8/18/2019 08 - Scan-1 Teorie OA

1/16

.

Diagramof

the

lightwave

spectrum

lnfra-red

Figure 1

. Electromagnetic

spectrum

'D**n

trt

&

coucknt\

[

n,^f '*

il1u-ul etvL

d-dr*&

wu,a

*kC*)

,

*\

-a

\

vv-

I

I

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C* tIq

f^yr#<

. nU

a

,

Ct{t-1lu}<

waveleigth

at

the-moaimff;ffii'il*:lfi'ensJrn

^o

rs e ne

as

the

The

spectrar

width

is

given

by

the

standard

deviation

o.

For

a

gaussian

signal,

o is

the

half-width

at

0.6

times

the

maximum

power

lo,

or:

o

=

0.4. w

where

w

is

the

mid-way

spectrar

harf-width

A narrow

spectrum

signartherefore

has

a

ro*

ri..ti r*iotn.

Amplitude

1.0

Wavelength

Figure

2.

Emission

spectrum

of

a

gaussian

signal

8ASe0001

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to

be copied

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written

permission.

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The

consequence

of this

phenomenon

is

to

elitend

the

duration

of

pulses.

A

signal

has

a

certain

spectral

distribution.

lts

ditferent

component

wavelengths

travel

at

different

speeds.

After

propagation

through

the medium,

the

fastest

wavelengths

arrive

befoL

tne

 slowest ,

which

has

the

effect

of altering

and

extending

the

duration

of

the

pulse.

This

is

referred

to

as

pulse

broadening .

b

)

lllustration

of

how

a

pulse

is

extended

in

time

consider 1.1

,

L2

and

L3

three

component

wavelengths

of

a

signal.

After

propagation,

the

signal

is

broadened

and

its

time

width

has

increased.

Spectrum

of

the laser

emitter

1 1 +12

+13

Transmitted

Received

O

Broadening

of a light

pulse

Tima

by

chromatic

dispersion

'

rr

I re

Figure

5.

lllustration

of

chromatic

dispersion

s

b

/xt

or i

Le

I

-a)

i r

(tz

iol

ao I

I

w

\m

n

hW*i*

cn*.*o.h

cr)

frad{

-

{^,fo*

cl*ttro,l6

^ai

hJr{

,f

c^,\ferv>ddi

,ra-

I

*f*

' Tt

Bzpc-ullnAq

cre

e.t]-auw it

'yo'ul

.,-fuc(u

c

q

(

n^,..u'dou0srA

vwu

T6'( )

(

oUutl ntaa1

-

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permission.

8/18/2019 08 - Scan-1 Teorie OA

4/16

 .

.

rncpes

of

light

emission

and

energy

bands

An

isolated

atom

has

electrons

distributed

in

layers.

These

occupy

particular

energylevels.

The

atom

inserted

in

acrystal

(consisting

of

N

iorri

is

suoyect

to

the

influence

of

neighbouring

atoms,

which

slighily

itt.r.

tne

energy

levels

of

the

electrons.

ln

total,

for

the

N

atoms

of

the

crystal,

N

energy

levels

will

be

generated.

These

distinct

energy

levels,

close

to

one

iirtn.r,

form

a

continuum

called

an

energy

band.

Onlythe

bands

corresponding

to

the

outer

electron

layer

are

involved.

These

electrons provide

the link

between neighbouring atoms

(valence band)

and

when

they

become

free,

in

other

words

when

th6

atoms

ire ionted,

they

ar-

transferred

in

the

conductionband.

Theirtotal

movement,

underthe

etfect

of

an

electricalfield, gives

rise

to

an

electric

current.

Energy

Ec

Conduction

band

Emission

of

photons

by

e-hole

recombination

'vv'\_.'

AE=hV

Valence

band

Figure

10.

Energy

bands

2 .1

.4

Detection

principle

The

advantage

of

semiconductor

materiats (which

have

three

or

five

electrons

in

the outer

layer

and

are also

called

lll-V

materials)

is

that

to

pass

from

the

valence

band

to

the

conduction

band,

the

electron

must

cross

a

potential

barrier

known

as the

energy

gap

(Eg).

