Anti-resonant hollow core fibre is a new kind of optical fibre waveguide in which light is trapped in a hollow core surrounded by the capillary formed microstructured cladding. This fibre exhibits high damage threshold, low dispersion and ultra-low nonlinearity with relatively low loss of a few tens of dB/km. Its intrinsic feature of multimode delivery limits the applications with high requirements of single mode transmission. In my thesis, I demonstrate how the design of hollow core fibre can be improved with single mode guidance. S2 imaging measurement was used to analyse the mode content of the solid core fibres. In my research, I established S2 measurement to measure the mode contents in hollow core fibres for the first time. Two hollow core fibres with 8 capillaries and 7 capillaries in their claddings were fabricated in same fashion and showed differences in low attenuations. By comparing the mode contents in both of the fibres via S2 imaging measurement, 7-capillary HCF was demonstrated to give better performance on single mode guidance. Among the applications of the HCF, the property of delivering high power in HCF makes the gas filled HCF laser possible. In my research, a continuous-wave mid-infrared acetylene filled hollow core laser was built with a slope efficiency of 33% and an output power of over 1 watt at the wavelength region of 3.1~3.2 μm. The pump source is an Erbium-doped fibre amplified tunable laser diode which works at C-band wavelength. The fibre without the gain medium has two transmission bands with low attenuation of 0.037 dB/m and 0.063 dB/m at pumping and lasing wavelengths respectively. This laser system works in either cavity-based configuration or single pass ASE configuration. The latter configuration shows a better performance in high output power and high slope efficiency. The optimized laser system was studied experimentally with the proper fibre length and gas pressure. This laser system could be extended to be filled with other molecules to longer wavelengths and has potential for high power output.