Electrical characterization of sputter-deposited ZnO coatings on optical fibers

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 〈001〉 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 sputter-deposition 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.