Numerical Study of Processes in the Cavity of the 170 GHz Gyrotron for ITER Operating at the TE25.10 Mode

The opportunity of use of a TE_25.10 operating mode for CW 170 GHz/1 MW gyrotron for ITER is estimated. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. The influence of unwanted mode conversion at the output of the resonator, mode competition, electron beam potential depression, ion compensation of the space charge and beam energy recovery is investigated.     

径向线型模式变换器

 研究了一种通过改变波导内场分布的旋转对称性,可将高功率微波源输出的TEM模或TM01模转换为TE11模的径向线型模式变换器。介绍了该模式变换器的基本原理,即采用金属插板将同轴波导TEM模变换为4路90°扇形波导TE11模,各路扇形波导间所需的输出相位差通过将扇形波导转换为双层径向线传输来实现。基于这一原理,设计了一个中心频率为1.6 GHz的同轴TEM-TE11模式变换器,并进行了数值模拟计算,结果表明该模式变换器具有较高的功率容量,中心频率处反射系数为0.05,模式转换效率为99%,在1.52~1.68 GHz的频带范围内,模式转换效率大于90%。     

A cavity with reduced mode conversion for gyrotrons

Mode conversion at discontinuities of an ordinary gyrotron cavity is examined. It is shown how the production of unwanted spurious modes can be reduced substantially by introducing smooth transition regions between the individual parts of the cavity. The cavity of the 140 GHz KfK gyrotron operating in the TE_10,4 mode is used as an example. Cavity and adjacent tapered output waveguide are optimized as a single unit.     

The reflection of the fundamental torsional mode from cracks and notches in pipes

A quantitative study of the reflection of the T(0,1) mode from defects in pipes in the frequency range 10-300 kHz has been carried out, finite element predictions being validated by experiments on selected cases. Both cracklike defects with zero axial extent and notches with varying axial extents have been considered. The results show that the reflection coefficient from axisymmetric cracks increases monotonically with depth at all frequencies and increases with frequency at any given depth. In the frequency range of interest there is no mode conversion at axisymmetric defects. With nonaxisymmetric cracks, the reflection coefficient is a roughly linear function of the circumferential extent of the defect at relatively high frequencies, the reflection coefficient at low circumferential extents falling below the linear prediction at lower frequencies. With nonaxisymmetric defects, mode conversion to the F(1,2) mode is generally seen, and at lower frequencies the F(1,3) mode is also produced. The depth and circumferential extent are the parameters controlling the reflection from cracks; when notches having finite axial extent, rather than cracks, are considered, interference between the reflections from the start and the end of the notch causes a periodic variation of the reflection coefficient as a function of the axial extent of the notch. The results have been explained in terms of the wave-number-defect size product, ka. Low frequency scattering behavior is seen when ka < 0.1, high frequency scattering characteristics being seen when ka > 1.     

A High Conversion Efficiency Q-Switched Intracavity Nd:YVO_4/KTA Optical Parametric Oscillator under Direct Diode Pumping at 880nm

We report a high conversion efficiency Q-switched Nd:YVO_4/KTiOAsO_4(KTA) intracavity optical parametric oscillator(IOPO) operating near 3.5 um based on direct 880 nm laser diode(LD) pumping. A maximum average idler output power of 2.6 W with a pulse width of about 7.9 ns is achieved under an absorbed LD power of 45.4 W at a pulse repetition rate(PRR) of 10 kHz. The maximum optical-optical conversion efficiency from LD power to OPO mid-infrared(MIR) output of 6.74% is achieved. To our knowledge, this is the highest conversion efficiency for a KTA-IOPO by exploiting a Q-switched laser as the parent fundamental pump source. The beam quality factors M~2 of the MIR beam at the full output power with a PRR of 10 kHz are within 2.12 in both the horizontal and vertical directions, indicating a near Gaussian mode.     

High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser

We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6.     

Conical sum-frequency generation in a bulk anomalous-like dispersion medium

We observe conical sum-frequency generation in a bulk anomalous-like dispersion medium, which is attributed to complete phase-matching of one fundamental wave and the scattering wave of the other fundamental wave.In addition, efficient sum-frequency output is achieved making use of total internal reflection with conversion efficiency of 7.9% by only one reflection. The experiment proposes a new phase-matching mode under an anomalous-like dispersion condition, which suggests potential applications in efficient frequency conversion.     

