Transcript 16.711 Lecture 9 Optical Fiber Amplifier – PDL, Transient

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
Polarization Dependent Gain (PDG)
• Why PDG is important?
• Where the PDG originates?
(1) Polarization difference between Maximum ASE and the signal
PDG ~ 0.01dB
(2) Polarization difference between pump laser and the signal
PDG ~ 0.05dB
16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk
Polarization Dependent Gain (PDG)
• PDG and input signal Power
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:
q 2  S sq / Ein Ti ,
16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk
Transient effect
• Two important time scales in Optical amplifier:
(1) Stimulated emission: ~ ps
4
(2) Carrier life-time at energy level I13/ 2 ~ ms
ms
ps
16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk
Transient effect
• Population density of the upper level 4 I13/ 2
response only to the average signal power for bit rate >10Kb/s.
16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk
Transient effect
4
• Change of population density of the upper level I13/ 2
takes ~ 1ms to settle down.
16.711 Lecture 9 Optical Fiber Amplifier – PDG, Transient, Cross-talk
Transient effect
• Ways to reduce the Transient effect.
(1) Pump feedback to keep constant population density 4 I13/ 2
(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:
S
G 2 S min ' S
( ) EDFA 
 ,
N
N  N' N
S
G2N

 1,
'
S min N  N '

2
S  ASE
P in 
N '  s2 ASE 4RPin nspG (G  1)


,
2
N
T
4kT
 4PinnspG(G 1),
 T2 
4kT
,
R
kT
 470w,
Rn sp
Q2
exp(  )
1
2 ,
BER 
Q
2
Q
Is
GPin

,
2
2
N total (1)  N total (0)
 T   s  ASE
• 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:
itotal  ( Es  En ) 2 , P  Psignal  PASE  GPs ,in  S ASE v,
2
2
2
2
2
2
itotal
, noise   T   short S   short ASE   S  ASE   ASE  ASE ,
 T2 
4kT
, R is the resistance of the detector load resistor.
R
 S2 ASE  4PinnspG(G 1),
Total ASE power:
2
 short
 S  2eGPin B0 ,
Ps ,in
G
S
,
  
2eB 1  2nsp (G  1)
 N out
Ps ,in
S
,
  
N
2
eB
 in
( S / N )in 1  2nsp (G  1)
NF 

 2nsp ,
( S / N )in
G