The study of chromatic dispersion and chromatic dispersion slope of WI- and WII-type triple-clad single-mode fibers

In this paper, the chromatic dispersion and chromatic dispersion slope of two kinds of triple-clad single-mode fibers with a depressed index inner cladding named WI- and WII-type were examined. A feasible approach to calculate chromatic dispersion and higher-order dispersion was established successfully, and the influences made by the optical parameters and geometric parameters on the chromatic dispersion coefficient and its slope were analyzed in detail. The calculated results show that the optical parameter R₂, which symbolizes the third cladding effect, has a strong impact on the chromatic dispersion coefficient and the chromatic dispersion slope, and the degrees of such impact are closely related to the other parameters.     

Field-trial evaluation of the Q-factor penalty introduced by fiber four-wave mixing wavelength converters

The performance of tunable all-optical wavelength converters based on four-wave mixing in optical fibers is experimentally tested in a field-trial network. Two converters were built with two different fibers. The first one was made with a small variation in the zero-dispersion wavelength (ZDW) dispersion shifted fiber and the second one with a highly nonlinear fiber that presents great ZDW variations. In order to compare the tuning ranges obtained in both cases we present an experimental spectral analysis. Numerical simulations that consider the influence of both the dispersion slope and the long-scale ZDW variations of the fiber complement the experiments. The tuning bandwidth was larger in the highly nonlinear fiber case. For a set of different optical signal-to-noise ratios, the measurements of the Q-factor of the signal and those of the converted wave are our main results. These results show that the penalty imposed by the converters is different for each converted wavelength. The maximum penalty obtained for the Q-factor was ∼6 dB, but it was ?3 dB for most cases. In all experiments we used a technique based on a dynamic polarization controller in order to avoid power fluctuations in the converted wave caused by polarization induced variations in the signal.     

Dispersion characteristic of hexagonal and square lattice chalcogenide As2Se3 glass photonic crystal fiber

In this paper, we report a chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) for dispersion compensating application. We have used the improved fully vectorial effective index method (IFVEIM) for comparing the dispersion properties (negative and zero dispersion) and effective area in hexagonal and square lattice of As₂Se₃ glass PCF using different wavelength windows. It has been demonstrated that due to their negative dispersion parameter and negative dispersion slope in wavelength range 1.2-2.5 μm, both lattice structures of As₂Se₃ glass PCFs, with pitch (Λ = 2 μm), can be used as dispersion compensating fibers. Further, design parameters have been obtained to achieve zero dispersion in these fibers. It is also shown that As₂Se₃ glass PCF provides much higher negative dispersion compared to silica PCF of the same structure, in wavelength range 1.25-1.6 μm and hence such PCF have high potential to be used as a dispersion compensating fiber in optical communication systems.     

Cladding-ring-equivalent effective index method for analyzing the propagation characteristics of multi-cladding photonic crystal fibers

Cladding-ring-equivalent effective index method (CREEIM) is proposed to analyze the propagation characteristics in the multi-cladding photonic crystal fibers (MCPCFs). The effective indices and dispersions of the step index two and three-cladding structures are calculated with the Sellmeier formula. The accuracy of CREEIM is competitive to the results calculated by the multipole method (MPM). With different structure parameters, the differences between two methods are evaluated with the relative error R_E of effective index and relative difference R_D of dispersion. This method is also used to analyze the mode cut-off and nonlinear characteristics in the three-cladding structure PCFs by calculating the mode effective index n_eff instead of complex mode field distribution.     

Study of bending loss and mode field diameter in depressed inner core triple-clad single-mode optical fibers

In this paper, the bending loss and the mode field diameter (MFD) of the R-type depressed inner core triple clad single-mode optical fibers are investigated. The effects of the optical and geometrical parameters on the bending loss and the MFD are examined in these fibers. The simulation results indicate that with increasing of the core radius (a), which is desired from manufacturing point of view, the bending loss and MFD coefficients are decreased. Consequently, the large core radius can be used to optimize the bending loss in the foregoing fibers. In the meantime, simulation outcomes show that the Δ and Q have considerable impact on the bending loss in the RI and RII fibers, respectively. The MFD and bending loss is decreased with increasing of Δ, but the case is inversed for Q. Based on the presented simulations, it is found out that the bending loss strongly depends on the distribution profile of the electric field in the cladding region for a given MFD. In other words, the field amplitude and damping rate in the cladding region determine the fiber bending loss.     

