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Bias of integrated optics Pockels cell high-voltage sensors. Jaeger, Nicolas A. F.; Rahmatian, Farnoosh 1994

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Bias of integrated optics Pockels cell high-voltage sensorsNicolas A.F. Jaeger and Farnoosh RahmatianUniversity of British Columbia, Department of Electrical EngineeringVancouver, BC, Canada V6T 1Z4ABSTRACTThe results of measurements of the intrinsic phase-differences of titanium-indiffused lithiumniobate waveguides, for use in integrated optics Pockels cell high-voltage sensors, are presented.The dependencies of the intrinsic phase-differences of these waveguides on their lengths andwidths are investigated; a change of between 4.9 and 5.9°Im/mmwasobtained. Also, thechange in the intrinsic phase-difference as a function of both temperature and time wasinvestigated; a typical change of O.02°/°C/mm was measured and, following a small initialchange, the bias was found not to drift with time. Some suggestions for possible post-processingof the output signals, of the integrated optics Pockels cell high-voltage sensors, to increase thedynamic range and to compensate for small changes in the bias, are presented.1 .INTRODUCTIONOptical sensors have found various applications in industrial environments'. Several opticaldevices have been suggested and developed, over the last few years, for the measurement ofvoltage, current, or other parameters in high-voltage environments27. In particular, systemsusing fibre optics offer immunity to electromagnetic interference. This immunity may beimportant when a sensor is employed in the high-voltage environment, where very large transientfields can occur. Integrated optics Pockels cells (IOPCs), to be used as electric field sensors inhigh-voltage environments, have been previously introduced8'9. Here, several issues related tothe bias of these sensors are discussed; also, for completeness, a brief description of thesedevices is provided first. 2. DEVICE DESCRIPTIONBasically, an IOPC consists of a sensor-head, to be placed in the high-voltage environment,optical fibres to transmit light to and from the sensor-head, a laser diode as a light source, andan optoelectronic conversion unit to measure the optical signals. The sensor-head consists of awaveguide fabricated by diffusing a strip of titanium into a y-cut substrate of lithium niobate,the waveguide being parallel to the z crystallographic axis. The dimensions of the pre-diffusiontitanium strip are chosen so that the waveguide formed, after the diffusion, supports only twomodes: the fundamental TE-like mode and the fundamental TM-like mode. Polarized light, fromthe laser diode, is transmitted to the sensor-head using an input fibre which is polarizationmaintaining (PM). The input fibre is aligned with the end of the waveguide in the sensor-headsuch that the PM axis of the fibre is at about 450tothe x and y crystallographic axes of thelithium niobate substrate. This alignment is intended to result in nearly equal amounts of opticalO-8194-1337-2/94/$6.OO SPIE Vol. 2072 / 87Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termspower in the fundamental TE- and TM-like modes, supported by the waveguide, at the outputof the waveguide. The propagation constants of the two modes are slightly different; therefore,when no electric field is applied, the two modes have an intrinsic phase-difference at the outputof the waveguide. Application of an electric field parallel to the y crystallographic axis createsa change in the difference between the phase velocities of the two modes of the waveguide. Thepolarization state at the output of the waveguide will be elliptical with the major and minor axesof the polarization ellipse at about 45°tothe x and y crystallographic axis. The output of thewaveguide is interrogated using a birefringent PM optical fibre. The fibre is aligned with its PMaxes parallel to the principal axes of the polarization ellipse to interrogate the optical powersparallel to these two axes. The two optical powers can be separated and measured individually.The response is normalized by dividing one optical power component by the sum of the two,i.e. ,thetotal optical power detected. This normalization is done to make the sensor lesssusceptible to variations in the total optical power due to vibration or other factors. The applied-electric-field-in/normalized-optical-intensity-out transfer function of the sensor is1 a cos [(ITE I E1)+ 1] (1)2where S is the normalized output intensity, plus or minus depends on the choice of the outputcomponent for the numerator when normalizing, the constant a is close to, but smaller than, one(typically   0.99), E is the electric field parallel to the y crystallographic axis inside the sensor-head, ET the half-wave electric field, and is the intrinsic phase-difference (bias) betweenthe two modes supported by the waveguide. Theoretically, the half-wave electric field can bewritten as (2)2 n03 r22 Lwhere is the free-space optical wavelength, n0 is the ordinary refractive index for lithiumniobate, r22 is the relevant electro-optic coefficient, and L is the length of the waveguide.3. DISCUSSION OF FACTORS AFFECTING THE BIASFor the IOPC to be used as an electric field sensor, a nearly linear transfer function for thesensor is desirable. The transfer function (1) is nearly linear when p1 =rr/2(900)and(ir E IE) is sufficiently small. For 'p1 =n/2and small (ir E / Er), the a.c. component of S, s, canbe written as airs;E.