16.711 Lecture 9 Optical Fiber Amplifier – PDL, Transient, Cross-talk Last Lecture Amplifier Noise...
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Transcript of 16.711 Lecture 9 Optical Fiber Amplifier – PDL, Transient, Cross-talk Last Lecture Amplifier Noise...
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