We describe the history, guiding mechanism, recent advances, applications, and future prospects for hollow-core negative curvature fibers. We first review one-dimensional slab waveguides, two-dimensional annular core fibers, and negative curvature tube lattice fibers to illustrate the inhibited coupling guiding mechanism. Antiresonance in the glass at the core boundary and a wavenumber mismatch between the core and cladding modes inhibit coupling between the modes and have led to remarkably low loss in negative curvature fibers. We also summarize recent advances in negative curvature fibers that improve the performance of the fibers, including negative curvature that increases confinement, gaps between tubes that increase confinement and bandwidth, additional tubes that decrease mode coupling, tube structures that suppress higher-order modes, nested tubes that increase guidance, and tube parameters that decrease bend loss. Recent applications of negative curvature fibers are also presented, including mid-infrared fiber lasers, micromachining, and surgical procedures. At the end, we discuss the future prospects for negative curvature fibers.

Negative curvature fibers

We present and numerically characterize a closed-form multi-core holey fiber based plasmonic sensor. The coupling properties of the specific modes are investigated comprehensively by the finite element method. It is found that not only phase matching but also loss matching plays a key role in the coupling process between the fundamental mode and plasmonic mode. The coupling transforms from incomplete coupling to complete coupling with increasing analyte RI. An average sensitivity of 2929.39nm/RIU in the sensing range 1.33-1.42, and 9231.27nm/RIU in 1.43-1.53 with high linearity is obtained. The dynamic sensing range is the largest among the reported holey fiber based plasmonic sensors, to the best of our knowledge.

A multi-core holey fiber based plasmonic sensor with large detection range and high linearity

We numerically investigate the polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes, through a full-vector modal solver based on the finite-element method (FEM). Firstly, we investigate the fundamental coupling properties between the core guided light and surface plasmon polaritons (SPPs) excited on the surface of metal wire. Secondly, we show that we can obtain highly polarization-dependent transmission characteristics in PCFs by introducing several metal wires closely aligned into the cladding, and reveal the strongly polarization-dependent coupling properties between the core guided modes and the SPP supermodes, which consist of discrete SPP modes. Finally, we show the importance of arranging the metal wires close to each other for high polarization-dependence.

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Polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes

Two kinds of novel plasmonic high sensitivity of refractive index (RI) sensors based on analyte-filled photonic crystal fiber (AF-PCF) are proposed in this paper. The metallic gold and silver is used as the surface plasmon resonance activity metal. A full-vector finite-element method is applied to analyze and investigate the sensing and coupling characteristics of this designed AF-PCF with the gold or silver layer. Phase matching between the second surface plasmon polariton and fundamental modes can be met at different wavelengths as the analyte of the RI is increased from 1.40 to 1.42. The phase-matching wavelength of the designed AF-PCF with the gold layer is shifted to the longer wavelength direction compared with that with the silver layer, and the resonance strength is much stronger. The average sensitivities of 7040 and 7017 nm/RIU in the sensing are arranged from 1.40 to 1.42 with high linearity are achieved for the designed sensors with the gold and silver layers, respectively, which are almost the same. However, the figure of merit with the silver layer is much better than that with the gold layer.

High Sensitivity of Refractive Index Sensor Based on Analyte-Filled Photonic Crystal Fiber With Surface Plasmon Resonance

This article presents the photonic crystal fiber (PCF) based surface plasmon resonance (SPR) biosensor for early detection of malaria disease in humans by measurement of the variation of red blood cells (RBCs). In the proposed PCF, two layers of air holes are arranged in a hexagonal lattice structure and a thin film of gold-coating is used over PCF for the occurrence of SPR phenomena. It occurs when surface plasmon polariton (SPP)-mode coupled with the core-mode during phase-matching conditions. Malaria infected RBCs samples are filled into the PCF, which have own refractive index (RI) that shift the SPR resonance wavelength during confinement loss measurement. The resonance wavelength of malaria-infected RBCs samples is different from their normal RBCs samples due to the difference in RI of infected and normal RBCs samples. The proposed work is helpful in the detection of different stages of malaria-infected RBCs such as ring phase, trophozoite phase and Schizont phase by measuring the shift in resonance wavelength. The calculated wavelength sensitivities of the proposed sensor for the ring phase, trophozoite phase and Schizont phase RBCs are 13714.29 nm/RIU, 9789.47 nm/RIU, and 8068.97 nm/RIU, respectively in x-polarized direction and 14285.71 nm/RIU, 10000 nm/RIU, and 8206.9 nm/RIU, respectively in y-polarized direction with the maximum detection limit of 0.029. The proposed PCF-based SPR biosensor is suitable for the early diagnosis of malaria disease due to its enhanced sensing performance (low detection limit and high sensitivity).