(3)2E1Lithium niobate based integrated optics modulators used for communication applications are88/ SPIE Vol. 2072Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsoften biased using a separate pair of electrodes'°. In such devices, the bias can be controlled byapplying an appropriate d.c. voltage to the bias electrodes. As mentioned earlier, the sensor-head of an IOPC is basically a dielectric crystal (lithium niobate) and, preferably, has no metalelectrodes, in order to minimize the danger of corona discharge (flash over) when placed in largeelectric fields. As a result, it is quite inappropriate to use bias electrodes for controlling theintrinsic phase-difference of the sensor-head. Fortunately, the bias of the sensor-head of anIoPc can be set by controlling its fabrication parameters. The results of measurements of thedependence of the intrinsic phase-difference of the sensor on the waveguide' s length, waveguide'swidth, and change in temperature are given in the following sections. The bias drift and somesimple post-processing methods to compensate for a non-ideal bias are also presented.3. 1 .Theeffect of a waveguide's length on its biasThe intrinsic phase-difference of the IOPC is, theoretically, dependent on the length of thewaveguide in the sensor-head, L, and the difference in the propagation constants of the twomodes supported by the waveguide. It can be written as'pi =(BTE_B)x L (4)where TE and B, are the propagation constants for the fundamental TE- and TM-like modes,respectively. In order to fabricate properly biased waveguides, a long waveguide can befabricated first, and its intrinsic phase-difference can be measured. Then, it can be cut to alength for which the intrinsic phase-difference is expected to be close to n12.Experiments were conducted to examine the relation between the intrinsic phase-difference andthe length of the waveguides; the results are plotted in fig. 1.Thewaveguides used in this plotare divided into three clusters; each cluster contains waveguides having either 3.0, 3.5, or 4.0jm pre-diffusion titanium strip widths. This distinction is made in order to be able todiscriminate between the effect of the length and the effect of the width of a waveguide on itsintrinsic phase-difference. The straight lines in fig. 1 are least-squares-fits to the data points ineach cluster. It should be noted that the waveguides used in this plot were not all fabricated atthe same time or under identical conditions. The pre-diffusion titanium strip thickness, diffusiontime, and diffusion temperature are some factors that may affect BTE and BTM and may, in turn,affect the bias; small variations in the intrinsic phase-differences of the waveguides of similarlengths are observed in fig. 1 .Theresults presented in fig. 1 indicate that (4) holds reasonablywell for these waveguides, and can be exploited in fabricating well-biased devices. Fig. 1 showsthat a well-biased waveguide should be between 6 and 9 mm long, depending on its otherfabrication parameters.For nearly linear operation, see (1) and (3), q can be any odd multiple of 7r/2. However,it should be noted that the piezoelectric resonance frequencies of the lithium niobate substratesare inversely proportional to the substrate dimensions9"2; therefore, to obtain devices havinglarge useable bandwidths, their sensor-heads should be small. This is a reason for trying toobtain waveguides with cp= n/2,and not just any odd multiple of n/2.SPIEVol. 2072 / 89Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms500450400350(I)300250U)2000a150100500Fig. 1 .Theintrinsic phase-difference (bias) as a function of the length.3.2. The effect of a waveguide's width on its biasA waveguide's width also affects its intrinsic phase-difference. Using simple models, basedon channel waveguides having rectangular cross-sections, one can show that wider waveguideshave larger intrinsic phase-differences 12•Experimentswere conducted to investigate the effectof a waveguide's width on its bias. Fig. 2 shows a plot of the measured intrinsic phase-differences for several waveguides as functions of their pre-diffusion titanium strip widths.Again, in order to be able to differentiate between the effect of the width and the effect of thelength of a waveguide on its bias, the data presented in fig. 2 are arranged in three clustersaccording to the waveguide length: these waveguides were 8, 16, or 28 mm long. As predicted,the wider the waveguide the larger the intrinsic phase-difference. It should be noted that eachdata point in fig. 2 is an average intrinsic phase-difference for