Method for excitation of a high-order spatial mode in the input cavity of gyroklystron

Microwave-power input devices for a multimegawatt pulsed gyroklystron that works in the centimeter-wavelength range using a relativistic electron beam are considered. A device representing a supply waveguide with rectangular cross section that is tangentially interfaced with the side wall of a hollow cylindrical cavity is proposed for the excitation of a high-order rotating spatial mode as the working mode of the input cavity. The efficiency of the device is numerically optimized and measured to be 70% at a low power level using a broadband Bragg reflector for the input variant that provides the excitation of the TE_6,1,1 mode.     

Optical propagation of the HE11 mode and Gaussian beams in hollow circular waveguides

The propagation of the HE₁₁ mode and Gaussian beams in hollow oversized circular waveguides is analyzed using optical theories. Different types of waveguides are considered : hollow dielectric or conducting waveguides, dielectric-lined waveguides, corrugated waveguides. General formulas are derived which give the power transmission through these different guides. The best wall materials and structures are determined from a comparison of the waveguide transmissions, at the infrared and millimeter wavelengths. The question of the coupling between the HE₁₁ mode and Gaussian beams is discussed and from a review of coupling coefficients derived before, an optimum value is pointed out. The problem of matching a Gaussian beam into circular waveguides in order to achieve the maximum power transmission is analyzed. These results are of interest for infrared lasers or waveguide applications and for Electron Cyclotron Wave (ECW) systems at the millimeter wavelength.     

Coupling impedance for a rippled electron beam in a cylindrical guide carrying the E01 mode

A hollow beam of electrons, injected into an axial magnetic field from a field-free region, has a diameter which varies periodically at the cyclotron frequency. An expression is derived for the coupling impedance between such a rippled beam and the fast E01 mode in a cylindrical waveguide.     

The smallest possible whispering-gallery-like mode in the square lattice photonic-crystal slab single-defect cavity

The smallest possible whispering-gallery-like mode (WGM) formed in a free-standing square-lattice photonic-crystal slab single-defect cavity is studied by using the three-dimensional finite-difference time-domain method. The WGM is nondegenerate and the only existing resonant mode in the transverse electric mode bandgap of the square lattice. High-quality factors are achieved from this mode by suppressing both the in-plane and the out-of-plane optical losses. By optimizing the structural parameters, a total quality factor of 18 000 and a vertical quality factor of 40 000 are obtained from the WGM. The quality factor does not decrease significantly in the presence of a current flow path at the center of the cavity due to the of electromagnetic energy at the cavity center, which means this mode is advantageous to achieve electrically pumped photonic-crystal lasers. In addition, by combining the high-quality factor with wavelength-size small-mode volume, very high Purcell factors >8000 are achieved, the largest value ever reported. The WGM of the square-lattice single-defect cavity is promising for both cavity quantum electrodynamics research and the low-threshold microcavity laser.     

Profile-broaden ultraviolet lasing from whispering gallery mode cavity in crown-like zinc oxide

Based on the transfer matrix method,a detailed theoretical and numerical study on double-phase-shifted fiber Bragg grating(FBG)is investigated.Temporal responses of the double-phase-shifted FBG to optical pulse are analyzed and the influence of the two phase-shifts’position on the reflected output pulse is evaluated.Results demonstrate that very different temporal pulse waveforms can be achieved by adjusting the length ratio(α=L2/L1).Specifically,a transform-limited Gaussian input optical pulse can be shaped into flat-top square pulse(α=1.81)or two identical optical pulse sequences(α=1.93).     

Quansi-optical mode converter for Ka-band application on a gyrotron

A series of electron cyclotron masers (ECMs) has been designed and successfully operated in the frequency range of 6 to 200 GHz over the past 10 years at Strathclyde University. The current Mk. VI ECM operated between 35–200 GHz, the TE _{0 1} ⁰ mode (35.2 GHz) being the lowest order cavity mode excited. Investigation of this ECM and quasi-optical converter combination was made to establish to what extent a Gaussian beam could be produced from the TE _{0 1} ⁰ gyrotron cavity mode. An antenna of the Vlasov (step-cut radiator) configuration was chosen and successfully developed in a series of low-power experiments up to the W-band frequency regime. The low-power experiments demonstrated that such an antenna system could be constructed and operated in the K_a-band frequency region without resorting to high precision quasi-optical components. The final version of the quasi-optical antenna was externally coupled to the output window of the maser and a plane polarized pencil beam was successfully obtained from the combined maser/Vlasov antenna mode converter. A theoretical model involving a single TE _0n ⁰ input mode was chosen to analyse the output beam characteristics from the Vlasov antenna system. Comparison was made between the single mode theoretical model and the experimental results obtained from the maser.     

Study of whispering-gallery-mode in a photonic crystal microcavity

The whispering-gallery-mode (WGM) photonic crystal microcavity can be potentially used for miniaturized photonic devices, such as thresholdless lasers. In this paper, we use plane wave expansion (PWE) method and study the WGM of H2 photonic crystal microcavities which are formed by removing seven center air holes in a photonic crystal. The WGM in these large-size cavities has some advantages compared with single defect WGM in the view of real device applications. We analyze the nearby air hole e...     