Cerium (III) doping effects on optical and thermal properties of PVA films

Cerium chloride (CeCl₃) doped polyvinyl alcohol (PVA) films were prepared by casting technique. The effect of CeCl₃ concentrations on the structural, optical and thermal properties of the PVA films was studied by X-ray diffraction (XRD), FT-IR, UV-visible, transmittance (T), reflectance (R), differential scanning calorimetry (DSC) and thermogravimetry (TG). Both of the XRD and the DSC results affirm the increase in amorphousity. Absorption spectra of the doped films have shown an absorption band at 260 nm assigned to the trivalent state of cerium ions. Absorption, transmittance and reflectance spectra were used for the determination of the optical constants. The results indicate that the optical band gap (E_g) was derived from Tauc's extrapolation and decreases with the cerium content. The refractive index increases with monotonic behavior as the cerium content increases. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomernico model for obtaining the dispersion parameters. The obtained optical parameters were found to be strongly affected by CeCl₃ dopant. Thermal analysis showed that the thermal parameters of PVA are enhanced by CeCl₃. The dependence of the activation energy of the decomposition temperature on doping level was estimated.     

Investigations on PMD-induced penalties in 40 Gbps optical transmission link

Polarization-mode dispersion (PMD) in optical fibers and components can be a limiting impairment in high-speed, long-haul transmission link. In this paper, we have carried out investigations on PMD-induced penalties in a 40 Gbps optical transmission link and we obtained results for DGD, BER, Q² (dB) for different values of the PMD coefficient and different random seeds using computer simulation.     

Impurity-ligand distances derived from magnetic resonance and optical parameters

Non empirical methods for obtaining information from EPR, ENDOR and optical data on the true impurity-ligand distance,R, as well as on the true variations, ΔR, induced by chemical and hydrostatic pressures, phase transitions in the host material and temperature changes are discussed through this work. Special attention is addressed to spectroscopic parameters of d-impurities whose dependence onR can reasonably be calculated theoretically for the superhyperfine (shf) tensor or the lowest optical transitions but not for fine structure terms. In the case of impurities with unpaired σ-electrons it is shown that the isotropic shf constant,A _s, is specially sensitive toR changes. The microscopic origin of this fact is discussed in detail. Determination of trueR values from experimentalA _s has been carried out for impurities like Mn²⁺ or Ni⁺ with encouraging results. In the case of Mn²⁺ in fluorides, results obtained by this method coincide with those reached through EXAFS and the analysis of the experimental 10 Dq. Finally, for some selected complexes, a view is offered on the dependence of several EPR and optical parameters upon metal-ligand distances. The main conclusion is that ΔR values of the order of 0.1 pm can be detected using EPR and optical parameters thus improving by an order of magnitude the sensitivity reached through EXAFS. In particular the use of ENDOR allows one to measure ΔR values close to 0.01 pm.     

On the renormalization of the quantum field theory of point-like monopoles and charges

The quantum field theory of point-like monopoles and charges is first formulated on a euclidean lattice for a convenient regularization. The regularization preserves the peculiar features of the theory, namely those related to the invariance and to the quantization condition. The partition function is expressed as a path integral over the particle's closed paths and the action is constructed in terms of arbitrary surfaces having those paths as boundaries. The possible divergences of the continuum limit are discussed, in particular the vacuum polarization ones. It is found that, although both the electric charge Q and the magnetic charge G are renormalized as Q = Z_QQ_R and G = Z_GG_R, the quantization condition is preserved by the renormalization i.e. Z_QZ_G = 1 so that QG = Q_RG_R = 2πn. Due to the dual symmetry of the theory, then, for Q = G we get Z_Q = Z_G = 1.     

Investigations of the spin-Hamiltonian parameters,optical absorption bands,and local structure for the tetragonal Cu2+ center in Cu2+-doped ZnCdO nanopowder

The spin-Hamiltonian (or EPR) parameters of tetragonal Cu²⁺ octahedral centers in ZnCdO nanopowders are calculated from the high-order perturbation formulas based on the cluster approach. In these formulas, the contributions to spin-Hamiltonian parameters due to the admixture between the d orbitals of dⁿ ion and the p orbitals of ligand ions via covalence effect are considered. The crystal field parameters are calculated from the superposition model and so the optical absorption bands (related to the crystal field energy levels) and local structure of Cu²⁺ octahedral centers in ZnCdO nanopowder are also studied. The calculated spin-Hamiltonian parameters and optical absorption bands are in reasonable agreement with the experimental values. The tetragonal elongation ΔR (=R_//−R_⊥) of Cu²⁺ octahedron in ZnCdO nanopowder due to Jahn–Teller effect is acquired from the calculations. The results are discussed.     

