Design of Broadband Single Fundamental Mode Hollow Core Bragg Fibre

The condition of the single fundamental mode （HE11） transmission in hollow core Bragg fibres is investigated theoretically by the transfer matrix method. The influences of core size and cladding parameters on the single HE11 mode bandwidth are analysed, showing that the maximal bandwidth is more sensitive to the core size than the cladding. The numerical results show that sufficiently broad bandwidth of single HE11 mode transmission can be achieved by proper fibre design. A simple and fast method based on improved hollow metal waveguide model is proposed to optimize fibre structure parameters for the maximal single HE11 mode bandwidth.     

Design of a TE10-TE30 Rectangular Mode Converter for 4.6GHz LHCD Launcher in the Experimental Advanced Superconducting Tokamak

A compact rectangular TE10-TE30 mode converter is developed for the lower hybrid current drive （LHCD） launcher on the Experimental Advanced Superconducting Tokamak （EAST） at 4.6 GHz. The converter with periodic width perturbation aims to divide the microwave power into three sub-waveguides in the poloidal direction. We present the design and numerical calculation of the mode converter. Calculations are performed on the ripple wall converter by codes based on numerical solving the coupled-mode differential equations and on the simulation of the High Frequency Structure Simulator （HFSS） package. The resulting conversion efficiency from TE10 mode to TE30 mode exceeds 95% within the bandwidth from 4.56 GHz to 4.64 GHz, and the return loss of the oversized transducer can be considerably decreased to 0.068% by means of a capacitive button embedded in the E-plane of the waveguide.     

Phenomenological Scaling of Rapidity Dependence for Anisotropic Flows in 25 MeV/nucleon Ca＋Ca by Quantum Molecular Dynamics Model

Anisotropic flows （v1, v2, v3 and v4） of light fragments up to the mass number 4 as a function of rapidity are studied for 25 MeV/nucleon ^40Ca ＋ ^40Ca at large impact parameters by a quantum molecular dynamics model. A phenomenological scaling behaviour of rapidity dependent flow parameters vn （n = 1, 2, 3 and 4） is found as a function of mass number plus a constant term, which may arise from the interplay of collective and random motions. In addition, v4/v2^2 keeps to be almost independent of rapidity and remains a rough constant of 1/2 for all light fragments.     

Broadband nearly-zero ultra-flattened dispersion single mode index guiding holey fiber

This paper proposes and demonstrates a novel type of silica index guiding holey fibers (IGHFs) that has two cladding layers at the defective innermost structures. The proposed IGHFs exhibit remarkable chromatic dispersion properties such as nearly-zero and flattened dispersion over a wide spectral range and single mode guidance along with very low confinement loss. The numerical results indicate that 5 air-hole rings of nearly zero ultra-flattened dispersion single mode IGHFs can be designed with desire flattened dispersion of over a 340 nm bandwidth including the entire band of interest with low confinement loss of less than 10⁻⁶ dB/m.     

Design and Fabrication of Chirped Mirror

Chirped mirrors （CMs） are designed and manufactured. The optimized CM provides a group delay dispersion （ODD） of around -60fs^2 and average reflectivity of 99.4% with bandwidth 200 nm at a central wavelength of 800nm. The CM structure consists of 52 layers of alternating high refractive index Ta2O5 and low refractive index SiO2. Measurement results show that the control of CM manufacturing accuracy can meet our requirement through time control with ion beam sputtering. Because the ODD of CMs is highly sensitive to small discrepancies between the layer thickness of calculated design and those of the manufactured mirror, we analyze the error sources which result in thickness errors and refractive index inhomogeneities in film manufacture.     

Nonlinear analysis of a multi-cavity gyro-twystron

To preserve high gain, high efficiency and high power merits of gyroklystron, a gyro-twystron is designed using an electron beam with α(v_⊥/v_z) greater than unity. With a multi-cavity section of high gain, the length of the waveguide output section can be made shorter than the threshold length of the absolute instability without losing total system gain. Numerical simulations are carried out to analyze a ka-band gyro- twystron consisting of three TE₁₁₁ mode cavities and an output section of a TE₁₁ mode waveguide. Stability study is performed to ensure the tube without self-excited oscillations. With α=1.5, the 3-dB linear and saturated gain bandwidth in excess of 2 % can be obtained by stagger tuning for an 80 kV, 3 A electron beam with 5 % axial velocity spread. The maximum saturated gain is more than 55 dB at 33 % efficiency. By tapering the magnetic field of the last 2 cm of the interaction region, the efficiency can be increased to 43 % without degrading the bandwidth, which corresponds to an output power of 103 kW.     

耦合角度对平面波导叠加型光栅解波分复用性能的影响

利用玻璃材料的光敏性可在平面波导中制作出具有解波分复用功能的叠加型光栅,根据耦合模理论对这种平面波导叠加型光栅的解波分复用特性进行研究,发现了入射角度通过改变耦合系数会对光栅的反射率和带宽产生影响：当入射角度较大时,单个光栅的反射率虽稍有增大,但带宽变宽;而在入射角度较小时尽管其反射率稍有下降,但带宽却显著变窄.因此在光栅设计时必须综合考虑反射率和带宽的要求以获得更好的解波分复用性能.     

