Design and optimisation of micro-structured waveguides in nonlinear crystals

Abstract

Direct femtosecond laser inscription has emerged as one of the most efficient methods for direct three dimensional micro-fabrication of integrated optical circuits in dielectric crystals.Lithium niobate is one of the most widely used dielectric crystal for a wide range of optical functions. Using the direct femtosecond inscription technology, it is possible to produce almost circular tracks of 1-2:5μm diameters with negative refractive index changes up to -0:012 in lithium niobate crystals. Those tracks can be used as a cladding region to confine the propagating light inside a core region of a micro-structured waveguide. This dissertation is focused on the numerical investigation of the propagation properties of depressed-cladding,buried micro-structured waveguides in z-cut lithium niobate crystals which can be fabricatedby direct fs laser inscription method. First of all, we discuss how experimentally achievable parameters of cladding tracks such as their position, total number, refractive index contrasts between the low index cladding structure and the core region can be used to design buried micro-structured waveguides with good confinement properties and to achieve any control over the propagation properties of different polarisation modes specific to a wide range of applications of lithium niobate. Numerical analysis of micro-structured waveguides are implemented by using finite element method. The high nonlinear coefficient and wide transparency region of lithium niobate enable its use for frequency conversion applications towards mid-infrared wavelength ranges. In this thesis, optimisation of the guiding properties, specifically the confinement losses, of microstructured waveguides in lithium niobate is realised for both around telecom and mid-infrared wavelength regions. Optimisation is based on a practical approach which takes into account the variation of experimentally achieved track parameters over cladding region. It is shown that the spectral region where confinement losses are below 1 dB/cm can be extended up to a wavelength of 3:5μm. In recent years, a variety of design geometries for micro-structured waveguides has been a focus of research interest as a means of manipulating and controlling the properties of propagating light. The flexibility of writing tracks at various depths inside lithium niobatecrystals allows direct fabrication of micro-structured waveguides with advanced design geometries.The ability to write tracks at varying sizes by femtosecond laser inscription method enables the fabrication of micro-structured waveguides with highly complex spiral geometries. Here, we explore design issues of equiangular, Fermat and Archimedes spiral geometries in accordance with experimentally available track parameters. Optimisation of each geometry is separately implemented for telecom and mid-infrared wavelength ranges. The primary advantage of designing waveguides with spiral geometries is a much finer control and better manipulation of propagating light stemming from a higher number of parameters available for design. Also, it is found that the spectral region where confinement losses are below 1dB/cm can be further extended up to a wavelength of 3:66 μm.