Air Holes In Cladding Research Articles

Porous-Core Photonic Crystal Fiber for Low Loss Terahertz Wave Guiding

We report a novel porous-core octagonal photonic crystal fiber (POPCF) for practical low-loss terahertz (THz) wave guiding. The POPCF with a porous core surrounded by an air-hole cladding shows a low material absorption loss of ~0.07 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , or one third of that for the bulk material absorption loss at the operating frequency ~1 THz. In addition, the confinement loss, bending loss, and effective modal area properties of the POPCF are also reported and demonstrated to be relatively low. The proposed POPCF has potential applications for efficient transmission of broadband THz radiation.

Long period fiber gratings written in photonic crystal fibers by use of CO2 laser

Photonic crystal fibers are usually divided into two different types of fibers: solid-core photonic crystal fibers (PCFs) and air-core photonic bandgaps fibers (PBFs). We presented the fabrication methods and applications of long period fiber gratings (LPFGs) written in these two types of photonic crystal fibers by use of a CO2 laser. A stain sensor with a high sensitivity was demonstrated by use of an LPFG written in solid-core PCFs. An in-fiber polarizer based on an LPFG was fabricated by use of a focused CO2 laser beam to notch periodically on a PCF. A novel LPFG was written in an air-core PBF by use of a CO2 laser to collapse periodically air holes in the fiber cladding.

Acoustic-Birefringence-Induced Orthogonal Acousto-Optic Gratings in Grapefruit Microstructured Fibers

The acousto-optic mode coupling in grapefruit microstructured optical fibers (GMOFs) has been investigated. Due to the presence of grape-like large air holes in the fiber cladding, unequal acoustic modulation is generated, and orthogonal acoustic gratings come into being in the GMOF, which is verified by our experimental observation on the mode profile of resonance peaks. The experimental results indicate that two resonance peaks originating from the mode coupling in acoustic-birefringence-induced orthogonal acoustic gratings simultaneously exist in the transmission spectrum, and the wavelength spacing between them reaches as large as 227.4 nm, which indicates that a greatly enhanced acoustic birefringence has been introduced in the GMOF. And moreover, some important grating features, including frequency-dependent grating period, frequency responses of acoustic phase velocity, and acoustic dispersion, have been investigated for the orthogonal acoustic gratings. These studies would be of importance for understanding the acoustic birefringence mechanism in MOF-based acoustic gratings and development of related acousto-optic devices.

Acoustic birefringence suppression in a fiber acoustic grating employing solid-core photonic crystal fiber with hexagonal air-hole array cladding

An acoustic birefringence suppression scheme for acoustic fiber gratings has been proposed and experimentally demonstrated. Experimental observation of the transmission characteristics of the single-mode-fiber-based (SMF-based) acoustic grating confirms that introduction of one segment of solid-core photonic crystal fiber (PCF) with air-hole array in the cladding region helps to eradicate resonance peak splitting induced by acoustic birefringence, which has been explained by our theoretical analysis from the acoustic wave modulation method and fiber cross-section structure. Moreover, experimental results indicate that the introduction of the solid-core PCF does not have a considerable impact on the frequency dependence of resonance wavelength for the SMF-based acoustic fiber grating, which ensures the performances of the PCF-assisted SMF-based acousto-optic tunable filter for practical applications.

Surface Plasmon Induced Polarization Splitting Based on Dual-Core Photonic Crystal Fiber with Metal Wire

The polarization characteristics of a dual-core photonic crystal fiber (PCF) with a metal wire filled into the cladding air hole between the two cores have been investigated. Numerical investigation shows that the inclusion of the metal wire greatly changes the coupling characteristics of the modes in the two cores. In fact, the coupling lengths of the two polarizations show increased difference, which leads to the possibility of designing a dual-core PCF with a coupling length ratio of 1:2 for the two polarization states. The proposed polarization splitter shows extinction ratios as low as −20 dB with bandwidths as great as 146 nm.

