Erbium-Doped Barium Titanate Thin Film Waveguides for Integrated Optical Amplifiers
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Erbium-Doped Barium Titanate Thin Film Waveguides For Integrated Optical Amplifiers Andrew R.Teren1, Seong-Soo Kim2, Seng-Tiong Ho3, Bruce W.Wessels1,3 1 Materials Science and Engineering Department , Northwestern University, 2225 N.Campus Drive, Evanston, IL 60208 U.S.A. 2 Physics and Astronomy Department, Northwestern University, Evanston, IL 60208 3 Electrical and Computer Engineering Department, Northwestern University,2145 Sheridan Rd., Evanston, IL 60208 ABSTRACT The factors affecting optical gain were studied for Er-doped BaTiO3 thin film waveguides. Er-doped BaTiO3 with dopant concentrations of 0.3 – 9 at.% was deposited by metal-organic chemical vapor deposition. The luminescence efficiency was maximized by optimizing the growth temperature and erbium concentration as well as by post-deposition annealing. Stimulated emission was studied using the pump-probe technique over the spectral range of 1,520-1,550 nm. A maximum differential gain of 3 dB/cm was measured at 1,540 nm in an 8 mm long, 8 µm wide ridge waveguide. INTRODUCTION Ferroelectric films have received much attention for integrated optics due to their nonlinear optical properties. By depositing these films on low index substrates, a number of microphotonic devices can be fabricated. The ferroelectric BaTiO3 is an excellent non-linear optical material since it has one of the highest electro-optic coefficients (r42= 820 pm/V), making it useful for high-frequency modulators. BaTiO3 is also a promising host material for high gain, optical amplifiers because of its high solubility for rare-earth ions[1]. Previously BaTiO3 thin films have been doped with erbium to concentrations as high as 1 atomic percent and strong, characteristic luminescence at 1,540 nm was observed at room temperature[2,3]. Guided- wave fluorescence and stimulated emission have also been measured in a ridge waveguide fabricated from Er-doped BaTiO3 thin films deposited on a low index substrate[4,5]. Optical gain, however, was not achieved presumably due to both low radiative efficiency of the Er ions as well as scattering losses from the waveguide. Previously, it was found from the dependence of photoluminescence intensity on erbium concentration that the number of radiative erbium centers is sensitive to the growth conditions[6]. It has also been shown that the deposition conditions affect the radiative efficiency via the incorporation of luminescence quenching centers, such as OH impurities[7] and oxygen vacancies[8] . In this study, several factors influencing optical gain were systematically investigated for Er-doped BaTiO3 thin films and high contrast waveguides. These include erbium concentration, emission linewidth, and upconversion. Optical properties were studied in samples prepared by metal-organic chemical vapor deposition (MOCVD) using a liquid , metal-organic barium precursor, which offers greater process control than previously obtained with a solid source. To minimize inhomogeneous line broadening of the characteristic emission, lanthanum strontium aluminum tantalate
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