Optical Amplifier: EDFA

1
Optical Amplifier EDFA
2
Outline

• Performance
• Tradeoffs
• Number of channels
• Channel speed
• Channel spacing
• Total spectral width
• Amplifier spacing (gain per stage)
• Regenerator spacing ( of stages)
• Total costs
• Items 1-4 impact system capacity
• Item 5 impacts total distance w/o regeneration ?
  cost
• Item 6 ? cost

3
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)

4
EDFA Challenges

• Gain Flattening
• Gain Transient
• Gain Bandwidth Widening

5
Energy Levels

• Stark splitting
• t321us, t2110ms
• gain bandwidth 1525nm 1570 nm
• gain peak at 1532nm

980nm
1480 nm
1530 nm
6
Amplifier Noise

• Signal-Spontaneous beat noise dominates the
  output noise of the amplifier

7
Amplifier Noise

• Even for ideal amplifiers, population inversion
  factor 1, the noise figure is 3dB.
• For EDFA, NF is around 4-7dB.
• With coupling loss at the beginning, NF is worse.

8
N-Stage Cascaded Amplifiers
Loss L1
Loss L2
G1
G2
NF F1
NF F2
9
Two-Stage design

• 1st stage
• high gain, low noise
• 2nd stage
• high output power
• 2 pumps to be more robust if one fails
• noise performance of amplifier is determined by
  the 1st stage

10
Two-Stage Design
When L1L21 (no loss)
11
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

12
Two-Stage EDFA

• With 2-stage design, low noise, high gain with
  flat gain spectrum can be achieved.

13
Gain Spectrum

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

14
Gain Spectrum

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

15
Non-Uniform Gain Accumulation
16
Gain Flattening

• Passive equalization
• Pre-equalize the input signal
• Add dopant fluoride based EDFA
• Broadband filter
• Hybrid pump
• Active equalization
• Acousto-Optic Tunable Filter (AOTF)

17
Gain flatness

• Silica fiber of 20 db gain
• 1dB variation over 20nm, 2.5 dB over 30nm
• Fluoride fiber of 20db gain
• 1.1 dB over 30 nm

18
Fluoride-Based EDFA

• Naturally Flat.
• Pumped only at 1480 nm due to ESA at 980 nm
• Noisier, brittle, difficult to splice with
  typical fiber

19
Passive Gain Equalization

• Cannot respond to dynamic change in the network
  link loss, routing, reconfiguration...
• Must know the exact spectral shape of gain

20
Long-Period Fiber Grating Filter

• Different length of fiber has different gain
  spectrum. Need separate design.
• Wider the flatness, the higher the loss for some
  wavelength. 3times higher for 40nm wide gain
  spectrum compared with 33nm.
• Penalty for higher filter loss is higher NF and
  lower output power

21
Long-Period Fiber Grating Filter

• Index grating period 100mm provides coupling
  between the core and cladding modes

22
LPG Design and Result

• Over 40nm gain flattened

23
Hybrid EDFA at 1.55um

• By optimizing the length of each fiber, gain
  flatness and low noise can be achieved

24
Hybrid EDFA at 1.55um

• gain excursion less than 0.9 dB

25
Acousto-Optic Tunable Filter
26
Active AOTF
27
Active AOTF

• Gain tilt due to pump power change
• Active gain flattening (lt0.7dB) independent of
  input power with 35nm bandwidth

28
EDFA Gain Trasient

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

29
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

30
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.

31
EDFA Transient Dynamics

• plot

32
Single EDFA

• For single EDFA, transient response is slow (on
  the order of ms)

33
Cascade EDFA

• The transient time reduces to ?s range for large
  number of cascading EDFAs.

34
Power and SNR Fluctuations
35
Optical Attenuation Compensation

• Every EDFA needs compensation
• Same idea applies for pump power compensation

36
Control Channel Method

• By adding/dropping the corresponding control
  channel when a channel is drop/add, power and SNR
  transient can be suppressed.

37
EDFA for L-Band
S
S
L
C
S
Fiber loss
1300
1525
1565
1600
1400

• Expand the total bandwidth
• utilize dispersion shifted fiber without the FWM
  penalty

38
C-Band v.s. L-Band

• 6.3db/mw gain coefficient and max power
  conversion efficiency (PCE) 77.2 with 1.48 pump
  at 1.55 band
• gain coefficient is smaller for 1.58 band due to
  smaller stimulated cross section
• PCE is higher in the 1.55 band. This is because
  1.58 amplification occurs from the 1.55 ASE
  generated from 1.48 pump
• Greater pump power is needed for 1.58 band

39
Parallel Type EDFA
40
Parallel Type EDFA
41
Ultra-Wide Silica EDFA

• 80nm bandwidth!

