16.711 Lecture 9 Optical Fiber Amplifier – PDL, Transient, Cross-talk

Last Lecture

• Amplifier Noise• OSNR and BER• System Applications

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Today

• Polarization dependent gain (PDG)• ASE induced Time Jitter• Cross-talk, spectral hole burning and gain clamping• Transient• Amplifier Examples

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

• Where the PDG originates?

Polarization Dependent Gain (PDG)

(1) Polarization difference between Maximum ASE and the signal

(2) Polarization difference between pump laser and the signal PDG ~ 0.01dB

PDG ~ 0.05dB

• Why PDG is important?

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

• PDG and input signal Power

Polarization Dependent Gain (PDG)

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Polarization Dependent Gain (PDG)

• Solution to reduce PDG: Polarization scrambling

• Improve Q factor by 2dB

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

ASE induced time jitter • What is time jitter? • Why time jitter is important? • ASE induced time jitter

(1) ASE generated carrier density fluctuation

(2) Index change due to the carrier density fluctuation --- time jitter• Analysis is complex, approximately: ,/2

iinsq TESq

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Transient effect • Two important time scales in Optical amplifier:

(1) Stimulated emission: ~ ps

(2) Carrier life-time at energy level ~ ms 2/134I

ps

ms

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Transient effect • Population density of the upper level response only to the average signal power for bit rate >10Kb/s.

2/134I

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Transient effect • Change of population density of the upper level takes ~ 1ms to settle down.

2/134I

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Transient effect • Ways to reduce the Transient effect.

2/134I(1) Pump feedback to keep constant population density

(2) Gain clamping, --- keep the constant input power.

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Cross-talk, multi-channel amplifying • Any multi-channel cross-talk when all channel present?

• Why not, (valid Only for EDFA) ?

• Cross-talk when a channel is turned on and off.

16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk

Examples • Preamp --- receiver sensitivity improvement by EDFA:

,'

')( min

2

N

S

NN

SG

N

SEDFA

,1'

2

'min

NN

NG

S

S,

4

)1(4'2

2

kT

GGnRP

N

N spin

T

ASEs

),1(42 GGnP spinASES ,42

R

kTT

,470 wRn

kTP

spin

,)

2exp(

2

1

2

Q

Q

BER

,)0()1( 22

ASEsT

in

totaltotal

s GP

NN

IQ

• Preamp --- receiver sensitivity improvement not valid for Pin arbitrary small.

16.711 Lecture 8 Optical Fiber Amplifier – noise and BER

Receiver noise

Total photo-detector current:

,)( 2nstotal EEi

Total ASE power:

,42

R

kTT R is the resistance of the detector load resistor.

,, vSGPPPP ASEinsASEsignal

,222222, ASEASEASESASEshortSshortTnoisetotali

),1(42 GGnP spinASES

,2 02 BGPe inSshort

,)1(212

,

Gn

G

eB

P

N

S

sp

ins

out ,2

,

eB

P

N

S ins

in

,2)1(21

)/(

)/(sp

sp

in

in nG

Gn

NS

NSNF