Square Lattice Photonic Crystal Fiber Research Articles

A square-lattice D-shaped photonic crystal fiber sensor based on SPR to detect analytes with large refractive indexes

With the booming of nanotechnology, surface plasmon resonance (SPR) technology has become one of the most active research hotspots in optical sensing. In this work, a square-lattice photonic crystal fiber (PCF) sensor based-SPR with indium tin oxide (ITO) coating is investigated to detect analytes with large refractive indexes (RIs) varying from 1.380 to 1.405. To confirm the dependence of sensing characteristics on the geometrical parameters, the finite element method (FEM) is applied to modeling and numerical simulation. The big air holes in cladding cause a birefringence effect, which is stronger with y-polarization mode. In this way, the maximum and average spectral sensitivity of 60,000 and 18,400 nm/RIU can be achieved with resolution in 10−6 order. Furthermore, this sensor which provides a high-sensitivity detection in large analyte RIs with near-infrared region (1500–2800 nm) shows excellent figure of merit (FOM), signal-to-noise ratio (SNR) and detection limit (DL). There will be more extensive space and prospect for progression in biochemical safety with DNA hybridization, blood glucose analysis and organic chemical samples detection involved.

Proposal for dispersion compensating square-lattice photonic crystal fiber

A dispersion-compensating square-lattice photonic crystal fiber for broadband compensation which covers the S, C and L communication bands, i.e. wavelength ranging from 1 460 nm to 1 625 nm is proposed in this paper. Theoretically, it is shown a negative dispersion coefficient of about −595 ps/(nm·km) to −1 288 ps/(nm·km) over S to L bands and −975 ps/(nni·km) at the operating wavelength 1 550 nm. The relative dispersion slope is perfectly matched to that of conventional single-mode fiber of about 0.003 6 nm−1. Besides the proposed photonic crystal fiber shows the large nonlinear coefficient of 61.88 W/km at the operating wavelength of 1 550 nm. Moreover, the variation of structural parameters is also studied and discussed here.

Design of two miniaturized and broadband polarization filters based on two types of square-lattice gold-filled photonic crystal fibers

Two ultra-broadband polarization filters based on Structure 1 and Structure 2 are reported and characterized in terms of attenuation, crosstalk, bandwidth, and polarization filtering properties by the finite element method. Structure 1 is formed by introducing partial elliptical air holes in the first layer of the square-lattice gold-filled photonic crystal fiber. Based on the design solution of Structure 1, Structure 2 is arranged by using circular air holes with the changed transverse pitch to replace elliptical ones. Numerical results show that high resonance intensities of 2265.07 and 1831.83 dB/cm for y polarization, together with the corresponding x polarization losses of 0.50 and 0.18 dB/cm at respective communication wavelengths of 1.55 and 1.31 μm can be achieved by varying the gold film thickness for Structure 1.Moreover, the design of Structure 2 with all circle holes can also realize a high resonance loss of 1942.54 dB/cm for y polarization with a low loss of 0.43 dB/cm for x polarization at 1.55 μm. Structure 1 and Structure 2 with the fixed fiber length of 300 μm can obtain the crosstalk values of −590.09 and −506.07 dB at 1.55 μm, together with the bandwidths of 1500 and 900 nm, respectively. Additionally, two types of designs represent good produce tolerances. These results show a high potential of the suggested fiber for application in polarization-dependent filtering and other polarizing devices.

Design of an ultrabroadband and compact filter based on square-lattice photonic crystal fiber with two large gold-coated air holes

An ultrabroadband and compact square-lattice photonic crystal fiber polarization filter based on surface plasmon resonance is proposed. Four large air holes are introduced near the core to destroy the symmetry of the original fiber structure so that the resonance wavelengths of x- and y-polarized core modes can be well separated. The inner walls of two large air holes along the x polarization direction are coated with gold film to generate the surface plasmon mode. In addition, to further enhance the coupling resonance strength between the core mode and the surface plasmon polariton mode, the six air holes near the gold-coated air holes are replaced by solid glass rods. Then the dispersion characteristics and loss spectrum of the proposed polarization filter are studied by the finite-element method. The results show that when the fiber length of the proposed filter is only 700 μm, ultrawideband y-polarized light transmission with a bandwidth of up to 730 nm in the wavelength range from 0.97 to 1.70 μm can be achieved. The effective filtering range covers all optical fiber communication bands from O to U. Moreover, the filter has excellent structural tolerances. These excellent properties are expected to make it a good candidate for a broadband polarization filter.