Polarized Micro-Raman Study of the Temperature-Induced Phase Transition In 0.67 Pb（Mg1/3Nb2/3）O3-0.33PbTiO3

Polarized Raman spectra of ferroelectric relaxor 0.67Pb（Mg1/3Nb2/3）O3-0.33PbTiO3 （0.67PMN-0.33PT） single crystal are systematically investigated in a wide temperature range from -196 to 600℃ by micro-Raman scattering technique. The results clearly reveal that there are two structural phase transitions in such composite ferroelectric relaxor： the rhombohedral-tetragonal （R- T） phase transition and the tetragonal-cubic （T- C） phase transition. The former occurs at about TR-T =34℃, corresponding to the vanishing of the soft A1 mode at 106cm^-1 recorded in the parallel polarization. The latter appears at about TT-C = 144℃, which can be verified with the vanishing of mode at 780cm^-1 measured in the crossed polarization.     

Polarization-Independent Guided-Mode Resonance Filters under Oblique Incidence

We present the dispersion relation of guided-mode resonances in planar periodic waveguides, both for s-polarized （TF, mode） and p-polarized （TM mode） incident waves. For a fixed homogeneous planar waveguide, dispersion curves of the TE eigenmode cannot cross that of the TM eigenmode at all. That is to say, at a certain wavelength, TE and TM modes cannot be excited with the same propagation constant. Due to Bragg reflection in the planar periodic waveguide, dispersion curves of the TE leaky mode may intersect with that of the TM leaky mode in the first Brillouin zone. We employ these intersections to achieve polarization-independent guided-mode resonance filters.     

Ultrawide-bandwidth gain in a coaxial dielectric Cherenkov maser

A coaxial waveguide partially filled with a dielectric as the slow-wave structure of a dielectric Cherenkov maser is investigated. The dispersion of the fundamental mode of this waveguide is very weak at phase velocities close to the velocity of light, and for this reason a very wide gain bandwidth is possible under conditions of an interaction with a relativistic electron beam. The dispersion equation for an infinitely thin tubular beam in a coaxial waveguide with a dielectric liner adjoining either the outer or inner conducting surface is derived. The gain bandwidth as a function of the parameters of the electron beam and the slow-wave system are investigated on the basis of numerical solution of the dispersion equation, and a comparison with similar dependences for the conventional configuration of a dielectric Cherenkov maser is made. The structural features of the coaxial configuration which enable novel approaches to the problems of matching the microwave signal at the entrance and exit of the system are discussed. Zh. Tekh. Fiz. 67, 66–72 (May 1997)     

Influence of the group-velocity on the pulse propagation in 1D silicon photonic crystal waveguides

We present a detailed analysis of the influence of the group velocity (GV) on the dynamics of optical pulses upon their propagation in one-dimensional photonic crystal waveguides (PhCW). The theoretical model used in our analysis incorporates the linear optical properties of the PhCW (GV dispersion and optical losses), free-carrier (FC) effects (FC dispersion and FC-induced optical losses) and nonlinear optical effects (Kerr nonlinearity and two-photon absorption). Our analysis shows that, unlike the case of uniform waveguides, the GV of the pulse, dispersion coefficients, and the waveguide nonlinear coefficient are periodic functions with respect to the propagation distance. We also demonstrate that linear and nonlinear effects depend on the group velocity, v _g , as v_g^-1v_{g}^{-1} and v_g^-2v_{g}^{-2}, respectively.     

Realization of MMI Power Splitter by UV-light Imprinting Technique Using Hybrid Sol-Gel SiO2 Materials

An efficient fabrication scheme of buried ridge waveguide devices is demonstrated by UV-light imprinting technique using organic-inorganic hybrid sol-gel Zr-doped SiO2 materials. The refractive indices of a guiding layer and a cladding layer for the buried ridge waveguide structure are 1.537 and 1.492 measured at 1550nm, respectively. The tested results show more circular mode profiles due to existence of the cladding layer. A buried ridge single-mode waveguide operating at 1550 nm has a low propagation loss （0.088 dB/cm） and the 1× 2 MMI power splitter exhibits uniform outputs, with a very iow splitting loss of 0.029 dB at 1549nm.     

1.5 μm Ti:LiNbO3电光波导强度调制器

用钛内扩散技术和常规光刻手段,研制出1.5μmLiN_bO₃电光波导强度调制器。它由单模波导非对称Mach-Zehnder干涉仪和三电极共平面波导组成。干涉仪的两个臂长相差波导波长的四分之一,使器件在最佳线性点工作,无需外加电偏压。集总调制和行波调制的理论带宽分别为1GHz和7.8GHz。对于集总器件,测量了调制特性,半波电压为9.5V,调制带宽为0.7GHz。     

Additivity Rule for Electron-Molecule Total Cross Section Calculations at 50-5000 eV： A Geometrical Approach

To quantify the changes of the geometric shielding effect in a molecule as the incident electron energy varies, we present an empirical fraction, which represents the total cross section （TCS） contributions of shielded atoms in a molecule at different energies. Using this empirical fraction, a new formulation of the additivity rule is proposed. Using this new additivity rule, the TCSs for electron scattering by CO2, C2H2, C6H12 （cyclo-hexane） and CsH16 （cyclo-octane） are calculated in the range 50-5000 e V. Here the atomic cross sections are derived from the experimental TCS results of simple molecules （H2, O2, CO）. The quantitative TCSs are compared with those obtained by experiments and other theories, and good agreement is attained over a wide energy range.     

