IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012 621
Light-by-Light Polarization Control of 10-Gb/sRZ and NRZ Telecommunication Signals
Julien Fatome, Philippe Morin, Stephane Pitois, and Guy Millot
Abstract—Controlling the state of polarization of a light beampropagating in a standard single-mode fiber by means of a loss-less and instantaneous interaction is a fundamental effect of greatinterest for telecommunication applications and all-optical signalprocessing. In this paper, we experimentally demonstrate light-by-light polarization control via a nonlinear interaction occurring insingle-mode optical fiber between a signal wave and a counter-propagating control pump beam. We observe a polarization at-traction and stabilization of a 10-Gb/s optical telecommunicationsignal around 1550 nm for either return to zero or nonreturn tozero modulation format. These experimental results confirm yetanother fascinating possibility to all-optical control the light prop-erties in optical fiber.
Index Terms—Nonlinear optics, optical fiber, polarization con-trol, signal processing.
I. INTRODUCTION
IN MANY fields of photonics, especially in optical fiber-based devices, the state of polarization (SOP) of light re-
mains so far one of the most elusive uncontrolled variable, whichcan dramatically affect the performances of telecommunicationsystems. Indeed, despite the significant progress in the manufac-turing process of optical fibers [1], [2], the stochastic residualbirefringence and its associated polarization mode dispersion(PMD) induce after only a few kilometers of propagation, un-predictable polarization fluctuations that can reach thousandsof radians per second [3]–[8]. Consequently, implementation ofpolarization-sensitive signal processing such as high-contrastintegrated silicon or microchip waveguides [9], [10], nonlin-ear fiber-based functions [11], [12], regeneration process [13],and photonic-crystal waveguides [14], [15] are so far limited,thus delaying the development of future transparent networks.In this context, developing new functions capable to control orstabilize an arbitrarily polarized optical signal has become ofa great interest in many fields of optics, first in telecommuni-cation systems but also in sensing, fiber laser, interferences, ormetrology. Fixing the SOP of light can obviously be ensuredby a standard polarizer, but in that case, the input polarization
Manuscript received November 12, 2010; revised January 19, 2011 andFebruary 17, 2011; accepted February 19, 2011. Date of publication April 5,2011; date of current version March 2, 2012. This work was supported in partby the Agence Nationale de la Recherche (ANR FUTUR Project: ANR-06-TCOM-016) and in part by the Conseil Regional de Bourgogne.
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSTQE.2011.2119467
fluctuations are transformed into large-intensity fluctuations atthe device output, which is unacceptable in many practical situ-ations, especially for nonlinear postprocessing applications. Inorder to combat these impairments, a second way to stabilizethe light SOP is an active polarization control based on opto-electronic elements coupled to feedback algorithms [16], [17].These systems have already demonstrated their efficiency butare thus limited by the electronic response time and are notfast enough to master strong polarization variations [8], [16].Moreover, they cannot be compatible with wavelength divisionmultiplexing (WDM) applications. On the other hand, becauseof their quasi-instantaneous features, nonlinear effects occur-ring in optical fibers have recently paid considerable attentionas a possible solution to all-optical master the polarization oflight. Typical and remarkable examples of such a system are thephotorefractive crystal-based nonlinear polarizer reported byHeebner et al. [18] or Raman pulling by Martinelli et al. [19],[20] as well as Brillouin effect [21]–[24] occurring in opticalfibers. These different works have reported convincing experi-mental proofs of concept but have not yet succeed in a real in situtelecommunication demonstration that could stimulate futureemerging applications. Beside these works, we have identifiedin some previous experiments another approach of polarizingprocess taking place in an isotropic optical fiber pumped by twocounterpropagating beams and based on a four-wave mixing(FWM) phenomenon [25]. More precisely, it has been shownthat a circularly polarized pump could act as a lossless polar-ization attractor for a signal beam propagating in the oppositedirection [25], [26]. More recently, in [27], we have extendedthis concept to low-PMD optical fibers and demonstrated theall-optical control of the SOP of a 10-Gb/s return to zero (RZ)telecommunication signal. In this paper, we study in detail thisnew signal-processing device and experimentally demonstratethat it is possible to all-optical control and stabilize the SOP of10-Gb/s telecommunication signals either modulated in RZ ornonreturn to zero (NRZ) formats. Thanks to experimental obser-vations, we highlight the exchange of entropy occuring betweensignal and pump beams that originate from the polarization at-traction process. We also study the influence of the pump andsignal wavelength mismatch as well as the role of the pumpspectral linewidth. Finally, in the last section, we demonstratethat our system is able to vanish strong and fast polarizationevents.
