Z⁺/Z^- lithium niobate optical waveguide sensitivity related to pyroelectric effect

Lithium niobate crystal is widely used for the design and fabrication of integrated electro-optic modulators. As a ferroelectric material, one sees its spontaneous polarization change with temperature variations. This phenomenon, known as the pyroelectric effect, induces strong waveguide transmission variations for waveguides realized on Z-cut wafers. Waveguides made by titanium in-diffusion either on the Z⁺ or Z^- side of the crystal show a significant difference in temperature behavior. Experimental data, enlightened by numerical simulations, help to show why Z^- waveguides are more immune to temperature changes than Z⁺ ones.