带高阶模谐振腔反射器的过模高功率毫米波源

针对过模系数为2.3的高功率毫米波发生器设计了工作于高次模的谐振腔反射器。谐振腔反射器工作模式为TM035模式,在58~62 GHz频带内对TM01模式的反射系数大于0.9。运用2.5维全电磁粒子程序模拟分析了器件中束波相互作用过程,通过调整慢波结构与谐振腔反射器间的漂移段长度得到了器件在570 kV,6.0 kA电子注量驱动下,在引导磁场为4 T时,能辐射出功率1.06 GW、频率为60.2 GHz的毫米波,主要工作模式为TM01模,效率约为31%,起振时间为3.3 ns。     

Observing ultrasonic wave propagation by stroboscopic visualization methods

The generation of various wave modes by single element devices working in the range 2 MHz to 5 MHz is illustrated and the unwanted production of spurious wave modes identified. A linear four-element phased array is described with which beam steering can be clearly demonstrated. The mode conversion of waves which can occur during reflection at component boundaries, and the interaction of compressional waves with slots and internal defects of various shapes are displayed. The diffraction of compressional waves around an obstacle has been given particular attention, both in a solid and in water by the complementary schlieren technique. The results suggest that the transparent solid effects are very much more complicated than had been anticipated. The future value of these comparisons is briefly discussed in relation to the immersion and direct contact methods of material inspection.     

Femtosecond high-power spontaneous mode-locked operation in vertical-external cavity surface-emitting laser with gigahertz oscillation

We realize a femtosecond high-power spontaneous mode-locked operation with gigahertz oscillation in a vertical-external cavity surface-emitting laser under the condition of eliminating the internal and external unwanted reflection. We find that the reflectivity of the output coupler has a significant influence not only on the output power but also on the output pulse duration. With an incident pump power of 20 W, we have achieved 2.35 W of average output power with 778 fs pulse duration at a repetition rate of 2.17 GHz. The shortest pulse duration was 654 fs at an average output power of 0.45 W.     

全腔输出相对论磁控管输出模式转换结构的理论设计和数值模拟

采用全腔输出结构后,相对论磁控管径向尺寸显著减小,轴向长度也有较大幅度的缩短.但是,由于输出结构为三个相对独立的扇形波导,实际应用时,一般需要对微波输出模式进行转换.针对全腔输出相对论磁控管,本文研究了两种输出模式转换结构并利用三维全电磁粒子模拟程序对其进行了研究.首先研究了将三个扇形波导角向增宽从而渐变或者突变为一个同轴波导的情况,研究结果表明,两种情况下输出微波功率均大于采用传统三个独立扇形波导输出时的90%,输出模式主要是TEM模.其次研究了输出区由三个扇形输出波导分别变换为三个截面大小与之接近的矩形输出波导的可行性,研究结果表明,注入扇形波导中的TE₁₁模式几乎全部转换为矩形波导中的TE₁₀模式.实际应用时,可根据需要选择上述输出模式转换结构.     

基于共面波导微带线的光探测器高频电参量提取

针对不能在片测量的光探测器芯片,本文提出了一个简单而有效的实验方案来提取其等效电路模型的高频电参量.首先设计了和微波探针匹配的共面波导微带线,测量其输出反射系数,测试结果与理论设计符合很好;然后将芯片装载到微带线上,测量包含光探测器的整个测试结构的输出反射系数.仿真中涉及光探测器测试结构的等效电路模型,包含光探测器、键合金丝和共面波导微带线等因素.通过拟合已测的整个测试结构的输出反射系数,提取了光探测器的高频电参量.     

Mode conversion based on dual-phase modulation utilizing interference of two-phase-modulated beams

To realize mode conversion with high accuracy and efficiency, we propose a new method based on dual-phase modulation (DPM) that utilizes the interference of two-phase-modulated beams. DPM realizes complex amplitude modulation with the interference of two-phase-modulated beams generated by phase-type spatial light modulators (PSLMs). DPM realizes highly efficient conversion using the interference of light rather than the diffraction that is conventionally used for complex amplitude modulation. Moreover, DPM enables the suppression of modal crosstalk (MXT) utilizing the destructive interference between two unwanted 0th beams that occur from PSLMs. In this paper, we experimentally confirmed that DPM can reduce the amount of the unwanted 0th beam. Then, we performed an experiment on mode conversion based on DPM and achieved a modal crosstalk of less than ??20 dB and an optical loss of less than 10 dB.     