Square-lattice alcohol-filled photonic crystal fiber temperature sensor based on a Sagnac interferometer.

A square-lattice alcohol-filled photonic crystal fiber temperature sensor based on a Sagnac interferometer (SI) is designed and analyzed by the finite element method. Alcohol is filled in all air holes in the cladding. The temperature-sensing properties of the proposed fiber sensor are investigated. Simulation results exhibit the transmission spectrums of the fiber SI will shift with the change of temperature, because the birefringence of the alcohol-filled fiber will change under different temperatures. The temperature sensitivity is obtained from the fitting line of the temperature and resonant wavelength. The average sensitivity can reach to 16.55nm/°C in the range from 45°C to 75°C. This designed fiber temperature sensor has advantages in simple structure and high sensitivity. It can be used to detect temperature in complex environments.

Design of an ultrashort single-polarization wavelength splitter based on gold-filled square-lattice photonic crystal fiber

A compact gold-filled square lattice PCF is proposed and analyzed by the finite element method, being capable of wavelength splitting the incoming light at wavelength bands of 1.31 and 1.55 μm with two orthogonal polarization states. The reported PCF based wavelength splitter can achieve 373.79 dB/cm resonance losses for y-polarized core mode at communication wavelength of 1.31 μm with low loss of 9.41 dB/cm for x-polarized core mode. Moreover, resonance loss of 214.31 dB/cm can also be achieved for x-polarized core mode at communication wavelength of 1.55 μm with low loss of 3.88 dB/cm for y-polarized core mode. Additionally, the proposed wavelength splitter possesses an ultrashort length of 0.3 mm with bandwidths of 40 and 165 nm around the wavelengths of 1.31 and 1.55 μm, respectively.

Fiber Sagnac interferometer based on a liquid-filled photonic crystal fiber for temperature sensing

A fiber Sagnac interferometer based on the square-lattice photonic crystal fiber (PCF) for temperature sensing is designed. All the air holes are assumed to be filled with temperature sensitive liquid. The temperature sensing characteristics are analyzed by finite element method (FEM). The phase birefringence B, the group birefringence Bg, and the sensitivity dependence on the operable wavelength λ are studied, and the temperature varies from 25°C to 85°C. The birefringence sensitive coefficient of ∂B/∂T is −10−7/°C. The sensitivity could reach to −11.9nm/°C (−29750nm/RIU) at 1.8μm as the temperature is 25°C. The sensitivity is also studied by the transmission spectrum, and the wavelength spacing between the resonance wavelengths at 85°C can reach to 492nm. The average sensitivity could reach to −7.54nm/°C (−18850nm/RIU). The diameter d1 on the sensing characteristics of the temperature sensor is also analyzed. The temperature sensor shows high sensitivity and broad detecting window.

Hybrid Square-Lattice Photonic Crystal Fiber with Broadband Single-Mode Operation, High Birefringence, and Normal Dispersion

In this study we propose a new photonic crystal fiber (PCF) design that simultaneously offers broadband single-mode operation, high birefringence, and large normal dispersion in the optical-communication wavelength regime. The waveguide is based on a hybrid square-lattice PCF (HS-PCF) that has circular air holes of two different diameters alternating in the cladding, plus a pure silica defect at the center. The optical properties of the guided modes are analyzed numerically by the finite-element method (FEM) with a perfectly matched layer as the boundary condition.

Surface plasmon resonance biosensor based on large size square-lattice photonic crystal fiber

A surface plasmon resonance biosensor based on large size square-lattice photonic crystal fiber has been designed and simulated by finite element method. The square-lattice airholes are first coated with a calcium fluoride layer to provide mode confinement, then a nanoscale gold layer is deposited to excite the plasmon mode, and finally, the sample is infiltrated into the holes. The numerical results reveal that the resonance properties are easily affected by many parameters. The refractive index resolution of corresponding sensor can reach 4.3 × 10−6 RIU when the optimum parameters are set as the radius of curvature of the airhole r = 2 μm, the thickness of the core struts c = 200 nm, the auxiliary dielectric layer s = 1 μm, and the gold film d = 40 nm. In addition, the effective area and nonlinear coefficient are calculated.