Highly Birefringent Extruded Elliptical-Hole Photonic Crystal Fiber

Highly birefringent extruded elliptical-hole photonic crystal fibers (PCFs) with single defect and double defect are proposed and investigated, which are supposed to be achieved by extruding conventional triangular-lattice circular-hole PCFs. Comparative research on the birefringence and the confinement loss of the proposed PCFs with single defect and double defect is presented. Simulated results show that the proposed extruded elliptical-hole PCFs with single defect and double defect can be with high birefringence up to the order of 10-2. Confinement loss increases when the ellipticity of the air hole of the proposed PCFs increases, which nevertheless can be overcome by increasing the ring number or the area of the air holes in the fiber cladding.

S+C+L波段色散斜率补偿光子准晶体光纤

针对光纤通信中的色散补偿,尤其是密集波分复用系统的多信道同时补偿的需求,提出了一种用于宽带色散斜率补偿的光子准晶体光纤.该光纤结构包层空气孔为准晶体排列,并且在纤芯引入了中心缺陷.通过对光纤特性的数值分析表明,在1 460-1 625 nm的光通信波段,该结构光纤的相对色散斜率为0.004 4-0.002 9 nm^-1,相对色散斜率与标准单模光纤近似相等,且负色散值达-2 476 ps·nm^-1·km^-1.对标准单模光纤进行色散补偿后,在S＋C＋L波段色散值为（-0.5-0）ps·nm^-1·km^-1.该光纤可用于对当前光纤通信系统进行宽带色散补偿,实现多信道同时补偿,简化结构,并降低补偿成本.

Generation of visible and infrared broadband dispersive waves in photonic crystal fiber cladding

The optical properties of photonic crystal fiber cladding knot among the three air holes are analyzed. The mode area, nonlinear coefficient and dispersion characteristics of the core and cladding knot are contrasted. Cladding knot of photonic crystal fiber has a small core and highly nonlinear characteristics. For larger cladding air holes, double zero dispersion curves are obtained. According to the dispersion curve, phase-matching features are analyzed for dispersive wave generation. Variation rules of the central wavelength of the dispersive wave with pump power and wavelength are achieved. The photonic crystal fiber designed is fabricated. The visible and infrared broadband dispersive waves above 300 nm are obtained in experiment. Experimental and theoretical results are completely consistent with each other. These are foundation for wavelength conversion and supercontinuum broadband light source.

Wavelength-selective characteristics of high birefringence photonic crystal fiber with Au nanowires selectively filled in the cladding air holes

Filter characteristics of a designed gold-filled high birefringence photonic crystal fiber are investigated based on the finite element method. The wavelength filter resonances in the high birefringence photonic crystal fiber occur at different points for different polarized directions, and the resonance strength in the x-polarized case is much weaker than that in the y-polarized case. The much more obvious splitting filter characteristics and different resonance strength imply the study and application values in splitting and single polarization fiber devices. The simulation results show that increasing the number of the gold wires only enhances the resonance strength when there is no surface plasmon supermode formed. With the diameters of the gold wires increasing, the response wavelength moves to a longer wavelength, and the strength becomes stronger. When the diameter is increased to 1.4 μm, the response wavelength in the x-polarized case can be tuned to 1.318 μm, which is the communication wavelength. The strongest resonance occurs at 1.2375 μm in the y-polarized case, and the peaking loss can reach 435.83 dB/cm.

Design of Highly Birefringent and Low Confinement Loss Photonic Crystal Fibre by Introducing Asymmetric Defect Structures

Photonic crystal fibres –micro structured fibre consisting of air hole arrays running along its lengthhave attracted much attention for fibre device application because of its unusual optical properties that are not realized in standard optical fibre, such as high birefringence, high nonlinearity, low confinement loss and tailorable chromatic dispersion. High birefringent PCF can be designed by breaking the circular symmetry and implementing asymmetric defect structures such as dissimilar air hole diameter, varying the number of circular and elliptical air holes. This paper proposes a highly birefringent PCF with ultra-low confinement loss by introducing four ring solid core hexagonal structure which having both elliptical and circular air holes and introducing large air hole diameters near the core region for making the asymmetry. The modal birefringence, refractive indices, confinement loss and chromatic dispersion are calculated by using Finite element method (FEM). An endlessly single mode, high birefringent (05.152x10) and a low confinement loss (7.85x10 dB/km) found at the excitation wavelength of λ=1550nm with only four rings of air holes in the fiber cladding.

