16.711 Lecture 9 Optical Fiber Amplifier

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

• Amplifier Noise
• OSNR and BER
• System Applications

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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

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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
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16.711 Lecture 9 Optical Fiber Amplifier PDG,
Transient, Cross-talk
Polarization Dependent Gain (PDG)

• PDG and input signal Power

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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

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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

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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
ms
ps
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16.711 Lecture 9 Optical Fiber Amplifier PDG,
Transient, Cross-talk
Transient effect
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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.

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

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

• Preamp --- receiver sensitivity improvement by
  EDFA

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

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16.711 Lecture 8 Optical Fiber Amplifier noise
and BER
Receiver noise
Total photo-detector current
R is the resistance of the detector load
resistor.
Total ASE power