Electrical characterization of sputter-deposited ZnO coatings on optical fibers

Electrical characterization of sputter-deposited ZnO coatings on optical fibers

Sensors and Actuators A 63 (1997) 153–160 Electrical characterization of sputter-deposited ZnO coatings on optical fibers G.R. Fox U, D. Damjanovic E...

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Sensors and Actuators A 63 (1997) 153–160

Electrical characterization of sputter-deposited ZnO coatings on optical fibers G.R. Fox U, D. Damjanovic Ecole Polytechnique Fe´de´rale de Lausanne, Laboratoire de Ce´ramique, Lausanne, CH-1015, Switzerland Received 18 February 1997; revised 22 April 1997; accepted 5 May 1997

Abstract Piezoelectric ZnO coatings on optical fibers are of interest for active optical-fiber devices such as phase and wavelength modulators. Reactive magnetron sputtering has been used to prepare high-resistivity N001M radially oriented ZnO coatings on 85 mm long sections of Cr/ Au-coated optical fibers. The impedance spectra of 2 and 6 mm long transducers are analyzed between 1 kHz and 100 MHz by applying an electrical potential across the thickness of the ZnO coating. The capacitance of these devices exhibits a logarithmic frequency dispersion and a nearly constant dielectric loss of 0.006"0.002 between 1 and 100 kHz. Two radial-mode piezoelectric resonances, the first at approximately 22 MHz and the second at 66 MHz, are identified. The thickness distribution of the ZnO coating, which results from the magnetron sputterdeposition process, introduces parabolic dependencies of the capacitance and resonance frequencies of elements placed at different positions along the length of the fiber. Identification of the radial-mode resonances and the effects of ZnO thickness gradients on the piezoelectric resonances are made possible by the occurrence of the ZnO thickness distribution. Thickness-induced changes of the piezoelectric resonance frequencies also allow the observation of an ‘inversion’ of the resonance response for resonances that occur above the LCR resonance. q 1997 Elsevier Science S.A. Keywords: Zinc oxide; Piezoelectric thin films; Piezoelectric optical-fiber coatings; Radial resonance; Reactive sputtering; Optical phase modulator

1. Introduction Coatings of piezoelectric ZnO on optical fibers are of interest for the development of novel acousto-opic, actuator, and sensor devices [1,2]. The integration of optical fibers with piezoelectric coatings allows for the fabrication of active fiber devices that show promise for applications in telecommunications and fiber-optic sensing networks, as well as other specialized systems that utilize the combined capabilities provided by a fiber waveguide and an electromechanical transducer. Initial work on ZnO-coated optical fibers has concentrated on the development of piezoelectric fiber-optic phase modulators (PFOM) that use the converse piezoelectric effect of the ZnO coating to produce acoustic waves that strain modulate the optical-fiber core, resulting in a modulation of the core optical path length. The high-frequency optical and electrical response of devices based on non-concentric [3–5] and concentric fiber coatings [6,7] has been demonstrated for a variety of optical-fiber substrates including stanU

Corresponding author. Tel.: q41 21 693 2989. Fax: q41 21 691 5810. E-mail: [email protected]

dard telecommunications fibers [8,9]. The objective of this paper is to extend the understanding of the electrical behavior of concentric ZnO fiber coatings that were prepared by reactive magnetron sputtering without heating the fiber substrates. Investigations of the a.c. dielectric and piezoelectric resonance characteristics between 1 kHz and 100 MHz have resulted in a characterization of the coating material and effects related to device structure and geometry. The influence of actuator length, thickness, and position along the length of a coated optical fiber containing several actuators is described. These variables not only influence the capacitive response of the cylindrical ZnO actuators, but they also have a strong influence on the resonance behavior. Capacitance and dielectric loss measurements are used to characterize the dielectric response and uniformity of the ZnO fiber coating. Measurements of the fundamental and the second-order radial-mode resonances are used to demonstrate how actuator length, thickness, and position influence resonance behavior. The goal of this comparative study of ZnO-coated fiber actuators is to provide some guidelines for designing the electrical response of PFOM devices as well as give a more complete

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view of the dielectric and piezoelectric response of sputterdeposited ZnO fiber coatings.

