A model of large core and multimode photonic crystal fiber (PCF) with sealed air holes at its end face is proposed for the first time. The model indicates that this PCF can be replaced by another equivalent fiber with complete holes at a new end-face position. Instead of a segment of glass rod between the old and new end faces, light rays travel in a virtual medium with the loss rating of pure glass but the refractive index of the immersion medium. For a two-fiber structure, this segment of pure glass has the capability of enhancing the light collection efficiency, which is investigated using a specifically designed fiber probe and fluorescent samples with different concentrations.
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