An ultrabraoadband polarization splitter based on square-lattice dual-core photonic crystal fiber with a gold wire

An ultrabroadband polarization splitter based on square-lattice dual-core photonic crystal fiber with a gold wire is studied by the finite element method. Integrating a gold wire between two cores has effectively enhanced the bandwidth of the splitter. Numerical results demonstrate that the polarization splitter has an extinction ratio better than −20dB with a broad bandwidth of 430nm, covering almost all the O, E, S, C, L and U communication bands; Moreover, the extinction ratio at 1.55μm can reach −78.2dB.

Numerical analysis of high extinction ratio photonic crystal fiber polarization splitter based on ZnTe glass

A polarization splitter based on dual-core square-lattice ZnTe glass photonic crystal fiber has been designed and simulated by finite element method. The numerical results reveal that extinction ratio ERA=−92.16dB and ERB=−29.56dB at the communication wavelength 1.55μm, and the splitter with fiber length of 1.1452mm provides bandwidths with extinction ratio better than −20dB about 100nm for x-polarization mode and 45nm for y-polarization mode, respectively. At the communication wavelength 1.55μm, insertion loss is 4.8×10-3dB for x-polarization mode and 2.6×10-9dB for y-polarization mode respectively. We also compared the characteristics of ZnTe glass with those of silica glass.

Near-elliptic Core Triangular-lattice and Square-lattice PCFs: A Comparison of Birefringence, Cut-off and GVD Characteristics Towards Fiber Device Application

In this work, we report detailed numerical analysis of the near-elliptic core index-guiding triangular-lattice and square-lattice photonic crystal fiber (PCFs); where we numerically characterize the birefringence, single mode, cut-off behavior and group velocity dispersion and effective area properties. By varying geometry and examining the modal field profile we find that for the same relative values of <TEX>$d/{\Lambda}$</TEX>, triangular-lattice PCFs show higher birefringence whereas the square-lattice PCFs show a wider range of single-mode operation. Square-lattice PCF was found to be endlessly single-mode for higher air-filling fraction (<TEX>$d/{\Lambda}$</TEX>). Dispersion comparison between the two structures reveal that we need smaller lengths of triangular-lattice PCF for dispersion compensation whereas PCFs with square-lattice with nearer relative dispersion slope (RDS) can better compensate the broadband dispersion. Square-lattice PCFs show zero dispersion wavelength (ZDW) red-shifted, making it preferable for mid-IR supercontinuum generation (SCG) with highly non-linear chalcogenide material. Square-lattice PCFs show higher dispersion slope that leads to compression of the broadband, thus accumulating more power in the pulse. On the other hand, triangular-lattice PCF with flat dispersion profile can generate broader SCG. Square-lattice PCF with low Group Velocity Dispersion (GVD) at the anomalous dispersion corresponds to higher dispersion length (<TEX>$L_D$</TEX>) and higher degree of solitonic interaction. The effective area of square-lattice PCF is always greater than its triangular-lattice counterpart making it better suited for high power applications. We have also performed a comparison of the dispersion properties of between the symmetric-core and asymmetric-core triangular-lattice PCF. While we need smaller length of symmetric-core PCF for dispersion compensation, broadband dispersion compensation can be performed with asymmetric-core PCF. Mid-Infrared (IR) SCG can be better performed with asymmetric core PCF with compressed and high power pulse, while wider range of SCG can be performed with symmetric core PCF. Thus, this study will be extremely useful for designing/realizing fiber towards a custom application around these characteristics.

Geometric definition of the V-parameter in photonic crystal fibers

We propose a general geometric definition of the V-parameter in photonic crystal fibers (PCF). We argue that the fundamental modal field in both square-lattice and triangular-lattice PCFs can be approached by certain proper assemblies of the fundamental mode in a one-dimensional planar waveguide. This observation suggests a geometric definition of the effective core radius and the single-mode regime of V≤π/2 in various PCFs. The numerical results imply that the suggestion is valid for all of the one-rod, seven-rod, 19-rod, and 37-rod core triangular-lattice PCFs and the one-rod, nine-rod, and 25-rod core square-lattice PCFs.

Characteristics analysis of large solid-core square-lattice photonic crystal fibers with hybrid cladding

A photonic crystal fiber (PCF) with low confinement loss and nearly zero ultraflattened chromatic dispersion over ultra-broad wavelength is proposed. Some important PCF design parameters, such as the influences of the diameter of air holes and hole pitch on the confinement loss, chromatic dispersion has been thoroughly investigated by employing the full vectorial finite element method with anisotropic perfectly matched layers. The simulation results show that the dispersion coefficient is <0.6 ps/(km⋅nm) and is larger than −0.5 ps/(km⋅nm) from 1.32 to 1.64 μm, and the confinement losses are <10 −7 dB/m in the same wavelength range by introducing extra air holes in the same ring.