II. THEORETICAL CONSIDERATION
The physic involved in our system relies on the nonlinearinteraction based on FWM occurring between two counterprop-agating waves injected into an optical fiber [25], [26]. Indeed,
622 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012
Fig. 1. Schematic illustration of the attraction process occurring in an isotropicoptical fiber.
it was demonstrated in previous works that two waves, a pumpwave and a signal wave, injected with opposite directions in aperfectly isotropic fiber, tend to equalize their polarization el-lipticities all along the fiber length; whereas, in the same time,the angle between the principal axes of the pump and signal po-larizations is attracted toward a value that only depends on theinitial difference between the signal and pump ellipticities [25].In this particular system, as schematically described in Fig. 1, itwas theoretically and experimentally demonstrated that a pumpwave injected with a circular SOP constitutes an attractor orfunnel of that systems in such a way that the output polarizationof the signal is fixed and stabilized independently of its initialSOP [25], [26].
This remarkable polarization attraction effect has been exper-imentally observed in previous studies where it was highlightedthat fiber isotropy was a key element of the experiment success,in the sense that any perturbation of this isotropy inexorablylead to a dramatic decrease in the attraction efficiency. For thisreason, previous experiments have been restricted to short fiberlengths (a few meters only) with very intense nanosecond pumpand signal waves (more than 50 W) [28], [29], such parame-ters being incompatible with telecommunications applications.In a very recent work, we have reported the experimental ob-servation of such a phenomenon of polarization attraction in a20-km-long standard single-mode fiber [27]. In that experiment,it was then demonstrated for the first time that it was possible toall-optical control and stabilize the SOP of a 10-Gb/s RZ opticalsignal. As in early years experiments, the system was also basedon the FWM occurring in optical fiber between a polarization-scrambled signal and a pump beam with a fixed SOP, but thetechnological breakthrough was in the use of a standard low-PMD optical fiber. Indeed, in such a system, the whole fibercan be considered as a concatenation of short and perfect po-larization attractors as defined originally in isotropic fiber. Infact, if the local birefringence of the fiber is sufficiently lowenough, one can show that an FWM process occurs between thepolarization components of the two counterpropagating waves,whatever the wavelength mismatch between the signal and pumpwaves [30], [31]. In such a system, it was demonstrated that thisFWM process induces a unidirectional exchange of energy be-tween the two polarization components of the signal wave allalong the fiber length. A remarkable consequence of this non-linear interaction is that the signal polarization asymptoticallyconverges toward a fixed value at the fiber output, indepen-dently of its initial state [30]. As illustrated in Fig. 2, because
Fig. 2. Schematic illustration of the attraction process occurring in a low-PMDoptical fiber.
of the residual PMD and random evolution of the pump SOP,the system acts as a succession of short polarization attractorscharacterized by their own point of attraction. Meanwhile, asone goes along the fiber, the system adiabatically vanishes thepolarization fluctuations of the input signal. Consequently, theSOP of the output signal is fixed and stabilized independentlyfrom its initial state. Moreover, because of the residual PMD andassociated random-walk SOP evolution along the fiber length,the system is efficient even if initial pump and signal SOPs areexactly orthogonal. Finally, it is remarkable to note that, withthis configuration, the initial SOP of the pump wave can belocalized everywhere on the Poincare sphere.
