Contaminant free end-capped acetylene PMC for sub-Doppler spectroscopy

Gas-filled Hollow Core Photonic Cristal Fibres (HCPCF) or Photonic MicroCell (PMC) [1] has proven to offer an outstanding gas-cell for laser metrology and spectroscopy applications. Sub-Doppler spectral features, generated via electromagnetically induced transparency (EIT) or saturated absorption (SA), have been generated in molecular gases despite the weak strength of their optical transitions [1–4], and an optical frequency reference with excellent accuracy has been demonstrated [1]. So far, the reported stand-alone PMCs are either made by hermetically splicing the HCPCF tips to an all-solid optical fibre [1,2] or by encapsulating them to a glass cell [3]. All these techniques show limitations. For example, the splicing based all-fibre PMC technique is not adapted to all fibres as it requires matching the outer-diameter and/or mode-field-diameters of the two fibres. Also the use of fibre tapering to reduce this mismatch [2] remains too cumbersome for the obtained splice-loss figures, which are >2 dB and exhibit asymmetric loss. Furthermore, the use of Helium (He) gas during the "air-ambient" splicing process to avoid air pollution in the fibre core renders this technique too sensitive to residual gas contamination. Conversely, the technique of encapsulating the fibre tip in an enlarged borosilicate capillary [3] presents potential contamination during and after fabrication because of the use of Epoxy glue, and the ensued outgassing, which is detrimental for optimized observation of sub-Doppler features and for the life time of the PMC.