Hybrid Amplifiers
Hybrid Amplifiers
Part I
Combining Raman Amplification with
Amplification by EDFAs in Long
Haul WDM Systems
Outline
1. Overview of Raman and EDFAs combined
amplification
2. Hybrid amplifier issues
3. Comparison of different amplified system
configurations
4. Gain balance between Raman and EDFAs
Overview of Raman and EDFAs
Combined Amplification
Why use Hybrid Amplifiers
• Enlarges the transmission capacity of broadband systems
• “Upgrading” the existing systems built with EDFA amplifiers
with broader/flatter bandwidth
• Ability to carry more wavelength-multiplexed optical
channels at given spacing among the channels
• Raman amplification gives flexibility to the selected band
amplification
• Less sensitive to nonlinear effects – systems’ point-of-view
Basic Idea of Hybrid Amplifiers
Principle of working / Operating Mode
• Flatten EDFA gain by using Raman gain
• Improve gain performance at longer signal
wavelengths
Typical Configurations
Raman Amplifiers setup in different configurations:
• discrete
• distributed
• backward-, forward- and bidirectional pumped
• numerous types of fibers (DSF, DCF to be
used as SRS active media)
EDFAs setup considers:
• single or multiple fiber stages
• series or parallel configuration
• C- and L-band regions
Hybrid Amplifiers Issues
The Hybrid Amplifiers studies are concerned with:
• maximizing the span length and/or minimizing the
impairments of fiber nonlinearities
• enhancing the EDFAs’ bandwidth
• designing “optimal” hybrid amplifiers in order to
obtain flat and widest output gain performance
Comparing Different System
Configurations
EDFA only
RX
TX
EDFA + Raman
TX
RX
Raman only
TX
RX
Results Obtained After Comparing
the 3 Types of Systems
Considering in-line and pre-amplification functions:
• Long-haul EDFA-only systems are limited by OSNR and NL effects
• Raman-only systems tend to be limited by a reduction of SNR
caused by double Rayleigh backscattering
• Combination of distributed Raman and EDFAs present better
performance than conventional EDFA-only systems
• Raman complement EDFAs in terrestrial high capacity long haul
applications
Optimizing HFA in a System
Evaluate the optimum gain balance between the RAs
and EDFAs that enables maximum transmission distance*
* Carena et al., IEEE Photon. Techn.
Techn. Lett.
Lett. Vol. 13, No. 11, pp. 11701170-1172, 2001.
Gain Balance Between
Raman and EDFAs
1. Complex problem with several degrees of freedom
- optimization technique
2. Main considerations to determine the optimum gain
balance between Raman and EDFAs
3. Most important parameters
- OSNR, gain-flatness, bandwidth
- number of channels, number of spans, maximum
transmission capacity
Focusing on Extending the Bandwidth
• Increase the number of transmitted channels
• Gain flatness
• Optimize the hybrid amplifier performance
(gain, NF, OSNR)
Techniques that Enlarge Flattened
Gain-Bandwidth of “Discrete”
Fiber Amplifiers
I. New host materials
fluoride- and telluride-based EDFAs
thulium-doped fiber amplifier
II. Using EDFAs with GEQ + discrete Raman
amplifier
III. Different amplifier configurations
two-gain band
parallel/series configuration – multiple fiber stage
gain-equalizing (GEQ) filters
Typical Values Observed
in Hybrid Amplifiers
Bandwidth
> 40 nm
Gain
15 – 25 dB
Gain Ripple
< 11.3 %
Noise Figure
< 6 dB
OSNR
> 32 dB
Examples of Wide Flattened Bandwidth for
Discrete Raman/Hybrid Fiber Amplifier
Bandwidth (nm) G Flatness (%)
64
76
75
40
11.3
4.7
3.7
3.2
Reference
OAA 98, p. 103
OFC 98, paper PD7
Electron. Lett. 34, 897, 1998
IEEE PTL 9, 1343, 1997
Specifications
EDFA
1 stage
2 stage
Discrete Raman
2 stages
1 stage
Nr. Pump
2
3
NF(dB)
<6
<6
Part II
Designing Hybrid Amplifiers
Outline
1. Models required in the simulations
2. Hybrid amplifier designed to LAN
3. Gain and noise figure results
4. A hybrid amplifier setup in multiple fiber stages
5. Characterization and performance in a system
Models Required in the Simulations
EDFA Modeling:
Solve rate and propagating equations for pump, signal, and
ASE, considering non-linear effects present in the fiber
propagation.
Raman Amplifier Modeling:
Solve the rate and propagation equations for pump, signal,
and ASE in the Raman fiber amplifier.