2. Experimental procedure Standard telecommunications optical fibers with a 9 mm diameter core and a 125 mm outside diameter were coated with a concentric multilayer structure consisting of bottom electrode, ZnO piezoelectric, and top electrode layers as shown schematically in Fig. 1. Evaporation was used to deposit 13 nm thick Cr and 130 nm thick Au layers for the bottom electrode and 25 nm thick Cr and 400 nm thick Au layers for the top electrodes. The ZnO coating was deposited by reactive magnetron sputtering of a Zn target in a mixture of O2 and Ar using a d.c. cathode power of 250 W, oxygen partial pressure of 0.70 Pa, and total pressure of 1.50 Pa. The ZnO coatings were approximately 6 mm thick and they consisted of a columnar microstructure with an N001M radially preferred crystallite orientation. A full description of the fabrication process has been given in a previous publication [10].

The cylindrical capacitor coated-fiber structures were analyzed using an HP4194A impedance/gain phase analyzer. Electrical contact to the fiber was made by using the sample holder shown in Fig. 2, which provided a pressure contact between the fiber electrodes and a printed circuit board. Measurements were made by applying voltage between the inner and outer electrode coatings.

3. Results and discussion 3.1. Dielectric response The capacitance and dielectric loss between 1 kHz and 1 MHz are shown in Fig. 3 for a ZnO-coated fiber element with a 6 mm long top electrode. As is common with many solidstate dielectric materials, the capacitance generally exhibits a linear decrease with a logarithmic increase in frequency. Above 700 kHz, the capacitance becomes nearly constant due to an influence of the LCR resonance that occurs at high frequency, the frequency of which is dependent on the inductance, capacitance, and resistance (LCR) of the sample holder and sample. The dielectric loss exhibits a nearly constant value of 0.006"0.002 up to 100 kHz where the LCR resonance begins to cause a continuous increase in loss. Since the measurement of the dielectric loss is more sensitive to the influence of the LCR resonance than the capacitance measurement, the effect of the LCR resonance on the loss becomes evident at lower frequencies.

Fig. 1. Diagram showing structure and dimensions of PFOM device used for electrical characterization.

Fig. 2. Schematic diagram of fiber sample holder used for impedance measurements. The pressure applied could be adjusted with screws.

Fig. 3. Capacitance and dielectric loss spectra for a 6 mm long element with an applied r.m.s. voltage of 0.2 V.

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Both the capacitance and the loss curves exhibit low-intensity peaks or perturbations that are clearly visible for frequencies above 3 kHz. Tests with commercially available solid-state capacitors have revealed that these perturbations in the capacitance and loss spectra are due to a limitation of the resolution of the impedance analyzer and are not a characteristic of the ZnO coating. By comparing the capacitance of elements placed at different positions along the fiber, it was found that the capacitance exhibits a parabolic dependence on position, as shown in Fig. 4. The dependence of capacitance on position remains the same for all frequencies between 1 kHz and 1 MHz, and it can be explained by a parabolic change in the ZnO coating thickness along the length of the fiber. This parabolic thickness distribution is typical for thin films deposited from a circular magnetron cathode, and measurements of the thickness distribution have previously been reported for the deposition system used for coating optical fibers [10]. The minimum capacitance occurs at a distance zs40 mm from the inner electrode contact (zs0 mm position), and this position exactly corresponds to the position of the thickness maximum. From the capacitance and the coating thickness at the zs40 mm position, the dielectric constant is calculated to be 10.9"0.3 at 100 kHz. This is similar to values measured along the c-axis of ZnO single crystals [11]. As shown in Fig. 4, the dielectric loss remains nearly constant as a function of position for measurements at 100 kHz. Within the measurement error, the loss is independent of position for all frequencies between 1 and 100 kHz. Above 100 kHz, the dielectric loss increases on moving away from the bottom electrode contact, and the position dependence of the loss increases with increasing frequency. As explained above, this high-frequency increase in loss results from contributions of the LCR resonance. The position dependence of the loss above 100 kHz is due to an increased length for the inner electrode on moving away from the bottom electrode contact. Since the series impedance of the device increases with the length of the inner electrode, elements positioned close to the inner electrode contact have a lower series impe-