Very recently, Kozlov et al. have demonstrated that the mostimportant feature of that system is the length scale associatedwith the difference L′
B of beat lengths (distance for which thephase difference between the two fundamental modes becomes2π). Namely, L′
B = [L−1B (ωs) − L−1
B (ωp)]−1 , where ωp (ωs) isthe pump (signal) carrier frequency. The absolute condition forthe lossless nonlinear polarizer to be efficient is that L′
B shouldbe much longer than the total length of the fiber L. Otherwise,the mutual polarizations of the beams are rapidly depolarizedonto the Poincare sphere [30]. On the other hand, to observean efficient attraction process, the fiber length must exceed thenonlinear length LNL (1/γP, where γ is the Kerr coefficient andP the signal power) to allow the waves to interact in full strength.Typically, a length of 10 LNL should be used [30]. At this point,it is also important to note that the use of much longer opticalfiber than in earlier experiments does not restrict anymore thepolarization attraction process to only nanosecond high-powerexperiments. The system could now be compatible with rathercontinuous waves, pulse trains, or telecommunication signalswith average powers two orders of magnitude below previousexperiments obtained with quasi-continuous nanosecond pulsesin a few meters of isotropic fiber [29].
III. EXPERIMENTAL RESULTS
A. Experimental Setup
Fig. 3 represents the experimental setup that we have usedto observe the polarization attraction effect. The initial signalconsists of a 231−1 RZ or NRZ pseudorandom bit sequence(PRBS) cadenced at 10 Gb/s. The central wavelength of thatsignal can be tuned all over the C-band. Note that the initialcontinuous wave was first phase modulated at a frequency of100 MHz in order to limit the Brillouin backscattering effectoccurring within the optical fiber. A polarization scrambler wasthen inserted after the intensity modulator in order to introducerandom polarization fluctuations at a rate of 0.65 kHz. Before
FATOME et al.: LIGHT-BY-LIGHT POLARIZATION CONTROL OF 10-Gb/s RZ AND NRZ TELECOMMUNICATION SIGNALS 623
Fig. 3. Experimental setup used to observe the polarization attraction effect.PRBS: pseudorandom bit sequence, EDFA: erbium-doped fiber amplifier, NZ-DSF: nonzero dispersion-shifted fiber, BER: bit error rate, Pol: linear polarizer.
injecting into the optical fiber, the signal is finally amplifiedby means of an erbium-doped fiber amplifier (EDFA). At theopposite end of the fiber, the counterpropagating pump beam,which acts as the attractor wave, consists of a polarized inco-herent wave having a fixed arbitrary SOP, a spectral linewidthof 100 GHz and a central wavelength that can be tuned all overthe C-band. Note that the spectral linewidth of the pump wavewas large enough to prevent any Brillouin backscattering ef-fect within the optical fiber. The optical average power of thepump wave can be tuned between 0 and 1 W. Two optical cir-culators were inserted at both ends of the fiber so as to injectand collect the pump and signal waves. The optical fiber whichacts as the nonlinear Kerr medium was a 6.2-km-long nonzerodispersion-shifted Fiber (NZ-DSF) with a chromatic dispersionD = −1.5 ps/nm/km at 1550 nm and a PMD of 0.05 ps/km1/2 .Note that the normal dispersion regime was carefully chosen inorder to avoid any nonlinear impairment induced by modulationinstability effect during the propagation, especially for the NRZmodulation format. At the system output, the signal SOP wasanalyzed using the usual Stokes vectors formalism and was vi-sualized onto the Poincare sphere by means of a commerciallyavailable polarization analyzer. In order to monitor the polariza-tion fluctuations of the signal in the time domain, we simulatethe presence of a dependent polarization device by inserted aninline polarizer just before detection by a 28-GHz bandwidthoscilloscope associated with a bit error rate (BER) or an opticalsampling oscilloscope (OSO).
