Impact of material parameters on LiNbO3 waveguide devices

Abstract

The formation of optical waveguides in LiNbO3 by indiffusion of titanium was first demonstrated in 1973, and since then many laboratories have actively pursued this technology. However, the usability of Ti-indiffused LiNbO3 devices in signal-processing and sensor applications has been compromised due to photorefractive effects which occur in the otherwise advantageous visible and near-IR regions of the optical spectrum. These effects cause unacceptable instabilities in the devices. The dominant effect in LiNbO3, when exposed to high-intensity radiation, is the photoionization of iron impurities in the Fe2+ state to the Fe3+ state, creating a mobile electron which drifts preferentially in the +z(+c) direction (photovoltaic effect). The mobile electron drifts out of the illuminated region and is trapped, thus setting up an internal electric field. This electric field perturbs the refractive index of the illuminated region through the electrooptic effect and changes the phase velocity of the propagating light.1 The results of these two types of photorefractive effect depend on wavelength and optical power. Degradation in device properties ranges from a slow drift in the optical index of the waveguide to large losses because of light being scattered out of the waveguide. We will discuss methods of evaluating these effects2,3 and relate them to device performance.