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All-fiber electrooptical polarization control

0. Tarasenko, N. Myren and W. MargulisAcreo, Electrum 236, 16440, Stockholm, Sweden

walter :matgulisCa_ac;eose

I. C. S. CarvalhoDept. Fisica, Pontificia Universidade Catolica do Rio de Janeiro, 22453-900, Rio de Janeiro, Brazil

Abstract: Optical fibers gain a second order nonlinearity through thermal poling. The voltageinduced birefringence is small. An all-fiber Mach-Zehnder interferometer was then constructedenabling electrooptical polarization control with 78 V and sub-microsecond response time.©2005 Optical Society of AmericaOCIS codes: 190.4370 Nonlinear optics fibers; 230.2090 Electrooptical devices; 260.5430 Polarization

1. Introduction

Polarization control is required in PMD mitigators, in fiber lasers, amplifier pumps and in general applicationsinvolving fibers. Adjustment is necessary since the polarization state of light is generally affected by simple bendsof the fiber leads used, and because of the polarization dependence that some components exhibit. Ideally,polarization control should be accomplished at moderate cost, low loss, without having to resort to bulk elements orfree-space propagation, be fast and free from resonances, consume low electrical power and be compatible with highoptical intensities. Here, we exploit the possibility of using the electrooptical effect in optical fibers as a means ofcontrolling the polarization. Potentially, the desired characteristics named above can all be achieved. Fibers naturallyonly exhibit a very weak (quadratic) electrooptical coefficient, but poling can be used to bias the response foroperation in a linear electrooptical regime. However, even after poling we find that the polarization dependent indexchange is very weak (i.e., the voltage induced birefringence is small). Therefore, rather than actuating on the twoorthogonal polarization eigenstates of a single fiber, we split the input signal into two paths with independentpolarization, adjust their relative phase and recombine the signal at the output. To this end, we constructed an all-fiber electrooptical Mach-Zehnder interferometer for polarization control, capable of scanning a complete circle onthe Poincare sphere in less than 100 ns driven by a signal 0-78 V.

2. Electrooptical fiber

Several -1 m long pieces of twin-hole Ge-doped silica fiber were used, and provided with 70-90 cm long AuSnalloy electrodes by pumping the alloy into the holes as a liquid at 300 'C. The electrodes were subsequently used asa solid (<280 'C) [1]. For initial experiments, Fiber 1 was chosen, with core diameter 3.5 gim, An = 0.03 and edge-to-edge core hole separation 7 tm (anode) and 10 tm (cathode), respectively. The PDL in pieces of Fiber 1 filledwith 70 cm long metal electrodes was 1.26 dB and the loss (best case) 0.8 dB. The Mach-Zehnder interferometerwas constructed with Fiber 2, which had a core diameter 3.6 gim, An = 0.025 and the edge-to-edge core holeseparation 5.5 gm (anode) and 13 gm (cathode), respectively. Because of the smaller core-metal separation, Fiber 2behaved as a polarizer. Thus, a 70-cm metal filled piece exhibits 10 dB and >60 dB loss for P and S polarizations,respectively. In order to create a second-order electro-optic response, the pieces of fiber were thermally poled [2-4]at 255 'C and 4.3 kV for 40 minutes, so that a strong electric field was recorded in the glass. For characterization,each poled fiber was coupled as the active arm of a fiber Mach-Zehnder interferometer, and the electric fieldrecorded and the switching voltage (V,) of the interferometer at 1.55 gm were measured. Typically, for Fiber 2 thefield created by the displaced charges (-0.35 x 109 V/m) was equivalent to applying an external voltage of 7 kVbetween the electrodes, and V, - 80 V. This results in an effective second order nonlinearity 0.21 pm/V. For Fiber 1,V, was typically 250 V.

We studied the input polarization dependence of the poled fibers, when the polarization was parallel (P) ororthogonal (5) to the direction of the applied field. From a simple tensor analysis one should expect that the electricfield induced refractive index change in poled fibers should be significantly more efficient for P- than 5- polarizedlight. This polarization dependence is in fact observed in the efficiency of frequency doubling in poled fibers.However, previous studies in poled waveguides [5] and in fibers subjected to a DC electric field [6] indicated that

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this polarization dependence was absent because of electrostriction effects. Fig. 1 shows the phase shift induced in apoled piece of Fiber 1 for P and S input polarization. The value of the phase shift is measured in units of 7r-radians.Both for low and high applied fields the dependence is seen to be very small (A10%). A similar result was alsoobtained for Fiber 2 (V7, differed by 7°0 for P and S polarizations). This observation indicates that the process ofpoling - and the associated permanent displacement of charge - does not modify the lack of a polarizationdependence of the index change, as anticipated in [6]. The conclusion is that in the absence of additionalbirefringence, the refractive index ofpoledfibers can be altered with voltage without causing a significant change ofthe polarization state of light. Although this is an attractive feature for switching and modulation, it makespolarization control with poled fibers more difficult.

16-164 - P polarization (30.5 m\N)

r 12- S polarization (24.7 nW

coVCU 8

0

34-

0 500 1000 1500 2000Volts [V]

Fig. 1 Phase shift measured for input light polarized parallel (P) and perpendicular (S) to applied field. A 70-cm piece ofFiber 1 poled for 40 minutes at 255 °C under a bias 4.6 kV was used. The voltage induced birefringence is small.