Structural, morphological and optical properties of CuAlS2 films deposited by spray pyrolysis method

The structural, morphological and optical properties of CuAlS₂ films deposited by spray pyrolysis method have been investigated. CuAlS₂ in the form of films is prepared at different deposition conditions by a simple and economical spray pyrolysis method. The structural, surface morphology and optical properties of the films were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and absorbance spectra, respectively. The films were polycrystalline, crystallized in a tetragonal structure, and are preferentially orientated along the (1 1 2) direction. Grain size values, dislocation density, and d% error of CuAlS₂ films were calculated. The optical band gap of the CuAlS₂ film was found to be 3.45 eV. The optical constants such as refractive index, extinction coefficient and dielectric constants of the CuAlS₂ film were determined. The refractive index dispersion curve of the film obeys the single oscillator model. Optical dispersion parameters E_o and E_d developed by Wemple-DiDomenico were calculated and found to be 3.562 and 12.590 eV.     

Structure and optical analysis of Ta2O5 deposited on infrasil substrate

Electron beam gun technique was used to prepare Ta₂O₅ thin films onto infrasil substrates of thicknesses 333 and 666 nm. The structure characterization was investigated using X-ray diffraction patterns. Transmittance measurements in the wavelength range (240-2000 nm) were used to calculate the refractive index n and the absorption index k depending on Swanepole's method. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transparent region. The analysis of the optical absorption data revealed that the optical band gap E_g was indirect transition. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple-DiDomenico model, from which the dispersion parameters (E_o and E_d) and the high frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration to the effective mass ratio were estimated according to the model of Spitzer and Fan. Graphical representation of the relaxation time as a function of photon energy was also presented.     

Dispersion compensation in optical transmission systems using high negative dispersion chalcogenide/silica hybrid microstructured optical fiber

In this paper, a new hybrid microstructured optical fiber (H-MOF) based upon photonic bandgap (PBG) light guiding mechanism which can be used for dispersion compensation in optical transmission systems is designed and simulated. The H-MOF core is made up of silica glass and the holes in the cladding network are filled with As₂Se₃ chalcogenide glass. By selecting an appropriate geometrical parameters for the structure, the dispersion and confinement losses of the proposed H-MOF at 1.55 µm are calculated to be ?6700 ps/nm/km and 6?×?10^?4 dB/m, respectively. Relative dispersion slope (RDS) of the H-MOF at 1.55 µm is about 0.00347 nm^?1. The proposed H-MOF is suitable for use in wavelength division multiplexing and dispersion compensating systems in optical fiber transmission networks.     

Structural and optical studies of thermally evaporated CoPc thin films

The structure of the thermally evaporated cobalt phthalocyanine (CoPc) thin film in the β-form is investigated, and shows a single strong peak indicating preferential orientation in the (1 0 0) direction. Some structural parameters such as crystallite grain size, dislocation density and the number of crystallites per unit surface area are determined.The spectral parameters are determined by applying the electronic orbital transitions.But the optical parameters are deduced using band-model consideration for thin films of Pc.The spectral and optical parameters have also been investigated by using the spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm.The absorption spectra recorded in the UV–VIS region show two absorption bands of phthalocyanine (Pc) molecule, namely the Soret band (B) and the Q-band. The Q-band shows its characteristic splitting (Davydov splitting) with ΔQ=0.23 eV.Some of the important spectral parameters, namely optical absorption coefficient (α), molar extinction coefficient (_molar), oscillator strength (f), electric dipole strength (q²) and absorption half bandwidth (Δλ) of the principle optical transitions have been evaluated.The fundamental and the onset indirect energy gaps could be estimated as 2.90 + or − 0.05 and 1.51 eV, respectively.The refractive index showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. From analysis of dispersion curves, the dielectric constants, the dispersion parameters and the molar polarizability were obtained.All the above parameters were obtained for films as deposited and as annealed. No remarkable annealing effect on many parameters was observed.     

Optical properties of <Emphasis Type="Italic">R</Emphasis>Ni<Subscript>5</Subscript> intermetallic compounds (<Emphasis Type="Italic">R</Emphasis> = Y,La, Ce)

The paper reports on an ellipsometric study of the optical properties of RNi₅ (R = Y, La, Ce) paramagnetic intermetallic compounds. The dispersion measurements of the refractive index n and of the absorption coefficient k were conducted within a spectral interval 0.083–5.64 eV at room temperature. The behavior of optical interband conductivity with energy is analyzed. The results obtained are discussed in the frame of available information on the electronic band structure of these compounds. The plasma and relaxation frequencies of conduction electrons have been determined.     

