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 WOCC 2007 1
Outlines • EDFA Transient Control. • Network Transient Control • Simulation System • Summary WOCC 2007 2
Outlines • EDFA Transient Control. • Network Transient Control • Simulation System • Summary WOCC 2007 3
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 4
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. Express Express … … ROADM … ROADM Drop Drop WOCC 2007 5
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 6
Automatic Gain Controller EDF P i (n) P o (n) PD Pump Controller PD AGC model of EDFA • 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 P i (n) and P o (n), respectively. – The real gain at time n can be obtained as G(n) = P o (n) / P i (n). The gain error is ∆ = G(n) – G. – • With proportional controller, the pump power of the EDFA should be adjusted to P p (n) = P p (n-1) + a * ∆ , where P p (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. WOCC 2007 7
Automatic Level Controller EDF #1 EDF #2 P i (n) P o (n) VAT Pump Pump PD Controller PD PD Controller ALC PD Controller Automatic level control model of EDFA • 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. WOCC 2007 8
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 kP o directly to EDFA. – Weak portion (1-k)P o 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. 39 channels added: 1. With AGC 2. Without AGC 39 channel dropped: 1. With AGC 2. Without AGC Y. Ben-Ezra, etc., IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 12, DECEMBER 2006 WOCC 2007 9
Gain Control with Saturable Absorber • Optical gain control (OGC) can lock the inversion of 14 m 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. 1 m EDF – Gain error is reduced. – Gain tilt is compensated. 1527 nm • Transient performance can be improved by using • 8 wavelengths from 1530 – 1560 nm. absorbers with fast dynamics. • Per-channel power –10 dBm. – Short lifetime Er fiber. • 7 channel add/drop operation. – Semiconductor type. Chia-Chi Wang, et. al., IEEE PTL, VOL. 12, NO. 5, MAY 2000 WOCC 2007 10
Gain and Power Control with Power-Stabilized Control Channel • Reliable gain control solution to prevent uncontrolled EDFA Power transients with ALC 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 AGC 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. ALC AGC and ALC AGC Hirotaka Ono , et. al., JLT, VOL. 20, NO. 8, AUGUST 2002 WOCC 2007 11
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. 32 -> 3 EDFA transients under ALC EDFA transients under AGC Kuniaki Motoshima, et. al., JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 19, NO. 11, NOVEMBER 2001 WOCC 2007 12
Outlines • EDFA Transient Control. • Network Transient Control • Simulation System • Summary WOCC 2007 13
AGC Optimization for Cascaded EDFA Networks 80ch -> 20ch • 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 P p is determined as: 160 us P p (t+1) = P p (t) + P pr (t) + P int (t) • Simulation shows that optimized control parameters for a single EDFA does not lead to optimal performance in multiple 0.5 dB margin cascaded EDFAs. • Lower proportional gain should be used to prevent power oscillation in systems with cascaded EDFAs. 80ch -> 20 ch 5 us S. Pachnicke, JWA15, OFC/NFOEC 2007 WOCC 2007 14
Add/drop plan 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. 1 st order transients – Transient power excursion of different orders can be observed in the network. • Rise time of the surviving 2 nd order transients channels decreases linearly along the network in the 1 st order transient events. 3 rd order transients 2 nd and 3 rd order transient • power excursion decreases as transients are transferred to multiple wavelength groups. D. C. Kilper, et. al., OTuK6, OFC/NFOEC 2006 WOCC 2007 15
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 16
Outlines • EDFA Transient Control. • Network Transient Control • Simulation System • Summary WOCC 2007 17
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 18
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