Ultrafast laser assisted etching of fibre optic probes for optical biopsy instruments
Abstract
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.