In this work, an octagonal Penrose-type photonic quasi-crystal fiber (PQF) with dual-cladding is proposed. By optimizing three geometric degrees of freedom, the PQF exhibits ultra-flattened near-zero dispersion of 0.014±0.293 ps/nm/km, ultra-low order confinement loss of 10 dB/km, and large effective mode area of over 16.2 μm in a broadband of wavelength from 1.27 to 1.67 μm, covering almost all optical communication bands. At the common communication wavelength 1.55 μm, completely opposite trends of the dispersion and the confinement loss varying with the air-filling factor in the inner cladding are demonstrated. In addition, the robustness of optical properties including dispersion, confinement loss, and effective mode area in this PQF is discussed, assuming a deviation ±3% of all air holes.
A dual-cladding photonic quasi-crystal fiber is theoretically proposed to robustly transmit orbital angular momentum (OAM) modes. This fiber, based on conventional silica background, can lift the effective index separation of constituent vortex modes to the order of 10 −2 in the band of >200 nm, surpassing two orders of magnitude than that of common silica-based fibers. Moreover, the effective index separations in first order group (TE 01 , HE 21 , and TM 01 ) present an opposite and near-linear variation with the increase of air-filling fraction of inner and outer cladding. The mode TE 01 can simultaneously realize an ultra-flattened dispersion with a maximum variation of 19.736 ps/nm/km and low confinement loss of order of 10 −2 dB m −1 within a 400 nm bandwidth covering the communication bands of E, S, C, L and U. Detailed investigation on bend-induced influence shows that this fiber can remain well OAM propagation property with the bending radius being more than 30 μm. In addition, the robustness of OAM property is also stressed with assuming ±3% deviation of diameter of all air holes due to imperfect fiber preparation. This design is expected for the application of OAM optical communication system and to provide guidance for engineering the large-indexseparation fiber.
A novel polarization filter based on a sunflower-type photonic quasi-crystal fiber (PQF) is proposed in this paper. We also discuss different methods to tune the filter wavelength. The proposed filter can efficiently produce polarized light with visible wavelengths by using the resonance between the second-order surface plasmon polariton mode and the core mode of the PQF. The filtered wavelength can be tuned between 0.55 µm and 0.68 µm by adjusting the thickness of the gold film. When the thickness of the gold film is 25.3 nm, the resonance loss in the y-polarized direction reaches 11707 dB m−1 for a wavelength of 0.6326 µm, and the full width at half maximum is only 5 nm. Due to the flexible design and absence of both polarization coupling and polarization dispersion, this polarization filter can be used in devices that require narrow-band filtering.
A highly sensitive refractive index sensor is proposed that consists of a silicon ring surrounding a cavity in a circular photonic crystal (CPC). The bandgap of the CPC is determined from the transmission spectra. Relationships between the performance parameters and the structure parameters of the sensor are investigated. Transmission spectra of the optimized sensor filled with different analytes are analyzed, and a sensitivity of 1054 nm/RIU (refractive index units) and a quality factor of 18,206 are obtained. This work is expected to facilitate the application of sensors based on CPC.
The one-way rotating state is realized in circular photonic crystal (CPC) with yttrium-iron-garnet rods. Adopting 5 ring rods, eight one-way rotating mode frequency bands with periodic distribution and their corresponding eight one-way rotating resonant modes are found within a specific frequency range and their distribution laws are obtained. Adopting 2–6 ring rods, the best bandwidth and localization for the one-way rotating state are realized, respectively, when adopting 4 and 6 ring rods. The research results will provide reference for the designs of the new one-way rotating state devices based on CPC.
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