Gold-Immobilized Photonic Crystal Fiber-Based SPR Biosensor for Detection of Malaria Disease in Human Body

It is possible for core leaky mode to couple with cladding defect leaky mode when the cladding defect is close to fiber core. Dispersion properties and propagation loss of core mode will be affected in some extent once the coupling occurs. But a complete coupling between two leaky modes not always happens even the phase matching condition is satisfied. Leaky mode coupling in photonic crystal fiber with a hybrid cladding which includes low-index and high-index inclusions at the same time is numerically investigated based on a full vector finite element method. It is found that not only phase matching but also loss matching plays an important role in leaky mode coupling. The originally intersecting dispersion curves for the two leaky modes will split and become another two new curves due to the anti-crossing effect when both the real and imaginary parts of their mode effective refractive indices are equal. There is not splitting but some perturbation in dispersion curves for the two phase matching leaky modes when their losses are not equal. A theoretic explanation is also given to these phenomena.

Dependence of leaky mode coupling on loss in photonic crystal fiber with hybrid cladding

Chromatic dispersion of dual-concentric-core photonic crystal fibers (PCFs), which have an inner low-index core and an outer high-index core, is investigated based on a full vector finite-element method. Large negative dispersion can be obtained by introducing avoided crossing between modes of the two asymmetric cores. While increasing the negative dispersion, larger effective mode area is also possible by choosing a higher order mode in the outer core to couple with the fundamental inner core mode. During the analysis, a dual-core PCF with a dispersion peak value of -36 000 ps/(nmldrkm) and effective mode area of 34.8 mum2 is presented.

Large Negative Dispersion in Dual-Core Photonic Crystal Fibers Based on Optional Mode Coupling

Dual-core hybrid photonic crystal fibers in which high-index inclusions are introduced between the two cores are proposed and the directional coupling characteristics of them are investigated numerically. Alternative cutoffs of even and odd supermodes, decoupling effect, and maxima of coupling length are presented. It is demonstrated that the resonance and location of the high-index inclusions play an important role.

Resonance-Related Coupling of Symmetric Dual-Core Hybrid Photonic Crystal Fibers

Antiresonant guiding microstructured optical fiber (MOF), a new kind of photonic band gap fiber, has several attractive applications including tunable filters, optical modulators and sensors. This new kind of photonic bandgap fiber with low refractive index core surrounded by a finite lattice of high refractive index inclusions reveals several intriguing properties, which are mainly determined by the individual properties of high refractive index inclusions rather than their positions and number. In this paper the dispersion properties of these fibers with hexagonal lattice of cylindrical high refractive index inclusions are discussed based on a full vector multipole method. The influences of inclusion space, inclusion rings number and core size on dispersion properties are investigated with photonic band gap location almost unchanged by keeping the diameter and refractive index of the high refractive index inclusions fixed. In order to evaluate the dispersion property dependence on high refractive index inclusions, the comparisons between fibers with different diameters or refractive indexes inclusions are also done. Numerical results show that the waveguide dispersion can be adjusted in a wide range for individual band with the bandgap location almost unchanged. It is confirmed that the dispersion is more susceptive to inclusion space and fiber core size than inclusion rings number. Individual properties of high refractive index inclusions can affect not only the location of band gaps but also the dispersion properties remarkably. This research is useful to optical fiber communication, fiber nonlinear applications and tunable device designs.

Zhihua Zhang

Papers

Dependence of leaky mode coupling on loss in photonic crystal fiber with hybrid cladding

Large Negative Dispersion in Dual-Core Photonic Crystal Fibers Based on Optional Mode Coupling

Resonance-Related Coupling of Symmetric Dual-Core Hybrid Photonic Crystal Fibers

Dispersion property analysis of antiresonant guiding microstructured optical fibers