Manufacturing of integrated optical devices assisted by ultra-short pulse lasers

Thesis

Integrated photonics is a key enabling technology for communications, sensing, and quantum information processing. Femtosecond laser direct writing (FLDW) offers unique advantages for this field, including three-dimensional structuring, rapid prototyping, and compatibility with diverse transparent substrates. This thesis investigates the design, fabrication, and characterisation of laser-written waveguides and devices, with particular emphasis on understanding how fabrication strategies and device geometry influence optical performance.

This thesis starts with an overview of the research field, theoretical background, and experimental setup for the studies. The research progresses are then reported through three main experimental studies. First, a fabrication method is introduced that compensates for edge aberrations without post-processing, reducing failure rates and enabling in-situ device writing. Second, multiscan waveguides fabricated with spherical phase-induced modulation (SPIM-WG) are investigated through simulations using COMSOL Multiphysics software, showing how geometry, separation, and symmetry govern supported waveguide modes. Agreement with experimental results demonstrate how these SPIM-WG structures can be tailored for coupling to fibre or to devices with asymmetric spatial modes such as ppKTP waveguides. Third, directional couplers are analysed theoretically and experimentally, demonstrating how their splitting ratios vary with geometry and input conditions, and extending this understanding to three-dimensional, out-of-plane configurations.

Together, these results show how geometry, refractive index modification, and characterisation can be harnessed as design parameters for integrated photonics. Beyond fundamental insight, they establish design strategies with direct relevance to industrial applications in optical communications, co-packaged optics, data-centre interconnects, precision sensing, and quantum photonics. Looking forward, the methods developed here point toward heterogeneous integration with other platforms and the use of additional degrees of freedom of light, such as spatial modes and polarisation, to realise scalable, applicationready photonic systems.

Authors: Pong, ZK

• DOI: 10.5287/ora-r15eexpjb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: femtosecond direct laser writing; integrated photonics; microfabrication
• Subjects: Optical engineering; Photonics; Laser ablation; Femtosecond lasers