OpComm Lightwave System

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communication system fiber optic principle

Text of OpComm Lightwave System

  • OPTICAL FIBER COMMUNICATION SYSTEMS

    1

    A Gholami

    Isfahan University of Technology. gholami@cc.iut.ac.ir

    Lightwave System

  • LIGHTWAVE SYSTEMS

    2

    The preceding three chapters focused on the

    three main components :

    optical fibers,

    optical transmitters, and

    optical receivers

    In this chapter we consider system design and

    performance when the three components are put

    together to form a practical lightwave system.

  • CONTENTS

    3

    various system architectures

    The power and the rise-time budgets

    long-haul systems

  • WIDE AREA NETWORK

    4

  • WIDE AREA NETWORK

    5

  • WIDE AREA NETWORK

    6

  • METROPOLITAN AREA NETWORK

    7

  • METROPOLITAN AREA NETWORK

    8

  • LOCAL AREA NETWORKS

    9

  • SYSTEM ARCHITECTURE

    10

    fiber-optic communication systems can be classified

    into three broad categories

    point-to-point links,

    distribution networks

    local-area networks

  • POINT-TO-POINT LINKS

    11

    The link length can vary from less than a Km (short

    haul) to thousands of Km (long haul), depending on

    the specific application.

    Short haul

    Loss is not important

    Bandwidth may be important

    Mostly used in local area networks

    Long haul

    Loss is important

    Bandwidth is important

  • POINT-TO-POINT LINKS

    12

    Long haul

    Loss compensating through regenerators

    Regenerator is a receivertransmitter pair that detects the

    incoming optical signal, recovers the electrical bit stream,

    and then converts it back into optical form.

  • POINT-TO-POINT LINKS

    13

    Long haul

    Loss compensating through Optical Amplifiers

    optical amplifiers amplify the optical bit stream directly.

    The advent of optical amplifiers around 1990 revolutionized

    the development of fiber-optic systems.

    Amplifiers are especially valuable for WDM lightwave

    systems as they can amplify many channels simultaneously.

    Optical amplifiers add noise

    signal degradation as fiber dispersion and nonlinearity

    keeps on accumulating over multiple amplification stages.

  • POINT-TO-POINT LINKS

    14

    Terrestrial systems use a combination of the two techniques and place

    an optoelectronic regenerator after a certain number of optical

    amplifiers.

    Submarine systems are often designed to operate over a distance of

    more than 5000 km using only optical amplifiers.

    The repeater spacing between regenerators or optical amplifiers is a

    major design parameter because determines the system cost.

    The bit ratedistance product, BL, is generally used as a measure of

    the system performance for point-to-point links.

    The first three generations of lightwave systems:

    0.85m, BL 1 (Gb/s)-km

    1.3m, BL 1 (Tb/s)-km

    1.55 m, BL 1 (Tb/s)-km

    It exceed 1000 (Tb/s)-km for the fourth-generation systems.(WDM)

  • DISTRIBUTED NETWORKS

    15

    Many applications require that

    information transmitted and also

    distributed to a group of subscribers.

    (telephone services, cable TV, Internet )

    Integrated Services Digital Network (ISDN)

    is a set of communication standards for

    simultaneous digital transmission of

    voice, video, data, and other network

    services over the public switched

    telephone network.

    L < 50 km, but B could be up to 10 Gb/s

    for a broadband ISDN.

  • DISTRIBUTED NETWORKS

    16

    In bus topology, power available at the Nth tap is given by:

    where PT , C and are transmitted power, fraction of power coupled

    out and insertion loss of each tap. This topology called passive

    optical network (PON)

    Example: = 0.05, C = 0.05, PT =1 mW, PN =0.1 W N=60

    N should not exceed 60

  • LOCAL AREA NETWORKS

    17

    The ring and star are the main topologies for LAN

    applications.

    In the ring topology, consecutive nodes are

    connected by point-to-point links to form a closed

    ring.

    ring topology for fiber-optic LANs has been

    commercialized with the standardized interface

    known as the fiber distributed data interface

    (FDDI)

    In the star topology, all nodes are connected

    through point-to-point links to a central node

    called a hub, or simply a star.

    Star topology sub classified as active-star or

    passive-star networks. In passive the power in a

    node is given by:

    Example: = 0.05, PT =1 mW, PN =0.1 W N=500

  • FIBER OPTIC SYSTEM DESIGN

    18

    The design of fiber-optic communication systems requires a clear

    understanding of the limitations imposed by the loss, dispersion,

    and nonlinearity of the fiber.

    Since fiber properties are wavelength dependent, the choice of

    is a major design issue.

    In this section we discuss how the bit rate and the transmission

    distance of a single-channel system are limited by fiber loss and

    dispersion.

    The power and rise-time budgets illustrate loss and bandwidth of

    the system.

    The power budget is also called the link budget, and the rise-time

    budget is sometimes referred to as the bandwidth budget.

  • LOSS LIMITED LIGHTWAVE SYSTEMS

    19

    Except for some short-haul fiber links, fiber losses play

    an important role in the system design.

    Consider an optical transmitter which launches an

    average power P and a receiver with a sensitivity of Prec

    at the bit rate B, the maximum transmission distance is

    limited by:

    where f is the net loss (in dB/km) of the fiber cable,

    including splice and connector losses.

  • LOSS LIMITED LIGHTWAVE SYSTEMS

    20

    The bit-rate dependence of L arises from the linear dependence

    of Prec on the bit rate B. Noting that Prec= NphB,

  • DISPERSION LIMITED LIGHTWAVE SYSTEMS

    21

    When the dispersion-limited transmission distance is shorter than the

    loss-limited distance, the system is dispersion limited.

    NTT Co. in 2010 established a 170 Gb/s WDM link over 240km and for

    432 channels.

  • POWER BUDGET

    22

    The purpose of the power budget is to ensure that enough power

    will reach the receiver.

    where CL is the total channel loss and Ms is the system margin.

    A system margin of 46 dB is typically allocated during the

    design process.

  • RISE-TIME BUDGET

    23

    The purpose of the rise-time budget is to ensure that the system

    is able to operate properly at the intended bitrate.

    For a RC circuit. The rise time is found to be given by:

    The bandwidth f of the RC circuit corresponds

  • RISE-TIME BUDGET

    24

    In the case of NRZ modulation format f B/2 and for RZ

    modulation format, f B then:

    The three components of fiber-optic communication systems

    have individual rise times.

    Ttr is a few ns for LED but can be shorter than 0.1 ns for lasers.

    The receiver rise time Trec is determined y by the 3-dB electrical

    bandwidth of the receiver.

  • RISE-TIME BUDGET

    25

  • LONG HAUL SYSTEMS

    26

    Fiber losses can be compensated by amplifiers periodically along

    a long-haul fiber link.

    Fiber dispersion (GVD) can be reduced by using dispersion

    management.

    For single-channel lightwave systems, the dominant nonlinear

    phenomenon that limits the system performance is SPM.

    To reduce the impact of SPM in lightwave systems, it is necessary

    that:

    Where NA is the amplifier numbers

    Performance-Limiting Factors

    Nonlinear effects: SPM for single channel

    Amplifier noise

    Polarization effect

  • LONG HAUL SYSTEMS

    27

    When regenerators are used: the SPM effects accumulate only

    over one repeater spacing Pin