Optical Amplifier

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Optical Amplifier
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Generic optical amplifier
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Optical Amplifiers
Semiconductor Optical Amplifier
Erbium Doped Fiber Amplifier

• Important Parameters
• Gain
• Saturation Output Power
• Noise Figure

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Optical Amplifier Applications
B. Verbeek, JDSU
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Applications of optical amplifiers
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Gain bandwidth of optical amplifiers
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Amplifier Comparison
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Gain Saturation

• Output saturation power is defined as the output
  power when gain drops by 3db
• Power amplifiers usually operate at saturation.
• Saturation gain is lower than the unsaturated one.

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Amplifier gain versus power
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Gain versus Amplifier length
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Gain versus pump level
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Noise Sources
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Noise Figure

• Noise Figure definition is similar as for
  electrical amplifiers. Essentially a degradation
  of signal.
• However, we do not use the optical SNR, but
  rather the SNR that would be measured with an
  ideal square-law detector at the input and
  output of the amplifier.

Where EElectric Field, IDetector
Current, ASignal amplitude, x,yAmplifier
Spontaneous Emission
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Noise Figure

• NF definition assumes shot-noise limited source.
  Laser noise is ignored.
• Detector thermal noise is ignored/negligible.

Noise Figure
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2.5
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1.5
1
0.5
0
5
10
15
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Gain (dB)
3 dB NF limit, for complete inversion, high gain
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EDFA

• EDFA has revolutionized optical communications
• All optical and fiber compatible
• Wide bandwidth, 2070 nm
• High gain, 2040 dB
• High output power, gt200mW
• Bit rate, modulation format, power and wavelength
  insensitive
• Low distortion and low noise (NFlt5dB)

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Pump Source

• 980 nm
• low ASE, low noise amplifier
• 1480 nm
• higher power pump laser
• high output power
• not as efficient
• degree of population inversion is lower

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Gain Spectrum

• Amorphous nature of silica and the codopants
  inside the fiber affects the spectrum
  considerably.

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Gain Spectrum

• Population at different levels are different
  resulting gain dependence on wavelength
• Different pumping level has different spectrum

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EDFA configurations
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EDFA Gain Transient

• Channel turn-on, re-routing, network
  reconfiguration, link failure.

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Gain Transient

• Power may become too high (nonlinearity) or too
  low (degrade SNR) when add/drop channels
• transient happens in us to ms
• transient penalty depends on data rate, number of
  EDFAs and number of channels.
• power increase degrades performance due to SPM

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EDFA Transient Dynamics
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Semiconductor Optical Amplifier
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Operating Principle

• device physics same as EEL.
• difference is that Rlt 10-5 AR, angled stripe,
  window region
• SOA can be operated in saturation, or
  unsaturated. gain clamping
• single pass chip gain Gexp (g_modal L)
• packaging TEC, high coupling efficiency,
  isolators

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Gain vs. Wavelength
Single SOA

• 40-80 nm, InGaAs/InGaAsP. Spanning from
  1250-1650 nm

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Gain vs. Output Power

• An SOA has a Saturation Output Power

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Output Power

• SOAs are linear for small input powers.

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Gain Dynamics
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Saturation Output Power

• Saturation Output Power decreases for higher
  energy photons.

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Noise Figure
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Noise Figure

• SOAs are noisier than EDFAs because the coupling
  efficiency is lower. Otherwise, they have the
  same theoretical limitations.
• Thus, integrated SOAs should be less noisy.

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Cross Gain Modulation

• Saturating the SOA with a signal affects the
  overall gain spectrum. Thus, all wavelengths
  will be slightly modulated.

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Cross Gain Modulation solutions

• low input power (linear regime). SOA not in
  saturation. 8x20 Gbs 160 km. Spiekman et al,
  1999
• reservoir channel, SOAs in saturation. 32x2.5
  gbs. 125 km. Sun et al 1999
• Gain-Clamped SOA
• Solution Fixed Gain SOA
• want fixed gain to eliminate XGM
• Note a laser has fixed gain above threshold
  (gain clamping)

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Gain Clamped SOA

• gain medium is shared between SOA and a laser.
  lasing at a different wavelength.

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In-Line Optical Amplifier
Distance

• Noise Figure can limit performance of links

B. Verbeek, JDSU
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