Calculation of radiation from a quasi-optical reflector antenna for whispering gallery mode

We propose a method to calculate the radiation characteristics of a quasi-optical reflector antenna for whispering gallery mode called Vlasov's antenna. Correction of the shape of the reflector, which was a parabolic cylinder in the original paper by Vlasov et al. (1975), is performed using wave-normal rays. The radiation field is calculated by means of an image source which is obtained with the geometrical optics. Some numerical results show that the radiated beam is gaussian-like in far zone. A scheme of high-power application (500kW, 106·4GHz, TE_12,2) to a beam transmission system is also presented     

全反射氧化锗空芯光纤弹性弯曲半径的计算与验证

金属的塑性形变特性使金属毛细管空芯光纤在激 光传输过程中容易产生不可逆塑性形变,依据金属 管材的弹塑性弯曲理论和液相沉积金属毛细管氧化锗(GeO₂)空芯光纤的工艺要求, 本文对铍铜和不锈钢毛细管GeO₂全反射空芯光纤的弹性弯曲半径进行了计算并通过实验验证。结果表明,铍铜和不锈钢 空芯光纤的最小弹性弯曲半径的计算值与实验值相吻合。     

Record‐Low‐Threshold Lasers Based on Atomically Smooth Triangular Nanoplatelet Perovskite

Single‐crystalline perovskites are ideal candidates for lasing and other optoelectronic applications. Although significant efforts have been made to grow both bulk single‐crystalline perovskites in liquid solution, their dimensions are still too large to make nanoscale whispering‐gallery‐mode (WGM) resonator based lasers that possess high quality (Q) factor and small volume. Besides, most reported perovskite resonators do not possess atomically smooth surfaces and facets, which limits the Q and thereby increases the lasing threshold. Here, atomically smooth triangular PbI₂ templates are fabricated on a mica substrate by the vapor phase deposition method and are converted to atomically smooth perovskites which have regular and unwrinkled facets with average surface roughness less than 2 nm. By using a CH₃NH₃PbI₃ nanoplatelet with a side length of 27 µm and thickness of 80 nm, room temperature WGM lasing with a Q up to 2600 is demonstrated, the highest reported for hybrid organic–inorganic perovskite nanoplatelets. In addition, the volume of the WGM mode is reduced significantly in comparison with the prior reports. The realized high‐quality triangular CH₃NH₃PbI₃ perovskite nanoplatelets with high Q factor and small volume are expected to perform as ideal cavities for long pulse durations lasers and would find potential applications in integrated optoelectronic devices.     

Design of a quasi-optical mode conversion system with variable output beam size

The quasi-optical system consisting of a quasi-optical antenna and two parabolic mirrors can convert the TE₀₃ mode output (f = 301 GHz) of the Gyrotron FU IV into a well collimated, linearly polarized beam. This system offers the possibility of tuning the size of the beam produced from 5 mm up to 24 mm by rotating and moving the two parabolic mirrors.     

Generation of millimeter wave Gaussian beams and their propagation in a dielectric waveguide

Linearly polarized fundamental mode Gaussian beams were generated and coupled to a hollow circular oversized dielectric waveguide by placing the waist of the beam at the guide entrance. The transmission properties of the waveguide were characterized as a function of frequency for a variety of coupling conditions. These conditions included changes in the input beam waist radius, angle of incidence, and displacement perpendicular and parallel to the guide axis. It has been found that: 1.) power transmission is maximized when the waist of the input beam is centered at the guide input, injected normally, and has a radius of 0.43 times the waveguide radius, 2.) power transmission decreases rapidly with increasing angle of incidence and the rate of that loss increases with frequency, 3.) the waveguide preserves the linear polarization of the input beam, 4.) power transmission in the fundamental waveguide mode is not greatly affected by moderate displacements in the input beam position, and 5.) upon exit from the waveguide the launched EH₁₁ mode propagates as a fundamental mode Gaussian beam in the quasi-far field. The results compare favorably to the transmission theory of Belland and Crenn and approximately to the near and far field mode pattern theory of Degnan.     

X波段同轴腔多注速调管的研究

开展了具有同轴谐振腔互作用电路和双模工作杆控电子枪的X波段同轴腔双模多注速调管的研究工作.结合数值计算和冷测实验,对工作于TM₃₁₀高次模的同轴谐振腔模式分布和特性参数进行研究,获得了可满足多注速调管要求的谐振腔特性阻抗和良好的模式稳定性.采用具有双控制极的新型杆控多注电子枪及电子光学系统,可使多注速调管具有双模的新工作特性,通过数值模拟获得了优化的几何参数和具有良好层流性和波动性的空心多电子注.对采用6个电子注和5个谐振腔的X波段多注速调管进行了注波互作用大信号计算,结果表明当电子注电压为21.5kV,脉冲电流为14.4A时,可在30MHz频带范围内获得的100kW左右的脉冲输出功率,互作用效率大于30%,增益大于36dB.     