Design and First-principles Study of a Fullerene Molecular Device

By using open-ended armchair （6, 6） single-wall carbon nanotubes as electrodes, we investigate the electron transport properties of an all-carbon molecular junction based on the C82 molecule. We find the most stable system among different isomers by performing structural optimization calculations of the Cs2 isomers and the C82 extended molecules. The calculated results show that the C82 -C2 （3） isomer and the C82 extended molecule with C82-C2 isomer are most stable. For the all-carbon hybrid system consisting of C82-C2 extended molecules, it is shown that the Landauer conductance can be tuned over several orders of magnitude both by changing the distance between two electrodes and by changing the orientation of the C82 molecule or rotating one of the tubes around the symmetry axis of the system at a fixed distance. Also, we find the most stable distance between two electrodes from the total energy curve. This fact could make this all-carbon molecular system a possible candidate for a nanoelectronic switch. Moreover, we interpret the conductance mechanism for such a molecular device.     

Structural and Thermodynamic Properties of M3W3N （M=Fe,Co, Ni）

Ternary transition metal nitrides, Fe3 W3N, Coa W3N, and Nia WaN~ are studied by the use of interatomic potentials acquired from lattice inversion. The study indicates that Fe3 WaN would be more stable than the other compounds in the family of intermetallic tungsten nitrides. The investigation of phonon density of states indi- cates that the lower frequency modes are mostly excited by the metal atoms, and the higher frequency modes are mostly excited by the nitrogen atoms. A qualitative analysis is carried out with the relevant potentials for the phase stability and vibrational modes.     

Dispersive Properties of tunnelling—induced transparency in an asymmetric double quantum well

We have investigated the dispersive properties of tunnelling-induced transparency in asymmetric double quantum well structures where two excited states are coupled by resonant tunnelling through a thin barrier in a three-level system of electronic subbands. The intersubband transitions exhibit high dispersion at zero absorption, which leads to the slow light velocity in this medium as compared with that in vacuum (c=3×10⁸). The group velocity in a specific GaAs/AlGaAs sample is calculated to be v_g=c/4.30. This structure can be used to compensate for the dispersion and energy loss in fibre optical communications.     

Determination of nonlinear coefficient and group-velocity-dispersion of bismuth-based high nonlinear optical fiber by four-wave-mixing

Nonlinear coefficient and group-velocity-dispersion of bismuth-based nonlinear fibers were determined by four-wave-mixing measurements. The wavelength dependence of refractive index of bulk bismuth-based glasses was also measured to estimate the material dispersion and waveguide dispersion. A newly developed bi-directional four-wave-mixing configuration enabled us to determine all fiber parameters simultaneously. The obtained fiber nonlinearity γ ∼ 1000 W⁻¹ km⁻¹ of bismuth-based nonlinear fiber is the highest one in the step-index fiber made of oxide glasses as expected from a high refractive index at 1550 nm. Dispersion analysis reveals that the both material dispersion and waveguide dispersion affect to the large group-velocity-dispersion of bismuth-based nonlinear fiber.     

High Power Continuous-Wave and Acousto-Optic Q-Switched Nd：GdVO4 Laser Operated at 912 nm

We present a high power and efficient operation of the ^4F3/2 → ^4I9/2 transition in Nd：GdVO4 at 912nm. In the cw mode, the maximum output power of 8.6 W is achieved when the incident pump power is 40.3 W, leading to a slope efficiency of 33.3% and an optical-optical efficiency of 21.3%. To the best of our knowledge, this is the highest cw laser power at 912nm obtained with the conventional Nd：GdVO4 crystal. Pulsed operation of 912nm laser has also been realized by inserting a small aeousto-optie （A-O） Q-Switch inside the resonator. As a result, the minimal pulse width of 20ns and the average laser power 1.43 W at the repetition rate of lOkHz are obtained, corresponding to 7.1 kW peak power. We believe that this is the highest laser peak power at 912nm. Furthermore, duration of 65ns has also been acquired when the repetition rate is 100 kHz.     

GAIN AND BANDWIDTH ANALYSIS OF A VANE-LOADED GYRO-TWT

Theoretical investigation of the peak-gain and 3-dB bandwidth of the vane-loaded gyro-traveling wave tube (gyro-TWT) amplifier in the small-orbit TE₀₁ waveguide mode configuration at 35 GHz has been presented. The vane-loaded gyro-TWT enjoys higher gain and bandwidth compared to that of the smooth-wall device. In the analysis, the azimuthal harmonic effects generated due to the angular periodicity of vanes in the wedge-shaped metal vane-loaded cylindrical waveguide interaction structure have been taken into account in the cold (beam-absent) dispersion relation only.