Three-Dimentional Laser Writing of Mid-Infrared Waveguides Circuits in Lithium Niobate Crystal

Abstract

Lithium Niobate (LiNbO3) is a well-known dielectric material with extensive characteristics: excellent electro-optic, piezo-electrical, piezo-optic, and non-linear optical properties, and a broad transparency from the visible to the mid-infrared range. Three-dimensional ultrafast Laser-Written (3DLW) depressed-index cladding waveguides (CLWs) in LiNbO3 have offered potential applications in mid-infrared integrated photonic active devices. However, development of mid-infrared LiNbO3- CLWs has met a serious bottleneck. The fundamental problem is related to the high complexity of laser-written structures which originates from the anisotropy of the material and the stress-induced effects. For this reason, low-loss single-mode midinfrared guiding for both orthogonal polarisations has not yet been achieved. All reported mid-infrared LiNbO3-CLWs have been limited to propagation losses of ~3 dB/cm, which is far from the value for practical uses. In this Thesis, the complex midinfrared refractive index (RI) profiles of LiNbO3-CLWs microstructured under various 3DLW conditions are reported for the first time. The RI profiles were evaluated by a comprehensive finite element method based simulation model along with a heuristic mode-matching approach. The extracted index profiles allow for the first time to design high performance LiNbO3-devices using CLW circuit architectures. CLWs were fabricated in transversal writing geometry and under two thermal accumulation regimes: the low-repetition rate (1 kHz) and the critical-repetition rate regime (100 KHz). Singlemode guiding for both polarised lights with low propagation of 0.5 dB/cm has been achieved for the first time to our knowledge. Both straight and s-bend structures were successfully implemented. Simple functional devices such as beam splitters were designed, demonstrating that straightforward on-chip applications in mid-infrared light generation (i.e. using PPLN) and mid-infrared sensing (i.e. using Mach-Zehnder interferometers) are now feasible. Lastly, a novel 3DLW approach was developed using the longitudinal laser writing scheme, which has certain advantages over the transversal scheme, due to the cylindrical symmetry of the modification profiles.