An improved design for quasi-optical mode conversion of whispering gallery mode gyrotron radiation

Results are reported of a theoretical and experimental investigation of a quasi-optical mode converter for the transformation of whispering gallery mode gyrotron output into a linearly polarized Gaussian like beam. The mode converter consists of a helically cut waveguide launcher, similar to that originally proposed by Vlasovet al, followed by a focusing mirror. Theoretical results using aperture field methods indicate that the length of the waveguide launcher is of critical importance in providing a confined radiation pattern. Experimental results on the radiation pattern were obtained for several launcher lengths using a 0.6 MW, 149 GHz pulsed gyrotron operating in the TE_16,2 mode. Radiation pattern results for the optimum launcher length agree well with theoretical calculations using the Stratton-Chu aperture radiation theory for unperturbed waveguide modes. A mirror focusing in the azimuthal direction was designed by a geometrical optics approach to focus the radiation coming from the launcher. Good focusing with 91.4% efficiency (power in the focused beam divided by gyrotron power) was found experimentally using the combined launcher and mirror with the pulsed gyrotron. These results indicate that quasi-optical antennas are useful for transforming high order, high frequency gyrotron modes into directed beams in free space.     

Simple Scheduling Algorithms for Use with a Waveguide Grating Multiplexer Based Local Optical Network

As the need for greater bandwidth in local-area networks grows, wavelength-division multiplexing (WDM) is gathering attention as a viable successor to Gigabit Ethernet technologies. In this paper we introduce a new WDM optical LAN architecture based on the waveguide grating multiplexer (WGM) rather than the traditional passive star coupler (PSC). An N-port WGM allows N ² × N ² connectivity via only N physical wavelengths, due to its inherent space-division multiplexing property. Wavelength-routed networks based on the WGM promise to be significant components of future LAN and WAN technologies not only because of their efficient use of wavelengths, but also because they have been implemented as integrated devices. We propose simple, low-complexity TDM schedules for interconnecting MN nodes (M nodes per port) with a WGM in a local environment. Each node is equipped with a single tunable transmitter and a single tunable receiver (both of which can access all N wavelengths). Various transmitter and receiver tuning latencies are considered. We show that, for negligible tuning latencies, aggregate network throughput approaching min(MN, N ²) can be achieved, and for tuning latencies on the order of a packet length or more, throughput on the order of N can be achieved. Since these performance metrics are vastly superior to that of an equivalent PSC-based system (whose maximum throughput is limited by the number of wavelengths, N), we propose that the WGM be considered as an alternative to the PSC for enabling WDM LANs and multiprocessor interconnects.     

Theoretical analysis of whispering-gallery mode dielectric resonator in MM-wave MIC

The whispering-gallery mode (WGM) disk type dielectric resonator used in millimeter wave microstrip integrated circuits has been computed rigorously by the radial mode matching method. The dependence of resonant frequencies of WGM's on the size of shielding box and Q factors of WGM's relating to leakage loss in the case of parallel-plate shielding structure have been analyzed.     

影响柱形微腔回音廊模激光抽运阈值能量的因素

将石英光纤浸入染料溶液中形成圆柱形微腔,并采用近轴向消逝场抽运的方式,激发染料溶液的激光增益。柱形微腔回音廊模(WGM)激光抽运阈值能量的高低与抽运激光的消逝场和回音廊模消逝场的空间重叠程度有直接关系,同时也需要考虑低阶与高阶回音廊模损耗的影响。通过改变染料溶液的折射率以及光纤直径,来改变回音廊模的消逝场分布,从而改变两种消逝场空间重叠的体积以及回音廊模的损耗。实验结果表明,柱腔直径不变时,存在一个最佳的溶液折射率值;同时,溶液折射率不变时,也存在一个最佳的柱腔直径,使得一阶回音廊模激光抽运阈值能量最低。     

The measurement of the phase constant of EH11 mode in the hollow dielectric waveguides

In this paper, we presented the results of experiments of propagation phase shift in the hollow dielectric waveguides of the radius a～20λ with farinfrared laser (wave length λ=0.337mm). The experimental results agree well with theoretical expecting values. The results also show that it is to exist a coupling length between TEM_oo mode and the funtamental hybrid waveguide EH₁₁ mode. The coupling length is obtained from our measurement.     

Design of hollow and strip beam guns

This paper covers a study of strip and hollow beam guns. An automatic electron trajectory tracer has been used to study the properties of the slit-anode lens common to these types of guns. A thin beam approximation was made so that a flat-bottom electrolytic tank could be used. The results of this study have been incorporated into gun design charts. Eight hollow beam guns have been built and tested experimentally. The experimental data of these guns compare favorably with the data obtained from the anode-lens study. The limitation of thermal velocities on the maximum area compression of strip and hollow beam guns is discussed. The experimental results of a focusing scheme which uses a uniform magnetic field to focus a hollow beam down concentric drift tubes are discussed.