Low-loss bent channel waveguides in lithium niobate thin film by proton exchange and dry etching

Abstract

We propose and demonstrate an efficient method combining proton exchange with dry etching for the fabrication of low-loss bend channel waveguides in lithium niobate (LN) thin film. Our proposed method introduces the chemical etching caused by F+ ion to increase the etching rate. Our fabricated straight and bent channel waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of ~900 nm, and a slope of ~20° with respect to the vertical direction. To the best of our knowledge, this is the largest etching depth but with a small slope reported up to now. Mode intensity distributions and insertion losses were measured at 1.55 µm wavelength and bending losses were deduced. The results show that our fabricated bent channel waveguide with a radius of 20 μm can achieve low bending losses of 0.455 dB/90° and 0.488 dB/90° for the fundamental quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. Compared with the fabrication methods reported so far, our method can realize a faster etching rate and a larger etching depth while maintaining a high etching quality.