Optical frequency references are devices providing well-defined and stable optical frequency responses to incoming radiation for applications such as high-precision spectroscopy and optical fibre communications. To stabilise the emission frequency of lasers, which drifts with time mostly because of fluctuations in temperature and mechanical vibrations, atomic and molecular optical transitions can be used since they show precise and well-defined frequency responses to incoming radiation. However from a practical standpoint conventional gas cell devices cannot be easily integrated into existing optical systems because of their bulky dimensions. To replace conventional absorption cells, photonic crystal fibres filled with gas-phase material are promising devices owing to their robustness, reliability, and portable characteristics. In addition they can be directly embedded into existing optical systems and they perform well in harsh environments. In this experimental study, optical gas-phase frequency references based on photonic crystal technology are realised. The gas-sensing properties of different photonic crystal fibre samples are studied and the long-term stability and reliability of fibre gas cells are demonstrated. In addition an analytical model predicting the gas-filling time in photonic crystal fibres (PCF) is developed and can be applied to any type of fibre, fibre geometry, or length. Then fibre gas cells filled with acetylene gas, a recognised frequency reference gas-phase material, have been prepared to conduct fundamental research on slow & fast light generation in optical fibres to verify the possibilities of slow light in enhancing light-matter interaction. The group velocity of light is controlled by modifying the material and structural dispersive properties of the PCF absorption cells through stimulated Brillouin scattering and cavity ring resonators, respectively. We could demonstrate that material slow light has no impact on the molecular absorption effect whereas structural slow light has an impact on the absorption efficiency scaling linearly with the group index. Such radically different responses to slow light suggest that group velocity is not the universal physical quantity scaling light-matter interaction, and that the optical absorption of molecules is more closely related to the velocity of the electromagnetic energy. Finally the impact of slow light on the molecular absorption efficiency is also evaluated in dispersion-engineered photonic crystal waveguides. We demonstrate that in planar photonic crystal waveguides the field enhancement and its evanescent fraction have more impact on the absorption efficiency than the reduction of the group velocity of light.