Transient Control in Dynamically Reconfigured Networks with Cascaded - - PowerPoint PPT Presentation

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Transient Control in Dynamically Reconfigured Networks with Cascaded Erbium Doped Fiber Amplifiers

Lei Zong, Ting Wang lanezong@nec-labs.com NEC Laboratories America, Princeton, New Jersey, USA

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Outlines

• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary

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Outlines

• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary

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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

• ther 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.

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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-

• nly 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 ROADM Drop Express ROADM

…

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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

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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.

Controller Pump

EDF

PD

Po(n) Pi(n)

AGC model of EDFA

PD

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Automatic Level Controller

• ALC is usually realized a variable optical attenuator (VAT) in the middle of a two-
• r 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

PD PD

Pi(n) EDF #1

Controller Pump PD PD

EDF #2

VAT ALC Controller

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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 AGC
• 2. Without AGC

39 channel dropped:

• 1. With AGC
• 2. Without AGC

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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

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Gain and Power Control with Power-Stabilized Control Channel

• Reliable gain control solution to prevent uncontrolled EDFA
• peration.
• Two light sources are used for the control channel for failure

protection. – Output of the two light sources are combined with an

• rthogonal 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

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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

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Outlines

• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary

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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
• scillation in systems with cascaded EDFAs.
• S. Pachnicke, JWA15, OFC/NFOEC 2007

80ch -> 20 ch 0.5 dB margin 80ch -> 20ch 160 us 5 us

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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

• bserved 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

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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

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Outlines

• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary

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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.

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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

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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.

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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.

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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.

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

1 2 3 4 5

Power (dB) Time x 10 μs

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Outlines

• EDFA Transient Control.
• Network Transient Control
• Simulation System
• Summary

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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

• ptimal system performance.

– Interaction between EDFA control and ROADM attenuation adjustment causes severe network performance degradation.

• Simulation system and results are presented.

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Thank You!

Contact: lanezong@nec-labs.com