faq-how ngwdm amplifier works and how to calculate the osnr

5/22/2018 FAQ-How NGWDM Amplifier Works and How to Calculate the OSNR

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Optical Fibre Communication Systems

Optical Amplifier

Signal Reshaping and Amplification

In long distance communications, whether going through wire, fibre or wave, the signal carrying the

information experience: Power loss and Pulse broadening which requires amplification and signal

reshaping, In fiber optics communications, these can be done in two ways:

Opto-electronic conversion

ll optical

!epending on its nature, a signal can also be regenerated"

digital signal is made of #$s and %$s: it is possible to reconstruct the signal and amplify it at

the same time"

n analog signal however, cannot be reconstructed because nobody &nows what the original

signal loo&ed li&e"

Why the Need for Optical Amplification?

'emiconductor devices can convert an optical signal into an electrical signal, amplify it and

reconvert the signal bac& to an optical signal" (owever, this procedure has several

disadvantages:

)ostly

*equire a large number over long distances

+oise is introduced after each conversion in analog signals which cannot be

reconstructed

*estriction on bandwidth, wavelengths and type of optical signals being used, due to

the electronics

.y amplifying signal in the optical domain many of these disadvantages would disappear/

Optical Amplification

mplification gain: 0p to a factor of #%,%%% 12% d.

In 3!4: 'everal signals within the amplifier5s gain 6 bandwidth are amplified, but not to

the same extent

It generates its own noise source &nown as mplified 'pontaneous 7mission '7 noise"

2014-07-08 HUAWEI Confidential Page1, Total11


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Optical Amplification - Spectral Characteristics

Optical Amplification - Noise Figure

*equired figure of merit to compare amplifier noise performance

!efined when the input signal is coherent

+8 is a positive number, nearly always 9 I"e" ; d.

6ood performance: when +8 < ; d.

+8 is one of a number of factors that determine the overall .7* of a networ&"

Optical Amplifiers ypes


-,rationoisetosignalOutput

-,rationoisetosignalInput,+8-8igure+oise

o

i

SNR

SNR

=


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here are mainly t!o types"

'emiconductor =aser optical mplifier '= 'O

ctive-8ibre or !oped-8ibre

#rbium $oped Fibre Amplifier %#$FA&

8ibre *aman mplifier 8*

>hulium !oped 8ibre mplifier >!8

S'A - (rinciple Operation

*emember diode lasers?

'uppose that the diode laser has no mirrors:

- 3e get the diode to a population inversion condition

- 3e in@ect photons at one end of the diode .y stimulated emission, the incident signal will be amplified/

.y stimulated emission, one photon gives rise to another photon: the total is two

photons" 7ach of these two photons can give rise to another photon: the total is then

four photons" nd it goes on and on"""

(roblems"

Poor noise performance: they add a lot of noise to the signal/

4atching with the fibre is also a problem/

(owever, they are small and cheap/



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7!8 is an optical fiber doped with erbium"

7rbium is a rare-earth element which has some interesting properties for fibre optics

communications"

Photons at #2A% or BA% nm activate electrons into a met stable state

7lectrons falling bac& emit light at #CC% nm"

.y one of the most extraordinary coincidences, #CC% nm is a low-loss wavelength

region for silica optical fibers"

>his means that we could amplify a signal by using stimulated emission"

7!8 is a low noise light amplifier"



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D vailable since #BB%5s:

D 'elf-regulating amplifiers: output power remains more or less constant even if the input

power fluctuates significantly

D Output power: #%-; d.m

D 6ain: ;% d.

D 0sed in terrestrial and submarine lin&s

D 4ost of the pump power appears at the stimulating wavelength

D Power distribution at the other wavelengths changes with a given input signal"



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Optical Amplifiers" )ulti-!a*elength Amplification

Optical Amplifier - )ain (arameters

6ain PoutEPin

.andwidth

6ain 'aturation

PolariFation 'ensibility

+oise figure '+*iE'+*o

6ain 8latness

>ypes

.ased on stimulated emission 7!8, P!8, 'O

.ased on non-linearities *aman, .rillouin



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Optical Amplifier - Optical +ain %+&

GG S OutputE S Input +o noise

Input signal dependent:

Operating point saturation of

7!8 strongly depends on

power and wavelength of

incoming signal

6ain H as the input power

Pin +ain Pout

-% d.m ;% d. 1#% d.m

-#% d.m C d. 1#C d.m

+ote, Pinchanges by a factor of ten then Poutchanges only by a factor of three in this power range

Optical SNR

8or .7* J #%-#;the following O'+*s are required:

< #; d. for '>4-#K E O)-2A "C 6bps

< #A d. for '>4-K2 E O)-#B #% 6bps

Optical power at the receiver needs to bigger than receiver sensitivity

Optical mplifiers give rise to O'+* degradation due to the '7 generation and

amplification



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+oise 8igure G O'+*inEO'+*out

>herefore for a given O'+* there is only a finite number of amplifiers that is to say a finite

number of spans

>hus the need for multi-stage O design

System (erformance" OSNR 'imitation

C 'pans x C d.

; )hs" L "C6bEs with #; d. O'+*

.7* J #%-#;

)hannel )ount E 'pan =oss >rade-Off: C spans x d.

K2 chs L "C6bEs with #; d. O'+* and .7* J #%-#;

Optical Amplifiers" )ulti-Stage

#st ctive stage co-pumped: nd stage counter-pumped: optimiFed for high power

OptimiFed for low noise figure

1 NF#stEnd stage GPin- SNRo Md.N - #% =og hcE ; andNFtotalGNF#1NFEG#



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In a transmission system with several cascaded 7!8s, similar to the original signals, the

'7 noise can also be attenuated or amplified" s the entered '7 noise in each optical

amplifier is amplified and overlapped, the total '7 noise power is increased in proportion to

the number of optical amplifiers" >he '7 noise power might be higher than the signal power"

>he '7 noise spectrum varies with the system length" 3hen the '7 noise from the first

optical amplifier is sent to the second one, the gain spectrum of the second one also causes

'7 noise because of gain saturation" s a result, the gain spectrum varies" 'imilarly, the

valid gain spectrum of the third one also varies" 'uch an effect is passed to the next and next

one downstream"

7ven the narrow band filters cannot avoid the '7 noise, because the '7 noise exists in the

same band where the original signal exists"

>he '7 power in m3 per unit frequency interval for an optical amplifier is given by I>0-

> 6"KB :

+'P: spontaneous noise factor, +'P#

2014-07-08 HUAWEI Confidential Page, Total11

hGNPSPASE -#,: =


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6: amplifier gain

h: Planc&$s constant K"KKx#%-;2 3s

: optical frequency in (F

>he amplifier noise figure in d. is given by I>0-> 6"KB :

,N" input coupling loss for the amplifier in d.

(AS#%d.m& / -01 %d.m& 2 NF %d.& 2 + %d.& 3 ,N / 4

If the total output power including accumulated '7 power is equal after each amplifier, and

that the gain 6 99 #, then the O'+* is given approximately by I>0-> 6"KB :

Pout: output power per channel in d.m

=: span loss between amplifiers in d.

+8: noise figure in d.

Q%: optical bandwidth

+: number of spans in the chain, and we have assumed that all the span losses are

equal"

In the #"CCRm band, #%=og h Q% G -CAd.m at %"#nm optical bandwidth"

Raman Amplifier


IN

SP

SP

G

NNNF +

=

#::=og#%

[ ]%=og#%=og#% = hNNFLPOSNR out


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D Offer C to S d. improvement in system performance

D 8irst application in 3!4


faq-how ngwdm amplifier works and how to calculate the osnr

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