Abstract
Fused coupler forming with a single-mode fiber (SMF) and a photonic crystal fiber (PCF) is one of the solutions for optical coupling from a light source to a PCF. In this paper, we presented coupling characteristics of a fused fiber coupler formed with an ordinary SMF and a PCF having air hole collapsed taper. A prototype of SMF-PCF coupler with air hole collapsed taper was fabricated using CO2 laser irradiation. The coupling efficiency from SMF to PCF was −6.2 dB at 1554 nm wavelength in the fabricated coupler. The structure of the SMF-PCF coupler to obtain high coupling efficiency was theoretically clarified by beam propagation analysis using an equivalent model of the coupler with simplification. It was clarified that appropriately choosing the prestretched or etched SMF diameter and the length of air hole collapsed region was effective to obtain high coupling efficiency that was a result of high extinction ratio at cross port and low excess loss. We also demonstrated that the diameter of prestretched SMF to obtain high coupling efficiency was insensitive to the air hole diameter ratio to pitch of the PCF in the air hole collapsed SMF-PCF coupler.
Highlights
A photonic crystal fiber (PCF) that has air holes around the center of its cross-section exhibits various interesting characteristics not seen in conventional optical fibers, such as an endlessly single mode, an extremely high/low nonlinearity, a flexible dispersion controllability, and a high birefringence [1]
Using an ordinary single-mode fiber (SMF) is another solution for the optical coupling to the PCF, where the SMF is spliced to the PCF and the light is coupled to the PCF through the SMF
Theoretical studies were performed to obtain high coupling efficiency in the air hole collapsed SMF-PCF coupler by beam propagation analysis using an equivalent model of the coupler with simplification
Summary
A photonic crystal fiber (PCF) that has air holes around the center of its cross-section exhibits various interesting characteristics not seen in conventional optical fibers, such as an endlessly single mode, an extremely high/low nonlinearity, a flexible dispersion controllability, and a high birefringence [1]. Because of those unique properties, PCFs have been applied to functional fibers especially for dispersion compensations [2, 3] and nonlinear fiber optics [4,5,6,7]. It is expected that the prestretched SMF diameter to obtain high coupling efficiency is insensitive to the PCF parameters since the air holes are collapsed in coupler taper. The possibility of etching for SMF diameter reduction as an alternative technique of prestretch is examined
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