Sideband suppression in free electron lasers using a grating rhomb

The phase shift produced by a grating rhomb is included in free-electron laser (FEL) pulse calculations to investigate whether or not grating rhombs can be used to suppress the sideband instability. The idea is that because the group travel time through a rhomb is an increasing function of the laser wavelength, an FEL oscillator can be designed such that the optical pulse at a chosen central wavelength and the pulse of electrons overlap spatially when they enter the wiggler. Over many passes, light in a small bandwidth about the chosen wavelength receives the greatest amplification because it overlaps the electrons, and light at sideband instability wavelengths that does not overlap the electrons is suppressed by losses in the oscillator cavity. For a 5-m tapered wiggler, the range in rhomb dispersion and cavity loss that yields acceptable FEL performance is defined. At low values of cavity loss, for example 15%, a wide range of rhomb dispersion exists for which both the sideband instability is largely suppressed and the energy extracted from the electrons is high. At larger values of cavity loss, for example 30%, a critical value for rhomb dispersion exists below which the laser pulse is compressed by the rhomb, leading to reduced energy extraction     

Measurement of the electric field pattern of a Fabry-Perot resonator used in quasi-optical gyrotrons

The field pattern of the resonator used in a quasi-optical gyrotron operating in the millimetre wave range is measured. Two resonators are studied: one composed of a spherical mirror and an ellipsoidal grating and the other symmetric using two mirrors with annular slots. The measurements indicate that the electric field distribution is gaussian, in spite of the complex geometry of the resonator, and thus provide an experimental basis for the assumption often used to compute the efficiency of quasi-optical gyrotrons.     

Sub-nanosecond pulselength millimetre wave generation with a phase modulated quasi-optical gyrotron

It is shown that a quasi-optical gyrotron operating with a current modulated second beam can produce a train of phase-locked millimetre wave pulses.     

A mm-wave four-port quasi-optical circulator

A four-port quasi-optical circulator has been designed, built and tested for operation in the millimetre wave band. The specific circulator reported here was investigated over a 20 GHz bandwidth, centred on 94 GHz. Good performance characteristics were obtained over this bandwidth. Isolations of better than 30 dB and insertion losses of around 0.5 dB were achieved. The input vswr was found to be less than 1.35 over the whole bandwidth investigated and vswr's of less than 1.1 were achieved for narrower bandwidths. It is possible of course to configure the 4-port circulator as a 3-port circulator or indeed as 2-port isolator. Further optimization and consequent improvements in performance are almost certainly obtainable.     

3-mirror quasi-optical power combining of millimetre-wave sources

In order to achieve high power readily for applications such as plasma heating in fusion, it is required that the outputs of several gyrotron tubes be combined. A new kind of 3-mirror quasi-optical power combiner has been presented and studied in this paper. Both theoretical and experimental study point out that the proposed power combiner has attractive features. It seems that this 3-mirror quasi-optical power combiner may not be used only for high power millimetre gyrotron power combining, but also for millimetre and submillimetre solid state power combining     

Astigmatism in tapered slot antennas

A series of scale model measurements of transverse electromagnetic mode tapered slot antennas are presented. They show that the beam launched by this type of antenna is astigmatic. It is shown how an off-axis spherical mirror can be used to correct this astigmatism to allow efficient coupling to quasi-optical systems. A millimetre wave antenna and mirror combination is described and, with the aid of solid state noise diodes, the coupling of the launched beam to a quasi-optical spectrometer is shown to be in good agreement with that predicted by the scale model measurements.     

Quasi-optical gyro-peniotron at high cyclotron harmonics

This paper proposes a new kind of device operating at short millimetre wavelength, in which RF radiation generation is based on the interaction of a beam of sub-relativistic gyrating electrons and the RF standing-wave field of a quasi-optical resonator. This device can be called ‘quasi-optical gyro-peniotron’ because it is similar in electron-beam configuration, microwave circuitry and cyclotron resonance properties to the quasi-optical gyrotron and in its operational mechanism to the peniotron. Starting from relativistic equations of motion for the electrons, the calculation formulae are derived. Using computer simulation, a beam-wave interaction efficiency of 43% is obtained for the third cyclotron harmonic at frequency 100 GHz, beam voltage 80 kV and the applied magnetic field 13·3 kG, indicating that this tube will be potentially a high-efficiency high-power source not requiring a superconduction magnet system even within the 150 GHz band.     