several waveguides having thesame length and the same pre-diffusion titanium strip width, but possibly differing in their otherfabrication parameters; the symbol height represents the difference between the maximum andminimum measured values. From fig. 2, the change in the intrinsic phase-difference per unitwidth per unit length of a waveguide is between 4.9 and 5.9°/Mm/mm. These results, together90/ SPIE Vol. 207220Length(mm)Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termswith the results presented in fig. 1 ,canbe used in designing well-biased waveguides. Forexample, several waveguides with pre-diffusion titanium strips 3 to 4 jm wide, varying by 0.1m, can be made in a single substrate. Then, the waveguides can be cut to about 7 mm. It canbe deduced from fig. 's 1 and 2 that one of these waveguides should have an intrinsic phase-difference which is within a few degrees of 9Ø0•Hence,the intrinsic phase-differences of all ofthese waveguides can be measured, and the one with the intrinsic phase-difference closest to 900canbe chosen to be used for the sensor.3.5Pre—diffusion Ti strip width (,am)Fig. 2. The intrinsic phase-difference (bias) as a function of the pre-diffusion titanium strip width.As mentioned before, the wider the waveguides are the larger their intrinsic phase-differenceswill be. It is, therefore, appropriate to make a waveguide as wide as possible to be able tominimize the length of a well-biased waveguide. It should be noted, however, that there is alimit to how wide a waveguide can be; if the waveguide is too wide, it will support more modesthan just the two fundamental ones. In the experiments conducted, waveguides having pre-diffusion titanium strip widths  4.5 mgenerally supported more modes than the two0: 8—mm long16—mm longO : 28—mm long500450400350(I)30O25O,,2000150-100-50-0 3.0 4.0SPIE Vol. 2072 / 91Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms(1)Q)t:1)C!)0QJfundamental ones when 670nm; thus, they were not used in the sensors.3 .3.Theeffects of temperature on the biasA change in temperature can also affect the intrinsic phase-difference of a waveguide.Therefore, experiments were conducted to observe the dependence of a waveguide's bias ontemperature. Several samples, containing waveguides, were placed on a heating stage, and theintrinsic phase-differences of the waveguides were measured as the stage temperature wasincreased from room temperature, 20°C, to70°C. For each reading, once the temperatureof the stage was determined, 10 minutes were allowed to elapse before the measurement wastaken and recorded, so that the sample could reach the nominal stage temperature. The resultsfor nine waveguides are plotted in fig. 3. The straight lines are least-squares-fits to the datapoints for individual waveguides. All of these waveguides were 28 mm long. The pre-diffusiontitanium strip widths were 3.0, 3.5,and4.0 jm for the waveguides represented by the bottomthree, the middle three, and the top three best-fit lines, respectively. Fig. 3 shows that a typicalchange in the bias per degree change in temperature per unit length of a waveguide is0.02°/°C/mm. *42540037535032530010II III II II II II II I20 30 40 50Temperature111111 111160 70 80(°c)Fig. 3. The intrinsic phase-difference (bias) as a function of temperature.92 / SPIE Vol.2072Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms3.4. StabilityThe change in a waveguide's intrinsic phase-difference with time can affect the accuracy ofthe measurements made using the device. The bias drift is mainly attributed to the opticaldamage in the waveguide. The optical damage for x- and y-propagating waveguides in lithiumniobate are reported to be significant for optical powers in excess of a few jW when operatingwith visible light'3. For z-propagating waveguides, however, the optical damage is expected tobe very small as compared to those for x- or y- propagating waveguides11. Fig.'s 4a and 4bshow the transfer functions for one waveguide after the laser was on for one minute and for twohours, respectively; the output optical power was 200 j.W. A bias drift of about 2° after 2hours was measured. No further noticeable bias drift was observed as the laser was left on forseveral hours (> 20 hours). In the applications for which these sensors are intended, e.g.,metering high-voltage a.c. signals, the sensor is rarely (if ever) off. Therefore, this initial biasdrift should not affect the performance as long as the stabilized bias is close to its intended valueof 90°. Even though this drift is very small, practically negligible, it can be further reduced bymaking systems which operate at longer wavelengths, e.g., 1.3 j.m. It is reported'3 that opticaldamage is negligible for optical powers less than 20 mW when operating at =1.3jim.Fig. 4. The transfer function of an IOPC (a) oneminute and (b) two hours after turning the laser on.SPIE Vol.2072 / 93Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms3 .5. Post-processiflgThe dynamic range of the IOPC, i.e.,therange within which the sensor can be used, isrestricted to 8° of a bias point of 900,forthe error introduces by (3) to be less than 0. 