Structure functions in nuclei: Quark clusters and size effects

Two of the most interesting ideas to explain the EMC effect, clusters of quarks and confinement size factors, are unable to fully explain the existing data. In this paper we argue that the existence of clusters with different numbers of quarks are in the origin of different size parameters controlling the lowQ ² limit in the QCD evolution and thus we combine the two ideas. As a result we obtain a model for structure functions with two basic effective parameters: the average numberi of nucleons per cluster and the corresponding size parameterR _i . We achieve good agreement with the existing data for a wide range ofQ ²(?2 GeV²) and for all values ofx. Simple parametrizations of the Sea, Gluon and Valence distributions, for pion, nucleon and nuclei, are given. Some applications of the model, in particular to Drell-Yan processes, are discussed.     

Optical properties of excitons in semiconductor superlattices and microcavities

The optical response of Mott–Wannier excitons is investigated in semiconductor superlattices and microcavities. p-Polarized light is considered to calculate the reflectivity R_pand dispersion relation of the collective normal modes in superlattices accounting for extrinsic Morse potential wells, andR_p in microcavities. Results of R_pexhibit well-defined peaks of the exciton bound states in the Morse potentials for both transverse and longitudinal modes. Comparisons ofR_p with experimental reflectivity data of light for semiconductor microcavities exhibit good qualitative agreement as well as Rabi splitting.     

三种典型结构光子晶体光纤基本特性的比较和分析

应用多极法比较和分析了相同结构参数下的正六边形、正八边形和正十边形光子晶体光纤的色散系数、色散斜率、非线性系数和限制损耗.正六边形光子晶体光纤更适合用于色散补偿和高非线性的研究,在波长0.8 μm处的非线性系数达到了0.37 m^-1·W^-1;正十边形光子晶体光纤更适合用于色散平坦和低限制损耗的研究,在波长0.8 μm处的限制损耗相对正六边形光子晶体光纤减小了约3000个数量级,在1.4—1.65 μm波长范围内,正十边形光纤的色散系数介于-0.07—0.17 p     

Determination of magnetic anisotropy of complex rare-earth compounds from Mössbauer and NMR spectra

Experimental data and theoretical papers on the magnetic anisotropy (MA) of rare-earth-transition metal intermetallic compounds are reviewed. Discrepancies between the experimental data obtained by different authors, as well as between these data and the theoretical calculations of the MA constants, are indicated. A technique is proposed for determining the crystal-field parameters and the effective charges Q _i ^* of ions in intermetallic compounds. Using experimental Mössbauer and NMR spectroscopic data, possible values of Q _i ^* are determined for R ₂T_17?x and R ₂T_17?x Ti_x compounds, which allow one to find the MA constants of these systems with different R and T in a unified way. The problem of the sign of the contribution from the rare-earth metal sublattice to the MA is discussed. The heavy x dependence of this contribution in the R ₂T_17?x Ti_x system is explained to be due to the contribution to the crystal field from Ti ions in the dumbbells.     

One step towards the fabrication of a nanoscale Si-nc based laser cavity

In this paper, we report on the design of two major components of a laser architecture using Si-nc embedded in SiO₂ as the optical gain medium and sub-wavelength periodic structures to form the resonant cavity. Dimensions of the structures have been matched to near-infrared wavelengths (∼850 nm) of the maximum photoluminescent emission where optical gain has been observed from Si-nc. Both the front (FM) and rear (RM) mirrors have been fabricated by the implantation of Si ions (50 keV, 2×10¹⁷ Si⁺/cm²) through a mask, in order to produce a Bragg reflector by optical index contrast between the implanted and the non-implanted zones. Two closely spaced Bragg reflectors are used in the FM structure to allow a narrow bandpass (partial transmission) centered at 850 nm. The implanted structures have been annealed to produce Si-nc and passivation. Scanning electron microscopy (SEM) images show that the design dimensions of the structure have been obtained. Characterization of the structures by laser excitation reveals an optical gap in both mirrors between 825 and 870 nm, as per the design parameters. A quality factor Q∼95 and a reflectivity R∼0.2 have been measured for the FM. These results support the concept that a complete Si-nc based laser cavity can be built to emit coherent light.