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dance than elements positioned far from the inner contact. An increase in the series impedance causes the LCR resonance to be shifted to lower frequency, resulting in the position dependence of the loss at frequencies above 100 kHz. 3.2. Piezoelectric fundamental radial resonance behavior The first identifiable piezoelectric resonance occurs between 20 and 25 MHz (Fig. 5). By comparing the resonance frequency, shape, and magnitude of various actuator elements, qualitative information about the vibration mode, mechanical quality, mechanical coupling coefficient, and piezoelectric coefficients can be obtained. For both 2 and 6 mm elements positioned near the thickness maximum, zs40 mm, the general shape and center frequency of the resonances are the same, as can be seen by comparing Figs. 5 and 6. The resonance center frequency, fc, is defined as the frequency of the phase angle maximum, while the frequencies of the minimum and maximum impedance are designated as fm and fn, respectively. Both 2 and 6 mm elements exhibit a parabolic dependence of the piezoelectric resonance frequency on the position of the top electrode element, as indicated by the plot of fn versus position in Fig. 7. The lowest resonance frequency occurs at zs40 mm, and the resonance frequency shifts by nearly 2 MHz for elements positioned towards the ends of the fiber. The parabolic dependence of the resonance frequency on position is in good agreement with the parabolic dependence of the capacitance, but whereas the capacitance distribution

Fig. 5. Impedance and phase angle spectra showing the fundamental radialmode resonance of a 2 mm long element measured with an applied r.m.s. potential of 0.5 V.

Fig. 4. Dependence of capacitance and dielectric loss on position of actuator element along the length of the fiber. The position zs0 corresponds to the edge of the ZnO coating where the inner electrode contact is made. The capacitance is normalized with respect to top electrode length to allow comparison of 2 (open symbols) and 6 mm (filled symbols) long elements.

Fig. 6. Impedance and phase angle spectra showing the fundamental radialmode resonance of a 6 mm long element measured with an applied r.m.s. potential of 0.5 V.

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Fig. 7. Dependence of fundamental radial-mode resonance frequency on distance from bottom electrode contact. Open symbols indicate 2 mm long elements and filled symbols indicate 6 mm long elements.

is defined by the ZnO thickness distribution, the resonancefrequency distribution is determined by the mass and thickness distributions of the ZnO and electrode coatings along the length of the fiber. Since the electrode coatings are thin and have a small mass and thickness relative to the ZnO, position variations in the thickness of the electrode coatings should have a much smaller effect on the resonance frequency than the thickness distribution of the ZnO. Although the thickness distribution (and corresponding mass distribution) of the ZnO can explain the capacitance and resonance-frequency dependence on position, several other material properties that have not been fully characterized should not be completely discounted from having an influence on the electrical response. Porosity, the degree of preferred orientation, grain size, and defect density could all have distributions along the length of the fiber, resulting in contributions to the capacitance and the resonance-frequency measurements. For the capacitance and resonance frequency measurements of samples investigated in this study, these variations in material properties are expected to be of secondary importance in comparison with the ZnO thickness distribution. The most important conclusion that can be drawn from the fact that the resonance frequency is dependent on thickness and independent of top electrode length is that this resonance is a radial-mode resonance of the composite fiber and coating structure. Recent work on modeling the behavior of these PFOM devices confirms this conclusion [12]. Since the mechanical coupling coefficient of a piezoelectric material is related to the resonance peak width, defined as D fsfnyfm [13], it is of interest to observe the general behavior of the resonance peak with respect to position and top electrode length. The position dependence of the width of the resonance peak is found to be different for the 2 and 6 mm element lengths (Fig. 8). For 2 mm elements, the resonance width exhibits a maximum at the thickness maximum (zs40 mm), while for 6 mm elements the resonance width shows a minimum. The amplitude of the resonance peak is inversely proportional to the mechanical quality factor, capacitance, and peak width of the resonator [14]. Since the relationship describing resonance amplitude is also dependent on resonator geome-

Fig. 8. Dependence of fundamental radial resonance width on distance from bottom electrode contact for 2 and 6 mm long elements.