量子点半导体光放大器波长转换的Q因子特性

为了改善全光波长转换器的转换性能进而提高输出信号质量,研究了波长转换器的Q因子特性。采用牛顿迭代法和四阶龙格库塔法解光场传输方程和跃迁速率方程,分析了输入信号光功率、脉冲宽度、最大模式增益和有源区长度4个因素对全光波长转换器的Q因子特性的影响,并将得到的结果与相同条件下的输出消光比比较。结果表明:增大输入信号光功率,Q因子先增大后减小,并且在-12 d Bm时取得最大值8.819 d B;Q因子随着脉冲宽度的增加而不断下降;增大最大模式增益和有源区长度,Q因子增大。在实现波长转换的基础上,优化各参数数值,得到的Q因子达到16.680 d B,输出信号质量较好。要同时获得高的消光比和Q因子,提高输出信号的质量,必须选取适当的输入信号光功率、脉冲宽度、最大模式增益和有源区长度。     

高功率准连续波腔内和频全固态黄光激光器

报道了高功率准连续波腔内和频全固态黄光激光器的研究结果.为获得高功率的黄光输出,首先,激光器采用准连续方式运转,在保持抽运水平的条件下降低热效应,从而提高光束质量和光光转换效率;第二,采用热近非稳腔腔型设计,双棒串接补偿热致双折射技术,获得大基模体积高光束质量的基频光;第三,通过优化腔型,采用L型共折叠臂平-凹对称腔,使两束基频光达到空间重合且满足功率配比.通过这些方法,得到了输出功率7.6W,重复频率1.1kHz的准连续波黄光输出.据我们所知,这是目前腔内和频方案所获得的最高功率全固态黄光输出. 关键词： 黄光激光 腔内和频 Nd：YAG激光 全固态激光器     

双频回旋管内置准光模式变换器设计

回旋管一般使用准光模式变换器实现高阶腔体模式到高斯波束的转换。结合标量衍射理论、KS迭代算法、几何光学、最小均方法等方法设计了工作频率为140 GHz(TE24,9)和105 GHz(TE18,7)的双频准光模式变换器。仿真结果显示所设计的准光模式变换器工作频率为140 GHz(TE24,9)时能量传输效率99.0%、高斯含量99.7%,工作频率为105 GHz(TE18,7)时能量传输效率97.3%、高斯含量98.0%。能够满足MW级双频回旋管的应用需求。     

A Stable Wavelength Operation Ho:YAG Laser with Orthogonally Polarized Pump

A stable wavelength operation Ho:YAG laser dual-pumped by two orthogonally polarized Tm:YLF lasers is reported. Under the cw operation mode, a laser output power of 24 W is measured. The corresponding opticaloptical conversion efficiency is 44.75% and the slope efficiency is 50.12%. Under the Q-switched operation mode,the output maximum average power is 22.8 W at the re-frequency of 6 kHz. The corresponding optical-optical conversion efficiency and slope efficiency are 42.64% and 48.01%, respectively. The output central wavelength is 2090.73 nm, the linewidth is 0.40 nm, and the beam quality is M~2 1.6. Moreover, the shift of the output central wavelength is less than 0.01 nm, and the linewidth shift is also less than 0.01 nm.     

CW single transverse mode all-fiber Tm3+-doped silica fiber laser

The CW 25.6 W output power with a slope efficiency of 30.6% respected to the pump power from a CW single transverse mode all-fiber Tm³⁺-doped Silica Fiber Laser is reported. The all-fiber laser is made up by progressively splicing the pigtail fiber, matched FBG fiber and Tm fiber. The reflective FBG and Tm³⁺-doped fiber end Fresnel reflection build up the laser resonance cavity. Due to the multi-mode FBG as the reflective mirror, the output laser spectrum is multi-peaks at high output power, but the spectrum width is less than 2 nm at 1.94 μm. We estimate the beam quality to be M ² = 2.39, clearly indicating nearly diffraction-limited beam propagation.     

宽间隙反射器相对论返波管同频高阶模式抑制

实现输出模式控制和纯化是过模相对论返波管振荡器物理设计的关键。结合数值仿真和粒子模拟,对一种宽间隙反射器过模相对论返波管内的同频高阶模式激励及其抑制进行了研究,结果表明:模式转换和激励是同频高阶模式的直接来源,合理优化慢波结构、反射器和二极管参数,抑制由工作模式向高阶模式的转化效率,调整由二极管区反射回束波互作用区的模式相位,实现了过模相对论返波管的高效率工作和高模式纯度微波输出;所设计的宽间隙反射器过模相对论返波管输出微波功率中TM01模式的比例高于98%,功率转换效率约40%,工作频率为9.87GHz。     

Characterization of an Intra-Cavity Pumped P2O5-Doped Silica Raman Fiber Laser

We examine the characteristics of a P₂O₅-doped Raman fiber laser pumped by a double-clad ytterbium-doped fiber laser in an intra-cavity configuration. The double-clad fiber laser consists of two high reflection fiber Bragg gratings at 1092 nm, with an active length of 17 m, where the output grating is located at the end of the 400m P₂O₅-doped fiber. The Raman cavity is formed by a 1278-nm-high reflection fiber Bragg grating spliced to the double-clad fiber, and a 50% output coupler. The maximum conversion efficiency at 1278 nm with respect to the input pump power at 915 nm was 26.9%.