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WOCC 2007 1
Transient Control in Dynamically Reconfigured Networks with Cascaded Erbium Doped Fiber Amplifiers
Lei Zong, Ting Wang
NEC Laboratories America, Princeton, New Jersey, USA
WOCC 2007 2
Outlines
• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary
WOCC 2007 3
Outlines
• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary
WOCC 2007 4
Transient Effects
• In a WDM network, where multiple wavelengths are amplified by EDFAs, amplifier transient effects appear when the channel count changes.
– EDFAs normally operate in saturated mode.– Changes in channel count result in power
variation.– Due to cross-saturation effect, the power of
other channels increase or decrease accordingly.
• Signal quality can be severely affected during transient period.
– Nonlinearity – high power-excursion causes strong non-linearity in the fiber.
– OSNR degradation – gain tilt changes gain spectrum and results in smaller gain for some channels.
– Power fluctuation – channel power fluctuation during the transient period lead to additional BER.
WOCC 2007 5
Transients in Reconfiguration Networks
• In dynamically switched WDM networks, the influence of amplifier transients becomes more severe.
– Network failures, such as fiber cut, triggers EDFA transients.– Normal add/drop and cross-connection operation causes transients.– Interaction between EDFA control and ROADM attenuation adjustment
results in channel power instability, which lasts much longer than EDFA-only transients.
• Investigation of system response and behavior is necessary.• Novel solutions are required for:
– EDFA control optimization.– Coordination of EDFA transient control and ROADM attenuation
adjustment.
…
…
Drop
Express
ROADMDrop
Express
ROADM
…
WOCC 2007 6
Transient Control in EDFA
• During the last two decades, various control schemes have been proposed, tested and implemented.– Automatic gain control (AGC)– Automatic level control (ALC)– Gain-clamp
WOCC 2007 7
Automatic Gain Controller
• Assume the target gain of the amplifier is G, and the controller tries to maintain the target gain by adjusting the pump power of the EDFA.
• At time n, the monitored input and output power is Pi(n) and Po(n), respectively.– The real gain at time n can be obtained as G(n) = Po(n) / Pi(n).– The gain error is ∆ = G(n) – G.
• With proportional controller, the pump power of the EDFA should be adjusted to Pp (n) = Pp(n-1) + a * ∆, where Pp(n-1) is the previous pump power, a is the feedback coefficient.
• Integral and deviation controllers can also be combined with proportional controller to improve the control speed and accuracy.
• Both electronic and optical AGC solutions are available.
ControllerPump
EDF
PD
Po(n)Pi(n)
AGC model of EDFA
PD
WOCC 2007 8
Automatic Level Controller
• ALC is usually realized a variable optical attenuator (VAT) in the middle of a two-or three- stage EDFA.
– Each stage has an independent AGC controller.• ALC controller adjusts the attenuation of VAT to maintain constant output power.
– Total or individual channel output power can be controlled, according to the monitoring unit in the feedback control loop.
– Tone channel or control channel, which transmits with data channels, is usually used for ALC function.
Controller
Pump
Po(n)
Automatic level control model of EDFA
PDPD
Pi(n)EDF #1
Controller
Pump
PDPD
EDF #2
VAT
ALCController
WOCC 2007 9
Automatic Gain Control with SOA and Control Channel
• A semiconductor optical amplifier (SOA) based negative feedback loop is used to adjust EDFA pump power.
• Control channel wavelength at 1515 nm, pump wavelength 1480 nm.• SOA gain bandwidth is 80 nm centered at 1525 nm.• Pump laser’s output power is split into two portions:
– Strong portion kPo directly to EDFA.– Weak portion (1-k)Po passes saturated SOA and coupled with the
control channel (CC).• The two portions interfere before pumping the EDFA.
– The weak portion experience XGM and XPM with CC in SOA.– In stable condition, constant phase shift exists between the
strong portion and the chirped weak portion.– During transients, beating occurs but can be neglected since the
phase relaxation time is much shorter than transient duration.
Y. Ben-Ezra, etc., IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 12, DECEMBER 2006
39 channels added:1. With AGC2. Without AGC
39 channel dropped:1. With AGC2. Without AGC
WOCC 2007 10
Gain Control with Saturable Absorber
• Optical gain control (OGC) can lock the inversion of EDFA, but also cause spectral hole burning due to inhomogeneous gain medium.
• A saturable absorber is placed in the OGC laser cavity to adjust cavity loss.
– Gain error is reduced.– Gain tilt is compensated.
• Transient performance can be improved by using absorbers with fast dynamics.
– Short lifetime Er fiber.– Semiconductor type.
1527 nm
14 m
1 m EDF
• 8 wavelengths from 1530 – 1560 nm.• Per-channel power –10 dBm.• 7 channel add/drop operation.
Chia-Chi Wang, et. al., IEEE PTL, VOL. 12, NO. 5, MAY 2000
WOCC 2007 11
Gain and Power Control with Power-Stabilized Control Channel
• Reliable gain control solution to prevent uncontrolled EDFA operation.
• Two light sources are used for the control channel for failure protection.
