1220IEEE PHOTONICS TECHNOLOGY LETTERS,VOL.21,NO.17,SEPTEMBER 1,2009
38-krad/s 3.8-Grad Broadband Endless Optical Polarization Tracking Using LiNbO 3Device
Reinhold Noé,Member,IEEE ,Benjamin Koch ,Student Member,IEEE ,Vitali Mirvoda,Ariya Hidayat,and
David Sandel ,Member,IEEE
Abstract—We demonstrate automatic endless optical polariza-tion tracking over 3.8Grad at up to 38-krad/s control speed with mean/maximum polarization errors of 0.068/0.185rad.Without polarization fluctuations,mean/maximum polarization errors are 0.05/0.1rad.Small-signal control time constant is about
2s.Func-tion is maintained over the wavelength range 1505–1570nm.Index Terms—Optical fiber communication,optical fiber polar-ization,quadrature pha-shift keying (PSK).
I.I NTRODUCTION
P
OLARIZATION-MULTIPLEXED (PDM)transmission systems with direct detection avoid the high-speed,high-power electronics required for coherent polarization-diversity detection at high bitrates.On the other hand,optical polarization demultiplexing requires automatic polarization control [1]–[4],which should be fast and endless:In the field [5],significant polarization changes have been obrved within periods of about
100s.Much faster changes are possible if a dispersion-compensating fiber reel is hit hard.Endless means that the instantaneous polarization mismatch is bounded and small at all times,even if the tracked polarization moves around the Poincarésphere many or an unlimited number of times.Short glitches would already cau a large number of bit errors that could not be corrected by forward error correction.
Starting
from 0.1rad/s achieved in 1987[6],the past two decades have brought the maximum endless polarization tracking speed to 15krad/s [7].Averaged over this long period,speed has doubled approximately子非鱼安知鱼之乐
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every months.Other authors have reported a 4.9-krad/s speed [8]with ten periods of one particular repetitive endless Poincarésphere trajectory tracked in 20ms.A 12.6-krad/s speed was given in [9]while tracking emingly finite rather than endless polarization changes.To the best of our knowledge,more high-speed optical polarization control experiments have not been reported by other authors.
结婚典礼仪式Manuscript received April 26,2009;revid May 18,2009.First published August 07,2009;current versi
on published August 19,2009.This work was supported in part by Deutsche Forschungsgemeinschaft.
R.Noé,B.Koch,V.Mirvoda,and D.Sandel are with the University of Pader-born,33098Paderborn,Germany (e-mail:**********;************.de).A.Hidayat was with the University of Paderborn,33098Paderborn,Ger-many.He is now with Nokia,Qt Software,0402Oslo,Norway.
Color versions of one or more of the figures in this letter are available online at ieeexplore.ieee.
Digital Object Identifier
10.1109/LPT.2009.2024549
Fig.1.Setup for endless polarization control with integrated-optical LiNbO component containing eight electrooptic wave plates.
We have recently tracked endless polarization changes:
Step 1)In a 112-Gb/s PDM differential quadrature pha-shift keying (DQPSK)field trial at 800-rad/s speed [1];
Step 2)In a tup like Fig.1,from 0C to 70C,from 1520
透明泳装秀to 1564nm,and on a 2.5-Grad-long trajectory,all at 14-krad/s speed [10].
As will be en,the latter speed is not the limit.
II.E NDLESS P OLARIZATION C ONTROL S ETUP
Endless optical polarization control requires at least one Soleil–Babinet compensator (SBC),i.e.,a rotary linear wave plate with adjustable retardation [6],[7],[10].In our control system (e Fig.1),a commercial polarization transformer (EOSPACE)contains a cascade of eight such integrated-optical wave plates implemented in X -cut,Z -propagation
LiNbO .
V
oltage
shifts the TE versus TM pha difference,while
voltage
converts TE into TM and vice versa .Both voltages allow to realize a retarder with adjustable retardation and linear eigenmodes.
In the experimental tup,the controller is ud to stabilize the polarization-scrambled signal of an unmodulated lar source.The output signal of the polarization controller is fed into a polarization beam splitter (PBS).One of its output signals is detected and ud for feedback to the controller.By modu-lating the control voltages,a gradient algorithm,implemented in a field-programmable gate array (FPGA),eks and reaches a global intensity minimum.As a conquence,the other PBS output provides full intensity.
We found that controlling more than one wave plate simul-taneously by the gradient algorithm reduces required voltages,and greatly increas tolerance against device-inherent nonideal
1041-1135/$26.00©2009IEEE
NOÉet al.:38-krad/s 3.8-Grad BROADBAND ENDLESS OPTICAL POLARIZATION TRACKING USING LiNbO DEVICE
1221
Fig.2.Relative intensity error (RIE)versus time for 38-krad/s trajectory with controller switched on at t =0 s.
behavior and variations of the fixed output polarization.The reachable control speed is,therefore,improved.
