphorous Oxychloride

    2BBr3- Boron Tribromide

  • FIBER FABRICATION MCVD

    MODIFIED CHEMICAL VAPOUR DEPOSITION

  • MODIFIED CHEMICAL VAPOUR DEPOSITION

    Most widely method.

    Clear glass tube as clad taken.

    Metal halide with oxygen is flown into it.

    Soot deposited uniformly as tube is rotated.

    Burner sinters the soot to clear glass continuously.

    Later hole tube is heated strongly to collapse it to solid

    rod.

    Equations same as OVPO

  • FIBER FABRICATION PCVD

    PLASMA ACTIVATED CHEMICAL VAPOUR

    DEPOSITION

  • PCVD - PLASMA ACTIVATED CHEMICAL VAPOUR

    DEPOSITION

    Gas molecules or atoms turn into a plasma containing

    charged particles, positive ions and negative electrons,

    when heated or under strong electromagnetic field.

    The presence of a non-negligible number of charge

    carriers makes the plasma electrically conductive.

    Very small grains of silica within a gaseous plasma will

    also pick up a net negative charge.

    They act like a very heavy negative ion components of

    the plasma.

  • PCVD - PLASMA ACTIVATED CHEMICAL VAPOUR

    DEPOSITION

    Moving microwave resonator at 2.45GHz generates

    plasma inside tube to activate chemical reaction.

    Gaseous ions escape through exhaust while silica heavy

    plasma move along with M/W resonator and get

    deposited inside tube.

    Silica tube at 1000 to 1200C to reduce mechanical stress in growing glass film.

    Deposits clear glass directly on tube wall till desired

    thickness achieved.

    No soot, no sintering.

    At end tube is collapsed into a Preform.

  • DIRECT FIBER FABRICATION

    DOUBLE CRUCIBLE METHOD

  • DOUBLE CRUCIBLE METHOD

    Glass rods of core and clad material are separately

    made by melting mixtures of purified powders of

    required composition.

    Rods used as feedstock for two concentric crucibles.

    Fibers are drawn from molten state through orifices of

    crucibles.

    Has advantage of being continuous process.

    Requires careful attention to avoid contamination.

    Contamination can be from furnace environment or

    crucible.

    Glass crucible used to make rods.

    Platinum crucibles used in furnace to melt and draw

    fiber.

  • FIBER DRAWING

  • FIBER DRAWING

    Preform is softened in drawing furnace till it is

    possible to draw thin filament.

    Turning speed of drum decides thickness of fiber.

    Speed regulation is done by thickness monitor in

    feedback loop.

    A thin elastic coating is applied to protect from dust

    and water vapour.

    These fibers are later bound into cable.

  • FIBER TO FIBER COUPLING

    If all modes are equally excited, optical beam fills

    entire NA of emitting fiber.

    Perfect mechanical alignment required.

    Geometrical and waveguide characteristics must

    exactly match.

    In case of equilibrium state, energy in central region.

    Fills only equilibrium NA of next fiber.

    No joint loss for Slight misalignment or slight

    variation in characteristics.

    Further power loss in new fiber after new steady state.

  • TYPES OF MISALIGNMENT

  • AXIAL OR LATERAL MISALINGMENT

  • MECHANICAL MISALIGNMENT AXIAL

    OR LATERAL MISALIGNMENT

    STEP INDEX FIBER-( Constant NA)

    Most common in practice.

    Greatest power loss.

    Assuming uniform modal power distribution

    Coupled power proportional to common area.

    Coupling efficiency is ratio of common core area

    to receiving core end face area.

    F step = Acomm/a2

  • MECHANICAL MISALIGNMENT AXIAL

    OR LATERAL MISALIGNMENT

    GRADED INDEX FIBER-( Variable NA)

    Power coupled restricted by NA of transmitting and

    receiving fiber whichever is smaller at that point.

    For uniform illumination optical power accepted by core

    is that power that falls within the NA of that fiber.

    Optical power density p(r ) at a point r on the fiber end

    face is proportional to the square of local NA.

  • AXIAL OR LATERAL MISALIGNMENT

    GRADED INDEX FIBER-( Variable NA)

    In area A1

    NA of transmitting fiber is more than receiving fiber.

    Receiving fiber will accept only part of transmitted

    power that falls within its own NA.

    In area A2

    NA of receiving fiber is more than transmitting fiber.

    Receiving fiber will accept all of transmitted power in

    this region.

  • LONGITUDINAL SEPERATION

  • LONGITUDINAL SEPERATION All higher order modes optical power emitted in the

    ring of width x will not be intercepted by receiving fiber.

