High-performance and high-power applications of nested anti-resonant nodeless hollow-core fibres

Abstract

The use of hollow-core fibres (HCFs) for transmitting high-power, high-brightness laser light presents significant advantages over the current standard technology based on solid core fibres (SCFs). HCFs can deliver much higher optical power levels over longer distances while maintaining a near single-moded beam-quality. This approach has already shown promising results, and this thesis presents important findings on highly efficient coupling and delivery of such laser beams, as well as a relevant application that takes advantage of the hollow core. The studies conducted in this thesis cover a range of subjects. Initially, the coupling tolerances of an in-house developed, state-of-the-art Nested Antiresonant Nodeless Fibre (NANF) were examined and quantified. This led to the experimental verification of the fundamental limits governing free-space coupling of a near-Gaussian beam into the fibre’s fundamental mode. Exploring the opportunity of utilizing high average power lasers, the inevitable thermal load on the optical elements responsible for the input coupling was analysed. The effect of thermal lensing induced by the laser light on the coupling optics led to the degradation of the beam-quality above a certain characteristic threshold (in our experiments ~100W). The studies showed that using a high-purity lens pair over commercially available alternatives significantly decreased this parasitic effect, resulting in more efficient transmission and reduced thermalisation. Additionally, thermal lensing also modified the input spatial beam distribution, resulting in an increased excitation of unwanted higher order modes. Coupling light into these modes that exhibit greater amount of leakage increases the temperature of the fibre noticeably. A non-invasive technique to extract the excess light not coupled into the fundamental mode was designed and implemented. The decreased thermal load on the fibre coating in turn also reduced the possibility of thermal damage, allowing the further scaling of the coupled optical power. Based on the findings, coupling efficiency values of ~95% into NANFs are achieved and maintained at high average powers. This allowed the demonstration of the record delivery of 1 kW average optical power through 1km of HCF. In addition, the implementation of the improvements mentioned above allowed an enhanced setup to couple 2 kW of average optical power stably, as well as to deliver it beyond 10m efficiently. The results achieved in this thesis are limited by the equipment available, and simulations indicate that further scaling in power and transmitted length should be possible. By combining the efficient methods to couple light into HCFs with the low attenuation offered by NANF technology, the Thesis also examines the optical propulsion of micron sized particles inside the hollow core of such fibres. Initial experiments demonstrate the guidance of 10 μm diameter particles through 1m of fibre orientable in any spatial direction. By further upscaling the power and fibre length, this preliminary set of experiments indicates the possibility of a remote sensing solution, which can take advantage of the preservation of the high-quality optical beam to trap the microparticles over extended fibre lengths. Furthermore, extrapolations of the measurement results suggest the possibility, in future work, of accelerating such particles beyond 100m/s in air-filled HCFs.

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Identifiers

ORCID for Hans Christian Mulvad: orcid.org/0000-0003-2552-0742

ORCID for Francesco Poletti: orcid.org/0000-0002-1000-3083

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

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