As a multifunctional optical material with huge potential, lithium niobate thin film (LNTF) is considered as a revolutionary photonic integrated platform for on-chip optical manipulation and processing. Lithium niobate itself is an anisotropic crystal, which makes devices on LNTF intrinsically polarization dependent. Other than polarization-division multiplexing applications, the electro-optic effect and second-order nonlinear wave mixings are also polarization sensitive.  Therefore, polarization modulation on lithium niobate wafers is crucial in photonic integrated circuits (PICs). To these ends, high-capacity yet compact polarization manipulators like polarization beam rotators (PBR) and polarization beam splitters (PBS) that harness the polarization degree of freedom are greatly sought after for LNTF PICs. Polarization devices on LNTF will greatly expand the toolbox on the novel platform. So far, several PBR on LNTF with different structure and design have been reported, but fewer PBS have been demonstrated. And they still rely on complex design and structures.

Here, we demonstrate compact polarization beam splitters (PBS) on LNTF based on two asymmetric directional couplers with an effective length of only 180 µm. In terms of design, the cross-coupling performance of different polarization modes which is unique to the ridge waveguide is used to separate the TE mode and the TM mode to different output ports. The PBS is fabricated via a single-step electron beam lithography (EBL) and inductively coupled plasma (ICP) etching process with good fabrication tolerance. Our device shows efficient PBS operation at the telecom bands with an extinction ratio over 26 dB and 13 dB for fundamental TE and TM modes, respectively. The device holds promise for efficient on-chip polarization division multiplexing and polarization manipulation, which would become an indispensable component in future LNTF PICs.

Fig. 1. (a) Schematic illustration of the PBS on LNTF with numerically simulated eigenmodes in each waveguide. (b) Cross section and (c) top view of the PBS.

Fig. 2. (a) Efficiencies of coupling from the TM0 mode in WG1 to the TE1 mode in WG2 with respect to the directional coupling length Lc at different coupling gaps Wg. (b) Coupling efficiency with respect to Lc when Wg= 765 nm. (c) Light propagation for the TE (through) and TM (cross)modes at 1550 nm. (d) Transmission spectra of the through and cross ports.

Fig. 3. (a) Cross section of the waveguide. (b) Enlarged view of the corresponding areas marked in (c). (c) Optical microscopy image of the PBS device and the characterization experimental setup.

Fig. 4. (a) The intensity distribution and the mode profiles observed in the infrared CCD at different input polarization states, i.e., TE, TE&TM, TM input. (b) Measured transmission spectra of the TE and TM ports for TE or TM mode input.

The work was published in “Yinan Wu, Xuerui Sun, Hao Li, Chuanyi Lu, Yuting Zhang, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, Lithium niobate thin film polarization beam splitter based on asymmetric directional coupling, Journal of Lightwave Technology, 40(24), 7843-7847 (2022)”。

Link: https://ieeexplore.ieee.org/abstract/document/9873932/authors