A comparative analysis of TE and TM guided modes in Tidiffused Er:LiNbO3 waveguide lasers

Abstract

A detailed comparative analysis of TE and TM guided-modes in Z-cut Ti-diffused Er:LiNbO₃ strip waveguide lasers has been performed at laser emission wavelengths 1532, 1563, 1576 nm and pump wavelengths 1480 and 980 nm by the variational method. First, the mode size and corresponding effective refractive index at these five wavelengths as a function of initial Ti-strip width W, diffusion temperatures T, intial Ti-strip thickness H and diffusion time t were calcualted for several lower-order TE and TM modes; their characteristics and the single mode condition were discussed comparatively. Then, the plots of effective pump area and coupling efficiency between the laser and the pump modes under different pump and output mode polarizations were analysed. Finally, the coupling loss of a joint between the waveguide and a fibre in terms of these diffusion parameters were also calculated at these wavelengths. Numerical results are in good agreement with previously published experimental data.     

A study of the flux density distribution in type II superconductors rotating in a magnetic field

Torque measurements on type II superconductors rotating in a magnetic field H ₀ directed perpendicular to the rotational axis can be used to determine pinning and viscous friction of vortices. The exact analysis of these measurements requires a knowledge of the vortex configuration in the rotating specimen, which is studied for the case of a circular cylinder of infinite length. A method is developed to calculate numerically the vortex configuration if the dependence of critical current density J _c and flux flow conductivity _f on flux density B and if the equilibrium magnetization curve are known. An inverse procedure also allows us to determine these material parameters from the measured torque and magnetization of the rotating sample. Results obtained by applying this procedure to different materials are reported.     

Loss Reduction in a Mesh Waveguide Star Coupler

Abstract

A straight mixing waveguide has been introduced into a mesh waveguide star coupler for the purpose of reducing its excess loss. A preferable structure for a 4 × 4 star coupler has been found by a ray-tracing method. Loss reduction by 3·5 dB compared with a previous structure is demonstrated in an ion-diffused waveguide.     

Exergy maximization of cascade refrigeration cycles and its numerical verification for a transcritical CO2–C3H8 system

Analysis of an endoreversible two-stage cascade cycle has been implemented and optimum intermediate temperature for maximum exergy and refrigeration effect have been obtained analytically. Further, the heat reservoir temperatures has been optimised independently. A comprehensive numerical model of a transcritical CO₂–C₃H₈ cascade system was developed with intent to verify the theoretical results. It is seen that the simulation results agree well for optimal T_L but deviate modestly from the theoretical optimum of T_H. It has also been observed that system performance improves as T_H increases and unlike theoretical predictions, no optimal T_H is present within feasible working temperatures.     

Focusing properties of shadow deposited lens‐like waveguide

Abstract

A shadow deposited lens‐like waveguide using rf reactively sputtered Ta₂O₅ films onto Corning 7059 glass substrate has been fabricated. Laser beam can be successfully taper‐coupled into the waveguide from the backplate of the substrate. The focusing properties of the fabricated waveguide were observed from the photographs of multiple exposure of the laser beam ray trajectories. The effective index profiles of the deposited waveguide can be approximated by the sech(gx) function. Calculated results of the distances between focal points from the sech(gx) index profiles can fit the experimental data very well with a deviation of less than 1.8%.     

Possibility of excitation of azimuthal surface waves in a magnetized plasma by annular ion beams

Excitation of the azimuthal surface eigenmodes with the extraordinary polarization for the ion plasma component is shown to be possible in cylindrical waveguides with metal walls, filled partially by a cold magnetoactive neon plasma. The interaction of the oscillations with the flows of alpha particles rotating around the plasma column in a narrow layer that separates the plasma from the waveguide walls is studied. If the external magnetic field is strong enough, the resonance interaction of the beam with the waves can be realized for three minimum values of the azimuthal mode number when the wave frequency normalized by neon ion cyclotron frequency is close to the factor of five (m _Ne/m _He ≈ 5).     

Thermoelastic waves in a waveguide

The propagation of thermoelastic waves in a waveguide occupying the Cartesian space x₁ ϵ [0, L], x₂ ϵ [− H, + H], x₃ ϵ [− ∞, + ∞] is discussed. The analysis is based on the generalized theory proposed by Lord and Shulman. The solution of the problem is expressed in terms of the Lamé's scalar and vector potentials and the frequency equations are derived. For the special case of very short waves numerical results for the wave motion characteristics are presented and the role of the various parameters entering into the problem is extensively discussed.     

The surface mode of a dielectric waveguide with metal substrate

The conditions of excitation and the waveguide characteristics are determined in an analytical form for the TM₀ mode of a planar waveguide with metal substrate. This mode has a surface character, in contrast to other (bulk) modes excited in the system. The propagation and damping constants and the energy flux density distributions are compared for the TM₀ and TE₀ modes.     

