双芯光子晶体光纤中的模式干涉

应用全矢量超格子叠加模型分析了双芯光子晶体光纤(PCF)的模式特征.双芯PCF的基模和二次模分别由偏振方向不同的两个偶模和两个奇模组成，讨论了这四个模式的模式电场奇偶性.基于对双芯PCF模式电场奇偶性的认识，讨论了光纤中的矢量模式干涉问题.分析表明，不同偏振态模式的干涉不会引起芯间的功率转移，双芯PCF的芯间功率耦合是由相同偏振模式的干涉引起的.在相同偏振模式干涉过程中，光功率在两个芯子中呈余弦振荡，光纤的两个芯子一般存在约π／2的相位差.还讨论了两个不同偏振方向的耦合系数与波长的关系. 关键词： 光子晶体光纤 双芯光纤 模式 干涉 耦合     

双芯高双折射光子晶体光纤耦合特性研究

设计了一种双芯高双折射光子晶体光纤,采用多极法（multipole method）和光纤的模式耦合理论研究了光纤的双折射、耦合长度以及色散特性.数值研究发现,对于空气孔节距 Λ=1.2　μm,空气孔直径d=1.0　μm的光纤,在1.55　μm处双折射度为1.24×10^-2;对应x偏振模的耦合长度为21.6　μm,对应y偏振模的耦合长度为24.3　μm. 这种具有高保偏度和极短耦合长度的双芯光子晶体光纤对于微型光子器件的研制具有重 关键词： 光子晶体光纤 双芯 双折射 耦合长度     

A novel polarization splitter based on dual-core hybrid photonic crystal fibers

Highly birefringent dual-core photonic crystal fibers (PCFs) can be used as a polarization splitter because the orthogonal polarization modes with dissimilar coupling lengths are easily separated from each other. Different from the traditional methods achieving high birefringence, a new highly birefringent hybrid PCF that guides light by both index guiding and bandgap guiding is proposed. Firstly, a novel polarization splitter based on this kind of dual-core hybrid PCF is designed. The transmission modes, coupling lengths for the two orthogonal polarizations and performance of the proposed polarization splitter are investigated and numerically analyzed. The results demonstrate that it is possible to obtain a 4.72-mm-long polarization splitter. The splitting ratio is better than −20 dB in a large wavelength range of 1.53-1.72 μm. Its bandwidth is about 190 nm.     

Nonlinear dual-core photonic crystal fiber couplers

We study nonlinear modes of dual-core photonic crystal fiber couplers made of a material with the focusing Kerr nonlinearity. We find numerically the profiles of symmetric, antisymmetric, and asymmetric nonlinear modes and analyze all-optical switching generated by the instability of the symmetric mode. We also describe elliptic spatial solitons controlled by the waveguide boundaries.     

Integrating liquid crystal based optical devices in photonic crystal fibers

Liquid crystal photonic bandgap fibers form a versatile and robust platform for designing optical fiber devices, which are highly tunable and exhibit novel optical properties for manipulation of guided light. We present fiber devices for spectral filtering and polarization control/analysis.     

Bending dual-core photonic crystal fiber coupler

In this paper, we systematically study a designed structure of a bending dual-core photonic crystal fiber (PCF). We propose the controllable wavelength-selective coupling PCF. This coupler allows highly accurate control of the filtering wavelength. The different wavelengths can be selected by controlling the bending radius of the fiber. Coupling characteristics of novel bending wavelength-selective coupling PCF are evaluated by using a vector finite element method and their application to a multiplexer demultiplexer (MUX–DEMUX) based on the novel coupler is investigated. When the fiber length is 4168 μm, the bending radius of PCF couplers for 1.48/1.55 μm, 1.3/1.55 μm, 0.98/1.55 μm, and 0.85/1.55 μm is calculated, respectively, and the beam propagation analysis is performed. Different from the traditional wavelength-selective coupling PCF, the dual-core PCF is bent and it can realize the separation of multiple wavelengths.     

Coupling and decoupling of dual-core photonic bandgap fibers

Coupling characteristics of dual-core photonic bandgap fibers with triangular photonic crystal cladding are investigated by use of a vector plane-wave expansion method and a vector finite-element method. We demonstrate the eigenmodes and the coupling length for two orthogonal polarizations. A decoupling phenomenon is found at a certain wavelength in this fiber configuration. The decoupling effect is attributed to the effect of surface modes on the eigenmodes. The decoupling wavelength decreases as the ratio of core radius to cladding air-hole pitch increases from 1.05 to 1.15.     

Calculation of optical properties of hollow-core photonic crystal fibers

We propose a method for calculation of the bandgaps in the cladding of hollow-core photonic crystal fibers, based on the Floquet theorem. The effectiveness of the approach is confirmed by estimates of the effect of the number of air channels in the cladding on attenuation of the fiber modes due to mode energy leakage from the core. We have studied the conditions for the existence of eigenmodes in the indicated fibers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 311–320, March–April, 2009.     

Design of two kinds of dual-core high birefringence and high coupling degree photonic crystal fibers

We propose two kinds of dual-core high birefringence and high coupling degree photonic crystal fibers (DHBHCD-PCFs) in this paper. The characteristics of birefringence and coupling are studied by multipole method. Numerical results show that the birefringence and the coupling length reach an order of 10^− 2 and 10^− 5 m at 1.55 μm, respectively. It is found that the birefringence and the coupling intensity increase with the increase of air-filling fraction, which is different from other dual-core fibers. The DHBHCD-PCFs with high degree of polarization-maintaining and high coupling degree are helpful for manufacturing minitype photonic apparatus.     