– Output of the two light sources are combined with an orthogonal multiplexer or a 3-dB coupler.
– The total power of the control channels is stabilized.– Two portions of the multiplexed control channel are sent to two
directions, respectively.• EDFAs with both AGC and ALC are tested.
– AGC is realized by pump power control.– ALC is realized by VOA attenuation adjustment.
• System transients are studied under failures of one control channel.
Hirotaka Ono, et. al., JLT, VOL. 20, NO. 8, AUGUST 2002
AGC AGC and ALC
Power transients with ALC
AGC
ALC
WOCC 2007 12
AGC and ALC in EDFA
• A tone signal at wavelength 1547.72 nm is used for ALC.
– VAT compensates tone signal power fluctuation.
– Data channel power is kept constant by keeping tone channel power constant.
– Tone signal is 2 MHz.• Transient duration and power excursion can be
suppressed by reducing the response time of AGC feedback circuits.
Kuniaki Motoshima, et. al., JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 19, NO. 11, NOVEMBER 2001
EDFA transients under ALC EDFA transients under AGC
32 -> 3
WOCC 2007 13
Outlines
• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary
WOCC 2007 14
AGC Optimization for Cascaded EDFA Networks
• Proportional and integral controller are used for electronic feedback gain control to suppress EDFA transients.
• Similar to AGC analysis in page 8, the pump power Pp is determined as:Pp(t+1) = Pp(t) + Ppr(t) + Pint(t)
• Simulation shows that optimized control parameters for a single EDFA does not lead to optimal performance in multiple cascaded EDFAs.
• Lower proportional gain should be used to prevent power oscillation in systems with cascaded EDFAs.
S. Pachnicke, JWA15, OFC/NFOEC 2007
80ch -> 20 ch
0.5 dB margin
80ch -> 20ch
160 us
5 us
WOCC 2007 15
Transient Gain Dynamics• In ROADM networks with cascaded EDFAs, different
group of wavelengths has different ingress and egress points.
– When fiber is cut, power excursion propagates downstream and affects other groups of wavelengths.
– Transient power excursion of different orders can be observed in the network.
• Rise time of the surviving channels decreases linearly along the network in the 1st order transient events.
• 2nd and 3rd order transient power excursion decreases as transients are transferred to multiple wavelength groups.
Add/drop plan
1st order transients
3rd order transients
D. C. Kilper, et. al., OTuK6, OFC/NFOEC 2006
2nd order transients
WOCC 2007 16
Channel Power Stability Control in ROADM Networks
• Gain tilt oscillation arises from competing adjustments of multiple ROADMs in networks with gain-controlled EDFAs.– ROADMs usually contains a spectral
measurement unit and power leveling devices (e.g., WB and WSS).
• Channel power is monitored.• Channel power is equalized by
attenuation adjustment.– Independent adjustment would result in
gain tilt ripples or oscillation.– Coordinated, sequential node-by-node
adjustment is preferred.
D. C. Kilper, et. al., PDP11, OFC/NFOEC 2007
WOCC 2007 17
Outlines
• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary
WOCC 2007 18
Simulation System
• Simulation systems are realized with VPI and MatLab platforms.– EDFA and ROADM models are designed.– Multiple cascaded EDFAs and ROADMs are studied.– System response and behavior, as well as interaction between
EDFAs and ROADMs, are investigated.– Novel control algorithms are proposed and investigated.
WOCC 2007 19
System Optimization
• adsf
1e-3
3.3e-3
6.6e-3
1e-2
• AGC controller uses proportion control with feedback coefficient P.• P is optimized with simulation so that EDFA output power becomes stable in
shortest duration.• Various network scenarios are simulated.• Based on the simulation results, P = 6.6e-3 is selected as the optimal control
coefficient.
1e-3
3.3e-3
6.6e-3
1e-2
WOCC 2007 20
System Optimization
• However, the optimal P coefficient for a single EDFA results in power oscillation in a network with multiple cascaded EDFAs.
• Network stability is significantly improved by changing the AGC proportional coefficient to 5e-4.
WOCC 2007 21
10 Cascaded RXA with P = - 5e-4
• System performance can be improved by decreasing P to -5e-4.• This effect has just been confirmed in an OFC/NFOEC’ 07 paper JWA15.
WOCC 2007 22
Interaction between EDFAs and ROADMs
• With 30 ROADMs and 60 EDFAs, the system response to an input power variation is simulated with two simulation systems. The interaction between EDFAs and ROADMs are investigated in the simulation.
0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0
-1
0
1
2
3
4
5Power(dB)
Timex 10 μs
WOCC 2007 23
Outlines
• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary
WOCC 2007 24
Summary
• EDFA transients and its influence on signal quality is introduced.
• EDFA control methods, such as automatic gain control and automatic level control, are analyzed with multiple realization examples.
• Transients and suppression solutions in dynamic networks are introduced:
– EDFA control parameters must be modified to achieve optimal system performance.
– Interaction between EDFA control and ROADM attenuation adjustment causes severe network performance degradation.
• Simulation system and results are presented.