An

electron

in

the

valence

band

that receives

an

energy

E

>

Eg

(in

electrical

or

photon

form)

can move

to the

conduction

band.

This

free

electron

can

then

participate

in

creating

an electrical

current.

Photodetection

uses

this

phenomenon.

When an

electron becomes

free,

it

leaves

a

vacant

space

on

the atom

that

it

was

on. This

creates

a

hole

in

the

valence

band.

This

is

commonly

referred

to

as

an

electron-hole

pair.

8AS

90001

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permission.

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A

photon

can induce

the recombination

of

an electron-hole

pair.

ln

this

case,

the

electron

loses

energy and restores

it

in

the form

of

a

photon

that

has

exactly

the

same

characteristics

as

the

photon

that

induced

the

recombination.

Thii

is

the

principle

of

stimulatedemission.

ln

short,

one

photon

gives

rise

to

another

photon

that

has

the

same

characteristics,

which

makes

iipossible

to

obtain

very narrow

spectral

widths.

This

is

the

basic

principle

of

optical

amplification.

Diagrams

illustrating

light

emission

Stimulated

emission

Figure 11.

Principle

of

spontaneous

and

stimulated

emission

Light

emitting

diodes

(LED)

Conduction

'

band

-l-

I

I

Gap

I

I

I

Valence

band

2 .2 .1

Light

emitting

diodes

(LEDs)

Lightemitting

diodes

(LEDs)

basically

consist

of

a

PN

junction.

Atthe

interface

of the

junction,

a current

creates electron-hole

pairs

which

recombine

to

generate

light

by spontaneousemission.

2.1

.6

2.2

lncident

photon

,%

lncident

photon

%

l+)

^rJ

tnguced photon

Spontaneous

emission

14146

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rgE

8AS

90001

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-

rrsrrvrrrsrr

e .

s,

photons

must

be confined

in

a

region

containing

a high

density

of

electron-hole

pairs.

A

photon

confined

in

such

a region

will

induce

the

emission

of

a second

photon

which

will

be in

every

respect

iden1c4to

it.

step-by-step,

through

successive

stimulated

emissions,

a

signal

consistingof

identical

photons

is

created.

The

signal

created

in

this

way

has

a narrower

spectralwidth

(limiting

chromatic

dispersion).

A

cavity

formed

by

two

semi-

reflecting

mirrors

provides

the

confinement

for

the

photons.

Part

of

the light

is

transferred

to the

active region

(light

confinement),

the other

part

is

transmitted

and forms

the

laser

emission.

This

cavity known

as

the

Fabry-perot

cavity,

selects

a

number

of

wavelength.

Confinement

in the

active medium

can

be

supplemented

by

different

techniques

(index guiding,

gain

guiding,

etc)

defining

different

Vpes

of

laser.

3

)

Diagram

of laser

cavities

Semi-reflecting

mirror

[

___-_----_:

h==_=_=_:_;.-----

Figure

1

4.

Fabry-

Perot

cavity

Semi-reflecting

mirror

rf>

Laser

emission

8AS

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17146

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7/16

is

referred

to

as

an

n

x,

if,u;;;Y''r'r'vr

- . rr

I

n s

.

lnsertion

losses

Acouplerwill

introduce

insertion

losses

expressed

in

dB

which

are

generally

less

than

1

dB.

.

Poweroutputdistribution

A

coupler

is

used

to

tap

part

of

the

signal.

This

is

expressed

in

dB

or

as

a

percentage

with

respect

to

the

output

power.

A

coupler

would

be

referred

to

as a

50/50

or 9S/5

coupler

or

a

-3

dB

coupler.

.

Reflection rate

For a

coupler,

the reflection

rdte

is

also

defined

equivalent

to

the fraction

of

powerreflected.

4

.2

.2

Example

of

using

a

coupler

lnput

signal

Outputsignal

OJ

-*'

Power

measurement

photodiode

Figure

28.

