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 isotropi