Grating-assisted coupling of optical fibers and photonic crystal waveguides

We propose and analyze a highly efficient method of coupling light from optical fibers to two-dimensional photonic crystal waveguides. Efficient coupling is achieved by positioning of a tapered fiber parallel to the linear defect, where the photonic crystal's cladding functions as a grating coupler and provides field confinement as well. Numerical simulations indicate that better than 90% transmission is possible with a full width at half-magnitude bandwidth of 12nm. It is shown that one can increase the bandwidth by increasing the field overlap between the two waveguides.     

Polarization-controlled dispersive wave redirection in dual-core photonic crystal fiber

The complex study of the polarization-controlled supercontinuum generation in a dual-core square lattice photonic crystal fiber made of multicomponent glass was accomplished. The fiber was excited by 100-fs pulses at a 1250-nm wavelength in the anomalous dispersion region and the registered spectra exhibited soliton fission and Raman-induced self-frequency shift processes. The study also involved a detailed analysis of the infrared-to-visible light conversion exhibiting a dispersive wave origin. The special dual-core properties were investigated by separate spectral analysis of each of the two cores and the near-field profile registration. The emphasis was on the visible part of the spectrum where the input energy and polarization direction dependences were studied. The increase in the input energy allowed for the tuning of the wavelength of the visible spectral features, a further rotation of the polarization direction had an effect on the spectral dependence of the light distribution between the cores. The dual-core fiber exhibited a significant coupling performance and the spectral dependence of the visible light distribution is in good agreement with the simulated coupling length spectral characteristics. Single-core excitation in the linear regime revealed the possibility of coupling 50% of the energy to the other core and the same polarization-controlled redirection possibilities as that at the nonlinear experiments. Dual-core excitation of the fiber enhances the light redirection effect with the application potential for the polarization-controlled directional coupler accompanied by nonlinear frequency conversion.     

Generation of optical vortices in multihelical optical fibers

It is theoretically demonstrated that multihelical fibers are capable to change the topological charge of the incoming field by l units in the transmitted and reflected light. The magnitude of the change in the topological charge coincides with the number of coaxial helicoids forming the fiber core. This can be used in designing generators of optical vortices (OVs) from Gaussian beams: broadband ones in transmitted light and narrowband ones in reflected light.     

Elliptical-hole photonic crystal fibers

We study the dispersive properties of photonic crystal fibers (PCF's) with elliptical air holes. The unusual guidance of PCF leads to novel behavior of the birefringence, group-velocity walk-off, and dispersion parameters, including the possibility of zero walk-off with high birefringence in the single-mode regime. A number of these effects are closely tied to the underlying radiation states of the air-hole lattice.     

Hydrophobic photonic crystal fibers

We propose and demonstrate hydrophobic photonic crystal fibers (PCFs). A chemical surface treatment for making PCFs hydrophobic is introduced. This repels water from the holes of PCFs, so that their optical properties remain unchanged even when they are immersed in water. The combination of a hollow core and a water-repellent inner surface of the hydrophobic PCF provides an ultracompact dissolved-gas sensor element, which is demonstrated for the sensing of dissolved ammonia gas.     

Wavelength-selective coupling of dual-core photonic crystal fiber and its application

We investigate the coupling characteristics of photonic crystal fiber composed of two cores with different index profiles. The index curves of the fundamental modes of the two cores can cross at a desired wavelength by choosing appropriate index profiles of the cores. As a result, wavelength-selective coupling is achieved. The designed coupler can be applied as an oDtical fiber bandstop filter with 26-nm bandwidth.     

Twisted optical fibers sustaining propagation of optical vortices

The mode structure (the orbital number l = 1) of a few-mode weakly guiding optical fiber with high linear birefringence and a regular twist of its anisotropy axes is investigated. It is shown that, for certain values of twist pitch, the modes with l = 1 are almost pure linearly polarized optical vortices in the local coordinate system associated with the anisotropy axes. The range of values of twist pitch in which twisted fibers sustain propagation of linearly polarized optical vortices is determined numerically.     

Enhanced sensitivity of hemoglobin sensor using dual-core photonic crystal fiber

A Hemoglobin (Hb) biosensor based on dual-core photonic crystal fiber is proposed and analyzed. In this paper, the effective refractive index dependency on the Hb concentration within a blood sample is utilized in obtaining the transmission spectrum variation. The results are calculated using full vectorial finite element method. Through this study, the effect of the structure geometrical parameters on the sensor performance is optimized to maximize the sensor sensitivity. The numerical results show a sensitivity of 8.013 nm/g/dL for the X-polarized mode at 3.7 cm fiber length and 7.68 nm/g/dL for the Y-polarized mode at 3.2 cm fiber length of the proposed sensor.     

Group-velocity dispersion in photonic crystal fibers

The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 mum when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 mum.     

Nonlinear copropagation of two optical pulses of different frequencies in photonic crystal fibers

A theoretical investigation of the nonlinear copropagation of two optical pulses of different frequencies in a photonic crystal fiber is presented. Different phenomena are observed depending on whether the wavelength of the signal pulse is located in the normal or the anomalous dispersion region. In particular, it is found that the phenomenon of pulse trapping occurs when the signal wavelength is located in the normal dispersion region while the pump wavelength is located in the anomalous dispersion region. The signal pulse suffers cross-phase modulation by the Raman shifted soliton pulse and it is trapped and copropagates with the Raman soliton pulse along the fiber. As the input peak power of the pump pulse is increased, the red-shift of the Raman soliton is considerably enhanced with the simultaneous further blue-shift of the trapped pulse to satisfy the condition of group velocity matching. PACS 42.65.Tg; 42.81.Dp