High-power transverse flow CW CO2 laser for material processing applications

A transverse flow transversely excited (TFTE) CW CO₂ with a maximum output power about 15 kW has been developed. This is excited by pulser sustained DC discharge applied between a pair of multi-pins anodes and a common tubular cathode. Though the laser power in convective cooled CO₂ laser scales proportionally with the volumetric gas flow, it did not increase in this laser when the volumetric gas flow was increased by increasing the electrode separation keeping the flow velocity constant. The discharge voltage too remained almost unchanged with increase of the electrode separation. These observations are explained considering the electrical discharge being controlled by ionization instability. Laser materials processing applications often demand programming facilities for laser power modulation. A four-stage cascaded multilevel DC–DC converter-based high-frequency switch mode power supply has been developed to modulate the output power of the laser. Laser was operated up to 15 kW output power in four different modes viz. continuous wave mode, pulse periodic mode, single shot mode and processing velocity-dependent power mode with 1.2 kHz modulation bandwidth. We describe briefly the laser system, the SMPS, and the temporal behavior of laser beam.     

Temperature and composition dependence of vibrational modes in Sm1−xLaxAlO3

At room temperature and for x₀ ～ 0.43, the system Sm_1?xLa_xAlO₃ undergoes a first order phase transition in which an orthorhombic structure transforms into a rhomboedral structure as x increases. This phase transition was investigated by Raman scattering experiments on polycrystalline compounds in the range 0 ? x ? 1. The dependence of low frequency modes on x is reported and compared with their well known temperature dependence. Soft modes have been observed in the orthorhombic (D_2h¹⁶) and rhomboedral (D_3d⁶) phases. It was found that the first order phase transition is probably driven by a double degenerate mode whose two components in the low symmetry phase display a linear composition dependence of their squared frequencies. The source of anomalous behaviour of Raman bands near the phase transition is discussed.     

Synthesis, growth and optical spectroscopy studies of BaBiBO4 and CaBi2B2O7 crystals

Single-crystalline species of the novel compounds BaBiBO₄ and CaBi₂B₂O₇ with Pna2₁ structure were successfully grown by the top-seed solution growth (TSSG) method and solid-state recrystallization, respectively. X-ray diffraction methods were used to confirm the symmetry of the crystal structure and to determine unit cell parameters. Normal-mode classification and assignment were performed from the results of polarized Raman and IR absorption measurements on the obtained crystals. The high-frequency part of the vibrational spectra of both materials (580–1400 cm^?1) is reminiscent of the normal modes of the isolated [BO₃]^3? group. The expected crystal-field induced changes in these modes from group-theoretical considerations provide important clues for the normal-mode assignment of the high-frequency spectral lines.     

Raman scattering in TlInS2xSe2(1−x) layered mixed crystals (0.25≤x≤1): Compositional dependence of the mode frequencies and line widths

The Raman spectra of TlInS_2xSe_2(1−x) layered mixed crystals were studied for a wide composition range (0.25≤x≤1) in the frequency region 10-360 cm⁻¹ at room temperature. The shift of Raman-active phonon frequencies versus mixed crystals composition x were established. The effect of crystal disorder on the line width broadening of three high-frequency Raman-active modes is reported.     

Mode frequency shifts and Q-factor Changes in 2D microflower cavity and its deformed cavity

Mode frequency shifts and Q-factor changes in 2D microflower cavity and its deformed cavity are analyzed. The effective mode-splitting of double-degenerate WG modes is obtained and the Q-factor changes of matched and mismatched modes are discussed for the microflower cavity. The Q-factor stability of the splitted WGH_(8,1) modes due to two types of local deformations is studied, showing that the local deformations can badly spoil the mode Q-factor if the deformations are not controlled properly. The output directionality of the splitted WGH_(8,1) modes due to the local deformations also is presented, and a basically unidirectional light output of OO mode under local deformation DA (deformation happens at one “valley” of the microflower cavity) is obtained.     

