Acoustooptic collinear TE-TM mode conversion in a two-layer Ti-indiffused and proton-exchanged waveguide structure in LiNbO3.

In a recent publication (1), we showed that acoustooptic TE-TM mode conversion in LiNbOsub3, in the 0.8 mym optical wavelength range, can be very efficient. Only 20 m W RF power was necessary to achieve more than 90 percent conversion efficiency for planar propagating surface acoustic waves (SAW). In addition, a very narrow bandwidth was obtained. The 0.8 mym optical wavelength range is especially interesting for several sensor applications. Recently, we have developed a two-layer structure in LiNbOsub3 for refractive index sensing applications (2) consisting of a Ti-indiffused channel waveguide on top of which a thin proton-exchanged (PE) layer is fabricated. In the latter the sign of the optical birefringence is reserved compared with the bulk (and Ti-indiffused) crystal owing to a large increase of the extraordinary and a small decrease of the ordinary refractive index (3). As a result, the fundamental extraordinary mode of this structure becomes highly sensitive to refractive index changes in the superstrate, whereas the ordinary mode is a factor of 10high3 less sensitive (2). This allows the application of a new acoustooptic sensing principle (4), making use of the change of the waveguide birefrigence that results from a change of the superstrate refractive index. The waveguide birefringence can be detected with an acoustooptic TE-TM mode converter since the SAW phase-matching frequency is proportional to it. In this way the SAW frequency becomes a measure of the (absolute) refractive index of the superstrate. The acoustooptic sensing principle is especially suitable for gas sensing applications. Having this idea in mind we investigated collinear acoustooptic TE-TM mode conversion in the two-layer structure. We have obtained efficient narrowband mode converters well suited for sensor applications.