Fig. 9. Dependence of fundamental radial resonance impedance amplitude on distance from bottom electrode contact for 2 and 6 mm long elements.

try, the resonance amplitude is only used here for a semiquantitative comparison of the mechanical quality factors of fiber actuators. Unlike the resonance frequency, the amplitude of the resonance peak, defined as the difference in the impedance between fm and fn, does not exhibit a smooth parabolic dependence on position (see Fig. 9). The amplitude exhibits a maximum at approximately zs40 mm for both 2 and 6 mm elements, but the amplitude decreases linearly on either side of the corresponding amplitude and ZnO thickness maximum. The linear decrease in amplitude is observed for positions within a distance of 30 mm from the maximum, i.e., between zs10 and 70 mm. Beyond this distance the amplitude remains nearly constant. It should be noted that the positions zs10 and 70 mm correspond to the position of the erosion track of the Zn target, suggesting that the incident angle of the depositing species and/or the ejection of high-energy or ionized species may influence the ZnO piezoelectric properties and the resulting resonance amplitude. In general, the resonances of 6 mm long elements exhibit lower amplitudes than those of 2 mm long elements at approximately the same position along the fiber. The amplitude of the resonance is highly dependent on the pressure applied for making electrical contact, and for this reason caution must be exercised when comparing the amplitude of resonances at different positions. A contact pressure dependence of the amplitude is expected since contact with the resonator provides an additional damping mechanism. For elements positioned close to the ZnO thickness maximum, the resonance exhibits one sharp peak in the phase

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angle and one corresponding minimum and maximum in the impedance spectrum, typical of an ideal resonance. On moving away from the thickness maximum, the resonances of 6 mm long elements split into multiple peaks, suggesting the occurrence of a superposition of resonances rather than one specific resonance. Perturbations of the resonance curve also occur for 2 mm elements that are not positioned at the thickness maximum, but these perturbations generally remain small in comparison to the amplitude of the resonance and they are often not observed for elements that exhibit lowamplitude resonances positioned close to the ends of the fiber. These perturbations can be associated with asymmetries or microstructural defects in the actuator structure and may be a result of coupling of other vibration modes that are not the principle cause of the resonance. Both the resonance amplitude and width dependencies on position for the 2 and 6 mm elements are believed to be influenced by edge effects introduced by the finite lengths of the elements and asymmetries introduced by thickness gradients, but these behaviors are not fully understood. It is clear that the finite length of the top electrode causes the standingwave radial-mode vibration to be coupled to travelling shearmode vibrations. It is likely that the shear-mode coupling to the radial mode will be dependent upon the ZnO thickness and thickness gradient, and the top electrode length. Some qualitative understanding of the position dependence of the resonance amplitude and width can be attained if the effects of finite size and asymmetry are kept in mind. Thickness distribution, which is believed to be the predominating factor influencing the position dependence of the resonance amplitude, can affect the amplitude in several ways. One thickness-dependent factor that will influence the resonance amplitude is the change in capacitance with position. In addition, coupling of shear-mode vibrations is expected to increase on moving away from the zs40 mm position due to the enhanced thickness gradients, i.e., enhanced asymmetry. This enhancement of travelling shear-mode vibrations provides an extra energy-loss mechanism for the device. As stated above, distributions of the material properties may also be partially responsible for the observed behavior. The reduced amplitude of 6 mm elements with respect to 2 mm elements is believed to result from length-dependent changes in piezoelectric induced stress and shear-mode coupling. Two competing effects appear to be responsible for the position dependence of the resonance width since the 2 and 6 mm elements show opposing trends. For 6 mm elements, the broadening and splitting of the resonance on moving away from the thickness maximum is clearly dominated by the increasing asymmetry introduced by the thickness gradient. Since the variation in thickness due to a thickness gradient is smaller for a 2 mm element than for a 6 mm element, asymmetry due to a thickness gradient is less important for 2 mm elements. For this reason, the inverted resonance-width behavior for 2 mm elements, in comparison with 6 mm elements, is believed to result from reductions in resonance amplitude that are linked to changes in thickness or other

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material properties that reduce the piezoelectric response of the ZnO coating. The three-fold difference in the width of the resonances of 2 and 6 mm elements positioned close to zs40 mm is still a mystery, but it is believed to result from differences in the influence of top electrode edge effects and shearmode coupling and their relation to length-dependent changes in the stored energy of a device. 3.3. Piezoelectric second radial resonance behavior A second radial-mode resonance is observed at approximately 66 MHz for both 2 and 6 mm long elements as shown in Figs. 10 and 11. This resonance also exhibits a parabolic dependence of the resonance frequency on position (Fig. 12), similar to the fundamental radial resonance occurring around 22 MHz. The shift of the second resonance for

Fig. 10. Impedance and phase angle spectra showing the second radial resonance of a 2 mm long element with an applied r.m.s. potential of 0.5 V.

