Integrated Optical Ti:LiNbO3 Ring Resonator for Rotation Rate Sensing

Design, fabrication, packaging, and characterization of a high finesse Ti:LiNbO3 integrated optical ring resonator are reported. First results of rotation rate sensing are presented. Introduction Optical rotation rate sensors utilizing the Sagnac effect are attractive devices, which in contrast to their mechanical counterparts have no moving parts [1]. Active ring laser gyroscopes and passive fiberoptic gyroscopes of high resolution are already used successfully for navigation of aircrafts and ships. However, for consumer needs with low and medium resolution like in car navigation and robotics, less complex and cheaper sensors systems are needed suited for volume production. Integrated optical ring resonators based on planar microfabrication technologies have the required potential. If fabricated on an electrooptic substrate like LiNbO3 (LN), even optical signal processing components can be monolithically integrated. In contrast to ring resonators for wavelength filtering, devices for rotation rate sensing must have a much larger diameter as the Sagnac effect is proportional to the area enclosed by the ring. Up to now, only a few integrated optical ring resonators have been reported for rotation rate sensing. They have been fabricated by chemical vapour deposition of silica on silicon [2-4]. In this paper we report the design, fabrication, packaging, and characterization of the first ring resonator fabricated in LN for rotation rate sensing with a diameter of 60 mm. Theoretical modelling and design The overall waveguide structure to be investigated is shown in Fig. 1. It consists of a single mode (at λ ~ 1550 nm), Ti:LN ring resonator of 60 mm diameter. Fig. 1: Scheme of the integrated ring resonator connected with an adjacent straight waveguide via a directional coupler. Light can be coupled to the ring via a directional coupler connecting a straight waveguide and the resonator. The directional coupler determines to a large degree the properties of the resonator and – as a consequence – the properties of the rotation rate sensor. It is formed by the straight and the curved waveguides approaching each other. As the Ti:LN waveguides are anisotropic with polarization dependent mode field dimensions also the properties of the directiononal coupler will be polarization dependent. Nevertheless, we will consider in the following the TE-polarization only as the corresponding waveguide losses are smaller than those of the TM-mode. Therefore, for a given polarization (and wavelength) there is only one parameter to be adjusted, namely the smallest gap d between both guides (see Fig. 2). Fig. 2: Directional coupler formed by straight and curved channel guides. In a first step a power coupling coefficient K is calculated as function of d at the wavelength λ = 1550 nm (see Fig. 3 on the left). K is equivalent to the reflectivity of one of the two mirrors of a conventional Fabry-Perot cavity with a second mirror of reflectivity 1. Moreover, the dependence of K on the wavelength is analyzed and presented on the right of Fig. 3 for d = 4.9 μm as fixed parameter. This result shows that the coupling can be fine tuned even after fabrication by selecting the right wavelength.