**3. Applications of Lithium Niobate Waveguides**

Titanium-diffused lithium niobate waveguide devices are suitable for electric-field detection since their sensors will not perturb the field to be measured. Paper [6] studied photonic electric-field sensors using a 1 × 2 Y-fed balanced-bridge Mach-Zehnder interferometer modulator composed of two complementary outputs and a 3 dB directional coupler based on the electro-optic effect and titanium diffused lithium niobate optical waveguides.

Proton-exchange (PE) is one of the waveguide fabricating techniques. In the research in paper [7], authors simulated and analysed a proton-exchanged E-O Mach-Zehnder interferometer in an x-cut lithium niobate on insulator, LNOI. Based on the full-vectorial finite-difference method, the single-mode conditions, mode size, and optical power distribution of PE waveguides were investigated. The bending losses the Y-branch structures were analysed and propagation losses of the PE waveguides with different separation distances between electrodes were simulated. The half-wave voltages of the devices were calculated using the finite difference beam propagation method (FD-BPM).

In paper [8] it was confirmed that the nano-domains in lithium niobate thin films are thermally unstable even at a temperature of the order of ~100 ◦C, which can be easily reached due to light absorption. The thermal instability of nano-domains could be very detrimental to practical applications, such as periodically poled lithium niobate (PPLN) microcavities, PPLN ridge waveguides, and ferroelectric domain memories. Thermal stability of nano-domains can be greatly improved when the lithium niobate thin film undergoes a pre-heat treatment before the fabrication of nano-domains. This thermal stability improvement is attributed to the generation of a space charge field during the pre-heat treatment, which is parallel to the spontaneous polarisation of nano-domains.

The wide range of topics covered by the papers in this special issue shows that the field of lithium niobate research is very much alive and that we can continue to expect new developments in this research area.

**Conflicts of Interest:** The authors declare no conflict of interest.