菲涅耳菱体隔光器的实验研究

一、引言 在用于激光加热等离子体研究的高功率激光系统中,为了防止从靶面及等离子体反馈回激光放大器序列的反向激光被进一步放大到足以毁坏光路中光学元件的功率密度,已经采用相应孔径的普克耳斯盒电光开关和磁光开关等光学隔离器(也称为隔光器),达到良好的效果。进一步扩大隔光器的通光孔径时,我们为六束高功率激光装置LGJ—Ⅱ研制成60毫米孔径的菲涅耳菱体隔光器,作为末级隔光器。本文描述有关的实验结果。     

光相位延迟器延迟量的智能化测量

从琼斯矩阵理论出发，利用偏振镜、分束器和锁相放大器以及微处理器，搭建了测试系统，实现了数据的自动化处理。利用闭环反馈控制系统和高精度步进马达，对旋转角度进行了反馈控制，利用Si光电探测器，使用范围覆盖近紫外到近红外，创建的测量系统具有测量精度高，重复性好，自动化程度高，操作简单的特点。经过对菲涅耳菱体型相位延迟器件的测试，测量误差小于0．3％。该系统也可以用来测量其他类型器件的相位延迟，在偏光测试领域有着广泛的应用。     

Gyrotron with quasi-optical cavity of special configuration†

A quasi-optical cavity of special configuration suitable for use in a high-power gyrotron operating at short millimetre and submillimetre wavelength has been developed. A gyrotron using this new type of cavity is under development. Both the linear and non-linear analyses are worked out. Digital calculation predicts efficiency of 37% at V ₀ = 50 kV.     

An Adjustable Quasi-Optical Bandpass Filter---Part I: Theory and Design Formulas

A quasi-optical bandpass filter suitable for millimeter and submilliieter wavelengths and in the far infrared region is described. It consists of three or more wire-grid polarizers with quarter-wave spacings. The filter has the advantage over conventional quasi-optical filters, e.g., Fabry-Perot filters, that its bandwidth and the shape of its frequency response are adjustable. This is achieved by changing the angular orientations of the wires of the different polarizers. The filter requires the input electric field to be linearly polarized in a direction perpendicular to the wires of the first grid. The theory of operation is presented and design formulas for the filter are given, under the assumption that ideal wire-grid polarizers are employed. The effects of using realistic grids on the performance of the filter are dealt within another paper.     

Cold tests and modeling of low-Q quasi-optical resonators

Measurements are reported on low-Q open resonators at 85 GHz with a large centered coupling hole in each mirror. This is the first study of open resonators with large diffraction losses and efficient input coupling at millimeter wavelengths. The input resonator for a quasi-optical gyroklystron is described with quality factor Q=2000 and 13% round-trip losses where approximately 65% of the incident power in the waveguide is coupled to the fundamental TEM₀₀ mode. Two models have been developed to calculate the coupling from the rectangular waveguide through a circular aperture into the open resonator. Both models properly account for the increased diffraction loss of the resonator when coupling apertures are present. Measured results are in reasonably good agreement with values calculated using both scalar diffraction theory and an equivalent-circuit model     

Quasi-optical antenna duplexers

The development of antenna duplexers for transceiving quasi-optical systems of millimeter and submillimeter wavelengths are considered for two groups of devices — passive and active action. Duplexers, using properties of quasi-optical hybrids, belong to the first group, and duplexers on the base of semiconductor or ferrite devices are included to the second group. Results of developments and investigations of considered devices in the millimeter wave range are presented.     

