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A novel class of wavelength-selective coupling photonic crystal fiber (PCF) that operates by a hybrid light-guiding mechanism has been proposed. Different from the traditional PCF coupler operating principle, this fiber coupler shares properties of both the total internal reflection index-guided and the photonic bandgap mechanism. This coupler allows highly accurate control of the filtering wavelength; both bandstop and bandpass filters can be easily implemented. The spectral transmission results demonstrate that the bandpass characteristic of the coupler is very narrow and free of sidelobes. Moreover, the operating wavelength and the coupling length can be continuously tuned by changing the refractive index of the filling material. This research gives a physical insight into the propagation mechanism in the PCF coupler and is crucial for future applications of the proposed device.

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We present and analyze a novel hollow-core photonic quasi-crystal fiber (HC-PQF) constructed with 12-fold quasi-periodic structure. The quasi-periodic cladding is shown in this letter to be able to guide light in a large-size hollow core. This HC-PQF provides two photonic bandgaps in the 3 1 region, thus two spectral windows of low loss guidance can be achieved. The behavior of guided modes that depends on the size of the defect core is studied, and the dispersion and confinement loss are also investigated. This research gives a physical insight into the propagation mechanism in HC-PQF and is crucial for future transmission applications.Index Terms-Finite-element method (FEM), holey fiber, photonic bandgap (PBG), photonic quasi-crystal fibers (PQFs).

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