As one essential property of electromagnetic field, the state of polarization can be used to encode, calculate, and process information using light and reflects a wealth of applications in optical perception and operation. Effective modulation of optical polarization states can encode and multiplex information in different systems has exploited superb capabilities in a wide breadth of sciences and applications, such as optical communication, optical sensing, quantum entanglement and so on. However, conventional polarization operation approaches often depend on large volume bulk optical elements, including waveplate-type and optical fiber circular polarization controller. It is a priority to implement polarization manipulation on the integration under the urgent demands of compactness, device stability and high-speed response. Currently, the complexity and maturity of photonic integrated circuits (PICs) is steadily increasing and PICs are finding their way into more and more applications. Thin-film lithium niobate (TFLN) has attracted considerable attention both in academics and industry, which benefits from the large electro-optical coefficient of lithium niobate. Generally, the modulation performance of most polarization operators are highly dependent on the quality of the polarization diversity device, i.e., the PSR. The perfect implementation of PSR requires special design and precise accuracy, which aggravates the difficulty in fabrication. Additionally, the PSR with lengths of several hundred microns consequentially leads to a large device size.

In this letter, we exhibit a TFLN-based polarization modulator utilizing an electro-optic phase modulator and a two-dimensional grating coupler (2D GC). The modulator avoids polarization diversity processor elements, which enormously improves the technological tolerance. We demonstrate effective modulation of polarization states with a polarization extinction ratio (ER) that is in excess of 16 dB for all the polarization output states and up to 35 dB for a subset of the operating points. In addition, mutually switching specified polarization states is also precisely realized. The structure of the polarization modulator, as shown in Fig. 1(a), The schematic of the 2D GC is shown in Figs. 1(b) and 1(c). The two orthogonal waveguides are symmetrical about the optical axis, and θ is the diffraction angle. The simulated normalized filed intensity at the 2D GC is shown in Fig. 1(d). The experimental setup is depicted in Fig. 2(a). To quantitatively analyze the modulation performance of the device, the output polarization states are adopted whose results are described by the Poincaré sphere and Stokes vectors, as shown in Figs. 2(b) and 2(c). Fig. 2(d) indicates the experimental polarization ER, which is in excess of 16 dB for all the output polarization states, and the maximum polarization ER achieves 35 dB. The experimental results of adjusting the right-handed and linear polarization states produced by the specified DC voltage drive device, as well as the corresponding Stokes parameter measurements, are summarized in Fig. 3, Notably, this work provides novel scheme of modulating polarization states on the TFLN platform, exhibiting huge potential for realizing ultra-compact and miniaturized optical systems at specified scenario, such as polarization encoded quantum key distribution.

Fig. 1. (a) Schematic of the TFLN-based polarization modulator. (b) Schematic of the 2D GC, which is symmetric about the optical axis. θ: the diffraction angle. (c) Cross-sectional view of the 2D GC. (d) The simulation of wave propagation in the 2D GC at 1550 nm.

Fig. 2. (a) Experimental setup for the EO polarization modulation. The generated polarization states as plotted by (b) the Poincaré sphere and (c) the Stokes parameters. (d) Polarization extinction ratio as a function of the applied voltage.

Fig. 3. The Stokes parameters of (a) the right-handed polarization state and (b) the linear polarization state. (c) The switching result that plotted on the Poincaré sphere. (d) The conversion of the Stokes parameters with the switch of the polarization states.

This research is published by “Xuerui Sun, Yinan Wu, Chuanyi Lu, Hao Li, Xiaona Ye, Yuting Zhang, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, "Thin-film lithium niobate polarization modulator without polarization diversity," Optics Express 30 (17), 30592-30599 (2022)”.

Link: https://doi.org/10.1364/OE.468533