Performance characteristics of deep violet InGaN DQW laser diodes with InGaN/GaN superlattice waveguide layers

The effect of the indium (In) composition of In_xGa_1−xN (GaN) waveguide layers on the performance of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) emitting at 390 nm output emission wavelength has been numerically investigated. Simulation results indicated that by increasing In composition of the In_xGa_1−xN waveguide layers, the threshold current decreases, the slope efficiency, and differential quantum efficiency (DQE) increase, whereas the output power decreases. The increase in the In composition of the InGaN waveguide layers increases the refractive index and consequently increases the optical confinement factor (OCF) which result in the increase in the slope efficiency and DQE and the decrease in the threshold current. The decreasing movement of electron and hole carriers from the bulk waveguide layers to the active regions also causes to decrease the output power. A new LD structure with InGaN/GaN superlattice (SL) waveguide layers has been proposed to exploit the increased OCF of InGaN waveguide structures, and the enhanced electron and hole mobilities and the tunneling effect of the periodic structure of the SL structures. The results also showed that the use of InGaN/GaN SL waveguide structures effectively improves the output power, slope efficiency and DQE and decreases the threshold current of the LD compared with (In)GaN bulk waveguide structure.     

Confinement of polar optical phonons in quasi-one-dimensional Wurtzite GaN-based quantum well wires: propagating and half-space phonon modes

With the aid of the macroscopic dielectric continuum and Loudon’s uniaxial crystal models, the propagating (PR) and half-space (HS) optical phonon modes and corresponding Fröhlich-like electron-phonon interaction Hamiltonians in a quasi-one-dimensionality (Q1D) wurtzite quantum well wire (QWW) structure are derived and studied. Numerical calculations on a wurtzite GaN/A_l0.15Ga_0.85N QWW are performed, and discussion is focused mainly on the dependence of the frequency dispersions of PR and HS modes on the free wave-number k _z in the z-direction and on the azimuthal quantum number m. The calculated results show that, for given k _z and m, there usually exist infinite branches of PR and HS modes in the high-frequency range, and only finite branches of HS modes in the low-frequency range in wurtzite QWW systems. The reducing behaviors of the PR modes to HS modes, and of the HS mode to interface phonon mode have been observed clearly in Q1D wurtzite heterostructures. Moreover, the dispersive properties of the PR and HS modes in Q1D QWWs have been compared with those in Q2D quantum well structures. The underlying physical reasons for these features have also been analyzed in depth.     

The effect of oxygen plasma generated by different microwave modes on NdBa2Cu3O6+δ superconductor: structure and IR studies

The effect of longitudinal and transverse modes of oxygen plasma generated by microwave source with frequency 2.45 GHz has been used to characterize polycrystalline samples of NdBa₂Cu₃O_6+δ (Nd-123) high T_c-superconductor. For this purpose we have developed a simple oxidation technique, used by us earlier, to investigate the effect of different modes on the appearance of superconductivity in Nd-123. Our main results show that the mode of microwave plasma and time of exposure can control the local arrangement of oxygen vacancies in CuO_δ basal plane. Samples exposed to the transverse mode reveals a well ordered orthorhombic phase transition while the longitudinal modes caused the samples to have a non-superconducting tetragonal structure for all of the exposure time. The T_c vs. δ curve does not show the typical 60 K plateau corresponding to the ortho-II phase. The IR spectrum show bands at shorter wavelengths which corresponding to the metal oxides.     

HEM11模磁绝缘线振荡器的高频分析

作为小型化和紧凑型的高功率微波源,磁绝缘线振荡器(MILO)在过去十几年里得到了广泛的研究和发展.在大多数研究中,最低的对称模一直被当作器件的主模.然而,由于结构的对称性或者电子发射均匀度不理想等原因,很容易激励起非对称模式.计算了MILO同轴结构中同时包含对称模和非对称模的本征方程.在此基础上,通过对结构的优化设计,提出了一种HEM₁₁模工作的MILO,并开展了原理性实验.在二极管的电压为480kV,电流为39kA条件下得到了功率为1.2GW,脉冲宽度为40ns的微波输出,功率转换 关键词： 磁绝缘线振荡器 高频特性 11模')" href="#">HEM₁₁模 开放腔     

Diode-end-pumped passively Q-switched Nd:Y0.8Lu0.2VO4 laser with Cr:YAG crystal

A diode end-pumped passively Q-switched Nd: Y_0.8Lu_0.2VO₄ laser with a Cr⁴⁺: YAG crystal is first demonstrated in this paper. The maximum continuous wave (CW) output power of 5.59 W is obtained at the incident pump power of 13.07 W with the output transmission T = 20%, resulting in an optical-to-optical efficiency of 42.7%. For Q-switching operation, the measured pulse duration of 8.5 ns, the pulse energy of 45.24 μJ and the peak power of 5.32 kW are respectively obtained for the output transmission of 50% when the Cr⁴⁺: YAG crystal is used with an initial transmission (T₀) of 60%.     

