Application of Electroceramic Thin Films to Optical Waveguide Devices
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Application of Electroceramic Thin Films to Optical Waveguide Devices David K. Fork, Florence Armani-Leplingard, and John J. Kingston Introduction Electro-optic devices such as fast (>20 GHz) modulators are one application1 of ferroelectric-oxide thin-film waveguides. A compact, blue laser source of a few milliwatts power capable of lasting thousands of hours is of great interest as applied to optical data storage and xerography.2 Ferroelectric-oxide thin films offer several potential advantages over bulk materials for optical waveguides, though no electroceramic thinfilm devices have replaced bulk devices yet. Bulk waveguides are defined by ion exchange, which produces only a small index difference. Thin films therefore permit higher intensity per unit power in the guide, and hence larger nonlinear effects and shorter interaction lengths.
Second Harmonic Generation Figure 1 illustrates the theoretical advantage of the strong internal optical confinement of thin-film heterostructures. Therein the theoretical secondharmonic-generation (SHG) efficiency versus film thickness is plotted for lithium-niobate and lithium-tantalate thin films on sapphire using firstorder quasi-phase matching (QPM) in a 2,000-nm-wide waveguide. Quasi-phase matching is a technique that modulates nonlinear materials' ferroelectric domains.3 Typical conversion efficiencies in bulk lithium-niobate waveguides are several hundred %/W cm2. In fairness the conversion efficiency in bulk structures is limited by many factors other
MRS BULLETIN/JULY 1996
than mode confinement, such as degradation of the nonlinearity due to proton exchange. In Figure 1, one sees that the conversion efficiency peaks at a film thickness of about 400 nm. The decreased performance at the low end reflects the intensity drop due to the mode
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Film Thickness (nm) Figure 1. Theoretical internal second-harmonic-conversion efficiency for first-order quasiphase matching in LiNbO3 and LiTaO3 films on sapphire.
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Application of Electroceramic Thin Films to Optical Waveguide Devices
have difficulty surviving the same thermal cycles as bulk devices. Conveniently the films assume a poled ferroelectric state as grown. To be effective, the thin-film devices must meet several challenges. Cost and performance in comparison to bulk devices are only two. One major challenge awaiting all frequency-doubled blue sources is the alternative technology provided by direct blue-laser sources, particularly in light of the report of efficient gallium-nitride light-emitting diodes5 and the more recent report of gallium-nitride lasers.6 Electro-optic Devices Several interesting articles on optical guided-wave modulators provide an excellent overview.7 Waveguide electrooptic modulators require a relatively low drive power in comparison to bulk electro-
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