Fused coupler forming with a single-mode fiber (SMF) and a photonic crystal fiber (PCF) is one of the solutions for optical coupling\nfrom a light source to a PCF. In this paper, we presented coupling characteristics of a fused fiber coupler formed with an ordinary\nSMF and a PCF having air hole collapsed taper. A prototype of SMF-PCF coupler with air hole collapsed taper was fabricated\nusing CO2 laser irradiation.The coupling efficiency from SMF to PCF was âË?â??6.2 dB at 1554nm wavelength in the fabricated coupler.\nThe structure of the SMF-PCF coupler to obtain high coupling efficiency was theoretically clarified by beam propagation analysis\nusing an equivalentmodel of the coupler with simplification. It was clarified that appropriately choosing the prestretched or etched\nSMF diameter and the length of air hole collapsed region was effective to obtain high coupling efficiency that was a result of high\nextinction ratio at cross port and low excess loss. We also demonstrated that the diameter of prestretched SMF to obtain high\ncoupling efficiency was insensitive to the air hole diameter ratio to pitch of the PCF in the air hole collapsed SMF-PCF coupler.
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