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This paper presents a study focusing on a fabrication process to shape an optical fibre tip capable of producing a photonic nanojet.
Photonic nanojet generated in the near field of a dielectric bead have been comprehensively studied. Contradicting the usual laws of optical geometry, this propagative, slightly diverging, light beam can have a Full Width at Half Maximum smaller than a half-wavelength, subsequently going beyond the diffraction limit. Thanks to these useful properties, the photonic nanojet is an ideal tool for near-field optical microscopy and subwavelength micromachining.
Nevertheless, for a repeatable and industrial processes, one need to bypass the use of dielectric beads that are tedious to manipulate. A photonic jet can also be generated at the end of a shaped optical fiber tip. Optical fibres are commonly shaped by various methods revolving around thermoforming. Unfortunately, the thermoforming process has some limitations: controlling the curvature of the tip is hazardous and the effect on the diffusion of the dopant in the core and cladding is unknown. Nonetheless, other approaches to optical fibre shaping have been investigated by the scientific community. A promising method, further investigated in this work, is chemical etching with hydrofluoric acid. The process, often labelled as highly hazardous due to the nature of the chemical involved, offers a great way to carefully shape the tip of silica based-fibre.
The optimisation of the fibre tip shape, in order to get the desired photonic jet, was made by 2D modelisation, by finite elements method. The wanted tip shape is then obtained by thermoforming, chemical etching and the combination of two methods. The thermoforming was done by electric arc. The isotropical etching was performed in a solution of 24 % HF. The characterisation of the fibre tips was carried out by the direct ablation of silica and photosensitive resin insulation using respectively 1064 nm and 514 nm lasers. The results were studied by optical microscopy and white light scanning interferometry. Insulated spots of 3.5 µm were measured on SU8. Ablated spots of 685 nm were observed on silica. These encouraging results prove that the combination of thermoforming and chemical etching can lead to highly accurate fibre tip shaping for Photonic NanoJet generation and thus open the way to enhanced sub-wavelength Material Laser Processing.
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