High-power continuous-wave doubly resonant all-intracavity sum-frequency mixing deep blue laser at 447 nm

In this paper, a high-power continuous-wave deep blue laser at 447 nm with intracavity tripling was achieved. The deep blue laser at 447 nm is obtained by using a doubly cavity, and type-II critical phase matching KTP crystal for intracavity sum-frequency mixing. Through designing of the cavity, the optimum matching of modes and gains for the two wavelengths was obtained. With incident pump power of 30 W for the Nd:YVO₄ crystal and 16 W for the other Nd:YVO₄ crystal, the deep blue laser output of 3.5 W at 447 nm with TEM₀₀ mode was obtained, the beam quality M² value was equal to 1.8 in both horizontal and vertical directions at the maximum output power, and the power stability is better than 3% at the maximum output power during half an hour. The experimental results show that the intracavity sum-frequency mixing by doubly resonant is an effective method for high-power blue laser.     

Growth, optical properties and laser performance of NdxLu1−xVO4 series crystals

Nd_xLu_1−xVO₄ (x = 0, 0.005, 0.02, 0.023, 0.03 and 0.05) series crystals have been grown by the Czochralski method. The results of X-ray powder diffraction experiments show that these crystals belong to the zircon-type structure. The cutoff wavelength of LuVO₄ is 330 nm and the transmittance is about 80% in 540-3010 nm. The functional relation of absorption coefficient α at 808 nm on Nd³⁺-doped concentration N_d in units of atomic % is obtained to be α = 7.649 N_d. The results of Judd-Ofelt analysis show that there is no obvious concentration quenching effect below 3 at.% doped concentrations. The highest output power at 1.06 μm is 1090 mW for Nd_0.02Lu_0.98VO₄ at the pumping power of 6.58 W of a laser diode, the optical-optical conversion efficiency is 16.6% and slope efficiency is 19.4%.     

Small signal gain and saturation intensity of a Yb:Silica fiber MOPA system

In this work, the small signal gain (γ₀) and signal saturation power (P_sat) of a continuous wave (CW) single-frequency ytterbium-doped (YD) double-clad (DC) fiber master oscillator power amplifier (MOPA) array has been determined in the optimum length. At first, we have obtained output versus input signal powers in the amplifier, using the coupled steady-state rate equations for the various end diode pumping such as co/counter propagation and bidirectional modes. On the other hand, the steady-state amplification relation was employed at a presumed γ₀ and P_sat as initial guess, to generate output signals. Then, using the least square method (LSM), those input/output signal powers were fitted by the best regression curve obtained from the amplification relation to determine the converged γ₀ and P_sat values. The analytical formulae for γ₀ and P_sat were derived using the rate equations as well.     

Investigation of Nd:YVO4/YVO4 composite crystal and its laser performance pumped by a fiber coupled diode laser

Thermal effect control is critical to scale the output power of diode end-pumping solid lasers to several watts up and beyond. Diffusion bonding crystal has been demonstrated to be an effective method to relieve the thermal lens for the end-pumping laser crystal. The temperature distribution and thermal lens in Nd:YVO₄/YVO₄ composite crystal was numerically analyzed and compared with that of Nd:YVO4 crystal in this paper. The end-pumping Nd:YVO₄/YVO₄ composite crystal laser was set up and tested with z cavity. The maximum output power of 9.87 W at 1064 nm and 6.14 W at 532 nm were obtained at the pumping power of 16.5 W. The highest optical-optical conversion efficiencies were up to 60% at 1064 nm and 40% at 532 nm, respectively.     

