Ultrafast laser assisted etching of fibre optic probes for optical biopsy instruments
Ross, Calum Alexander
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Advances in photonic techniques, instrumentation and computation is enabling the development of new tools for medical diagnosis. One such tool is a Raman-based “optical biopsy” where Raman spectroscopy is performed in the body to detect molecular level differences between healthy and malignant tissues to diagnose cancer and other diseases. Optical biopsies are less invasive than traditional surgical biopsies and can, in principle, provide highly specific, instantaneous feedback for the clinician. Raman spectroscopy can be performed in hard-to-reach regions of the body such as the oesophagus by utilising optical fibres and distal-end optics. Distal-end optical systems are small and complex and often require labour-intensive manual aligned and intricate bonding of components– a process which is time-consuming and expensive and not suitable for industrial manufacture. The lack of readily available distal-end optical systems has hampered progress in transferring fibre-based Raman biopsies from a research setting into the clinic. The aim of this work was to develop a miniaturised Raman probe, suitable for industrial manufacture, by employing an advanced three-dimensional laser-processing technique known as ultrafast laser assisted etching (ULAE). ULAE is a subtractive manufacturing process which relies on focused femtosecond laser pulses to locally enhance the chemical etchability of certain transparent materials, including fused silica. Material modification is confined to the laser focus and so freeform three-dimensional structures can be inscribed and subsequently removed by chemical etching. Several components can be written pre-aligned on a single substrate, making ULAE perfectly suited to the fabrication of distal-end optical systems (DOS). During the project, we developed a novel, confocal Raman probe with a sub-millimetre diameter and a collection efficiency of 52.1% over a numerical aperture of 0.8. To enable highly repeatable fabrication, we conducted a thorough investigation into how several laser irradiation parameters affect the etching enhancement and achieved an etching selectivity of 955. The Raman probe was used to measure the Raman spectra of healthy and tumorous colorectal mouse tissue and successfully identified molecular peaks associated with relevant cancer biomarkers.