Negative curvature fibre optics for delivery of high-peak power light for manufacturing applications

Abstract

This thesis covers investigations into high peak power short (nanosecond) and ultrashort (picosecond and femtosecond) pulse delivery through novel hollow-core microstructured silica fibres - the Negative Curvature Fibre (NCF) and the Free Boundary Anti-Resonant Fibre (FBARF) for manufacturing applications. Both types of fibres provide light guidance in the NIR and green spectral regions via the Anti- Resonant Reflecting Optical Waveguiding (ARROW) mechanism. NCFs exhibit extremely low attenuations of 0.04 dB/m and 0.15 dB/m at 1030 nm and 532 nm respectively, not achievable with photonic bandap hollow-core fibres. These fibres have been demonstrated to deliver NIR and green picosecond pulses with peak powers at the level of 15 MW and 5 MW respectively, which significantly exceeds the capabilities of commercially available singlemode fibres. Optimised NCFs for transmission in both spectral regimes provide singlemode, high quality output and maintain the spectral and temporal properties of the original laser pulse. Furthermore, these fibres exhibit a low sensitivity to bending with useful bend diameters ≥ 5 cm, which is sufficient for most beam delivery applications. This was further improved by developing and introducing a dynamic fibre delivered power stabilisation system which enables efficient compensations of bend-induced power fluctuations. Practical implementation of NCF delivered pulses in manufacturing applications has been demonstrated in the precision micro-machining of metals, crack-free micromilling of fused silica and waveguide writing in borosilicate glass. The FBARF structure is a modification of the NCF design aimed for an improvement in energy handling capacity, light confinement and further reduction of bending loss. However, the fibres reported in this thesis (fabricated for guidance at 1 μm and green wavelengths) are preliminary designs, and so their full potential e.g. their energy handling capacities could not be fully established. Nevertheless, initial tests performed within this work indicated that further development of this fibre design should result in a reduction of the intrinsic attenuation and bend loss to a level comparable with the NCFs while increasing the optical damage threshold.