Fig. 11. Impedance and phase angle spectra showing the second radial resonance of a 6 mm long element with an applied r.m.s. potential of 0.5 V.

Fig. 12. Dependence of second radial-mode resonance frequency on distance from bottom electrode contact. Open symbols indicate 2 mm long elements and filled symbols indicate 6 mm long elements.

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elements at the ends of the fiber as compared with the central element is larger for the second resonance than for the fundamental resonance. As with the fundamental resonance, the resonance shift results primarily from the parabolic thickness distribution. Although the resonance frequency and position dependence of the resonance frequency is the same for 2 and 6 mm elements, there is a striking difference in symmetry for the impedance and phase angle spectra, as can be seen by comparing Figs. 10 and 11. Whereas the impedance spectrum for the 2 mm element first exhibits a minimum followed by a maximum, the 6 mm element exhibits a maximum followed by a minimum. A similar change in symmetry is observed when comparing the phase angle spectra for the 2 and 6 mm elements. The reversal of symmetry for the piezoelectric resonance curves is linked to relative changes in the frequencies of the piezoelectric resonance and the LCR resonance. If the piezoelectric resonance occurs on the low-frequency side of the impedance minimum that results from the LCR resonance, the piezoelectric resonance will exhibit a ‘normal’ symmetry with an impedance minimum followed by a maximum. Piezoelectric resonances that occur on the high-frequency side of the LCR-induced impedance minimum exhibit ‘inverted’ symmetry. The fundamental radial-mode resonance that occurs between 20 and 25 MHz for both 2 and 6 mm elements is always on the low-frequency side of the LCR-induced impedance minimum, and therefore this resonance always exhibits normal symmetry. For 6 mm elements, the secondorder resonance occurring around 66 MHz always lies on the high-frequency side of the LCR impedance minimum and therefore always exhibits inverted symmetry. Since the second-order piezoelectric resonance (at approximately 66 MHz) for 2 mm elements occurs at approximately the same frequency as the LCR impedance minimum and since the frequency for both the piezoelectric and the LCR resonances shifts with the position of the elements, the second-order piezoelectric resonance of 2 mm long elements can exhibit

normal or inverted symmetry. This behavior is clearly seen from the impedance spectra shown in Fig. 13. The element at position zs44 mm exhibits a normal resonance peak at 65.4 MHz and the LCR minimum occurs at 74.3 MHz. For zs76 mm, the piezoelectric resonance is shifted to 68.0 MHz while the LCR minimum is shifted to 64.2 MHz, causing an inversion of the symmetry of the piezoelectric resonance. The element at zs4 mm has a piezoelectric resonance that occurs at nearly the same frequency as the LCR minimum, resulting in a piezoelectric resonance that is nearly symmetric with no significant piezoelectric-induced impedance minimum on either side of the peak. Since the second-order piezoelectric resonance does not occur on the same side of the LCR impedance minimum for all elements, the position dependencies of the resonance amplitude and width become even more complicated due to position-dependent symmetry changes of the resonance peak. As observed in Fig. 14, the maximum resonance amplitude for 2 mm elements occurs at zs20 mm, while the maximum for 6 mm elements occurs at zs40 mm. For the 2 mm elements, the shift of the amplitude maximum away from the thickness maximum is explained by the position-dependent change in symmetry that occurs for these elements. In a similar fashion, the symmetry change of the 2 mm elements causes the maximum resonance width to occur at zs56 mm as shown in Fig. 15. The minimum width of the 6 mm elements occurs at the same position as the amplitude maximum at zs40 mm since all the elements exhibit inverted resonances that are at frequencies far away from the LCR-induced

Fig. 14. Dependence of the second radial resonance impedance amplitude on distance from the bottom electrode contact for 2 and 6 mm long elements.