Characteristics Analysis of Photonic Crystal Fiber with Hexagonal and Octagonal Structure

Aiming at the requirements of high performance of optical communication systems, a new type of photonic crystal fiber (PCF) is proposed. This kind of PCF consists of hexagonal air holes in the inner cladding and octagonal air holes in the others layers. Two big elliptical air holes are embedded to enhance the birefringence. Numeral results show that this kind of PCF exhibits high birefringence with the level of 10-3 and high nonlinearity with the level of 10-2m-1w-1. In addition, two zero dispersion points are obtained after optimization, which will find application in dispersion compensation PCFs.

Numerical Investigations on Birefringent Holey Fibers by Modified Elliptical Air Holes in Fiber Cladding

In this study, we numerically propose a high-birefringence and low-loss index-guiding photonic crystal fiber (PCF) and compare it with three other types of PCF. Four cases of PCFs are investigated by the finite element method. A high modal birefringence is obtained when the cladding is composed of elliptical air holes placed asymmetrically, with two half-size elliptical air holes on the top and bottom of the PCF core. The size control of these air holes is the key to reaching a high birefringence and a low confinement loss. Numerical results confirm that the proposed structure at an excitation wavelength λ= 1550 nm shows a birefringence of up to 2.085×10-2 and a confinement loss of less than 1.63×10-5 dB/km.

Numerical Investigations on Birefringent Holey Fibers by Modified Elliptical Air Holes in Fiber Cladding

In this study, we numerically propose a high-birefringence and low-loss index-guiding photonic crystal fiber (PCF) and compare it with three other types of PCF. Four cases of PCFs are investigated by the finite element method. A high modal birefringence is obtained when the cladding is composed of elliptical air holes placed asymmetrically, with two half-size elliptical air holes on the top and bottom of the PCF core. The size control of these air holes is the key to reaching a high birefringence and a low confinement loss. Numerical results confirm that the proposed structure at an excitation wavelength λ= 1550 nm shows a birefringence of up to 2.085×10-2 and a confinement loss of less than 1.63×10-5 dB/km.

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

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.

热激法PCFG制备工艺中热传导特性研究

The fiber gratings fabrication technology with the heating method in a photonic crystal fiber (PCF) based on structural change is examined. The principle of photonic crystal fiber gratings (PCFGs) is analyzed in theory. The heat transfer theory and finite element method are used to examine the thermal field distribution in the fiber and the influence of the air hole structure in the cladding, and the parameters of the laser beam in the process of grating fabrication are discussed. The results show that gratings can be formed by the periodic collapse of air holes in the cladding of PCFs. Under double-point heating condition, the energy is uniformly distributed in the radial direction and is approximate to Gaussian distribution in the axial direction. With the same size of the luminous spot, as the layers and radius of the air holes increase, the laser power needed to make the air holes collapse decreases. With the same laser power, as the luminous spot radius increases, the needed heating time increases. Moreover, the relationship between the laser power needed and the air filling rate is obtained as the number of layers of the air holes changes from 1 to 7. This kind of PCFG can overcome the long-term thermal instability of conventional gratings in substance and thus has great potential applications in the related field of optical fiber sensors.

Investigation of V-type photonic crystal fiber with high birefringence

According to the full-vector finite element method, a new kind of V-shape photonic crystal fiber with high birefringence is proposed. Simulation results show that the V-shape photonic crystal fiber has a birefringence of 1.239×10-2 at a wavelength of 1.55 μm, when its cladding air hole pitch, large air hole diameter, small air hole diameter are 1.0 μm, 0.95 μm and 0.6 μm, respectively. There are two zero dispersion wavelengths in the visible and the near-infrared region respectively, and the abnormal dispersion zone includes the operation wavelength of Ti:sapphire laser, which is advantageous for nonlinear generation of photonic crystal fibers. The difference in air filling fraction between fast and slow axis can be used as a sensor for the weak strain.