Efficient and compact intracavity-frequency-doubled YVO4/Nd:YVO4/KTP laser through analysis of the interaction length

In this paper, a stable end-pumped intracavity-frequency-doubled green laser was demonstrated. The interaction length of different pump systems before setting up the experiment was analyzed in order to find out an effective pump system. The experimental results indicate that the pump system in our configuration is beneficial to the efficient CW Nd lasers. A continue-grown composite crystal YVO₄/Nd:YVO₄, with Nd³⁺ concentration doping of 0.3 at.%, is used as laser medium. With an incident pump power of 27.5 W, as high as 7.2 W of CW output power at 532 nm was achieved. The optical-to-optical conversion efficiency of 26.2% was obtained in CW modes with a flat-flat cavity design.     

Parametric measurements on a doped CO2TEA laser

Parametric measurements have been performed on a CO₂TEA laser with a discharge volume of 6.5 × 6.5 × 45 cm³. The effect of a low ionization seed gas, tri-n-propylamine, upon amplification, power output and voltage-current characteristics has been measured. A small-signal gain of 4.8%/cm has been measured in a 1:1:3 mixture and a power output of 60J/? in a 3:11:21 mixture.     

Elasticity, band structure, and piezoelectricity of BexZn1−xO alloys

Lattice constants, elasticity, band structure and piezoelectricity of hexagonal wide band gap Be_xZn_1−xO ternary alloys are calculated using first-principles methods. The alloys' lattice constants obey Vegard's law well. As Be concentration increases, the bulk modulus and Young's modulus of the alloys increase, whereas the piezoelectricity decreases. We predict that Be_xZn_1−xO/GaN/substrate (x=0.022) multilayer structure can be suitable for high-frequency surface acoustic wave device applications. Our calculated results are in good agreement with experimental data and other theoretical calculations.     

Effects of SnO2 addition on the magnetic properties of manganese zinc ferrites

SnO₂ was added to high-permeability MnZn ferrites and MnZn ferrites for high-frequency power supplies. The effects of the SnO₂ addition were studied. Sn⁴⁺ ions can dissolve into the spinel lattice and form stable Fe²⁺–Sn⁴⁺ pairs and hence can compensate the magneto-crystalline anisotropy constant K₁ and improve the initial permeability effectively. The initial permeability of ferrites is also improved as abnormal grain growth caused by ion vacancy is controlled with SnO₂ doping. In addition, the SnO₂ doping also leads to a decrease in the relative loss factor and an increase in density. The power loss and minimum power loss temperature decrease with SnO₂ doping.     

AC-chopper application for CW CO2 laser

A microprocessor-based algorithm was developed to control the power supply to a CO₂ laser using an AC-chopper method. This system was connected directly to a CO₂ laser tube without the need for a dc-dc converter or the storage capacitance of a multilevel circuit. The typical CO₂ laser power supply system had a full-bridge series resonant inverter or voltage multiplier. AC-to-AC converter schemes using zero voltage switching (ZVS) can be used efficiently and economically for low and medium power applications.This paper reports the performance characteristics of a symmetrical AC-chopper technology that can maintain the quality of the ac output of a CO₂ laser tube, regardless of the amount of switching loss. The laser was operated to an output power, maximum system efficiency total gas mixture of 37.2 W, 92%, and CO₂:N₂:He=1:9:15, respectively. The laser system, AC-chopper power supply and its operation were examined.     

X波段过模弯曲圆波导TM01-HE11模式变换器研究

在波束波导和反射面天线的馈源应用中, 为了产生低副瓣且方向图等化的高斯波束, 需要将高功率微波转换为准高斯模HE₁₁模辐射. 本文利用弯曲圆波导可同时从TM₀₁模产生TE₁₁模和TM₁₁模的原理, 提出了采用双弯曲过模圆波导结构直接将TM₀₁转换为HE₁₁的模式变换器, 避免了常规微波领域中首先将TM₀₁转换为TE₁₁再用波纹式或半径渐变式TE₁₁-HE₁₁转换器转换为准高斯波束功率容量不足或尺寸过长的不足. 基于模式耦合理论和Taguchi优化算法对模式变换器的弯曲半径、相移直端长度及引入位置进行了优化, 使输出的TE₁₁和TM₁₁成一定比率, 以组成HE₁₁模式, 并对设计的模式变换器进行了全电磁波仿真分析, 结果表明输出波束的标量高斯含量在9.05–9.8 GHz范围内均高于99%, 理论功率容量可达4.5 GW. 关键词： 高功率微波 模式耦合理论 Taguchi优化算法 模式变换器