Optical waveguides and electrooptic modulators in lithium niobate using proton diffusion method

Abstract

Proton diffusion method using a self-aligned $SiO_2$-cladding structure for optical channel waveguide fabrication is proposed and the behavior of proton exchange and diffusion is investigated by infrared spectroscopy. Furthermore, electrooptic modulators with self-aligned electrode structure are fabricated and characterized. First, proton exchange is performed by using either pure benzoic acid or distilled water as a proton source. The microscopic behavior of proton exchange is investigated based on the O-H stretching region. Infrared spectroscopy in the O-H stretching region has been used to determine the extent of proton exchange in Y and Z-cut lithium niobate. We propose a chemical model for proton exchange and diffusion, and it explains process mechanism such as proton exchange in dilute melt composition and annealed proton exchange. According to this model, the proton exchange is composed of two types of reaction and infrared absorption data support it. To examine a surface reaction by water molecules, proton exchange is performed in distilled water as a proton source. The result shows that a surface reaction by water molecules should be considered as a part of proton exchange process. To improve the optical quality, the proton diffusion method is proposed. The proton diffusion (PD) method is composed of proton exchange at low temperature (150℃) and subsequent proton diffusion with a $SiO_2$ cap at higher temperature (400℃). The role of $SiO_2$ cap is inhibition of proton out-diffusion and promotion of indiffusion of protons. Consequently, the rate of initial proton exchange at the surface can be lowered due to proton conserving property of $SiO_2$ cap. The proton conserving property of $SiO_2$ cap is proved by infrared absorption measurement. Based on the proton conserving property, proton-diffused waveguides without surface damage are fabricated on Y-cut $LiNbO_3$ substrates. Channel waveguides are fabricated in $LiNbO_3$ by proton diffusion with a se...