opcomm lightwave system
DESCRIPTION
communication system fiber optic principleTRANSCRIPT
OPTICAL FIBER COMMUNICATION SYSTEMS
1
A Gholami
Isfahan University of Technology. [email protected]
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
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METROPOLITAN AREA NETWORK
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LOCAL AREA NETWORKS
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SYSTEM ARCHITECTURE
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fiber-optic communication systems can be classified
into three broad categories
point-to-point links,
distribution networks
local-area networks
POINT-TO-POINT LINKS
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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
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Long haul
Loss compensating through regenerators
Regenerator is a receiver–transmitter pair that detects the
incoming optical signal, recovers the electrical bit stream,
and then converts it back into optical form.
POINT-TO-POINT LINKS
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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
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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 rate–distance product, BL, is generally used as a measure of
the system performance for point-to-point links.
The first three generations of lightwave systems:
0.85μm, BL ∼1 (Gb/s)-km
1.3μm, BL ∼1 (Tb/s)-km
1.55 μm, BL ∼1 (Tb/s)-km
It exceed 1000 (Tb/s)-km for the fourth-generation systems.(WDM)
DISTRIBUTED NETWORKS
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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
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In bus topology, power available at the Nth tap is given by:
where PT , C and δ are transmitted power, fraction of power coupled
out and insertion loss of each tap. This topology called passive
optical network (PON)
Example: δ = 0.05, C = 0.05, PT =1 mW, PN =0.1 μW N=60
N should not exceed 60
LOCAL AREA NETWORKS
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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: δ = 0.05, PT =1 mW, PN =0.1 μW 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 λ
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
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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 α f is the net loss (in dB/km) of the fiber cable,
including splice and connector losses.
LOSS LIMITED LIGHTWAVE SYSTEMS
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The bit-rate dependence of L arises from the linear dependence
of Prec on the bit rate B. Noting that Prec= NphνB,
DISPERSION LIMITED LIGHTWAVE SYSTEMS
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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
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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 4–6 dB is typically allocated during the
design process.
RISE-TIME BUDGET
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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 Δf of the RC circuit corresponds
RISE-TIME BUDGET
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In the case of NRZ modulation format Δf ≈ B/2 and for RZ
modulation format, Δf ≈ 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
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LONG HAUL SYSTEMS
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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
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When regenerators are used: the SPM effects accumulate only
over one repeater spacing Pin<22 mW for L=100Km it is
not very important (γ =2 W−1/km, NA=1)
when in-line 10 cascaded amplifiers are used: Pin<2.2 mW for
L=1000Km. It could be dominant (γ =2 W−1/km, NA=10)
The SPM effects are most dominant inside dispersion-
compensating fibers for which Aeff ≈20 μm2.
Appropriate chirping of input pulses can also be beneficial for
reducing the SPM effects.
This feature has led to the adoption of a new modulation format
known as the chirped RZ or CRZ format.
the noise of optical amplifiers is quantified through an amplifier
noise figure Fn.
The polarization effects that are totally negligible in the
traditional “nonamplified” lightwave systems become of concern
for long-haul systems with in-line amplifiers.