3. The interferometer

Mach-Zehnder interferometers transform phase modulation into amplitude modulation. In order to obtain goodcontrast (total extinction) it is necessary to have the same polarization state in the two interfering arms, a conditioneasily satisfied in a planar waveguide. In a fiber Mach-Zehnder interferometer, however, the two arms are normallyquite long (AI m) and unless extra care is taken (e.g., using polarization preserving fibers) there is no automaticguarantee that the polarization is the same when light reaches the output coupler coming from both arms. In the caseof a Mach-Zehnder formed with fibers with internal electrodes where the birefringence is typically one order ofmagnitude larger than in standard telecom fiber, care needs to be taken to align the fibers for good contrast inamplitude modulators. Conversely, if the fibers are positioned in such a way as to introduce a relative polarizationrotation, it is possible to deliberately obtain low contrast [7]. If the polarization in the recombining arms isorthogonal, the contrast is zero, and the consequence of a pure phase change in one of the arms is that the sum of theoptical fields at the output coupler has the polarization rotated (and constant intensity). This is the principle of thedevice exploited here.

The fiber Mach-Zehnder interferometer (MZI) constructed operated in a push-pull configuration with two activearms to equalize the loss and reduce the switching voltage. Two poled pieces of Fiber 2 were chosen (length 95 cm,electrode length 85 cm) with VZ = 81 and 79 V, respectively. These were spliced between two 3 dB couplers. Sincethe polarization at the output of the arms of the interferometer was not exactly orthogonal, amplitude modulation aswell as polarization rotation was obtained. The value of V7, for the device was 38 V. To the best of our knowledge,this is the first time a push-pull MZI with poled fibers is reported, and the switching voltage is the lowest achievedby thermal poling of fibers. For stability, the device was packaged (Fig. 2 right). Upon application of voltage, thesignal analyzed with a polarimeter moved along the Poincare sphere along a major circle (Fig. 2, left). The repeatedtrajectory obtained for higher voltages than 78 V indicates that the device has a well-defined birefringence axis andno appreciable hysteresis. Note that a low-loss input-independent polarization controller here should be built usinglow loss fiber (e.g., Fiber 1) and polarization splitters. The fact that Fiber 2 acts as a polarizer allows us to combineat the output coupler two linearly polarized independent signals even with a 3 dB coupler at the input, but at theexpense of loss of generality (a polarization controller at the input need to select appropriate input states).

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Fig.2. Polarization rotation with an all-fiber push-pull Mach-Zehnder interferometer incorporating two electrooptical(poled) fibers. On the right, the actual packaged device

4. Time response

Poled fiber interferometers have previously been used for analog video signal modulation (at 6 MHz), and thefrequency response of such devices was determined to be flat with cut-off in the range tens of MHz withoutresorting to travelling wave propagation (where >500 MHz has been achieved). Here, we studied the response timeof the polarization controller using linearly polarized light from a tunable laser at the input and analyzing the outputwith a polarizer. A pulse generator delivering 16 V pulses drove the fiber Mach-Zehnder interferometer. In thismeasurement, the signal polarization is switched from linear (crossed with the analyzer) to elliptical (the drivingsignal is lower than needed for circular polarization). We found that the length of coaxial cable between the sourceand the polarization controller affected the risetime obtained, but an illustrative measurement is shown in Fig. 3. Theresponse time obtained (10-90%) is well under 100 ns. For the estimated capacitance (70 pF) and resistance of theelectrodes (250 Q), the expected response time 2.2xRC is 38 ns, in relatively good agreement with the measuredvalue. 6

5.

Trise (ns.2) ~~fall-

01

00 200 400 600 800 1 000 1200 1400

Time (ns)Fig.3. Time response of a poled fiber Mach-Zehnder interferometer used for polarization switching

5. Conclusion

In this paper we demonstrated the possibility of using poled fibers to accomplish fast (sub-microsecond) polarizationrotation exploiting the electrooptic effect. Because the voltage induced birefringence was small, an all-fiberinterferometer was employed. The device is potentially of moderate cost and low loss. It is completely spliced, hasno moving parts, it is fast and free from resonances, consumes low electrical power and is compatible with highoptical intensities.

6. References

1. M. Fokine et al, "Integrated fiber Mach Zehnder interferometer for electro-optic switching", Opt. Lett. 27, 1643 (2002)2. R. A. Myers et al "Large second-order nonlinearity in poled fused silica," Opt. Lett. 16, 1732 (1991)3. X. Long et al, "Measurement of linear E/O effect in poled optical fibers," Elect. Lett. 30, 2162 (1994)4. P. Kazansky et al, "High second-order nonlinearity in poled silicate fibers," Opt. Lett. 19, 701 (1994)5. Y. Ren et al, "Thermally poled channel waveguides with polarization independent electro-optic effect", IEEE Phot. Tech. Lett. 14, 639(2002)6. N. Godbout et al, "Measurement and calculation of electrostrictive effects in a twin-hole silica glass fiber", J. Opt. Soc. Am. B, 17, 1 (2000).7. A. D. Kersey et al, "Optimization and stabilization of visibility in interferometric fiber-optic sensors using input-polarization control", J.Lightwave Tech., 6, 1599 (1988).