Low-loss 3D-laser-written mid-infrared LiNbO3 depressed-index cladding waveguides for both TE and TM polarizations.

Huu-Dat Nguyen,Javier R. Vázquez de Aldana,Airán Ródenas,Maria Cinta Pujol,Francesc Díaz,Magdalena Aguiló,Guillermo Martín,Javier Martínez

doi:10.1364/oe.25.003722

Abstract

We report mid-infrared LiNbO3 depressed-index microstructured cladding waveguides fabricated by three-dimensional laser writing showing low propagation losses (~1.5 dB/cm) at 3.68 µm wavelength for both the transverse electric and magnetic polarized modes, a feature previously unachieved due to the strong anisotropic properties of this type of laser microstructured waveguides and which is of fundamental importance for many photonic applications. Using a heuristic modeling-testing iteration design approach which takes into account cladding induced stress-optic index changes, the fabricated cladding microstructure provides low-loss single mode operation for the mid-IR for both orthogonal polarizations. The dependence of the localized refractive index changes within the cladding microstructure with post-fabrication thermal annealing processes was also investigated, revealing its complex dependence of the laser induced refractive index changes on laser fabrication conditions and thermal post-processing steps. The waveguide modes properties and their dependence on thermal post-processing were numerically modeled and fitted to the experimental values by systematically varying three fundamental parameters of this type of waveguides: depressed refractive index values at sub-micron laser-written tracks, track size changes, and piezo-optic induced refractive index changes.

Highlights

Lithium niobate (LiNbO3) has been established as a versatile material for integrated electrooptic devices and nonlinear optical applications due to its high electro-optical and non-linear coefficients, its well-developed industrial manufacturing quality, and its large transparency window from the UV to the mid-IR [1,2,3,4]
Cladding waveguide design and numerical analysis were done by using a finite element computational method (FEM hereafter) using commercial COMSOL package, and which takes into account both constant refractive index changes inside the laser written tracks and its surrounding piezo-optic index changes, supposing a thermal expansion of the laser written tracks, as previously reported [20]
The distinctive behavior of the waveguide under heat treatments has been explored with a focus on optimizing the waveguide properties for both polarizations both in terms of achieving single modality and low losses

Summary

Introduction

Lithium niobate (LiNbO3) has been established as a versatile material for integrated electrooptic devices and nonlinear optical applications due to its high electro-optical and non-linear coefficients, its well-developed industrial manufacturing quality, and its large transparency window from the UV to the mid-IR [1,2,3,4]. The large developments on LiNbO3 devices for the telecom range in the last three decades enable LiNbO3 crystals to be a promising material for the mid-IR photonics as well, with wide applications in chemical and bio-medical fields, in atmospheric research, high-resolution on-chip vibrational spectroscopy, and astrophotonic instrumentation [2,3,4,5,6,7,8,9,10] Lithographic techniques such as ion in-diffusion, ion implantation and soft proton exchange have been employed to manufacture optical waveguide structures in LiNbO3 [11,12,13]. LiNbO3 still poses various serious challenges for the 3DLW technique to be reliable and efficient for photonic fabrication: its birefringence induces aberration of the writing focal volume when using high numerical aperture lenses, and its photo-refractive, piezo-optic and non-linear properties further complicate the 3DLW fabrication process to a very high degree [2,7]

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