Title: Optical Amplifier: EDFA
1Optical Amplifier EDFA
2Outline
- 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
3EDFA
- 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)
4EDFA Challenges
- Gain Flattening
- Gain Transient
- Gain Bandwidth Widening
5Energy Levels
- Stark splitting
- t321us, t2110ms
- gain bandwidth 1525nm 1570 nm
- gain peak at 1532nm
980nm
1480 nm
1530 nm
6Amplifier Noise
- Signal-Spontaneous beat noise dominates the
output noise of the amplifier
7Amplifier 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.
8N-Stage Cascaded Amplifiers
Loss L1
Loss L2
G1
G2
NF F1
NF F2
9Two-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
10Two-Stage Design
When L1L21 (no loss)
11Pump 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
12Two-Stage EDFA
- With 2-stage design, low noise, high gain with
flat gain spectrum can be achieved.
13Gain Spectrum
- Amorphous nature of silica and the codopants
inside the fiber affects the spectrum
considerably.
14Gain Spectrum
- Population at different levels are different
resulting gain dependence on wavelength - Different pumping level has different spectrum
15Non-Uniform Gain Accumulation
16Gain Flattening
- Passive equalization
- Pre-equalize the input signal
- Add dopant fluoride based EDFA
- Broadband filter
- Hybrid pump
- Active equalization
- Acousto-Optic Tunable Filter (AOTF)
17Gain 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
18Fluoride-Based EDFA
- Naturally Flat.
- Pumped only at 1480 nm due to ESA at 980 nm
- Noisier, brittle, difficult to splice with
typical fiber
19Passive Gain Equalization
- Cannot respond to dynamic change in the network
link loss, routing, reconfiguration... - Must know the exact spectral shape of gain
20Long-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
21Long-Period Fiber Grating Filter
- Index grating period 100mm provides coupling
between the core and cladding modes
22LPG Design and Result
23Hybrid EDFA at 1.55um
- By optimizing the length of each fiber, gain
flatness and low noise can be achieved
24Hybrid EDFA at 1.55um
- gain excursion less than 0.9 dB
25Acousto-Optic Tunable Filter
26Active AOTF
27Active AOTF
- Gain tilt due to pump power change
- Active gain flattening (lt0.7dB) independent of
input power with 35nm bandwidth
28EDFA Gain Trasient
- Channel turn-on, re-routing, network
reconfiguration, link failure.
29Gain 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
30Gain 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.
31EDFA Transient Dynamics
32Single EDFA
- For single EDFA, transient response is slow (on
the order of ms)
33Cascade EDFA
- The transient time reduces to ?s range for large
number of cascading EDFAs.
34Power and SNR Fluctuations
35Optical Attenuation Compensation
- Every EDFA needs compensation
- Same idea applies for pump power compensation
36Control Channel Method
- By adding/dropping the corresponding control
channel when a channel is drop/add, power and SNR
transient can be suppressed.
37EDFA 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
38C-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
39Parallel Type EDFA
40Parallel Type EDFA
41Ultra-Wide Silica EDFA
42Tellurite Based EDFA
43EDFA Challenges
- Gain Flattening
- Gain Transient
- Gain Bandwidth Widening
44SNR, 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
45Noise 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.
46NF Dependence on Pump Lambda
47Noise Sources
48Effect of Optical Filter Passband
For NF3dB and Signal at 5Gbps
49Amplifier Chains 2 Configurations
50Amplifier 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
51Amplifier Chains SNR
For a fixed Gtotal, what are optimum N and G?
52Cascaded 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)
53Required Input Power vs. Spacing
54NF Dependence
55Overall Length vs. Spacing
56Effect of ASE
57Nonlinear Effect in the Fiber
- Stimulated Brillouin scattering
- Stimulated Raman scattering
- Self-phase modulation
- Cross-phase modulation
- Four-wave mixing
58Effect 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
59SBS
- 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
60SRS
- 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
61SPM
- 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
62XPM
- 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
63FWM
- 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
64SRS Effect
Limited max power per channel imposed by
SRS. Channel spacing assumed 0.8 nm, and
amplifiers are 80 km apart.
65FWM Effect