填充等离子体的介质契伦柯夫脉塞

 利用自洽线性场理论，普遍讨论了电子在扰动场作用下的三维扰动，进而分别对薄环形相对论电子注和实心相对论电子注在填充等离子体的介质筒慢波波导中激励的契伦柯夫辐射进行了详细的分析，导出了其色散方程和波增长率。分析表明注波互作用是由于慢波系统中的波导模与电子注模耦合所致，填充等离子体后能大大提高注波互作用效率，并详细计算和分析了等离子体密度和电子注半径对波增长率的影响。     

空心介质波导契伦可夫辐射器的粒子模拟

用扩展的1((2)/(2))维相对论电磁模粒子模拟程序模拟了由一束相对论电子,当它以超介质中光速的速率通过一空心介质波导时,所产生的契伦可夫辐射过程。模拟结果表明契伦可夫过程是一种具有很大潜力的高功率短波长辐射源。     

准位相匹配铌酸锂波导倍频特性分析与优化设计

用标量有限元方法计算了周期性极化的铌酸锂光波导中模折射率和模场分布,并在计算中引入铌酸锂晶体折射率与温度变化的关系,分析了准位相匹配铌酸锂波导倍频效率与极化反转光栅周期、基频光波长、波导器件温度等关系.理论分析与实验结果符合得很好.在此基础上,分析了波导制作参数与倍频效率、光栅周期与晶体温度,以及温度带宽与光栅通光方向长度等关系,进而对铌酸锂波导倍频器件进行优化设计. 关键词： 铌酸锂 光波导 准位相匹配 有限元     

未磁化等离子体介质切连科夫脉塞的线性理论

利用线性自洽场理论，讨论了电子在扰动场作用下的三维扰动．在此基础上，对常用的薄环形相对论电子环束在填充未磁化等离子体的介质筒慢波波导中激励的切连科夫辐射进行了详细的分析，导出了此电子环束与慢波系统中任意波导模互作用的色散方程和波增长率，并对其进行了详细的分析和讨论．分析表明切连科夫辐射是由其波导模与电子注模耦合所致．最后详细计算和分析了等离子体密度对色散特性和波增长率的影响 关键词：     

双介质片平面波导契伦科夫自由电子激光

给出双介质片平面波导契伦科夫自由电子激光波导模式的色散关系,求出器件的单程增益并证明存在一组新的波导模式__奇模。讨论了器件的工作频率和单程增益随介质片厚度、电子束能量和电子束通道宽度的变化关系。还研究了这组奇模对器件的影响。     

圆波导契伦柯夫自由电子激光流体描述

用磁流体理论研究了契伦柯夫自由电子激光的性质,导出了空心束驱动的圆柱对称波导契伦柯夫自由电子激光的色散关系,利用色散关系计算了契伦柯夫不稳定性增长率,给出了饱和功率的估计值,考虑了束与介质间间隙对契伦柯夫自由电子激光的影响,并就空心束与实心束情况进行了比较。     

同轴周期慢波结构色散特性的通用数值解法

 运用场匹配法和傅里叶级数理论,提出一种原则上可数值求解任意同轴周期慢波结构色散特性的方法,给出了同轴慢波结构TM_0n模的色散方程。基于该方法,编制了Matlab程序,给出了同轴波纹波导和同轴双波纹波导色散特性,以算例形式分析计算了同轴盘荷波导的色散特性。数值计算结果与多维全电磁模拟软件结果进行比较,证明了该数值算法的准确性和可靠性。     

圆波导劈形端口辐射器的数值分析

 高功率微波运用的圆波导劈形端口辐射器(Vlasov-Nakajima辐射器)的辐射场可视为由平开口圆波导的辐射场和在该辐射场中的劈形圆波导段的散射场叠加而成。运用等效电磁流原理数值求解Kirchhoff-kottler积分获得圆波导TM₀₁模的辐射场,再应用物理光学方法计算劈形段的散射场,并将计算的远场方向图与实验结果比较,获得较满意的一致。     

高功率微波高斯馈源口面场分析

 用波导本征模展开方法对用于高功率微波发射系统的方角锥高斯馈源口面场进行分析，提出结合馈源远场辐射特性和避免高功率击穿折衷选定相应的高斯模注腰半径，进而确定多个波导模幅值，从而为运用模匹配或耦合波理论设计高斯馈源提供依据。     

螺旋波纹波导研究

从耦合波方程出发，利用阻抗微扰法对螺旋波纹波导的边界不规则性进行处理，得到螺旋波纹波导的一般性耦合波传输方程，及耦合系数表达式.根据螺旋波纹波导内模式耦合的规则,得出色散方程并分析其色散特性. 关键词： 螺旋波纹波导 耦合波方程 阻抗微扰 色散特性 本征模     

