Ferroelectic BaTiO 3 Thin Film Optical Waveguide Modulators
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Ferroelectic BaTiO3 Thin Film Optical Waveguide Modulators A. Petraru, M. Siegert, M. Schmid, J. Schubert and Ch. Buchal Institut für Schichten und Grenzflächen (ISG1-IT), Forschungszentrum, D- 52425 Jülich, Germany, - email: [email protected] ABSTRACT High quality BaTiO3 epitaxial thin films on MgO substrates have been grown by pulsed laser deposition (PLD). Both c-axis and a-axis orientated BaTiO3 were studied. Mach-Zehnder (M-Z) optical waveguide modulators have been fabricated by ion beam etching. The BaTiO3 waveguide propagation losses are 1-2 dB/cm. Electro-optic modulation has been demonstrated with Vπ = 6.3 V at a wavelength of 633 nm and Vπ = 9.5 V at a wavelength of 1550 nm for the aaxis samples and with Vπ = 8 V at 633 nm wavelength and Vπ = 15 V at 1550 nm for the c-axis samples. Modelling of the modulators gave the Pockels coefficient r51 = 80 pm/V for the c-axis film and an effective Pockels coefficient reff =734pm/V for the a-axis films at 633nm wavelength. INTRODUCTION The realization of electro-optical devices by thin film technology is of scientific and technological interest. BaTiO3 is an attractive candidate for thin film electro-optic switches due to its large electro-optical coefficients [1], its high optical transparency and its favorable growth characteristics. Straight channel waveguide phase modulators made from metalorganic chemical vapor deposition (MOCVD) grown BaTiO3 that operate between crossed polarizers have been reported by Gill et al.[2]. EXPERIMENTAL High quality BaTiO3 (BTO) single crystalline films have been grown on MgO substrates by PLD with a KrF – excimer laser [3]. We have succeeded in fabricating both c-axis (optical axis perpendicular to the sample plane) and a-axis orientation. For c-axis films, we used a high laser power of 1100mJ/pulse and a MgO substrate temperature of 800 °C. a-axis films were formed using a reduced laser power of 300 mJ/pulse and a substrate temperature of 850 °C. The growth rate was around 4 nm/s. Ridge waveguides 2 µm wide with a ridge step height of 50 nm on a 1 µm thick BaTiO3 film propagated a single optical mode at a wavelength of 633 nm. They were patterned by lithography and ion beam etching. The BaTiO3 waveguides in use for these experiments have been patterned "as grown" without additional poling steps. The M-Z fork angle is 1.7 °. A lithographical lift-off process with subsequent deposition of a metal layer of 10 nm Cr and 90 nm Au was used to prepare the electrodes. The length of the electrodes is 3 mm and the distance between adjacent electrodes is 10 µm. The modulator input and output faces have been cleaved. The optical propagation losses are 1-2 dB/cm for the c-axis films and 2-3 dB/cm for the a-axis films at 633 nm. The losses are 30% lower at 1550 nm. The set-up for the electro-optic measurements is sketched in Fig. 1. The output of a He-Ne laser (633 nm) or a laser diode at 1550 nm is coupled into a single mode fiber. A fiber polarizer has been used for defining the input polarization. The modulators themselves are polarizati
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