The efficiency of the polarization attraction effect was alsoevaluated by calculating the degree of stability (DOS) of theoutput signal polarization, which we define as
DOS =
√〈S1〉2 + 〈S2〉2 + 〈S3〉2
S0
where Si are the so-called Stokes parameters of the signal and〈 〉 were evaluated over 256 initial polarizations. This definitionof the DOS, close to the classical degree of polarization butcalculated on a much longer polarization fluctuation time scale,
Fig. 4. Evolution of the DOS of the signal wave at the output of the fiber as afunction of the pump power. The signal power was fixed to 26.3 dBm (430 mW),whereas the pump power was tuned between 0 and 800 mW.
allows us to characterize the spread of the polarization fluctua-tions on the Poincare sphere and thus, to quantify the efficiencyof the attraction process. In fact, low values of the DOS will cor-respond to large temporal fluctuations of the polarization stateall over the Poincare sphere, whereas a DOS value close to unityis associated with a nearly constant and stabilized polarizationstate.
B. Polarization Control of a 10-Gb/s NRZ Signal
In a first series of experiments, we have evaluated the evo-lution of the DOS of a 10-Gb/s NRZ signal at the output ofthe system as a function of the polarization attraction pumppower. Important fluctuations of the signal polarization wereintroduced by means of the polarization scrambler, whereas thepump wave was injected with a fixed polarization. The resultsare represented in Fig. 4. The signal power was fixed to 430 mWwhile pump and signal wavelengths are 1545 and 1563 nm, re-spectively. As can be seen in Fig. 4, the DOS of the signal polar-ization, which has initially a low value, near 0.4, due to its initialscrambling, strongly increases when the pump power is injectedinto the fiber, thanks to the polarization attraction effect. At lowand moderate pump power, the signal DOS increases almostlinearly with respect to the pump power and finally saturatesand reaches a value very close to unity for pump power above600 mW. Such a DOS value, close to 1, is then associated withan efficient polarization attraction process and demonstrates thecapability of our system to all-optical control and stabilizes theSOP of a 10-Gb/s NRZ signal.
This remarkable polarization attraction process can be moreunderstood by directly monitor the SOP of the output signalonto the Poincare sphere. Fig. 5 illustrates the polarization stateof the 10-Gb/s signal at the input of the system. Because ofthe polarization scrambling process, the points are uniformlydistributed onto the sphere [see Fig. 5(a)]. When the counter-propagating pump wave is injected into the optical fiber withan 800-mW average power, we clearly observe that most of thepoints are localized into a small area, indicating an attractionand stabilization of the polarization state of the signal wave [seeFig. 5(b)].
624 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012
Fig. 5. SOP of the initially scrambled 10-Gb/s NRZ signal plotted on thePoincare sphere (a) at the input of the system and (b) at the system output witha 430-mW signal average power and 800-mW pump power.
Fig. 6. Visualization of the 10-Gb/s NRZ pattern at the system output andmonitored behind a polarizer (a) without and (b) in presence of the counter-propagating pump beam.
The efficiency of the polarization attraction process is morestriking when directly monitored in the temporal domain. Tothis aim, we have inserted a polarization depending device, i.e.,a polarizer at the output of the system (see Pol in Fig. 3) soas to translate the polarization fluctuations of the outcomingsignal into intensity fluctuations. We have then recorded the10-Gb/s output signal NRZ pattern in persistent mode by meansof an OSO (see Fig. 6). In the pump-free configuration [seeFig. 6(a)], the polarization fluctuations are transformed intolarge-intensity fluctuations through the polarizer, leading to acomplete dramatic loss or even extinction of the signal. Byinjecting the counterpropagating pump wave [see Fig. 6(b)], aclear polarization stabilization is obtained. As can be seen, theNRZ pattern is totally recovered, all the outcoming pulses havenow almost identical polarizations, so that the “one” and “zero”levels are now clearly indentified.
C. BER Measurements
Finally, we have focused our experiments on the capabilityof our system to improve the BER of a random bit streamcarried by randomly polarized NRZ signal and detected behinda polarization-dependent component, i.e., a polarizer.