Synthesis and properties of a copper(I) cobalt(II) double sulfite

The Pourbaix diagrams of the Cu–H₂SO₄–H₂O and Co–H₂SO₄–H₂O systems have been refined and the stability regions of sulfite phases in the diagrams have been identified. Phase diagrams of copper(I) copper( II) and copper(I) cobalt(II) double sulfites have been mapped out. The double sulfites Cu₂SO₃ · СuSO₃ and Cu₂SO₃ · СoSO₃ have been isolated from an aqueous solution saturated with sulfur dioxide. We have obtained electron paramagnetic resonance spectra of the double sulfite Cu₂SO₃ · СoSO₃ and characterized it by X-ray diffraction, IR spectroscopy, particle size analysis, and thermogravimetry. A foundation has been laid for the thermodynamic prediction of the synthesis of the Cu₂SO₃ · MSO₃ (M = Cu, Co) double sulfites.     

Magnetization and torque measurements on rotating niobium cylinders and ellipsoids

The magnetization and torque of niobium samples of different shapes rotating in a magnetic field are studied with emphasis on the behavior near the upper critical field H _c2. The observed rotational magnetization is free of hysteresis and is found to represent a good approximation to the thermodynamic equilibrium magnetization of the samples if pinning is not too strong and the frequency of rotation is sufficiently low. The viscous contribution to the torque exhibits a step at H _c2whose dependence on sample geometry is in good agreement with predictions derived under the simple assumption of a homogeneous arrangement of parallel straight vortices in the rotating sample.     

Analysis and design of a rib-like-based slot waveguide for nonlinear silicon nanophotonics

Design of efficient nonlinear optical waveguides is an essential requirement for development of silicon nanophotonics. These waveguides should have a unique capability to play the main role in realization of both passive and active optical devices. A high-performance dielectric rib-like-based slot waveguide is proposed for nonlinear silicon nanophotonics. Its slot region can be filled with Si-nc:SiO₂ which exhibits a high third-order nonlinear effect. Study of numerical results shows that this new slot waveguide has a nonlinear parameter of the same order of magnitude as an equally sized silicon nanophotonics strip-based slot waveguide. The new waveguide can be fabricated easily by etching a slot in the core region of the silicon-on-insulator (SOI)-based rib-like waveguide.     

Magnetic Susceptibility and High Field Magneto-transport of Silver-Added Bi-2223 Superconductor: a Revisit

The effect of Ag addition on weak link behaviour of a Bi-2223 (Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀) polycrystalline sample has been investigated in terms of AC susceptibility, critical current density (J _c), electrical resistivity ρ(T)H and upper critical field (H _c2). A series of phase pure Bi-2223-Ag _x (x = 0.0–0.3) samples are prepared by the solid-state synthesis route. The phase purity, crystal structures and surface morphology are being studied using the X-ray diffraction and scanning electron microscopy (SEM) technique, respectively. The effect of Ag addition on inter- and intragranular coupling has been investigated by means of AC susceptibility and magneto-transport ρ(T)H measurements, and the results are compared with the pure Bi-2223 sample. Enhancement in granular coupling between the grains of the 20 wt% Ag-added Bi-2223 sample has been witnessed. Critical current density (J _c) has been estimated using the AC susceptibility technique, and the results are interpreted in terms of inter- and intragranular coupling of the investigated samples. The high field magneto-transport technique has been used to estimate the upper critical field (H _c2) and thermally activation flux flow (TAFF) activation energy. The ensuing results revealed that H _c2 increases for the 20 wt% Ag-added sample along with enhancement in grain alignment and intergrain connectivity.     

Preparation of a copper-polymer composite through the thermolysis of copper(II) succinate

A procedure has been proposed for the preparation of copper/polymer composites through Cu(C₄H₄O₄) · H₂O succinate thermolysis. Three types of copper nanoparticles have been incorporated into a fibrous carbon-polymer matrix of the composite: cubic (350 × 350 nm) and needle-like (40–80 nm in diameter and 4.5–5.5 μm in length) crystals and spherical copper particles (5 to 45 nm). We have compared copper succinate and copper maleate decomposition processes. The composites obtained through thermolysis have been shown to differ drastically.     

An evanescent field absorption gas sensor at mid-IR 3.39 μm wavelength

Trace gases such as H₂O, CO, CO₂, NO, N₂O, NO₂ and CH₄ strongly absorb in the mid-IR (>2.5 μm) spectral region due to their fundamental rotational and vibrational transitions. CH₄ gas is relatively non-toxic, however, it is extremely explosive when mixed with other chemicals in levels as low as 5% and it can cause death by asphyxiation. In this work, we propose a silicon strip waveguide at 3.39 μm for CH₄ gas sensing based on the evanescent field absorption. These waveguides can provide the highest evanescent field ratio (EFR)>55% with adequate dimensions. Moreover, EFR and sensitivity of the sensor are highly dependent on the length of the waveguide up to a certain limit. Therefore, it is always a compromise between the length of the waveguide and EFR in order to obtain greater sensitivity.     

