title: Heuristic modelling of laser written mid-infrared LiNbO3stressed-cladding waveguides.
abstract: Mid-infrared lithium niobate cladding waveguides have great potential in low-loss on-chip non-linear optical instruments such as mid-infrared spectrometers and frequency converters, but their three-dimensional femtosecond-laser fabrication is currently not well understood due to the complex interplay between achievable depressed index values and the stress-optic refractive index changes arising as a function of both laser fabrication parameters, and cladding arrangement. Moreover, both the stress-field anisotropy and the asymmetric shape of low-index tracks yield highly birefringent waveguides not useful for most applications where controlling and manipulating the polarization state of a light beam is crucial. To achieve true high performance devices a fundamental understanding on how these waveguides behave and how they can be ultimately optimized is required. In this work we employ a heuristic modelling approach based on the use of standard optical characterization data along with standard computational numerical methods to obtain a satisfactory approximate solution to the problem of designing realistic laser-written circuit building-blocks, such as straight waveguides, bends and evanescent splitters. We infer basic waveguide design parameters such as the complex index of refraction of laser-written tracks at 3.68 µm mid-infrared wavelengths, as well as the cross-sectional stress-optic index maps, obtaining an overall waveguide simulation that closely matches the measured mid-infrared waveguide properties in terms of anisotropy, mode field distributions and propagation losses. We then explore experimentally feasible waveguide designs in the search of a single-mode low-loss behaviour for both ordinary and extraordinary polarizations. We evaluate the overall losses of s-bend components unveiling the expected radiation bend losses of this type of waveguides, and finally showcase a prototype design of a low-loss evanescent splitter. Developing a realistic waveguide model with which robust waveguide designs can be developed will be key for exploiting the potential of the technology.
authors.forename: Huu-Dat
authors.initials: HD
authors.lastname: Nguyen
authors.forename: Airán
authors.initials: A
authors.lastname: Ródenas
authors.forename: Javier R
authors.initials: JR
authors.lastname: Vázquez de Aldana
authors.forename: Javier
authors.initials: J
authors.lastname: Martínez
authors.forename: Feng
authors.initials: F
authors.lastname: Chen
authors.forename: Magdalena
authors.initials: M
authors.lastname: Aguiló
authors.forename: Maria Cinta
authors.initials: MC
authors.lastname: Pujol
authors.forename: Francesc
authors.initials: F
authors.lastname: Díaz