Fabrication and characterisation of ultrafast direct laser written waveguides

Thesis

A novel ultrafast direct laser writing (DLW) system using adaptive optics is proposed and demonstrated. This system has the potential to generate high-quality three-dimensional (3D) optical waveguides and components. The experimental setup and procedures for the DLW process are studied, after which various optical waveguides are fabricated in different transparent materials. The resulting waveguides are characterised by the measurement of the near-field laser coupling profiles in combination with optical microscopy techniques. Quantum random number generation (QRNG) and the potential application of the DLW technique in quantum information is also discussed.

To completely understand the fabrication procedures for the DLW system, the experimental equipment and effects of different fabrication parameters are studied and analysed. With the use of a liquid-crystal spatial light modulator (SLM) in the DLW system, dynamic control of phase modulation can be provided to correct aberrations adaptively. An SLM can also make the cross-sectional profile of the written waveguides more circular and facilitate the fabrication of more complex 3D structures. Experiments reveal that the shape of the focal spot can be improved dramatically with adaptive optics, resulting in higher-quality optical waveguides.

The refractive-index information of the written waveguides and their optical properties are measured using the propagation-mode near-field method (PMNFM). Simulation results and experimental measurements of a commercial single-mode fiber and a waveguide sample are demonstrated and compared. Quantitative phase measurement is also applied via the transport of intensity equation (TIE) to monitor the refractive-index change during fabrication. The propagation losses of the waveguides are measured and discussed.

Different optical waveguides are fabricated using DLW in fused silica, potassium dihydrogen phosphate (KDP), and lithium niobate (LiNbO3) crystals. Different materials have different characteristics and properties, requiring different fabrication parameters and resulting in waveguides exhibiting different properties. Waveguides at various depths are demonstrated both with and without effects of adaptive optics. Experimental results indicate great improvements in the quality of the written waveguides after aberration correction. With an understanding of the optical properties of the straight waveguides using the characterisation methods, modelling and fabrication of bend waveguides and y-splitters are presented and studied.

A high-speed QRNG system is also demonstrated in this thesis, with potential implementation using the DLW technique for a more compact and stable system. Finally, the possibility of the DLW fabrication of complex 3D optical components and their applications are discussed for future work.

Authors: Huang, L

• DOI: 10.5287/ora-ppyoyqqn5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English