Fig. 7 represents the eye diagrams of the polarization-scrambled 10-Gb/s NRZ signal detected at the output of the po-larization attractor followed by a polarizer without [see Fig. 7(a)]and with [see Fig. 7(b)] injection of the counterpropagating con-
Fig. 7. Eye diagram of the initially polarization-scrambled 10-Gb/s NRZ sig-nal (430 mW) at the output of the polarization attractor and detected behind apolarizer (a) without counterpropagating pump wave and (b) with counterprop-agating pump (800 mW). (c) Corresponding evolution of the BER as a functionof the power incoming on the receiver: back-to-back (circles), scrambled signalwithout (triangles) and with pump wave (diamonds).
trol pump wave. In the pump-free configuration [see Fig. 7(a)],the initial signal polarization fluctuations are transformed intointensity fluctuations via the output polarizer, leading to a com-plete closure of the eye diagram and loss of the informationtransmitted by the signal. By injecting the 800-mW counter-propagating pump wave [see Fig. 7(b)], a clear polarization sta-bilization is obtained. As can be seen, all the outcoming pulseshave now almost identical polarizations, without additional in-tensity fluctuations, so that the opening of eye diagram is nowefficiently recovered.
In order to underline the practical compatibility of the devicewith telecommunication applications, we have also measuredthe corresponding BER of the NRZ 10-Gb/s signal as a functionof the average power incoming on the receiver [see Fig. 7(c)].The receiver was based on an EDFA booster working at a con-stant output power of 10 dBm and placed just in front of a28-GHz bandwidth photodiode. The reference is represented bythe back-to-back configuration (i.e., at the fiber input) in circles.At the output of the system, when the polarization of the signalis scrambled and the pump off, corresponding to the eye dia-gram of Fig 7(a), the BER is limited to 10−4 (triangles). Whenthe counterpropagating pump wave is applied (diamonds), thequality of the transmission is greatly improved by almost eightorders of magnitude on the BER measurements for a −30-dBmaverage power on the receiver with negligible power penaltiescompared to back-to-back measurements.
D. Polarization Attraction Imposes Entropy Exchange
Another important physical property of the polarization at-tractor that has to be underlined in this paper is that the total
FATOME et al.: LIGHT-BY-LIGHT POLARIZATION CONTROL OF 10-Gb/s RZ AND NRZ TELECOMMUNICATION SIGNALS 625
Fig. 8. Evolution of the pump DOS as a function of the 10-Gb/s NRZ signalpower, for a fixed pump power of 800 mW and the same pump and signalwavelengths as in Fig. 4.
Fig. 9. SOP of the 800-mW pump wave plotted on the Poincare sphere (a) atthe input of the system and (b) at the system output with a 430-mW 10-Gb/sNRZ signal average power.
polarization entropy is supposed to be conserved during theattraction process. As a consequence, the signal polarizationfluctuations are expected to be transferred to the pump polar-ization and finally evacuated from the system thanks to thecounterpropagating configuration.
This remarkable effect is clearly visible in Fig. 8, which showsthe DOS of the pump wave as a function of the signal power. Inthe absence of signal wave (signal power = 0 mW), the pumpwave polarization is not significantly affected by the propagationthrough the optical fiber and remains constant at fiber output,leading to a polarization DOS close to unity. At the opposite,when the polarization-scrambled 10-Gb/s signal is injected atthe other end of the fiber, as its average power increases, thepolarization attraction effect occurs and the signal polarizationfluctuations are transferred to the pump wave, leading to a spec-tacular decrease of its DOS.
This effect is also clearly visible onto the experimentalPoincare spheres depicted in Fig. 9 in which the initial fixed-polarized pump wave [see Fig. 9(a)] is completely scrambledthrough the polarization attraction process [see Fig. 9(b)].