Example

of

using

couplers

95/5

coupler

8AS

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5.3.2

Example

Figure

35. Example

of using

an optical

filter

for

filtering

optical

noise

Figure

36.

Example

ol

using

a tunable filter

for

selecting

a

wavelength

channel

ffiit-t

8AS 90001 011

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46146

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9/16

to the incident

signal,

generates

an

optical

noise

which

reduces

the

s

ig

nal

-to -

noise

ratio

an

d

therefore

transm

ission

q

ual

ity.

1

.1

.3

Main

physicalphenomena

Absorptlon

O

\

+

's

I

%

|

__t

-T

Pump

'rn-n+

I

photon

(dp)

a_t

Spontaneous

emission

o

I

lrs

l*-

lo

Stlmulatedemlssion

o

fs

+

fs

Figure

2.lllustration

of

absorption

and spontaneous

and

stimulated

emission

1 .1

.4 Basic

OA

configuration

An

amplifier

is

built

around

a

doped

fibre

which

is the

amplifying

medium.

A

wavelength

division

multiplexer is

used

to

inject

the

signalto

be

amplified

and

the

optical

pump

power

into

the

doped fibre.

lnput

connector

Output

connector

Pump

residue

Output signal

Figure 3.

Basic configuration of

an

OA

amplified

signal

/

lnput signal

8AS 90001

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1

.3

Types

of optical

amplifier

The

characteristics

of

amplifiers,

make

them

particularly

useful

modules

for

opticaltransmission

systems.

Their intended

application

leadstothe

selection

and

optimization

of

certain

parameters.

1 .3.1

Post-amplifiers

These

amplifiers are used at the transmitting

end

to

provide

a high

optical

power

(up

to

+30

dBm).

Their

characteristics

are

defined to optimize

output

power.

They are

characterized by

high

pump

powers

and

special

internal

confi

gurations (double

pumping).

Figure

7.

Functional diagram

of a

post-amplifier

1

.3

.2

Pre-amplifiers

Pre-amplifiers

are used at

the

receiving

end

to

amplify low

power

signals

beforethe

detection stage

(<

1

0

dBm).

They

are

optimized

to

provide

high

gain

(greater

than 20 dB) and very

low noise

(F

<

5 dB). They

can

be used

to

increase

the

sensitivity

of

the detector by

more

than

10

dB.

Figure 8.

Functional diagram

of

a

pre-amplifier

lifier

\

Pre-amp

8AS 90001

01

19

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rrE

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1

.3

.3 Line

amplifiers

1

.3

.4

Example

These

amplifiers

are

used for long

links

requiring

amplification

of

the

signal

during

transmission.

They are

characterized by low

noise

and

gains

varfing

from

10

to

20 dB.

Taking

into

hccount

installation

conditions-(for

exampl6

submarine

networks)

where reliability

is

particularly

important,

strategic

components

such

as

pump

lasers

undergo

rigorous

qualification

tests.

Figure

9.

Functional

diagram

of

a

line

amplilier

90

to

10

coupler

lsolator

Erbium

WDM

dopedfibre

WDM

Optical

filter

90to

10

coupler

Figure

10. Post-amplifier

on

5

GbiVs

submarine

system

.

Characteristics

This

amplifier

is

used

to

provide

a constant

output

power

regulated

to

+13

dBm. Part

of the

output

power

is

tapped

by

a

90/10

coupler

and

measured.

The

measurement

is

compared

with

a

set

point.

The

result

of

the

comparison

is

used

to

regulate

the

power

of

one

of the

pumps

(by

adjustingthe

bias

current)

to obtain a

constant

output

powerirrespective

of

the input

power.

The 90/1

0

input

coupler

is used

to detect

the

presence

of

the input

signal.

Line

amplifier

l.

=

1480nm

l.

=

1480

nm

V

@IrI

8AS

90001

01

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ZZAEd.l

2121

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4.3

Operatingprinciple

Figure

7.