Optimization of mixed Langmuir-Blodgett films of a water insoluble porphyrin in a calixarene matrix for optical gas sensing

The gas sensing capabilities of Langmuir-Blodgett (LB) mixed films of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) and p-tert-butylcalix[8]arene (C8A) have been studied in this work. Although EHO is known to be very sensitive to NO₂ gas, this study demonstrates that the C8A matrix improves the sensing properties of the porphyrin molecules in the solid state. After the exposure to NO₂ and the subsequent recovery, the UV-vis spectrum of a C8A:EHO film shows no aggregation of the porphyrin. In atomic force microscopy (AFM) images, C8A:EHO films appear with sharper surfaces than those made of pure EHO, allowing a better accessibility of the gas molecules to the active binding sites. Multilayer LB films of the C8A:EHO system ranging from 2 to 40 layers have been prepared to study their response to NO₂ by UV-vis spectroscopy, and their kinetics reveal an important thickness dependence. Through the analysis of AFM images, it has been found that the surface roughness increases until the sample reaches 20 layers and then remains almost constant, which is related to the response time. The optimum film thickness has been found to be 20 layers, for which both the speed of response and the surface roughness are maximum.     

Alkaline earth titanoniobates with a layer structure : A0.5H(TiNbO5)2.nH2O

The ion exchange properties of the layer oxide HTiNbO₅ in the presence of alkaline earth acetate solution A(CH₃COO)₂ has been investigated. The maximum exchange rate increases with the size of the A ion, leading to the hydrates : Ba(TiNbO₅)₂.4H₂O, Sr_0.75H_0.50(TiNbO₅)₂.4H₂O and Ca_0.50H(TiNbO₅)₂.4H₂O. The minimum exchange rate corresponds to the definite compounds A_0.5H(TiNbO₅)₂.4H₂O, labelled 4W-hydrates. The study of the thermal stability of the 4W-hydrates leads to the hydrates A_0.5H(TiNbO₅)₂.2H₂O and A_0.5H(TiNbO₅)₂.H₂O, and finally to the dehydrated oxides A_0.5H(TiNbO₅)₂. The reversibility of the ion exchange reaction and of the hydration phenomenon has been studied. The X-ray diffraction study of these compounds shows that the structure of the (TiNbO₅) layers is conserved whatever the nature of the A ion and of the hydrate may be. The structural evolution versus water content and size of the A ions is discussed.     

The growth characteristics of chemical vapour-deposited β-SiC on a graphite substrate by the SiCl4/C3H8/H2 system

Silicon carbide has been grown at 1300–1800°C by chemical vapour deposition using the SiCl₄/C₃H₈/H₂ system on a graphite substrate. The effect of C₃H₈ flow rate and deposition temperature on the growth characteristics and structure of the deposit has been studied. The experimental results show that the degree of film density is changeable from a dense plate to a porous one with increasing C₃H₈ flow rate. The activation energy increases with increasing C₃H₈ flow rate. The grain size of the polycrystalline β-SiC becomes coarser when the C₃H₈ flow rate and the deposition temperature are increased. The preferred orientation of the deposited SiC layers changes from (111) to (220) on increasing deposition temperature from 1300°C to 1400°C. The deposition mechanism is also discussed.     

Chemical dehumidification by liquid desiccants: theory and experimentDeshumidification chimique par des substances absorbantes liquides: theorie et expérimentation

Chemical dehumidification of air by a liquid desiccant in a packed tower has been investigated both theoretically and experimentally for air conditioning and industrial applications. A computer model of a packed tower, able to determine heat and mass transfer between air and desiccant, has been developed and a parametrical study was carried out considering the solutions H₂O/LiBr and H₂O/CaCl₂ to determine the optimum operative conditions. An experimental apparatus including a packed tower and a desiccant regenerator has been described together with experimental results: a set of 70 experimental runs with H₂O/LiBr. Data have been reported and compared against the results of the computer code simulations.     

Theoretical study of Ti:LiNbO3 strip waveguides in the Mid-infrared

Abstract

The transverse magnetic and transverse electric mode size and effective refractive index as functions of the Ti-strip initial width W, diffusion temperature T and Ti-strip initial thickness H in c-cut Ti-diffused LiNbO₃ waveguides have been theoretically studied at wavelengths of 3.0 and 3.39 μm on the basis of modified expressions for the waveguide surface index increment. The single-mode, multiple-mode and cut-off conditions have been identified from those effective refractive index plots. These mid-infrared numerical results are discussed comparatively with near-infrared data reported earlier.     

Input cavity for high-order asymmetric-mode gyroklystron

A new input cavity design for a high-order asymmetric-mode gyroklystron is proposed. Methods of the selective excitation of a resonant mode with a rotating field structure and the prevention of cavity self-excitation at harmonics of the gyrofrequency are proposed. Results of experimental investigation of the H₇₁₁ mode cavity for a multimegawatt pulsed gyroklystron are presented.