opcomm lightwave system

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OPTICAL FIBER COMMUNICATION SYSTEMS 1 A Gholami Isfahan University of Technology. [email protected] Lightwave System

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Page 1: OpComm Lightwave System

OPTICAL FIBER COMMUNICATION SYSTEMS

1

A Gholami

Isfahan University of Technology. [email protected]

Lightwave System

Page 2: OpComm 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.

Page 3: OpComm Lightwave System

CONTENTS

3

various system architectures

The power and the rise-time budgets

long-haul systems

Page 4: OpComm Lightwave System

WIDE AREA NETWORK

4

Page 5: OpComm Lightwave System

WIDE AREA NETWORK

5

Page 6: OpComm Lightwave System

WIDE AREA NETWORK

6

Page 7: OpComm Lightwave System

METROPOLITAN AREA NETWORK

7

Page 8: OpComm Lightwave System

METROPOLITAN AREA NETWORK

8

Page 9: OpComm Lightwave System

LOCAL AREA NETWORKS

9

Page 10: OpComm Lightwave System

SYSTEM ARCHITECTURE

10

fiber-optic communication systems can be classified

into three broad categories

point-to-point links,

distribution networks

local-area networks

Page 11: OpComm Lightwave System

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

Page 12: OpComm Lightwave System

POINT-TO-POINT LINKS

12

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.

Page 13: OpComm Lightwave System

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.

Page 14: OpComm Lightwave System

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 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)

Page 15: OpComm Lightwave System

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.

Page 16: OpComm Lightwave System

DISTRIBUTED NETWORKS

16

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

Page 17: OpComm Lightwave System

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: δ = 0.05, PT =1 mW, PN =0.1 μW N=500

Page 18: OpComm Lightwave System

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.

Page 19: OpComm Lightwave System

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

including splice and connector losses.

Page 20: OpComm Lightwave System

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= NphνB,

Page 21: OpComm Lightwave System

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.

Page 22: OpComm Lightwave System

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 4–6 dB is typically allocated during the

design process.

Page 23: OpComm Lightwave System

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 Δf of the RC circuit corresponds

Page 24: OpComm Lightwave System

RISE-TIME BUDGET

24

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.

Page 25: OpComm Lightwave System

RISE-TIME BUDGET

25

Page 26: OpComm Lightwave System

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

Page 27: OpComm Lightwave System

LONG HAUL SYSTEMS

27

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.