Temperature compensated,high permittivity dielectric resonators for millimeter wave systems

The properties of selected low loss, temperature compensated high permittivity microwave and millimetre wave dielectrics are summarised and experimental convenience suggests that planar hybrid microstrip is the preferred circuit technology for the initial studies of dielectric resonators at the millimetre wavelengths. Data are presented for temperature stabilised oscillators and some simple band pass filters are discussed.     

Theoretical relationship between rain depolarisation and attenuation

An approximate theoretical relationship between rain depolarisation and attenuation, valid for Earth-space paths at microwave and millimetre wavelengths, is presented. The relationship is sufficiently general that it allows the prediction of rain-depolarisation statistics from attenuation statistics available for other frequencies, polarisations and elevation angles.     

A New Type of Quasi-Optical Waveguide Component (Correspondence)

A new form of quasi-optical waveguide component has been designed that is based on use of the Brewster angle and on the rapid variation of reflectance and change of phase near the critical angle. An attenuator was built in S-band waveguide and its performance measured at 35 and 94 GHz. Diffraction is the limiting factor on the device operation, so practical device applications will likely be limited to 1-mm wavelengths and less.     

A novel quasi-optical monopulse-tracking system for millimeter-waveapplication

A novel, low-loss, polarization-independent quasi-optical monopulse comparator (QOMC) has been developed as an alternative to the commonly used waveguide comparator. It has been implemented as an experimental model with an aperture of 23×23 wavelengths for a 140-GHz monopulse tracking antenna fed by a beam waveguide. The QOMC consists of two cascaded identical stages (sections) to perform azimuth and elevation angle measurements. Each comparator stage is composed of only two plane-parallel reflectors and a low-loss quasi-optical 3-dB beam splitter symmetrically positioned in between them. Extensive model calculations of a single QOMC stage with square aperture dimensions between 10 and 25 wavelengths, and experimental investigations of the realized two-stage QOMC in a beam-waveguide field were performed. Owing to its sufficient high dynamic range at bandwidths of up to about 300 MHz, and particularly, its low absorptive loss of less than 3.5% per stage at 140 GHz, this QOMC is suited ideally for millimeter-wave applications     

Fresnel zone plate antennas at millimeter wavelengths

The performance characteristics of Fresnel zone plates are described with emphasis on applications at millimeter wavelengths. Recent results are included along with a summary from numerous earlier investigations. Parameters described include design information, efficiency, bandwidth, focal characteristics, off-axis performance, axial intensity dependence, aberrations, and far-field pattern. Both transmission and reflection configurations are discussed, mostly for cases where the focal length and diameter are comparable. This type of zone plate has advantages (compared to a lens or paraboloid) of reduced loss, weight, volume, and cost, as well as simple planar construction, with similar diffraction-limited beamwidth and major sidelobe performance, but lower efficiency.     

A two-dimensional quasi-optical power combining oscillator array with external injection locking

A quasi-optical power combiner for a 4/spl times/4 IMPATT oscillator array has been designed and experimentally investigated. The combiner consists of a bi-periodic dielectric phase grating which transforms the near field of a rectangular horn array into a pseudoplane wave. The horn array is excited by oscillators which operate uniformly in both amplitude and phase. A parabolic mirror with a superimposed surface relief couples the pseudoplane wave into a rectangular output horn antenna. In principle, the combiner has no restriction in inter-element spacing and is hence scalable up to submillimeter wavelengths without degradation of power combining efficiency. The quasi-optical design has been verified by scalar field measurements in several planes. The oscillator matrix is injection-locked by a master oscillator from the output port. A continuous wave output power of 1.3 W with an overall power combining efficiency of 70% has been measured at 65 GHz.     

Small-signal theory of the transistor transit-time oscillator (translator)

The operation of a junction transistor structure as a microwave negative resistance diode is described. Small-signal negative resistance, dynamic capacitance and noise measure are evaluated for a device with doping profiles representative of standard silicon planar technology. Useful operation at centimetre and millimetre wavelengths is anticipated.