III.E XPERIMENTAL R ESULTS
To achieve highest scrambling speeds,a fast rotary half-wave plate (HWP)is placed between two pairs of fiber-optic quarter-wave plates (QWP),which rotate endlessly at unequal rates of 0.88,5.2,5.68,and 0.96Hz.The HWP is realized by an ad-ditional
LiNbO polarization transformer that endlessly rotates the eigenmodes of the SBCs at a constant retardation
of .With linear polarization at the input,one HWP voltage period will
generate
polarization rotation at the output.At a drive fre-quency of 3.02kHz,the polarization changes have a speed of 38krad/s.Due to the two QWP pairs before and after the HWP,the output polarization mostly describes circles with dif-ferent sizes and orientations.Mean (30krad/s)and root-mean
square scrambling speeds
are
and times the max-imum speed,respectively.
In Fig.2,the QWPs are halted and aligned for linear HWP input polarization.This means that the HWP output polarization moves maximally fast,and pass through the points of min-imum and maximum polarization error.The normalized feed-back ,the relative intensity error (RIE),is recorded in the FPGA,while the polarization controller is switched on at
time
s.In this ca,it
took s for the controller to fully reach the global minimum.
From an analysis of another 100switch-on process from ar-bitrary polarizations but with initial
errors 1rad,we have as-sd the small-signal control time constant to be on the order of
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2s.If signal acquisition from worst polarizations were prob-lematic—which it is not—it could be speeded up by a systematic initial arch.
To quantify control accuracy,the feedback signal,including the deviations caud by the voltage modulation,is recorded in the FPGA about every 229ns,and put into a histogram.Fig.3shows the complementary distribution
function
(RIE)of the ,the probability that the RIE becomes wor than the value given on the abscissa.Results are shown for different scrambling speeds.Each curve stands for a mea-surement interval
during 5min.The leftmost trace shows a reference measurement without light to indicate measurement
noi and determine the point of zero intensity error
RIE
.When the scrambler did not move (or was slow—there was
not
Fig.3.Complementary distribution function 10F (RIE)of RIE for different maximum polarization scrambling
speeds.
Fig.4.RIE and polarization error that are surpasd only with the given prob-abilities,as a function of
scrambling speed.
much difference),the mean/maximum RIE were 0.06%/0.25%.The mean/maximum polarization errors derived from this RIE are 0.05/0.1rad.With the scrambler generating polarization changes up to 20krad/s,mean/maximum RIE were measured to be 0.08%/0.45%,respectively.With maximally fast polar-ization changes of 38krad/s,the errors were 0.11%/0.85%.The mean/maximum polarization errors corresponding to the are 0.068/0.185rad.A more accurate value of the small-signal control time constant is,therefore,calculated as
0.068rad/38
krad/s
s.The 38-krad/s measurement was extended to a duration of 35h to show long-term stability.Trajectory length during this last measurement is calculated
as 3.8Grad.Fig.4shows RIE and derived polarization error of the tracking experiments for various thresholds of the com-plementary distribution function.Since polarization-dependent outages typically last longer than a forward-error correction (FEC)frame,the bounded nature of the RIE that we measured is an indispensable ast.
cad填充The control behavior of the prent polarization controller at 14-krad/s speed and room temperature is shown in Fig.5.Over-laid with it are 15traces of tests of our earlier polarization con-troller at 15temperatures between 0C and 70C [10].This tracking behavior is very reproducible,irrespective of temper-ature.Obviously,the earlier and the new polarization control
1222IEEE PHOTONICS TECHNOLOGY LETTERS,VOL.21,NO.17,SEPTEMBER 1,
2009
Fig.5.Comparison of tracking experiments for 15equispaced temperatures from 0C to 70C with old t up (gray traces)against measurement at room temperature with new t up (thick black trace).Each curve corresponds to 30
min.
Fig.6.Tracking up to 38krad/s at 14wavelengths 1505;1510;...;1570nm.At 1570nm,there was slightly insufficient light power,which probably moved that trace to the left of the others.
system are of similar quality.A slight result discrepancy is still visible.It may be due to
a 2-dB position-dependent loss of the bulk-optic HWP that was ud in the old experiments instead of the electrooptic one.The position-dependent loss lets the inten-sity loss be underestimated.
The controller was also tested at 38-krad/s tracking speed for 14equispaced wavelengths between 1505and 1570nm,30min each (e Fig.6).Performance was again very uniform,benefit-ting from the prence of eight wave plates in the device.The 1570-nm trace ems to be a bit better than the others,but this is due to a slightly insufficient optical power at that wavelength,which in effect downscales the intensity.
IV .D ISCUSSION AND C ONCLUSION
Our polarization control speed is veral to many times higher than that in [8]and [9],and tracked endless polarization trajectories are completely general and more than ven decades longer.Compared to [1],[7],and [10],veral ingredients were changed:Since that controller is on loan for field trials [1],we built another unit with another
LiNbO device,suitable for higher tracking speed.Power consumption of the electrode voltage sources was halved to a total
of 5W.Tracking speed was incread from 14(or 15)to 38krad/s as follows:We improved the modulation scheme and simplified the basic control algorithm.And,we now u an electrooptic HWP in the scrambler with low position-dependent loss.Demonstrated wavelength tolerance range is now 65nm instead of the earlier reported value of 44nm.
With the aforedescribed technology,100Gigabit Eth-ernet [(GbE)28Gbaud],or even桂花雨
2100GbE (56Gbaud)PDM-DQPSK transponders can be developed today.
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