ELECTRICAL CHARACTERIZATION OF SPUTTER-DEPOSITED ZNO COATINGS ON OPTICAL FIBERS

Piezoelectric ZnO coatings on optical fibers are of interest for activ e optical-fiber devices such as phase and wavelength modulators. React ive magnetron sputtering has been used to prepare high-resistivity [00 1] radially oriented ZnO coatings on 85 mm long sections of Cr/Au-coat ed optical fibers. The impedance spectra of 2 and 6 mm long transducer s are analyzed between 1 kHz and 100 MHz by applying an electrical pot ential across the thickness of the ZnO coating. The capacitance of the se devices exhibits a logarithmic frequency dispersion and a nearly co nstant dielectric loss of 0.006 +/- 0.002 between 1 and 100 kHz. Two r adial-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-depositio n process, introduces parabolic dependencies of the capacitance and re sonance frequencies of elements placed at different positions along th e length of the fiber. Identification of the radial-mode resonances an d the effects of ZnO thickness gradients on the piezoelectric resonanc es are made possible by the occurrence of the ZnO thickness distributi on. Thickness-induced changes of the piezoelectric resonance frequenci es also allow the observation of an 'inversion' of the resonance respo nse for resonances that occur above the LCR resonance. (C) 1997 Elsevi er Science S.A.