Fiber-connectorized ultrafast-laser-inscribed K-band integrated optics beam combiner for the CHARA telescope array.

A fiber-connectorized K-band integrated-optics two-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ∼1.1d B over the 2-2.3 µm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ∼75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

Axi-Stack: a method for manufacturing freeform air-silica optical fibre.

We present a method with potential for fabricating freeform air-silica optical fibre preforms which is free from the stacking constraints associated with conventional stack-and-draw. The method, termed Axi-Stack, is enabled by the precision machining of short cross-sectional preform discs by ultrafast laser assisted etching; a laser-based microfabrication technique which facilitates near arbitrary shaping of the preform structure. Several preform discs are stacked axially and fused together via ultrafast laser welding to construct the preform, which can be drawn to fibre using conventional methods. To illustrate the Axi-Stack process, we detail the fabrication of a 30 cm long solid-core photonic crystal fibre preform with a square lattice of cladding holes and characterise fibre drawn from it.

Miniaturised gap sensor using fibre optic Fabry-Pérot interferometry for structural health monitoring.

A miniaturised structural health monitoring device has been developed capable of measuring the absolute distance between close parallel surfaces using Fabry-Pérot interferometry with nm-scale sensitivity. This is achieved by fabricating turning mirrors on two opposite cores of a multi-core fibre to produce a probe with dimensions limited only be the fibre diameter. Two fabrication processes have been investigated: Focused ion beam milling, which has resulted in a sensor measurement accuracy, sensitivity and range of ±0.056 µm, ±0.006 µm and ∼16000  µm respectively; and ultrafast laser assisted etching of the cleaved fibre end, where a sensor measurement accuracy, sensitivity and range of ±0.065 µm, ±0.006 µm and ∼7500 µm have been demonstrated.

Investigating focus elongation using a spatial light modulator for high-throughput ultrafast-laser-induced selective etching in fused silica.

Ultrafast-laser-induced selective chemical etching is an enabling microfabrication technology compatible with optical materials such as fused silica. The technique offers unparalleled three-dimensional manufacturing freedom and feature resolution but can be limited by long laser inscription times and widely varying etching selectivity depending on the laser irradiation parameters used. In this paper, we aim to overcome these limitations by employing beam shaping via a spatial light modulator to generate a vortex laser focus with controllable depth-of-focus (DOF), from diffraction limited to several hundreds of microns. We present the results of a thorough parameter-space investigation of laser irradiation parameters, documenting the observed influence on etching selectivity and focus elongation in the polarization-insensitive writing regime, and show that etching selectivity greater than 800 is maintained irrespective of the DOF. To demonstrate high-throughput laser writing with an elongated DOF, geometric shapes are fabricated with a 12-fold reduction in writing time compared to writing with a phase-unmodulated Gaussian focus.

A Miniature Fibre-Optic Raman Probe Fabricated by Ultrafast Laser-Assisted Etching.

Optical biopsy describes a range of medical procedures in which light is used to investigate disease in the body, often in hard-to-reach regions via optical fibres. Optical biopsies can reveal a multitude of diagnostic information to aid therapeutic diagnosis and treatment with higher specificity and shorter delay than traditional surgical techniques. One specific type of optical biopsy relies on Raman spectroscopy to differentiate tissue types at the molecular level and has been used successfully to stage cancer. However, complex micro-optical systems are usually needed at the distal end to optimise the signal-to-noise properties of the Raman signal collected. Manufacturing these devices, particularly in a way suitable for large scale adoption, remains a critical challenge. In this paper, we describe a novel fibre-fed micro-optic system designed for efficient signal delivery and collection during a Raman spectroscopy-based optical biopsy. Crucially, we fabricate the device using a direct-laser-writing technique known as ultrafast laser-assisted etching which is scalable and allows components to be aligned passively. The Raman probe has a sub-millimetre diameter and offers confocal signal collection with 71.3% ± 1.5% collection efficiency over a 0.8 numerical aperture. Proof of concept spectral measurements were performed on mouse intestinal tissue and compared with results obtained using a commercial Raman microscope.

Optimisation of ultrafast laser assisted etching in fused silica.

Ultrafast laser assisted etching (ULAE) in fused silica is an attractive technology for fabricating three-dimensional micro-components. ULAE is a two-step process whereby ultrafast laser inscription (ULI) is first used to modify the substrate material and chemical etching is then used to remove the laser modified material. In this paper, we present a detailed investigation into how the ULI parameters affect the etching rate of laser modified channels and planar surfaces written in fused silica. Recently, potassium hydroxide (KOH) has shown potential to outperform the more commonly used hydrofluoric acid (HF) as a highly selective etchant for ULAE. Here we perform a detailed comparison of HF and KOH etching after laser inscription with a wide range of ultrafast laser irradiation parameters. Etching with KOH is found to be significantly more selective, removing the laser modified material up to 955 times faster than pristine material, compared with up to 66 when using HF. Maximum etching rates for the two etchants were comparable at 320 µm/hour and 363 µm/hour for HF and KOH respectively. We further demonstrate that highly selective, isotropic etching of non-planar surfaces can be achieved by controlling the polarization state of the laser dynamically during laser inscription.