    Loss is given by--

  • FIBER RELATED LOSS

    Due to difference in geometrical and wave guide

    related characteristics as--

    Core diameter variation*

    Core area ellipticity.

    NA variation*

    RI profile variation

    Core cladding concentricity

  • FIBER RELATED LOSS COUPLING

    LOSS If aE aR

    And

    But

  • FIBER RELATED LOSS COUPLING

    LOSS

    If NAE(0) NAR(0)

    And

    But

    aR = aE

  • FIBER RELATED LOSS COUPLING

    LOSS

    And

    But aE =

    aR

    If E R

    For R

  • FIBER END FACE PREPARATIONS

    For splicing or connectorisation, end face must

    be : Flat

    Perpendicular to fiber axis

    Smooth

    Techniques: Sawing

    Grinding

    Polishing

    Controlled fracture

  • FIBER PREPERATION

    CONTROLLED FRACTURE TECHNIQUE

    Fiber scratched to create pressure concentration.

    Uniform Tension is applied to two ends of fiber kept on curved base.

    Maximum stress occurs at scratched point.

    Crack propagates through the fiber.

    Highly smooth and perpendicular end face can be achieved.

  • IMPROPERLY CLEAVED FIBER END

    Non uniform stress applied.

    Curvature of fiber not proper.

    LIP:

    Sharp protrusion from edge of cleaved fiber.

    Prevents proper contact with adjoining fiber.

    Can cause fiber damage.

    HACKLE:

    Severe irregularity across fiber face

    Smooth Surface

    Hackled Surface

  • IMPROPERLY CLEAVED FIBER END

    ROLL-OFF:

    Rounding off of edge of fiber, condition opposite to lip.

    Also called Break-over.

    Can cause high insertion or splice loss.

    CHIP:

    Localized fracture or break at end of cleaved fiber.

    MIST:

    Less severe hackle.

    SPIRAL or STEP:

    Abrupt changes on fiber end faces topology.

    SHATTERING:

    Due to uncontrolled fracture, fiber face has no definable cleave

    or surface characteristics.

  • FIBER SPLICING

    Fusion splicing

    V-Groove splicing

    Tube mechanical splicing

    Elastic tube splicing

    Rotary splicing

  • FUSION SPLICING

    Fiber end-face prepared and aligned microscopically.

    Joint then heated by electric arc or laser pulse.

    Joint momentarily melts and joins.

    Very low splice loss 0.06dB.

    Weak splice may result if end face not clean and

    prepared, and uncontrolled heating etc

  • V-GROOVE SPLICING

    Temporary splice needed during testing.

    V-shaped channel made of silicon, plastic, ceramic, metal substrate.

    Bonded together with adhesive or held in place with cover plate.

    Splice loss depends on outer dimension of fiber, eccentricity.

  • ELASTIC TUBE SPLICING

    Automatically performs, laterally, longitudinal and angular alignment.

    Splices multimode fibers with good accuracy.

    Less equipment and skills needed.

    Uses elastic tube with hole slightly smaller than fiber with taper on each end for easy insertion.

    Fibers to be joined need not have same outer dimensions.

  • OPTICAL FIBER CONNECTOR

    Requirements of a good connector are:

    Low coupling loss

    Interchangeability compatibility from manufacturer

    to manufacturer.

    Ease of assembly, even on field, independent of

    operator skill.

    Low environmental sensitivity- temperature, dust,

    moisture have no effect on connector losses.

    Low cost and reliable construction

    Ease of connection

  • CONNECTOR TYPES

    Single channel and multichannel assemblies in

    Screw-on

    Bayonet-mount

    Push-pull

    Basic mechanisms are

    Butt joint more common

    Expanded beam

  • BUTT JOINT-STRAIGHT SLEEVE CONNECTOR

    Metal, ceramic or molded-plastic ferrule for each fiber.

    Precision sleeve into which ferrule fits.

    Fiber epoxied into hole drilled in ferrule.

    SM and MM fibers.

    Length of sleeve and guide ring on ferrule determine

    the end separation of fibers.

  • BUTT JOINT-TAPERED SLEEVE CONNECTOR

    Metal, ceramic or molded-plastic ferrule for each fiber.

    Taper sleeve to accept and guide tapered ferrule.

    SM and MM fibers.

    Length of sleeve and guide ring on ferrule determine

    the end separation of fibers.

  • EXPANDED-BEAM CONNECTOR

    Lens on the end of fiber.

    Lenses collimate or focus expanded beam into

    receiving core.

    Fiber to lens distance is equal to focal length of lens.

    Connector less dependent on lateral alignment.

    Beam splitters and switches can easily be inserted into

    the connector.