Gas-filled Hollow-Core Photonic Crystal Fibers for sensing applications and ultrafast non-linear optics

The emergence of Hollow core fibers (HCF) has revolutionised the field of optical photonics in a number of ways. Firstly, it enhanced light/matter interaction in by providing a waveguide that not only confines light withing its core, but also allows for the interchangeability of this host gas. This therefore allows for numerous sensing application such as spectroscopy of gaseous and liquid compounds.

Additionally, and perhaps even more important, is that by changing this gas and tuning its pressure, the properties of the fiber such as nonlinearity and dispersion changes. This is a new concept that could not be achieved through with previous solid-core fibers, and since the core of these fibers is hollow it meant that intense level of light could be propagated in the core without fear of damaging the fiber.

The combination of these intriguing physical phenomena lead to HCF to be deployed in fields of Ultra-fast nonlinear Optics, Supercontinuum generation, pulse compression, gas sensing, delivery of high power lasers etc. In contrast to photonic crystal fibres (PCF), HCF are not restricted by the silica material, in principle.

This means their transmission can be tailored and be made to guide at wavelengths that silica has fallen short. These designs, such as the negative curvature fibers, have allowed to HCF to transmit light in a large bandwidth. Including and not limited to the Ultraviolet and mid-Infrared (mid-IR) regime.

In this thesis, we present a unique demonstration of these HCF, buy generating a multioctave spanning SC light, which spans from 200 nm in the UV to 4000 nm in the mid-IR, this was achieved through pumping the HCF itself in the mid-IR, thereby having a significant fraction of the light in this very important region.

Furthermore, through resonant dispersive waves (DW), light was able to be generated in the deepUV region, another very important region of the electromagnetic spectrum. This Deep-UV light has been is much interest in various fields such as lithography, semiconductor chip inspection, etc. Therefore the generated light was characterized and important features such as its coherence and noise was investigated in the thesis.

Besides, another type of lasers, called Raman lasers have been realized through the use of Raman active Gasses in the fiber core. To explore the full potential of these fibers in gas sensing, this thesis explores novel ways of sending gas into the cores of they were to be spliced. Unique micromachining methods such as Focused ion beam and femtsecond laser ablation were examined in making channels while maintaining minimal distortion the fiber’s guiding properties.

Other technique which relies on commercially-available components was used to demonstrate continuous detection of multiple gasses in HCF, this type of application is of primary importance especially in detecting and monitoring air pollution. Denmark, for example, has a huge demand for such gas sensors because of the Ammonia and Methane that is released in areas of intense agriculture like diary barns and pig farms.

Having photonic chips integrated with ammonia sensors will help greatly in monitoring the levels of pollutants released into the atmosphere. The presented ammonia and methane sensor is a step forward towards actualising this dream. We are confident that the work presented, not only confirms the future of HCF in next-generation UV and mid-IR lasers, but also demonstrated their suitability in making a more compact and low-cost sensors that could provide real-time, highly sensitive and reliable gas sensors.