42
Tellurite Based EDFA
43
EDFA Challenges

• Gain Flattening
• Gain Transient
• Gain Bandwidth Widening

44
SNR, Optical SNR and Q
When there is an optical amplifier in the system,
Bo filter bandwidth matters
For example Be 2.5 GHz Bo0.1 nm12.5 GHz
45
Noise Figure
Spontaneous power in a fiber amplifier is
expressed as
Here, ASE stands for amplified spontaneous
emission and Dn is the emission spectral width.
If the amplifier is filtered through an optical
filter before hitting the detector, then Dn can
be replaced by Bo.
46
NF Dependence on Pump Lambda
47
Noise Sources
48
Effect of Optical Filter Passband
For NF3dB and Signal at 5Gbps
49
Amplifier Chains 2 Configurations
50
Amplifier Chain

• Beating noise happens at detector only and the
  formulation applies to direct detection only
• Amplified spontaneous emission is amplified just
  like the signal ? optical filtering is essential
• Assuming equal G, L (loss) and Bo, for all
  amplifiers in an N amplifier chain and GL1

51
Amplifier Chains SNR
For a fixed Gtotal, what are optimum N and G?
52
Cascaded Amplifier Link Design

• For a fixed BER, SNR is set
• ? given Bo, bit rate of the system (Be), NF and
  distance Gtotal
• We can calculate Pin and N and distance between
  amplifiers (from G)

53
Required Input Power vs. Spacing
54
NF Dependence
55
Overall Length vs. Spacing
56
Effect of ASE
57
Nonlinear Effect in the Fiber

• Stimulated Brillouin scattering
• Stimulated Raman scattering
• Self-phase modulation
• Cross-phase modulation
• Four-wave mixing

58
Effect Area and Length

• Two key parameters that are needed to estimate
  the thresholds for the nonlinear effects
• Power will be assumed to be constant over the
  effective area and length
• Typical Aeff values are 80 and 50 um2 for regular
  and dispersion shifted fibers
• Typical Leff value is 20 km for a0.22 dB/km

59
SBS

• Interaction of photons with acoustic phonons
• Scattering of a photon into a photon of lower
  frequency (by 11 GHz) that propagates in the
  opposite direction plus a phonon.
• Brillouin bandwidth is narrow, 20 MHz
• The backward propagating downshifted photons are
  amplified with distance exponentially
• This process reduces SNR because
• Signal strength is reduced
• Random SBS process introduces noise
• Typical thresholds
• Proportinal to Aeff/Leff, laser linewidth,
  modulation pattern, etc.
• 4.2 mW for a narrow CW source for SMF 28
• 2.6 mW for dispersion shift fiber

60
SRS

• Interaction of photons with optical phonons
• Scattering of a photon into a photon of lower
  frequency (by 15 THz) that can propagate in the
  forward direction plus a phonon.
• Brillouin bandwidth is wider, 20 THz
• The co-propagating downshifted photons are
  amplified by the signal and can cause crosstalks
  with other WDM channels
• Typical thresholds
• Proportinal to Aeff/Leff, laser linewidth,
  modulation pattern, etc.
• 1.8 W for a narrow CW source for SMF-28
• 1.1 W for dispersion shift fiber

61
SPM

• Refractive index is intensity dependent
• High pulse intensity
• Short pulses
• Impairment comes from dispersion
• Need to use dispersion-shifted fiber or
  dispersion compesation at the receiver
• Probable spectral crosstalks for WDM adjacent
  channels

62
XPM

• Same physical origin as SPM
• Particular for WDM systems
• Intensity variations of one pulse alter the phase
  of another channel via nonlinear refractive index
  of glass ? spectral broadening
• As two pulses (different channels) traverse each
  other, one pulses time varying intensity profile
  will cause a frequency shift in the other
• Particularly bad if
• Collision length is long
• Pass thru an amplifier during collision

63
FWM

• Nonlinear interaction between several different
  channels in a WDM system
• When two waves are interacted, two other EM waves
  are generated that is proportional to the cube of
  the vector sum of the E fields.
• Dispersion-shifted fiber for 1.55 micron signals
  ? 25km
• N channels ? N2(N-1)/2 side bands are created,
  causing
• Reduction of signals
• Interference
• Cross talk
• Typical transmission today uses non-zero
  dispersion shifted fiber at 1-2ps/nm/km.

Frequency
64
SRS Effect
Limited max power per channel imposed by
SRS. Channel spacing assumed 0.8 nm, and
amplifiers are 80 km apart.
65
FWM Effect