Pwr Bal Cal

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    Optical Fiber Comn Sys(Principle Components)

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    1. Two major issues:(a) To launch Optical Power into a particular fiber.(b) To couple Optical Power from one fiber toanother.

    2. Launching of optical power entails aspects s.t. NA,Core size, RI profile, Core Cladding Indexdifference as well as the size, radiance and angular

    power distribution.

    3. Coupling Efficiency, = Pf / Ps,where, Pfis the power coupledinto the fiber and Ps

    power emitted from the source.

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    4. The launch efficiency depends upon(a) Source output pattern,(b) Reflectivity R = (n1 n)2 / (n1 + n)2(c) Wavelength of source(d) Equilibrium NA(e) Lensing Scheme

    5. Fiber to Fiber Joints(a) Splices permanent or semi permanentjoints.

    (b) Connectors

    demountable fiberconnectors are removable joints which alloweasy, fast, manual coupling and uncoupling offibers.

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    Loss (in dB) due to Fresnel reflection at a fiberinterface .

    The effect of Fresnel reflection can be reducedby use of index matching liquid.

    Problem areas at fiber fiber joint:

    Different core and/or cladding diameter Different NA and/or relative RI differences ()

    Different RI profile.

    Fiber faults i.e. core ellipticity, core concentricity.

    Losses due to above factors together withFresnel reflection are referred as intrinsiclosses.

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    losses at splices/ joints between two SMfibers.

    Three types: Transverse (or Lateral/ axial/ radial)


    Angular Misalignment

    Longitudinal Misalignment

    In absence of these losses, still thereare losses due to non-identical fielddistributions for dissimilar SM fibers.

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    (a) Transverse

    (b) Angular

    (c) Longitudinal

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    Optical losses due to three types ofmisalignment depend upon:

    Fiber type. Core diameter.

    Optical power distribution between propagatingmodes.

    Transverse misalignment gives significantlygreater losses per unit displacement thanlongitudinal one.

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    R Rx



    F F F


    Tx TransmitterRx ReceiverC ConnectorR RepeaterF - Fiber

    S - Splice



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    POINT TO POINT LINK1. The link is the simplest fiber optic comn link wherein atransmitter at one end sends information along an optical

    fiber link to a receiver at the other end.2. The design of such a system involves, apart from thetxn type, many interrelated aspects such as type of source(LED or LD),the kind of fiber (MM or SM) and

    photodetector (APD or PIN).3. The key System requirements for the link design are:

    (a) Data or bit rate / bandwidth.(b) Bit error rate BER / SNR

    (c) Transmission distance or link length.

    4. Apart from that, the choice of various componentsalso depends on the the cost, reliability and availabilityof components, modulation/coding scheme, possibility

    of upgrading etc.

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    5. Main objective - To meet the system requirementsin a cost effective manner.

    6. Several shorter lengths of fiber in tandem to coverlonger transmission distances and to maintainadequate quality.

    7. Each such short fiber is connected to the next one

    through a regenerative repeater.

    8. Regenerative repeater incorporates a line receiverin order to convert the optical signal back into

    electrical regime. In case of analog transmission, itis amplified and equalized before it is retransmittedas an optical signal via line transmitter.

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    1. The basic issues involved in carrying out analysisof the system performance are:

    (a) Attenuation which determines the power

    available at the photo detector input for a givensource power.(b) Dispersion which determines the limitingdata rate or usable bandwidth.

    (c) These are referred to as link power and timebudgets respectively.

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    2. Generally, the link power budget is made first inwhich one determine the power margin between the

    optical transmitter output and the minimum receiversensitivity needed to establish a specified BER. Iffound unsatisfactory, then some components mightbe changed or Amplification be incorporated.

    3. Once established, the designer can prepare timebudget to ensure that the desired overall systemperformance is achieved.

    4. Choice of type of fiber to be used depends on theamount of dispersion that can be tolerated and thepower to be coupled into the fiber.

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    5. LEDs are generally used with MM fibers and lasersource can be used with both SM and MM fibers.

    6. In making the choice of the detector, minimumoptical power required to satisfy the BER requirementat the specified data rate and receiver complexity areto be considered.

    7. PIN detector is stable and simpler, but APD has tobe used when the received optical power level is verylow.

    8. On the basis of the above considerations, source,fiber and detector are tentatively decided.

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    1) Required optical power level (Pr) at the receiverto meet the system requirements, coupling lossesetc. are required to be given.

    2) An allowance has to be made for the degradation

    of components with ageing, replacements,variations due to temperature fluctuations,improper termination/connection, field repairs,maintenance, variations in drive condition and soon.

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    3) A designer has to keep a safe margin to takecare of these factors.

    4) The loss model for a point-to-point optical fiberlink is shown:

    a) Lo is the factory unit length of fiber.

    b) L is the link distance.

    c) is the attenuation coefficient of fiber in dB/km.d) Pt is the source output power (dBm).

    e) Pr(min)is the minimum receiver power (dBm)

    f) Lsfis the source to fiber coupling loss (dB).

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    5) Splice loss:

    a) Number of splicesN = [ L/Lo] integer part.

    b) Lff - Fiber to fiber coupling loss (dB).

    c) Total splice loss is

    N* Lff (dB).

    d) Fiber loss = *L.

    e) Lfd - Fiber to detector coupling loss dB.