3 % 14•Forbetter signal-to-noise ratios, increasing the dynamic range of the sensor is desirable. Thedynamic range can be increased by using approximations other than sin(x) x, which was usedto obtain (3) from (1). For instance12, the approximation [sin(x) + ksin3(x)]x will introduceless than 0. 1 % error for a dynamic range of about a bias point of 900,ifthe constant kischosen properly (k =0.188). For implementing this technique, when metering a 60 Hz signalfor example, the output of the sensor can be a.c. coupled to eliminate the d.c. value in (1).Then, a digital processing unit can be designed to sample the output and to carry out the tasksof additions and multiplications in real-time to perform the calculations suggested above.A digital sampling unit, together with a micro-computer, can also be used for post-processingthe output. This post-processing can be used to compensate for bias changes and to increase thedynamic range of the sensor. For example, the sampling unit can sample the output of the IOPCand transfer the results to the computer. The parameters of the transfer function (1) can bemeasured, at the time of fabrication, and stored in the computer. Since both components of theoutput of the sensor-head are measured, the intrinsic phase-difference, ,canbe continuouslyupdated. The computer can use the transfer function and the values obtained by the samplingunit to calculate E. In this way, the dynamic range is increased to close to n.4.SUMMARYThe effects of a waveguide's length and width on its intrinsic phase-difference have beeninvestigated. Longer and wider waveguides have larger intrinsic phase-difference as comparedto shorter and narrower ones. Well-biased waveguides 6 to 9 mm long have been successfullyfabricated. The effect of temperature changes on the bias of the waveguides has also beenexamined; for the waveguides studied, a typical change in bias of O.02°/°C/mm was measured.Following a small initial change, the bias appeared to be stable with time. Post-processing canincrease the dynamic range of the sensor and/or compensate for any of the observed biaschanges.5 .ACKNOWLEDGEMENTThe authors wish to thank the Science Council of British Columbia and the NaturalSciences and Engineering Research Council of Canada for their financial support of this work.6.REFERENCES1. J. Dakin and B. Cuishaw, Optical Fiber Sensors: Principles and Components, ArtechHouse, Norwood, 1988.2. 5. Kobayashi, A. Horide, I. Takagi, M. Higaki, G. Takahashi, E. Mori, and T.Yamagiwa, "Development and field test evaluation of optical current and voltage transformersfor gas insulated switchgear," IEEE Trans. on Power Delivery, vol. 7, no. 2, pp. 815-821,1992.94/ SPIE Vol. 2072Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms3. J. Beatty, T. Meyer, and E.A. Ulmer, "Application and Field Trial of High AccuracyOptical Current Transducer for Electrical Power Systems, "presentedat the Conference onOptical Sensing in Utility Applications, San Francisco, CA, May 1991.4. R.F. Cook, "Optical sensing-metering systems," presented at the EEI-AEIC meter &service committee conftrence, pp. 1-4, Washington, D.C. ,April3, 1990.5. T.D. Maffetone and T.M. McClelland, "345 kV substation optical current measurementsystem for revenue metering and protective relaying, "presentedat the IEEE/PES WinterMeeting, pp. 1-8, New York, NY, 1991.6. T. Mitsui, K. Hosoe, H. Usami, and S. Miyamoto, "Development of fibre-optic voltagesensors and magnetic-field sensors," IEEE Trans. on Power Delivery, vol. PWRD-2, no. 1 ,pp.87-93, Jan. 1987.7. T. Sawa, K. Kurosawa, T. Kaminishi, and T. Yokata, "Development of Optical InstrumentTransformers," IEEE Trans. on Power Delivery, vol. 5, no. 2, pp. 884-891, 1990.8. N.A.F. Jaeger and F. Rahmatian, "Integrated Optics Pockels Cell as a High VoltageSensor, "Proceedingsofthe 8th Optical Fiber Sensors Conference, pp. 153-156, Monterey, CA,Jan. 29-31, 1992.9. F. Rahmatian and N.A.F. Jaeger, "Frequency Responses of Integrated Optics Pockels CellHigh Voltage Sensors," Proceedings ofthe IEEE/LEOS Annual Meeting, pp. 462-463, Boston,MA, Nov. 16-19, 1992,.10. APETh Mach-Zehnder modulator," United technologies Photonics Technical information.1 1 . S.Thaniyavarn, "Wavelength independent, optical damage immune Z-propagatingLiNbO3 waveguide polarization converter," Appl. Phys. Lett., vol. 47, no. 7, pp. 674-677, Oct.1985.12. F. Rahmatian, Integrated optics Pockels cell high-voltage sensor, M.A.Sc. thesis, Univ.of British Columbia, Vancouver, Canada, 1993.13. A. Neyer, "Integrated-optic devices in lithium niobate: technology and applications",SPIE, vol. 1274, Electro-Optic and Magneto-Optic Materials II, pp. 2-17, 1990.14. D. C. Erickson, "A Primer on Optical Current and Voltage Sensors and an Update onActivity," Engineering Symposium, Bonneville Power Administration, Mar. 31-Apr. 1, 1992.SPIEVol. 2072 / 95Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms


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