Polarization Coupling of $X$-Cut Thin Film Lithium Niobate Based Waveguides

Jingyi Wang,Liu Liu,Daoxin Dai,Pengxin Chen

doi:10.1109/jphot.2020.2995317

Jingyi Wang, Liu Liu + Show 2 more

Open Access

https://doi.org/10.1109/jphot.2020.2995317

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Abstract

Thin film lithium niobate (LN) shows great potentials for highly compact passive and active devices. As LN is an anisotropic material, waveguides made on it exhibit different mode properties from those on conventional isotropic materials. We study the effective refractive indices of fundamental modes of two polarizations in etched ridge waveguides on an X-cut LN thin film. Mode hybridization phenomenon, where the effective refractive indices of the two polarizations are close, is analyzed in detail with different structural parameters. Transmission through a 90° bend, which is a typical routing element for a photonic chip, is simulated. Significant polarization coupling related to the mode evaluation through the bend is observed, and becomes the dominant fact limiting the performance of this element. In order to ensure a low bending loss, the required bending radius is much larger than that for waveguides on an in-plane isotropic material, e.g. a Z-cut LN thin film. Mode hybridization also plays an important role in the performance of the 90° bend, which should be avoided. Generally, decreasing the thickness of the LN thin film, working at a longer wavelength, or confining the propagation angle on a chip would help to decrease the polarization coupling.

Highlights

In the past few decades, integrated photonics has developed rapidly and there exist many different materials, such as SiO2 [1], [2], Si3N4 [3], silicon on insulator (SOI) [4], [5], III-IV compound semiconductors [6], and lithium niobate (LN) [7], to fabricate different functional photonic elements
We study the effective refractive indices of fundamental modes of two polarizations in etched ridge waveguides on an X -cut LN thin film
High-performance and highly-integrated electro-optic modulators [9], [14], [15], micro-ring resonators [16]–[18], grating couplers [19], and photonic crystal devices [20], [21] fabricated on the thin film LN have been demonstrated on this LN on insulator (LNOI) platform

Summary

Introduction

In the past few decades, integrated photonics has developed rapidly and there exist many different materials, such as SiO2 [1], [2], Si3N4 [3], silicon on insulator (SOI) [4], [5], III-IV compound semiconductors [6], and lithium niobate (LN) [7], to fabricate different functional photonic elements. Benefited from the large refractive index contrast between the LN and the silicon oxide, as well as the robust dry-etching technology of LN, ridge waveguides fabricated on thin-film LN present a tight light confinement, and a small bending radius can be expected. High-performance and highly-integrated electro-optic modulators [9], [14], [15], micro-ring resonators [16]–[18], grating couplers [19], and photonic crystal devices [20], [21] fabricated on the thin film LN have been demonstrated on this LNOI platform. The propagation direction of the waveguide on a chip can largely affect the mode properties [24] This effect is further enhanced for a high refractive index contrast waveguide based on LNOI [10], [11]. Guidelines for designing an LNOI waveguide structure suitable for on chip routing is discussed

Mode Property

Bending Performance

Findings

Discussion and Conclusion