Fig. 13. Impedance spectra of 2 mm long elements showing the positiondependent change in the relative frequencies of the second piezoelectric resonance and the impedance minimum (vertical arrows) resulting from the LCR resonance. A change in symmetry of the piezoelectric-induced resonance peak is observed when comparing piezoelectric resonances that occur on opposite sides of the LCR resonance-induced impedance minimum.

Fig. 15. Dependence of the second radial resonance width on distance from the bottom electrode contact for 2 and 6 mm long elements.

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minimum impedance. Due to the influence of the LCR resonance on the symmetry of the piezoelectric resonance, it can be concluded that comparison of actuator response is more straightforward for elements that are not positioned near the impedance minimum of the LCR resonance. From these observations, it is clear that both element length and position along the length of the fiber strongly influence the shape and frequency of the piezoelectric resonance due to variations of the ZnO thickness and influences of the LCR resonance. This information is useful for understanding how the resonance frequencies of these ZnO-coated fiber devices can be tuned and for determining requirements for actuator driving electronics. It is important to have this detailed understanding of the electrical response of these PFOM devices since it will undoubtedly influence their optical phase shift capabilities.

4. Conclusions The electrical characteristics of 6 mm thick ZnO coatings deposited on optical fibers have been characterized in the frequency range between 1 kHz and 100 MHz. Between 1 and 100 kHz, the dielectric constant and loss of the ZnO coating were 10.9"0.3 and 0.006"0.002, respectively. Resonances occurring at approximately 22 and 66 MHz were identified as radial-mode resonances by confirming the resonance-frequency dependence on ZnO coating thickness and independence of top electrode length. Observations of the electrical characteristics of elements with different top electrode lengths and different positions along the fiber revealed that the PFOM device response is sensitive to device structure. A parabolic thickness distribution of the ZnO coating resulting from the magnetron sputtering process causes a parabolic variation of the capacitance and radial-mode resonance frequencies of elements positioned along the length of the fiber. For the fundamental radial-mode resonance, an amplitude maximum coincides with the thickness maximum of the ZnO coating. The position dependence of the width of the resonance peak depends on the length of the top electrode, and it is clear that thickness gradients introduce perturbations of the resonance modes that can lead to broadening of the piezoelectric resonance. Resonance width is likely to be influenced by shear-mode coupling at the ends of the top electrode, and this coupling appears to be dependent on thickness gradients and the length of the top electrode. For the second radial-mode resonance, at approximately 66 MHz, it was found that the symmetry of the piezoelectric resonance can be inverted depending on whether the piezoelectric resonance occurs at frequencies below or above the LCR resonance. Piezoelectric resonances on the low-frequency side of the LCR-induced impedance minimum exhibit a normal resonance behavior while those on the high-frequency side show an inverted resonance. For 2 mm long elements that exhibit piezoelectric resonances on both sides

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of the LCR resonance, the changes in symmetry complicate the comparison of the position dependence of the piezoelectric resonance width and amplitude. All the 6 mm long elements exhibit inverted symmetry of the second radial-mode resonance, and the position dependence of the resonance amplitude and width is similar to that observed for the fundamental resonance. The detailed characterization of the electrical response of ZnO-coated fiber actuators presented here highlights the geometrical factors that influence device operation and will aid the understanding of optical phase shifting capabilities of PFOM devices.

Acknowledgements This work was supported by the Optical Sciences, Applications, and Technology Priority Program of the Board of the Swiss Federal Institute of Technology.