Creation of multi-frequency terahertz waves by optimized cascaded difference frequency generation

A new scheme which generates multi-frequency terahertz (THz) waves from planar waveguide by the optimized cascaded difference frequency generation (OCDFG) is proposed. A THz wave with frequency ω_T1 is generated by the OCDFG with two infrared pump waves, and simultaneously a series of cascaded optical waves with a frequency interval ω_T1 is generated. The THz wave with a frequency of M-times ω_T1 is generated by mixing the m-th-order and the (m+M)-th-order cascaded optical wave. The phase mismatch distributions of cascaded difference frequency generation (CDFG) are modulated by changing the thickness of planar waveguide step by step, thereby satisfying the phase-matching condition from first-order to high-order cascaded Stokes process step by step. As a result, the intensity of THz wave can be enhanced and modulated by controlling the cascading order of OCDFG.     

A Simple and Effective Method for Calculating the Bending Loss and Phase Enhancement of a Bent Planar Waveguide

A simple and effective method is introduced to calculate the bending loss and phase enhancement of a bent planar waveguide. The wave field is represented in terms of Airy functions and an eigenvalue equation is derived by matching the boundary conditions and the radiation condition in the outer cladding layer. The complex propagation constant is obtained by solving the eigenvalue equation with the Newton-Raphson method, and the imaginary part of the propagation constant gives directly the bending loss of the bent waveguide. The results are compared with the previous experimental and numerical results and are shown to be highly accurate and effective. The phase enhancement due to the bending is also studied.     

Investigation of parameters of terahertz pulses generated in single-domain LiNbO<Subscript>3</Subscript> crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.     

Theoretical research on terahertz wave generation from planar waveguide by optimized cascaded difference frequency generation

A novel scheme for high-efficiency terahertz(THz)wave generation based on optimized cascaded difference frequency generation(OCDFG)with planar waveguide is presented.The phase mismatches of each-order cascaded difference frequency generation(CDFG)are modulated by changing the thickness of the waveguide,resulting in a decrement of phase mismatches in cascaded Stokes processes and an increment of phase mismatches in cascaded anti-Stokes processes simultaneously.The modulated phase mismatches enhance the cascaded Stokes processes and suppress the cascaded anti-Stokes processes simultaneously,yielding energy conversion efficiencies over 25%from optical wave to THz wave at 100 K.     

Photonic-crystal waveguide for the second-harmonic generation

The possibility of efficient second-harmonic generation in the optical range in a planar dielectric waveguide with the active region in the form of a one-dimensional photonic crystal has been theoretically shown. The true phase matching can be achieved by controlling wave dispersion in the photonic crystal. The dispersion equations for the photonic crystal and three-layer waveguide have been self-consistently solved. It is shown that the coherence length may exceed 10 mm.     

Cascaded waveguide phase-matching arrangement

We propose a new method of phase matching in nonlinear waveguides, using the higher-order waveguide mode as an intermediary for efficient second-harmonic or difference-frequency generation, and show that high conversion efficiencies can be achieved with this scheme.     

Compact surface plasmon amplifier in nonlinear hybrid waveguide

Surface plasmon polariton(SPP), a sub-wavelength surface wave promising for photonic integration, always suffers from the large metallic loss that seriously restricts its practical application. Here, we propose a compact SPP amplifier based on a nonlinear hybrid waveguide(a combination of silver, LiNbO_3, and SiO_2), where a couple of Bragg gratings are introduced in the waveguide to construct a cavity. This special waveguide is demonstrated to support a highly localized SPP-like hybrid mode and a low loss waveguide-like hybrid mode. To provide a large nonlinear gain, a pumping wave input from the LiNbO_3 waveguide is designed to resonate inside the cavity and satisfy the cavity phase matching to fulfill the optical parametric amplification(OPA) of the SPP signal. Proper periods of gratings and the cavity length are chosen to satisfy the impedance matching condition to ensure the high input efficiency of the pump wave from the outside into the cavity. In theoretical calculations, this device demonstrates a high performance in a very compact scheme(~3.32 μm) and a much lower pumping power for OPA compared with single-pass pumping. To obtain a comprehensive insight into this cavity OPA, the influences of the pumping power, cavity length, and the initial phase are discussed in detail.     

Nonlinear optical terahertz wave sources

We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG, process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz, waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.     

Second harmonic generation in a strontium barium niobate crystal with a random domain structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.     

Broadband generation of terahertz radiation in a waveguide

We report a novel geometry that allows for the phase-matched generation of broadband terahertz radiation in a polymer-based parallel-plate metal waveguide by means of optical rectification. Both the optical pump beam and the generated terahertz radiation propagate in the fundamental mode of the waveguide. This allows for noncritical phase matching over a broad range of terahertz frequencies. We demonstrate guided-wave interaction lengths of up to 3 mm.