Guidance characteristics correlated to the core refractive index in a hybrid photonic crystal fiber

The properties of light guiding mechanism and modes relating to the core refractive index in a hybrid photonic crystal fiber (PCF) are numerically investigated by the full-vectorial finite element method. The light guiding mechanism of core mode is the double-bandgap-guiding when the core refractive index is smaller than the effective index of the air-hole cladding in the hybrid-PCF. If the core refractive index is between the refractive indexes of the two kinds of hybrid-claddings, there is a co-operative action of the index-guiding and the bandgap-guiding on the guide mode in the core. Only the total internal reflection guiding occurs in the fiber when the core refractive index is greater than the effective index of the high-index rod cladding. Comparison of these guiding properties in the hybrid-PCF are also carried out. The results provide guidance for the design of hybrid photonic crystal fiber for special purposes.

Flat band slow light in asymmetric photonic crystal waveguide based on microfluidic infiltration

In this paper, an asymmetric photonic crystal waveguide is proposed for flat band slow light transmission. Two structural parameters of one cladding layer of the waveguide are carefully adjusted. They are the refractive index of the liquid infiltrated in the cladding on one side and the radius of the air holes in the infiltrated cladding. In such an asymmetric waveguide, we can achieve a very flat band corresponding to high group index such as 29.4 and low dispersion (10(4)ps(2)/km) for Deltaomega/omega=1.46%. It is found that the value of n(g)Deltaomega/omega of the proposed waveguide can reach 0.429 and this waveguide can also yield a significant increase in the group index-bandwidth product n(g)Deltaomega/omega and improve the bandwidth.

A COMPARATIVE STUDY OF HIGH BIREFRINGENCE AND LOW CONFINEMENT LOSS PHOTONIC CRYSTAL FIBER EMPLOYING ELLIPTICAL AIR HOLES IN FIBER CLADDING WITH TETRAGONAL LATTICE

We numerically compare the mode birefringence and conflnement loss with four patterns (case A{D) of index-guiding photonic crystal flbers (PCF) using the flnite element method. These PCFs are composed of a solid silica core surrounded by difierent sizes of elliptical air holes and a cladding which consist of the same elliptical air holes in flber cladding with tetragonal lattice. The maximal modal birefringence and lowest conflnement loss of our proposed case A structure at the excitation wavelength of ‚ = 1550nm can be achieved at a magnitude of 5:3 £ 10 i2 (which is the highest value to our knowledge) and less than 0.051dB/km (an acceptable value less than 0.1dB/km) with only four rings of air holes in flber cladding, respectively. The merit of our designed PCFs is that the birefringence and conflnement loss can be easily controlled by turning the pitch (hole to hole spacing) of elliptical air holes in PCF cladding.

Characteristics of nano-structured photonic crystal fibers with high birefringence

A kind of high birefringence and highly nonlinear nano-structured photonic crystal fibers （N-PCFs） is proposed. The birefringence, fundamental mode field, dispersion and nonlinear characteristics of the fiber have been studied by electromagnetic scattering theory of multipole. It is found that the birefringence becomes larger by increasing the dissymmetry level of fiber core and air holes in cladding, and the largest birefringence reaches up to 1.918×10-2. With this type of high birefringence and highly nonlinear N-PCFs, the peak of birefringence can be moved to the required wavelength by adjusting the pitch value. The non-linear coefficient of N-PCF with pitch 400 nm reaches up to 0.2 m-1·W-1 in the wavelength of 800 nm.Meanwhile, the birefringence value is also very large in the wavelength range of 750—1000 nm. A perfect combination of high birefringence and high nonlinearity is realized in N-PCF. It is useful for manufacturing new type of nano-photonic apparatus and development of integrated optics.