Dispersion-ultra-flattened square-lattice photonic crystal fiber with small effective mode area and low confinement loss

Proposed in this paper is a simple square-lattice photonic crystal fiber (PCF) with nearly zero flattened dispersion (NZFD) over a wide wavelength span. We make a trade-off between the coupling efficiency and the effective mode area in order to obtain relatively low confinement loss and high nonlinearity. Via full-vector finite element method with hybrid edge/node elements, over 1137–1710nm, the dispersion coefficient is 0.3±0.3ps/(kmnm), the confinement loss is relatively low, in level of 10−7–10−4dB/km and the effective mode area remains 5.88–6.59μm2.

A theoretical study of large solid-core square-lattice silica photonic crystal fibers with square air-holes

Large solid-core fiber designs constructed by omitting nine air-holes at the center of square-lattice silica photonic crystals with square shaped air-holes have been investigated theoretically and their characteristics have been compared with those of circular ones. The V-parameter was estimated by using the step-index fiber approximation and the effective mode area calculated for the structures considered. To estimate the effective core radius of the structure with square shaped air-holes an approximation based on the air-filling fraction was used. The dispersion and confinement losses were also obtained and compared for both of the structures.

A novel broadband dispersion compensating square-lattice photonic crystal fiber

In this paper we propose a novel square-lattice photonic crystal fiber (SPCF) for dispersion compensation in a wide range of wavelengths. Perfectly matched layer (PML) is considered for the boundary treatment and an efficient compact two dimensional finite-difference frequency-domain (2-D FDFD) method is employed to model square-lattice photonic crystal fibers (SPCF). It has been shown with selecting appropriate parameters for SPCF it is possible to obtain high negative dispersion coefficient, negative dispersion slope over E to L wavelengths, confinement losses less than 10−5 dB/m and splice losses less than 3.6 dB. The designed SPCF exhibits a relative dispersion slope (RDS) of 3.543 × 10−3 nm−1 which is closely matched to the RDS of conventional single mode fibers.

Large-solid-core square-lattice photonic crystal fibers

A large-solid-core fiber design comprised by omitting nine air-holes at the center of square-lattice photonic crystal is investigated for the constant air-hole diameter and for the constant pitch length of air-holes. The fiber characteristics are calculated for both structures, and the results are compared. In the calculations, the plane-wave expansion method is used for effective indices, and we utilized the step-index fiber and Gaussian mode profile approximations for obtaining the effective V-number and the effective mode area respectively.

Solid-core square-lattice photonic crystal fibers: comparative studies of the single-mode regime and numerical aperture for circular and square air-holes

In this paper we have studied solid-core square-lattice PCFs with circular and square shaped air-holes. We have estimated the V-parameter by using the step-index fiber approximation (SIFA) and calculated numerical apertures for the structures considered. In order to estimate the effective core radius of the structure with the square shaped air-holes we have used an approximation based on the air-filling fraction.

Design of new square-lattice photonic crystal fibers for optical communication applications

In this paper, two new square-lattice photonic crystal fibers (PCFs) are designed and proposed. Both fibers have identical structures with five air-hole rings, yet the diameter of air-holes differ in their structures. Finite-difference time-domain (FDTD) method with the perfectly matched layers (PML) boundary conditions has been used to investigate these fibers. The proposed photonic crystal fibers exhibit properties of ultra-flattened nearly zero dispersion and low confinement loss in wavelength range of 1.2 to 1.7 µm. Dispersion slope in both fibers is about 7 × 10-3 ps/(nm2.km). Since dispersion and confinement loss of the first photonic crystal fiber (PCF1) at the wavelength 1.55 µm are 0.00249 ps/(nm.km) and about 4 × 10-6 dB/km, respectively, its application as transmission medium in optical communications will be highly favored. Although, the zero dispersion wavelength of the second PCF (PCF2) is 1.45 µm, the confinement loss of this fiber is lower than PCF1 and below 10-7 dB/km in all of the studied wavelength range. Key words: Square lattice, photonic crystal fiber, dispersion, confinement loss.