Wavelength

dlvislon multlplexlng

principle

(example:

16

channels)

4.4

Example:Multiplexingeightwavelengths

Signal transmitted

on

line

Signalreceived

12

SNR

=

1O.8

dB

1558

1560

Wavelength

(nm)

1fi2

1552 1554

1556

1s58

Wav€length

(nm)

1

560

4000 km

-

60

Repeaters

Figure

8. Wavelengthdlvlsion multlplexlng

-ft*rtr

8AS 90001 0120

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4.5

Generatcharacteristics

--'

f

ru-ri''tttwtcqrd4

[#

c'szt'otu'rau-zt

F

,t*,r*u{ol7an

/

/

.

Pre-emphasis:

Amplifiers

do

not

have

a

gain

that

is

absolutely

flat

as

a

function

of

wavelength.

When

severalamplifiers

are connected

in

cascade

(as

in

long

distance links), it

is

necessary

to

provide

more

power

on

channels

for

which

the

gain

is

lower, in

order

to

obtain

the

same

SNR

for

all

channels.

This

technique

is

called

pre-emphasis.

.

Dropping

and adding

a

channel:

It is

possible

to

drop and

add

wavelengths

using

optical

dem u

lti

plexers/mu

lti

plexers(WDM)

.

0

-5

1554 1556 1558 1560

1562

wavelength

(

nm

)

1552 1554 1556

1558 1560

1552

wavelength

(

nm

)

WADMoutput

wlth

added

channel

(A)

1552 1ss4

1556

1ss8

1s60 1562

wavelength

(

nm

)

0

-5

^-10

E

-rn

It

-

-20

o

e

-25

CL

-30

-35

5

-10

^-

10

E

-r.

rv

t

-

-20

rtt

z

-25

CL

^

-15

E

-ro

E

:

-2s

o

E

-so

cL

-35

-40

552

Flgure

9. lllustration of

wavelengith

add/drop

functions

8AS 90001

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21

127

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3

.5 Example

ADM

+

Post-amplification

+

Pre-amplification

+

Transponder

(FEC)

+ Remote

pre-amplification

+

Remote

pre-amplification

Commercial

spans

at

2.5

GbiVs

110km

215

km

270km

Figure

6.

Repeaterless

submarlne

system contigurations

according

to

transmlsslondlstance

D

f=

[]_.

(lUp.urq\

A/Yt

?cq+

dutA'st'

re'rynJ

/ M

@

@

V

@

8AS

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8127

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2.1

2.2

2

VARIABLE

OPTICAL

ATTENUATOR

Functions

The

optical

attenuator

is

used

to

attenuate

a

signal.

'

lt

is

used

in

particularto

attenuate

and

adjustthe

signalatthe

output

of

a

high

power

source.

'

lt lets

you

simulate

the

attenuation

of

a transmission

fibre.

'

Positioned

in

front

of

a

receiver,

it

lets

you

measure

the

detection

threshold.

Operating

principles

This

instrument

consists

of

an input

port

and

an

output

port.

There

are

difierent

technological

solutions.

one

technique

commonly

used

is

based

on

the

following

principles.

On

a

light

path,

moving

the relative

position

of

two

prisms

consisting

of

an

absorbantmedium

alters the length

of

material passed

through

and

th6refore

the

attenuation.

2.3

Functionaldiagram

lnput

signal

Output

signal

Figure

2.

Diagram

of

an

optical

attenuator

V

rrlrr

8AS

90001

0122VH

ZZAEd.l

/13

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2 .4

Characteristics

An

optical

attenuator has

the

following

characteristics:

.

dynamic

attenuation

range

(from

5

to

50

dB),

.

resolution (0.1

to 0.01

dB),

.

precision

(of

the

order

of 0.1 dB),

.

attenuator insertion

losses

(from

2 to

5 dB),

.

operating

wavelength (can

be

adjusted

on certain

types

of attenuator),

.

reflection

rate

(from

30

to

60 dB),

.

type:

manual

or

with

mechanical or

digital control mechanism,

.

input

interface

connectors

(often

modifiable).

2 .5

Examples

of

applications

Laser

emitter

Fixed

power

Variable

attenuator

I

I

Figure

3.lmplementation

of

an adjustable

power

optical

source

Variable

power

->

8AS90oo1

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