Cylindrical waveguide as a power transmission medium—Limitations due to mode conversion

A cylindrical waveguide operating in the TE01mode can transmit powers of the order of thousands of megawatts in frequency ranges below 10 Gc/s with acceptable copper losses over distances of hundreds of miles, provided it can be made with sufficient accuracy to avoid excessive power transfer into other unwanted high-loss modes. The tolerances on the guide straightness appear to be the most severe limitation on the use of this type of guide as a high power system component. These tolerances are evaluated here and found to be of the order of 10 to 100 mils in distances of a few hundred feet for typical values of the other parameters such as 1.0 Gc/s in a 2-meter-diameter guide providing 1 dB power loss in a distance of 200 miles. Tolerances on diameter variations and cross-section ellipticity are also evaluated and found to be of the same order of magnitude as the deviations from straightness, but should be easier to control by the proper manufacturing process. It is also shown that low loss bends (about 0.01 dB/mile) with bending radii of the order of 0.5 mile are possible if the length of the bend is accurately controlled and the guide has a dielectric lining of the proper thickness.     

Analysis and correction of echo due to mode conversion in WC-281waveguide

Testing of the antenna/waveguide systems of a long-haul, high-capacity microwave radio route showed unexpected high levels of group delay ripple. Initial analysis of these measurement results incorrectly indicated echo levels as high as 35 dB below wanted direct signal, as interpreted using a simplistic analysis technique which was later shown to be as much as 10-15 dB too pessimistic. Application of special Fourier analysis techniques to the test results subsequently showed that many of the cases of concern were in fact well within established system requirements. In other cases, it was found that installation of TE₂₁ filters could readily be used to absorb the echo energy. The performance of the QAM radio used in this application was also characterized in terms of the residual BER tolerance to multiple echoes. The source of the echoes was isolated to TE₂₁ mode generation within the WC-281 waveguide. To ensure the quality of new installations without the need for the application of filters, a method of pre-installation testing of waveguide sections to determine their individual moding levels was also established     

Modal matching applied to a discontinuity in a planar surface waveguide

A modal-matching method is used to evaluate the transmission and reflection coefficients at a junction between two planar surface waveguides. Results are obtained for parameters which represent the junction between a solid-state laser and a planar optical waveguide. Favourable comparison is made with results obtained from an integral-equation method.     

Calculated mode conversion at the sunrise boundary in the Earth-ionosphere waveguide

Mode conversion of v.l.f. radio waves in the Earth-ionosphere waveguide is considered. It is indicated that the transition distance ?d between day and night may be deduced, in a semiquantitative manner, from the observed fading characteristics. Values of ?d in the range 300?700 km are suggested.     

Modal transforms in rectangular waveguide

Transformation matrices that allow conversion between normalised s-parameters in TE_z/TM_z, TE_x/TM_x and TE_y/TM_y format are presented. The transforms are simple to implement and are essential when comparing results expressed in terms of different modal solutions     

Mode conversion in helix loaded waveguide

Forward coupling of a fast TE-like mode is an interesting and desirable option in the gyro-TWT amplifier. We consider in this paper a mode conversion at the abrupt junction of two infinitely long sheath-helix-loaded waveguides. A helix mode traveling from the left to the right side excites fast TE-like modes are numerically calculated by taking dominant terms in the into TE-like modes are numerically calculated by taking dominant terms in the eigenmode-expansion method. The bandwidth (3 dB) for TE-mode-like coupling in the second (right-hand-side) waveguide is found to be over 40$.     

Modal analysis of a planar waveguide with gain and losses

In this study, we analyze the waveguiding properties of a planar waveguide amplifier in which losses and gain can be present simultaneously. It is found that the subsequent modes comprise both loss and gain modes. Also, the dependence of the gain on the state of polarization turns out to be significant for realistic dielectric structures. For strong losses or gain, the standard transfer matrix approach may become numerically unstable, therefore, a scattering matrices formalism is employed. A semiconductor-like gain profile enables us to study the gain as a function of ω for realistic laser amplifier structures     

Modal analysis of the `gap effect' in waveguide dielectricmeasurements

In waveguide measurements on dielectric slabs, small air gaps between the guide walls and the dielectric sample are found to be capable of radically altering the complex reflection and transmission coefficients of the excitation mode. The modal-analysis representation is used to compute these coefficients for low-loss and high-loss samples with air gaps. The `gap effect' is explained quantitatively by considering the influence of the dominant `slab mode,' which focuses its energy into the dielectric slab, and the dominant `gap mode,' which focuses its energy into the air gap. An experimental approach, which consists of filling the air gap with conducting paste, is shown to essentially correct the problem     

Efficient infrared-terahertz pulse conversion in waveguide semiconductor structures

It is shown that the use of a waveguide semiconductor structure provides a means for substantially increasing the degree of conversion of a high-power infrared pulse to a terahertz pulse by optical rectification of the infrared pulse in a medium with a quadratic nonlinearity of the permittivity. Using the example of the C/GaAs/C waveguide heterostructure, the optimal parameters of both the structure as such and the infrared pulse are determined and the duration and shape of the output terahertz pulse are calculated.     