A fascinating approach of this entropy exchange is that theattraction process allows to all-optical impose to a signal beam,a polarization trajectory onto the Poincare sphere fixed by an-other. An example of this original feature is depicted in Fig. 10.An initial 256-points eight-like trajectory is imposed on the in-put 10-Gb/s NRZ signal by means of the polarization scrambler[sampling rate of 0.65 kHz, see Fig. 10(a)] and injected into thefiber with an average power of 430 mW. At the opposite end, the
Fig. 10. SOP of the 430-mW 10-Gb/s NRZ signal wave (a) at the input of thesystem and (b) at the output. SOP of the 800-mW pump wave (c) at the input ofthe system and (d) at the output.
Fig. 11. DOS of the 10-Gb/s NRZ signal wave as a function of pump linewidthfor the following parameters: signal power 430 mW, pump power 1.1 W,wavelengths are the same as in Fig. 4.
pump wave has a fixed polarization [see Fig. 10(b)]. After in-teraction and entropy exchange in the polarization attractor, thevariations of the signal SOP are greatly reduced [see Fig. 10(c)]while the output pump wave now describes a trajectory sim-ilar to the initial signal [see Fig. 10(d)]. This original featureof the polarization attractor could find some applications in theall-optical control of the polarization evolution, for example, toimpose SOP trajectories or even scrambling in sensing field orpolarization-dependent loss test beds.
E. Influence of the Pump Linewidth
In a perfectly isotropic fiber, we have shown in a previousstudy that the FWM process responsible for the polarizationattraction process can occur whatever the degree of coherenceof the counterpropagating interacting waves [32]. To quantifythe robustness of this system property, we have studied theinfluence of the pump spectral linewidth on the polarizationattraction efficiency. The results are displayed in Fig. 11, whichshows the evolution of the DOS parameter as a function of pumplinewidth. Signal and pump powers are 430 mW and 1.1 W,respectively. Wavelengths are the same as in Fig. 4. As can be
626 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012
Fig. 12. Efficiency of the polarization attractor effect on a 10-Gb/s NRZsignal as a function of the pump wavelength. The signal wavelength was fixedto 1545.3 nm, pump and signal powers are 1.1 W and 700 mW, respectively.
observed, no significant influence of the pump linewidth wasobserved in the range of 20 GHz to 1 THz. Indeed, it was shownin a previous work that, in an isotropic fiber, the polarizationattraction process occurs whatever the spectral linewidth of thetwo counterpropagating waves [25]. These experimental resultsdemonstrate that this property is also conserved in a small PMDoptical fiber. In other words, for the range of parameters usedin our experiment, the nonlinear polarization attraction effectis much stronger than the linear depolarization, which could beinduced by PMD, indicating a great flexibility in the polarizationattractor design.
F. Influence of the Wavelength
Finally, we have also studied the influence of a wave-length mismatch between the pump and signal waves. Theexperimental results are shown in Fig. 12. The pump wave-length was tuned between 1540 and 1550 nm, whereas the sig-nal wavelength was fixed to 1545.3 nm. The pump and signalpowers were fixed to 1.1 W and 700 mW, respectively. As pre-dicted by theory, the FWM process involved in the polarizationattraction effect is phase matched whatever the frequency differ-ence between the pump and signal waves, so that the efficiencyof the nonlinear interaction is not affected by the wavelengthmismatch. We would like to point out that this property is ofgreat importance for WDM applications and Raman amplifica-tion considerations.
G. 10-Gb/s RZ Results
In [27], we have had demonstrated the proof of concept of theall-optical control of the SOP on a 10-Gb/s RZ signal. Here, weimprove the quality of the device by carefully selecting the op-tical fiber involved in the attraction process. As the propagatedsignal undergoes a strong nonlinear regime, the sign and valueof chromatic dispersion have to be designed in order to avoiddramatic impairments on the temporal profile due to the inter-action of chromatic dispersion and nonlinearity. It is the reasonwhy, compared to [27], the fiber involved in these experimen-tal results was shorter (6 km) and in normal dispersion regime(D = −1.5 ps/km·nm). The wavelength of the RZ signal wascentered at 1563 nm with Gaussian-like pulses of 30-ps tempo-ral duration and a 500-mW average power. The wavelength of
Fig. 13. SOP of the initially scrambled 10-Gb/s RZ signal plotted on thePoincare sphere (a) at the input of the system and (b) at the system output witha 500-mW signal average power and 1.3 W pump power.
the pump beam was 1544 nm and the optimum power was findto be 1.3 W.