References [1] G.R. Fox, C.R. Wu¨thrich, C.A.P. Muller and N. Setter, Piezoelectric coatings for active optical fiber devices, Ferroelectrics, 201 (1997) 13–22. [2] G.R. Fox, C.A.P. Muller, C.R. Wu¨thrich, A.L. Kholkin, N. Setter, D.M. Costantini, N.H. Ky and H.G. Limberger, Applications of active thin film coatings on optical fibers, in Materials for Smart Systems II, Mater. Res. Soc. Proc., Materials Research Society, Pittsburgh, PA, 1997, pp. 25–34. [3] A.A. Godil, D.B. Patterson, B.L. Heffner, G.S. Kino and B.T. KhuriYakub, All-fiber acoustooptic phase modulators using zinc oxide films on glass fiber, J. Lightwave Technol., 6 (1988) 1586–1590. [4] B.L. Heffner and B.T. Khuri-Yakub, Deposition of oriented zinc oxide on an optical fiber, Appl. Phys. Lett., 48 (1986) 1422–1423. [5] B.L. Heffner, W.P. Risk, B.T. Khuri-Yakub and G.S. Kino, Deposition of piezoelectric films on single-mode fibers and applications to fiber modulators, Proc. IEEE Ultrasonics Symp., Williamsburg, VA, 1986, pp. 709–713. [6] D.S. Czaplak, J.F. Weller, L. Goldberg, F.S. Hickernell, H.D. Knuth and S.R. Young, AO phase modulator for single-mode fibers using cylindrical ZnO transducers, Proc. IEEE Ultrasonics Symp., Denver, CO, 1987, pp. 491–493. [7] F.S. Hickernell, The characterization of coaxial ZnO thin-film BAW transducers on optical fibers, Proc. IEEE Ultrasonics Symp., 1988, pp. 417–420. [8] N.H. Ky, H.G. Limberger, R.P. Salathe and G.R. Fox, Optical performance of miniature all-fiber phase modulators with ZnO coating J. Lightwave Technol., 14 (1996) 23–26. [9] N.H. Ky, H.G. Limberger, R.P. Salathe´ and G.R. Fox, 400 MHzbandwidth all-fiber phase modulators with ZnO coating on standard telecommunication fiber, IEEE Photon. Technol. Lett., 8 (1996) 629– 631. [10] G.R. Fox, N. Setter and H.G. Limberger, Fabrication and structural analysis of ZnO coated fiber optic phase modulators, J. Mater. Res., 7 (1996) 2051–2061. [11] K.H. Hellwege and A.M. Hellwege, Landolt–Bo¨rnstein, Numerical Data and Functional Relationships in Science and Technology, Group III: Crystals and Solid State Physics, Vol. 2, Springer, Berlin, 1969, p. 71.

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[12] A. Gusarov, N.H. Ky, H.G. Limberger, R.P. Salathe´ and G.R. Fox, High performance optical phase modulation using piezoelectric ZnOcoated standard telecommunication fiber, J. Lightwave Technol., 14 (1996) 2771–2777. [13] R. Holland and E.P. Eernisse, Accurate measurement of coefficients in a ferroelectric ceramic, IEEE Trans. Sonics Ultrasonics, SU-16 (1969) 173–180. [14] B. Jaffe, W.R. Cooke and H. Jaffe, Piezoelectric Ceramics, Academic Press, New York, 1971, pp. 281–302.

Biographies Glen R. Fox received a bachelor’s degree in ceramic science and engineering in 1987 and a Ph.D. in solid-state science in 1993 from The Pennsylvania State University. Since 1993 he has worked in the Laboratory of Ceramics at the Swiss Federal Institute of Technology in Lausanne, where he is currently a research group leader. His research has concentrated on the processing and characterization of thin-film, thick-film, and bulk piezoelectric and ferroelectric materials

for microelectronic and microelectromechanical device applications. Dragan Damjanovic received his B.S. degree in physics from the University of Sarajevo in 1980, and his Ph.D. degree in ceramics science from The Pennsylvania State University in 1987. After working briefly at the Materials Research Center of Energoinvest Corp. in Sarajevo, he joined the Materials Research Laboratory at Penn State University in 1988, where he worked on piezoelectric properties of ferroelectric and relaxor ceramics, polymer–ceramic piezoelectrics, and pyroelectric properties of biological polymers. From 1991 to the present he has been with the Laboratory of Ceramics, Swiss Federal Institute of Technology, Lausanne. He currently works on piezoelectric and related properties of ceramics for high-temperature sensor applications, investigation of various contributions to the piezoelectric properties of bulk and thin-film ferroelectrics, and devices based on ferroelectric bulk and thin-film materials.

Journal: SNA (Sensors and Actuators A)

Article: 1687