非对称共面波导弯曲结构模式转换的研究

针对传统共面波导弯曲结构传输损耗较大的问题,研究了非对称共面波导弯曲结构.将时域多分辨率分析算法与从Y参数提取c模、π模特性阻抗的方法相结合,推导了二端口两种模式的散射矩阵计算公式.计算了非对称共面波导弯曲结构的传输系数.在理论分析的基础上,提出了一种测试模式间能量转换的实验装置.理论分析和实验结果证明：相对于传统共面波导弯曲结构,非对称共面波导弯曲结构的模式间能量转换更低,传输损耗更小.     

Mode conversion at discontinuities in finite-width conductor-backedcoplanar waveguide

The effect of overmoding in a conductor-backed coplanar waveguide is discussed, with special emphasis on mode conversion of discontinuities. The coplanar waveguide gap end and shorted end are studied by means of a fully electromagnetic application of the moment-method technique. Significant conversion occurs at the gap end. The results of a simple experiment which partially verify the analysis are described     

Mode conversion losses in highly overmoded waveguide cavities

A technique to quantify the effects of mode conversion losses in highly overmoded waveguide cavity resonators using the WKB method is presented. The WKB theory is accurate for weakly irregular waveguide cavities with small amounts of mode conversion. This approach to the problem has the advantage that the mode conversion losses in waveguide cavities can be quantified quickly and cheaply. The objective is to compute the amplitude and phase of a resonant mode in the cavity to zeroth order (i.e. without regard to mode conversion effects), and then use amplitude and phase of the computed zeroth-order fields to compute the amplitude of the converted mode. This approach is valid to the extent that the converted modes are not a major perturbation to the resonant mode. The results of the theory are presented in the context of the trade-offs associated with the design of an 85 GHz TE_1.3 phase-locked gyroklystron oscillator experiment. Mode conversion losses in the input cavity are a particular problem, and place limits on the amount of usable beam current (and therefore the output power capabilities of the gyroklystron). The isolation of the input and output cavities due to mode conversion must be carefully considered in any design     

Approximate mode conversion coefficients in the Earth-ionosphere waveguide

Constant impedance boundary conditions have been used by many authors to obtain mode reflection and transmission coefficients for propagation across a land-sea discontinuity. The impedance boundary conditions are here replaced by exact boundary conditions and similar formulae are obtained for propagation between a diffuse inhomogeneous isotropic ionosphere and a homogeneous isotropic earth. These formulae permit the use of any ionospheric model with vertical inhomogeneity although some simplification results if impedance boundary conditions are used at the ground. A numerical example is given to illustrate the significance of the formulae.     

Geometric phases in magnetooptic channel waveguide devices

Geometric phases, generated by a coupling process between TE and TM polarizations through anisotropic chiral media-magnetooptic channel waveguides-are determined. In fact, many of the conventional phase and frequency shifters are based on this geometrical effect, nevertheless, the physical origin of these phase factors (spatial and temporal ones) has never been explained. In this work a physical interpretation, based on the topological phases theory, is given. Likewise, an integrated interferometer is proposed for both generating and checking these phases by changing the characteristics of the channel waveguides. The relationship between geometric factors and phase and frequency shifters is shown in the work     

Modal dimensionality and dephasing effects in slab waveguide electrooptic filters

The voltage-dependent transfer characteristics of multi-mode slab waveguide electrooptic filters have been reexamined analytically in considerable detail, with special attention to differential modal phase delays, modal dimensionality, and attenuation. An important determination was the minimum number of modes (i.e., minimum dimensionality) needed for a reliable representation of the model. We find that a dimensionality of 10 is essentially adequate for the device modeled, having thickness approximately 130 μm, operating near a 0.475-μm wavelength. In addition, we find that inclusion of an initial dead length, with the attendant depbasing of modes, is an important factor in achieving good agreement with experimental data. Suitable allowance for mode-dependent attenuation also aids in improving this match.     

PPLN光波导中传输光波模场重叠因子分析

为了研究铌酸锂光波导中传输光波模场重叠因子对波长转换效率的影响,采用有限差分的方法进行了波导中传输光波模场及差频波长转换效率的研究,得到了波导中1560nm基频光和780nm倍频光光场分布以及引入模场匹配因子进行修正后的差频波长转换效率。结果表明,修正后的转换效率理论值与实验结果更加吻合,并且能够得到直接键合铌酸锂波导的最佳尺寸。这为周期极化铌酸锂脊形光波导的设计提供了理论依据。     

Analysis and measurement of polarization conversion in a periodically loaded dielectric waveguide

The full-vectorial beam-propagation method with the improved finite-difference formula is applied to the analysis of a TE/TM mode converter based on an asymmetric periodic loaded waveguide. The polarization conversion behavior is measured at a microwave frequency. It is demonstrated that the calculated complete conversion length is in agreement with the experimental result.