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    Power Margin

    Pm = ( Pt - Pr(min) -Lsf - N* Lff*L Lfd ) dB

    - Apower margin of Pm 4dB is acceptable,

    otherwise some components need to beupgraded.

    - With Pm

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    LED switched on and off for binary digital txn.

    Current Rgs several tens to several hundred mili A.

    Performed at high speed to logic input Volt.

    Common method to switching LED in common Emitter


    Single stg cct provides current gain as well as small volt

    drop across switch when txtr is in saturation.

    Max current flow through LED is limited by resistor R2. Independent bias to device in provided by R3.

    Switching speed of CE in limited by space charge and

    diffusion capacitance.


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    Light output from led source should follow accurately thetime varying input voltage waveform.

    LED out power should respond linearly to input voltageor current.

    Not always because nonlinearities in LEDs which createdistortion.

    LED limits performance of analog txn sys

    Hence suitable compensation ccts are incorporated intodrive cct.

    Simple transistor drive ccts may be utilized.

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    Major component and Choices

    1. Optical fber type and parameters

    a). SM or MM

    b). Size

    c). Refractive Indexd). Attn

    e). Mode and dispersion

    f). Coupling and jointing

    g). Strength

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    2. Source type and characteristic

    a). LED or Laser

    b). Optical power launched

    c). Rise and fall time

    d). Stability

    3. Transmitter configurationa). Design of digital or analog txn

    b). Input Impedance

    c). Supply voltaged). Dynamic Rg

    e). Optical feedback

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    4.Detector Type

    a). P-n, p-i-n, or Avalanche

    b). Response time

    c). Bias volt

    d). Dark current

    5. Receiver Configurationa). Preamplifier design

    b). BER

    c). Dynamic rg

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    6. Modulation and coding

    a). Source intensity modulation

    b). Pulse modulation digital (PCM, ADM), Analog (PAM, PFM, PWM,PPM)

    c). Encoding Schemes : - biphase, delay modulation

    d). Direct intensity modulation

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    By Summing up (in db)

    1. Fiber cable loss (k in db/Km)

    2. Jointing losses to source or splicing losses (j in db/Km)

    3. Coupling losses to source & detector (cr in db)

    Over call ch loss Cl

    Cl = (fc + j ) L + cr

    L in length of fiber in 1m

    Problem 11.5A 4 Km optical fiber link with fiber attenuation of 5 db/Km. spliceslosses are 2 db/km and connection losses at source and detectorare 3.5 and 2.5 db respectively. Determine total ch loss.

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    Regarding pulse dispersion in optical fiber ch.

    Dispersion equalization penalty Dl becomes significant in high bit rate multi

    mode fiber sys

    Dl =( Te/T )4 db

    Te = 1/e full width pulse broading due to dispersion on link

    T = bit interval or pd

    Tc= 2 2

    As bit rate Bt as reciprocal of bit interval T

    Dispersion Equalization penalty

    Dl= 2 (2 Bt 2 )4

    dbTotal ch loss including dispersion equalization

    Cld = (fc+ j ) L + cr + Dl db

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    Problem 11.6

    The rms pulse broadening from intermodal dispersion in

    0.6 ns/km. estimate the dispersion equalization penaltyover unrepeated fiber link of length 8 km at bit rate :-

    a) 25 M bit/s

    b) 150 M bits/s

    Also calculate penalty with and without mode coupling


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    2. Total sys rise time determined from rise times of individual syscomponents including source on Txtr,

    Fiber cables and detector (receiver).3. Defined in terms of Gaussian response as 10 -90 % rise (or fall)

    time of individual components

    total sys rise time is

    Tsys = 1.1 (Ts2

    + Tn2

    + Tc2

    + Td2


    Ts and Td = source & detector 10- 90% rise time

    Tn = Intermodal dispersion

    Tc = Intermodal or chromatic dispersion

    Rise time tr =2.2 RCTherefore 3 db BW

    B = 1 / 2 RC

    RC= 1/ 2B

    Tr= 2.2/ 2 B = 0.35/B = Tsys

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    For RZ pulse format,

    bit rate Bt = B = 1/,

    Hence BT (max) = 0.35 / Tsys

    For NRZ BT = B/2 =

    BT (max) = 0.7/Tsys

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    Problem 11.7

    An optical fiber sys in designed to operate over 8 Kmlength without repeaters the rise time of chosencomponents are :

    Source (LED) - 8 ns

    Fiber - 5 ns/km

    (Pulse broadening) interamodal 1 ns/Km

    Detector (p-i-n photodiode) 6 ns.

    From rise time consideration, Estimate max bit rate onlimits using NRZ and RZ ?

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    Similar to power budgeting of communication sys

    When txtr character, fiber losses, rxr sensitivityknown

    Allows evaluation of repeater spacing or max txndist

    Sys margin may be included in safety margin Ma,also takes into account possible source and modalnoise, rxr impairments, noise degradation, sys

    margin is in rg of 5 to 10 dB. Sys with injection Laser margin requires larger

    safety margin (8dB ) than LED source (6 dB)

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    Optical power budget for sys isPi = Po + CL + Mo dBPi = input optional power launched into fiber

    Po = input incident optical power at rxrCL = Total ch loss

    Pi = Po+ ( jc+ J ) L + Ma dB

    Dispersion equalized penaltyPi = Po+ ( jc+ J) L + cr + L + Ma dB

    Gives max link length without repeater

    ALSO DO EG 11.8 &11.9

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    Example 11.8J M Senior

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    Example 11.9

    J M Senior

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