Diamond micromachining of photonic materials: dynamics and the limits of precision

Abstract

This thesis investigates the ultra-precision diamond micromachining of brittle photonic materials, focusing on optimising machining processes to achieve high-quality surfaces for applications in optical and quantum devices. It explores the dynamics of physical machining in materials such as lithium niobate, silica, and silicon, which are crucial in photonics due to their optical properties. By physically machining within the ductile regime, chip-free surfaces were achieved, offering nanometre-scale roughness and high uniformity, without requiring costly post-processing. Ridge waveguides, optical facets, and alignment structures were fabricated with a focus on minimising surface roughness, form errors, and defects. Surface roughness measurements for ridge waveguide sidewalls reached as low as 0.16 nm when using a 100 µm square-profiled blade. The impact of feed rate on blade wear rate and resulting surface roughness was investigated over a range of 0.1 mm s¯¹ to 10 mm s¯¹, revealing a critical shift at 2 mm s¯¹ in blade wear rate and a similar shift in surface roughnesses at 4 mm s¯¹ in lithium niobate. A comparison of blade wear, surface finish, and waveguide performance provides insights into deterministic machining.

A novel confocal probe-based scanner has been developed to profile the long-term variation of ridge waveguide uniformity, indicating an approximate 1.2 µm curvature across all of the ridge waveguides fabricated. A comparison of ridge waveguides fabricated with 300 µm and 100 µm wide dicing blades was performed, resulting in a significant improvement in optical conversion efficiency — an increase of 226% for the ridges machined with a 300 µm wide blade over a 100 µm wide blade. Plotting the accumulative width variation using the confocal probe traces reveals that the ridges machined with the 300 µm wide blade were significantly more consistent in their widths, confirming the cause of the improved optical conversion efficiencies.

The development of a novel 3D passive alignment structure based around machined U-Grooves and optical fibres has been proposed; however, due to inconsistent machine stepper backlash, practical success has been limited. V-Grooves and U-Grooves have been compared from a general coupling alignment perspective, with U-Grooves resulting in a 30% improvement in fibre-to-fibre alignment over an unsupported gap of 3 mm compared to V-Grooves. This has resulted in improvement of coupling efficiency from 97% to 99%.

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Identifiers

ORCID for Matthew Peter D'Souza: orcid.org/0000-0002-0165-3244

ORCID for James Gates: orcid.org/0000-0001-8671-5987

ORCID for Paul Gow: orcid.org/0000-0002-3247-9082

ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

ORCID for Peter G.R. Smith: orcid.org/0000-0003-0319-718X

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