Analysis of Waveguides on Lithium Niobate Thin Films

Yiwen Wang,Hui Hu,Zhihua Chen

doi:10.3390/cryst8050191

Abstract

Waveguides formed by etching, proton-exchange (PE), and strip-loaded on single-crystal lithium niobate (LN) thin film were designed and simulated by a full-vectorial finite difference method. The single-mode condition, optical power distribution, and bending loss of these kinds of waveguides were studied and compared systematically. For the PE waveguide, the optical power distributed in LN layer had negligible change with the increase of PE thickness. For the strip-loaded waveguide, the relationships between optical power distribution in LN layer and waveguide thickness were different for quasi-TE (q-TE) and quasi-TM (q-TM) modes. The bending loss would decrease with the increase of bending radius. There was a bending loss caused by the electromagnetic field leakage when the neff of q-TM waveguide was smaller than that of nearby TE planar waveguide. LN ridge waveguides possessed a low bending loss even at a relatively small bending radius. This study is helpful for the understanding of waveguide structures as well as for the optimization and the fabrication of high-density integrated optical components.

Highlights

Lithium niobate (LN) is one of the best known ferroelectric crystals gifted with many advantages—such as excellent electro-optic, nonlinear-optic, acousto-optic, piezoelectric, and pyroelectric properties—which makes it an attractive material for photonic, electronic, and sensor applications [1,2]
Various photonic devices based on LN thin films have been reported, such as photonic crystals [5,6,7], electro-optical modulators [8,9,10,11], wavelength conversion devices [12,13,14,15], and heterogeneous LN photonic devices [16,17,18]
Nano-scale waveguides in LNOI is essential for photonic integrated circuits with low power consumption and high integration density

Summary

Introduction

Lithium niobate (LN) is one of the best known ferroelectric crystals gifted with many advantages—such as excellent electro-optic, nonlinear-optic, acousto-optic, piezoelectric, and pyroelectric properties—which makes it an attractive material for photonic, electronic, and sensor applications [1,2].In recent years, high-refractive-index contrast, single-crystal LN thin film on a low refractive indexSiO2 cladding layer (lithium niobate on insulator, LNOI) or other substrates, has been fabricated using ion implantation and wafer bonding technologies [3,4]. Nano-scale waveguides in LNOI is essential for photonic integrated circuits with low power consumption and high integration density. Many methods have been reported to fabricate waveguides with good performance on LN thin film, including dry etching [19,20], proton-exchange (PE) [21], and strip-loaded [22]. The transmission losses of LN ridge waveguides fabricated via dry etching were reported to be 0.027, 0.3, and 0.4 dB/cm, respectively [19,20,23]. SiN strip-loaded waveguides presented the transmission losses of 0.3, 1, and 7 dB/cm, respectively [14,25,26]. The transmission losses of Si strip-loaded waveguides were 0.8, 1.3, and 6 dB/cm, respectively [18,27,28]. Bent waveguide is a basic building block for many integrated optical

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Conclusion