Features of spontaneous ferroelectric domain nucleation in Ti:LiNbO3 modulators

Abstract

Integrated optical modulators based on lithium niobate Ti-indiffused waveguides (Ti:LiNbO3) are used extensively for broadband devices that have a high coupling efficiency with optical fibers at a 1.5 μm wavelength window. However, the ferroelectric nature of lithium niobate crystal results in some undesirable effects causing parameters fluctuations. For example, the pyroelectric effect leads to thermal dependent DC-drift and excess optical losses. Therefore, to develop effective compensation methods, it is important to understand the origin of instability. In the interelectrode gap of phase modulators we found needle-like domain defects with switched polarization along a surface layer of the monodomain X-cut lithium niobate substrate. Based on domain nucleation theory and polarization switching behavior, the spontaneous nature of domain nucleation caused by the pyroelectric field was shown and described for the interelectrode gap of the modulator along electrode edges. Considering the corona effect, models of domain nucleation and growth are proposed, and the morphology of needle-like domains is describes based on numerical simulation of the pyroelectric field distribution in the crystal volume. Experimental studies of industrial optical phase modulators characterize the shape and size of domains not only in the interelectrode gap, but also along the boundary of external electrodes proving the pyroelectric type of domain nucleation. Several methods were used for domain visualization-selective chemical etching, piezoresponse force microscopy, and electron scanning microscopy. The present work provides a theoretical and practical basis for future research on stabilization of the parameters for optical phase modulators.