Hybrid Amplifiers Designed for
Local Area Networks*
• LAN and MAN applications
- Gave ~ 20 dB
- Net gain > 10 dB
• Distributed Raman fiber amplifier in series with
a remotely pumped discrete EDFA
• C-band and L-Band
• WDM signal input
• Optimized design parameters
* Karasek et al., IEE Proc.-Optoelectron. Vol. 148, No. 3, p. 150, 2001
Hybrid Amplifier Layout
Pump Power Specification
Co-propagating pump power
Raman
EDFA
Signal Input Power
Raman
51 channels, −17 dBm/ch
EDFA
Raman Amplifier –
Fiber Specifications
EDFA – Fiber Specifications
Evaluating Results in the C-Band
Wavelength Range
35
Hybrid
40
25
20
35
Hybrid
EDFA
Raman
15
10
30
5
0
1530
1540
1550
1560
Wavelength (nm)
1570
25
1580
OSNR (dB)
Gain (dB)
30
Raman Output Power Spectra
Forward
Backward
Total Powers Along the EDFA
Signal
Pump
ASE+
ASE−
25
40
20
39
15
38
Hybrid
EDFA
Raman
10
37
36
5
35
0
1570
1580
1590
Wavelength (nm)
1600
34
1610
OSNR (dB)
Gain (dB)
Results in the L-Band
Wavelength Range
Additional Comments
• Hybrid fiber amplifier simulated for multi-wavelength
operation in LANs
• Distributed RFA and remotely pumped EDFA
• Net gain 10 – 20 dB
• EDFA is excited by the pump power unabsorbed in the
transmission fiber
• C-band and L-band results
Part III
Simulating Gain-Equalized
Hybrid Raman-EDF Amplifiers
Outline
•
Simulating hybrid amplifier with no filter
•
Spectral gain results
•
How to flatten the gain spectrum
•
Conclusions
Wide Band Gain-Flattening
Hybrid Amplifier*
• Wide and flatness bandwidth
• Short EDF + discrete Raman amplifier
• Two-stage Raman amplifier
• Bidirectional pumped at 3 different λp
• C-Band and L-Band signal wavelength
* Masuda et al., IEEE Photon. Techn. Lett. Vol. 11, No.6, p. 647, 1999
Hybrid Amplifier with no Filter
Pump Power Specifications
Raman 1
Bidirectional
EDFA
Co-pump
Raman 2
Bidirectional
Signal Input Power Specifications
16 channels spaced by 5 nm
Power (dBm)
EDFA
Signal Input Power Specifications
Raman 2
Power (dBm)
Power (dBm)
Raman 1
Spectral Gain Along the C-Band
and L-Band Wavelength Range
25
Gain (dB)
20
Hybrid
EDFA
Raman1 + Raman2
Raman1
Raman2
15
10
5
0
1530
1540
1550
1560
1570
1580
Wavelength (nm)
1590
1600
Spectral NF and OSNR to C-Band
and L-Band
10
40
NF (dB)
38
6
4
36
2
1530
1540
1550
1560
1570
1580
Wavelength (nm)
1590
1600
OSNR (dB)
8
Hybrid Amplifier Output
Pump
Power (dBm)
Signal
ASE
Hybrid Amplifier Output
Transmitted
Power (dBm)
Pump
Reflected
Signal
Pump
Signal
ASE
ASE
Comments
• Raman amplifier increases the gain at longer
wavelength signal;
• Design can be optimized;
• Considerable gain ripple which limits
usefulness for high capacity transport systems;
• Er doped fluoride fiber also presents the gain
ripple observed for silica fiber.
How to Flatten the Gain Spectrum
• Use a passive filter device such as thin film, fiber grating, etc.
Including a transmission filter
in the design
Gain Flattening Filter Specifications
Wavelength
Range
Specifies how
flat the filter
will be
Gain Flattening Filter Optimization
Power (dBm)
Power (dBm)
Spectra Observed Before
and After the Filter
Filter Curve
Filter Curve
In/Out Signal Difference
Intensity (dB)
8
6
4
2
0
1520
1540
1560
1580
Wavelength (nm)
1600
Comparing Gain Results
30
Gain (dB)
25
20
15
With Gain-Flattening Filter
With No Filter
10
5
1530
1540
1550
1560
1570
1580
Wavelength (nm)
1590
1600
1610
Comments
• Hybrid amplifier was simulated using one EDF
stage and two Raman amplifiers cascaded in
series;
• C-Band and L-Band wavelength range;
• Qualitative results’ analysis;
• Hybrid amplifier configuration was used as an inline amplifier;
• Low gain variation (3.1 dB) was observed under
strong input signal variation (– 30 dBm to –20 dBm);
• Gain bandwidth equal to 69 nm.
Conclusions
• Hybrid amplifiers simulations considered different
amplifier configurations.
• Pump wavelength selection, EDFA and Raman
specifications, and the configuration / combination of
both amplifiers are critical in the design of hybrid
amplifiers.
• Hybrid amplifiers compensate for the gain decay
at longer signal wavelength.
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