Analysis and measurement of optical polarization state modulation in thin-film lithium niobate waveguides

In recent years, quantum communication has received extensive attention due to its high security of transmitted information. Quantum key distribution (QKD), an important branch of quantum information, is developing rapidly and has been gradually moving toward practicality and networking. The use of phase coding through fiber optic channels is the basis for the implementation of QKD systems. In QKD systems, electro-optical modulation techniques are mainly used to change the photon phase through phase modulators to realize the phase coding scheme. Among them, lithium niobate is a common material for making phase modulators in QKD systems. Lithium niobate (LN) crystals are an optical material with excellent acousto-optical and electro-optical properties. It has good physical and chemical stability, a wide optical low-loss window, a large electro-optical coefficient and an excellent second-order nonlinear effect. It has a wide range of applications in high-speed electro-optical tuning, holographic storage, nonlinear frequency conversion, etc. Thin-film lithium niobate (LNOI), as a new integrated optical material, can well combine the excellent electro-optical, acousto-optical and nonlinear properties of the material with a compact optical waveguide. It also has the advantages of a small waveguide cross-section size, high electric field density, strong nonlinear effect, low half-wave voltage length product, and small size. It has significant advantages in the integration of optoelectronic devices. In the phase-coded QKD system, the coding object of the information is the phase of the optical signal. The polarization state of the optical signal can have a serious impact on the system. The phase-encoded QKD system based on the Faraday-Michaelson interference loop is able to self-compensate for the polarization variations in the system to remove the relevant effects of polarization variations on the QKD system. The application of a phase modulator based on thin film lithium niobate preparation in quantum key distribution can effectively enhance the rate of the quantum key distribution system. However, there is still a need to study the transmission and modulation characteristics of LNOI waveguides on polarized optical signals. In this paper, we develop a phase modulator based on thin-film lithium niobate for high-speed QKD systems. Simulation and analysis of the polarization mode of the optical signal transmitted in the optical waveguide. Test and study the transmission loss and modulation efficiency difference of a thin-film lithium niobate optical waveguide for TE and TM polarization state optical signals. To build a test system for application to the measurement and modulation test of the polarization state of optical signals in a high-speed phase-encoded QKD system.