The experimental results are quite similar to those describedin previous sections and prove that our system is transparent tothe modulation format (RZ or NRZ) but rather sensitive to theinput average power. Fig. 13 illustrates the SOP of the 10-Gb/ssignal at the input of the system [see Fig. 13(a)] and at the out-put of the polarization attractor [see Fig. 13(b)]. As the SOPof the initial signal is scrambled, the points are distributed ontothe whole Poincare sphere [see Fig. 13(a)]. When the counter-propagating pump wave is injected with a 1.3-W average power[see Fig. 13(b)], we then clearly observe an efficient attractionprocess accompanied by a strong localization of the points ona small area, indicating a great stabilization of the polarizationstate of the signal wave.
As in the NRZ results, in order to characterize the attractionprocess in the temporal domain, we have then detected the re-sulting 10-Gb/s RZ signal behind a polarizer. Fig. 14(a) showsthe resulting signal when the counterpropagating pump waveis OFF. As the polarization is scrambled, the SOP fluctuationsare transferred on the intensity profile through the polarizer re-sulting in large-intensity fluctuations and eye closure. At theopposite, when the control pump is injected [see Fig. 14(b)],corresponding to the polarization stabilization of Fig. 13(b),the eye diagram is greatly recovered with negligible intensityimpairments.
Finally, we have recorded the evolution of the BER of thepolarization-scrambled 10-Gb/s RZ signal after a polarizer asa function of the power incoming on the receiver. The resultsare depicted in Fig. 14(c), first, in back-to-back configurationand then after propagation through our device without and withthe counterpropagating pump wave. In absence of control pumpbeam, as the polarization of the signal is scrambled, correspond-ing to the configuration of Fig 14(a), all the polarization fluc-tuations are transferred into intensity fluctuations through thepolarizer resulting in many bit errors on the receiver. Conse-quently, in this configuration, the BER is thus limited to 10−5
for an average power of −30 dBm. At the opposite, when thecounterpropagating pump wave is applied, the quality of thetransmission is greatly restored with negligible penalties com-pared to the back-to-back configuration.
FATOME et al.: LIGHT-BY-LIGHT POLARIZATION CONTROL OF 10-Gb/s RZ AND NRZ TELECOMMUNICATION SIGNALS 627
Fig. 14. Eye diagram of the initially polarization-scrambled 10-Gb/s RZ sig-nal (500 mW) at the output of the polarization attractor and detected behind apolarizer (a) without counterpropagating pump wave and (b) with counterprop-agating pump (1.3 W). (c) Corresponding evolution of the BER as a functionof the power incoming on the receiver: back-to-back (circles), scrambled signalwithout (triangles) and with pump wave (diamonds).
Fig. 15. Intensity profile of the 6-ns polarization burst detected behind apolarizer by means of a low-bandwidth oscilloscope, without (dashed line) andwith (solid line) the 700-mW counterpropagating pump wave.
H. Burst Annihilation
In this section, we have characterized the ability of our sys-tem to annihilate a short polarization burst, i.e., a strong andfast variation of the signal SOP [8]. Such a dramatic event isfortunately rare but could be observed in a telecommunicationline and is difficult to master with present systems based on ac-tive electronic feedback [16]. To this aim, a polarization burst,having a temporal width of 6 ns, was introduced into the initial10-Gb/s RZ signal by means of a 15-GHz bandwidth optoelec-tronic phase modulator polarization switch.
Fig. 15 shows the intensity profile of the polarization burst,monitored by means of a low-bandwidth oscilloscope at the out-put of the system and detected behind a polarizer. In absenceof counterpropagating pump beam (dashed line), we observethe strong variation in the intensity profile, which could be dis-astrous for any polarization-sensitive or nonlinear processing
function. When the 700-mW counterpropagating pump beam isinjected (solid line), the polarization burst was efficiently ab-sorbed by the attraction process, leading to an error-free trans-mission (BER = 10−12).
IV. CONCLUSION
In conclusion, we have reported the experimental demonstra-tion of an all-optical polarization attraction process allowingcontrol and stabilization of the SOP of 10-Gb/s telecommunica-tion signals around 1550 nm. Our device is based on a nonlinearinteraction between an incident signal beam with an arbitrarySOP and a counterpropagating control pump wave taking placein a 6.2-km-long single-mode optical fiber characterized by lowPMD. Based on Kerr effect, the polarization attractor is quasi-instantaneous, with a large bandwidth and almost lossless. Wehave found that the polarization fluctuations of the 10-Gb/s sig-nal are vanished from the system by an exchange of entropybetween the signal and pump beam. We have also shown thatsuch a device is efficient for RZ or NRZ modulation formatsand that it could be tunable in the C-band. Finally, we havealso demonstrated that our optical device could strongly van-ishes intense and fast polarization events as short as 6 ns. Basedon these observations, we think that this powerful system couldfind many applications in all-optical signal processing for futuretransparent optical networks.
APPENDIX
A short movie of the lab experiment could be found athttp://www.youtube.com/watch?v=XC_9IvTB2bQ.
ACKNOWLEDGMENT
The authors would like to thank S. Wabnitz from the Univer-sity of Brescia as well as L. Marazzi from PoliCom in Milan forfruitful discussions and encouragement.
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Julien Fatome was born in Charleville-Mezieres, France, in 1978. After grad-uating from the engineering school ESIREM, Dijon, France, he received theDEA degree in 2000 and the Ph.D. degree in physics for studies of ultra-shortpulse propagation at 160-Gb/s in dispersion managed optical fiber lines in 2004,both from the University of Bourgogne, Dijon, France.
In 2005, he became a Research Engineer in the Centre National dela Recherche Scientifique (CNRS), Laboratoire Interdisciplinaire Carnot deBourgogne, University of Bourgogne. He is currently carrying out research innonlinear effects, pulse trains generation at ultrahigh bit rate as well as all-optical nonlinear processing and polarization control. He has published morethan 70 contributions in journals and conference proceedings.
Philippe Morin was born in Epinal, France, in 1982. He received the Masterdegree from the University of Burgundy, Burgundy, France, in 2009. He iscurrently working toward the Ph.D degree at the Laboratoire InterdisciplinaireCarnot de Bourgogne, University of Bourgogne, Dijon, France, and carryingout its research on the all-optical control of the polarization state of light andespecially, on the polarization attraction process in optical fiber.
Stephane Pitois was born in Beaune, France, in 1974. He received the Ph.D.degree in physics in the field of modulational instability and domain walls inoptical fibers from the University of Burgundy, Dijon, France, in 2000.
He carried out postdoctoral research in the Department of Optics and Acous-tics, Free University of Brussels, Brussels, Belgium, on nonlinear effects indynamical Bragg gratings. In 2001, he became a Researcher at the Centre Na-tional de la Recherche Scientifique (CNRS), Department of Physics, Universityof Burgundy. He has published more than 100 contributions in journals, books,and conference proceedings. His research interests include nonlinear effects andpulse trains propagation in optical fibers.
Guy Millot was born in Alligny-en-Morvan, France, in 1960. He received thePh.D. degree in laser Raman spectroscopy in gases from the University ofBurgundy, Dijon, France, in 1986.
Since 1994, he has been a Full Professor in the Department of Physics, Uni-versity of Burgundy. He has published more than 200 contributions in journals,books, and conference proceedings. His research interests include nonlineareffects in optical fibers, modulational instabilities, solitons, generation, prop-agation and characterization of optical pulse trains at high repetition rates,stimulated Raman scattering, frequency conversion, and applications in opticalcommunications.
