Prediction of TE-, TM-, and hybridmode transmission losses in gaps of oversized waveguides using a scattering matrix code

The transmission losses of different modes in gaps in oversized waveguides are calculated using a modified scattering matrix formalism. Measurements performed for the modes TE₀₁, TE₁₁, TM₀₁, TM₁₁ and HE₁₁ are in good agreement with the calculated numerical results. Both numerical and measured values are compared with existing analytical formulas.     

X波段过模弯曲圆波导TM01-HE11模式变换器研究

在波束波导和反射面天线的馈源应用中, 为了产生低副瓣且方向图等化的高斯波束, 需要将高功率微波转换为准高斯模HE₁₁模辐射. 本文利用弯曲圆波导可同时从TM₀₁模产生TE₁₁模和TM₁₁模的原理, 提出了采用双弯曲过模圆波导结构直接将TM₀₁转换为HE₁₁的模式变换器, 避免了常规微波领域中首先将TM₀₁转换为TE₁₁再用波纹式或半径渐变式TE₁₁-HE₁₁转换器转换为准高斯波束功率容量不足或尺寸过长的不足. 基于模式耦合理论和Taguchi优化算法对模式变换器的弯曲半径、相移直端长度及引入位置进行了优化, 使输出的TE₁₁和TM₁₁成一定比率, 以组成HE₁₁模式, 并对设计的模式变换器进行了全电磁波仿真分析, 结果表明输出波束的标量高斯含量在9.05–9.8 GHz范围内均高于99%, 理论功率容量可达4.5 GW. 关键词： 高功率微波 模式耦合理论 Taguchi优化算法 模式变换器     

Minimization of losses and calculation of the optical properties of Bragg fiber waveguides with a hollow core

A numerical method for solving the inverse problem of determining the geometry of the multilayer shell of a Bragg waveguide that has the lowest waveguide losses for a given mode has been developed with the use of the genetic algorithm. For the calculated designs of waveguides, the distribution of the coordinates of the boundaries of shell layers has been found to be aperiodic under the condition r ₁ ≤ λ due to the axial symmetry of the problem. Waveguide losses for the TE ₀₁, TM ₀₁, and HE ₁₁ modes satisfy the conditions \(\alpha _{TM_{01} } > \alpha _{HE_{11} } > \alpha _{TE_{01} } \). It follows from the dependence of n _eff obtained for these modes on the core radius and number of shell layers that any change in the structure of the waveguide leads to the violation of the optimal propagation regime for these modes. A Bragg fiber waveguide with a hollow core that is designed for the TE ₀₁ mode and directs light in the single-mode regime is presented. The main fraction of losses in this waveguide is attributed to material absorption.     

Spontaneous decay rates of the hyperfine structure atomic states into an optical nanofiber

Spontaneous decay rates of atoms into guided modes of an optical nanofiber are found for atomic transitions between the hyperfine structure sublevels. The decay rates are evaluated for the hyperfine structure transitions in Rb atoms. The efficiency of the guided mode excitation by spontaneous decay of the specific hyperfine atomic states is examined for both the fundamental fiber mode HE₁₁ and the higher-order modes TE₀₁, TM₀₁, and HE₂₁.     

Hybrid mode trasmission in 62-m corrugated waveguide

Experimental measurements by wavepacket reflectometry are carried out to verify the low transmission loss of hybrid mode on HE₁₁ in the corrugated waveguide with 88.9 mm in diameter. The HE₁₁ mode is injected into the 62-m straight circular corrugated waveguide and the beat between the HE₁₁ and HE₂₁ (including degenerated TE₀₁ and TM₀₂) modes is observed. The loss is too small to measure and inferred to be less than 2 db/km. The result shows that the use of HE₁₁ mode for electron cyclotron heating system is suitable.     

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.     

Limitations in the perturbation analysis of bent finite-clad fibres and waveguides

The transverse shift in the field distribution and the correction to the propagation constant of the fundamental and symmetric cladding modes on bent finite-clad single-mode fibres and slab waveguides are evaluated from perturbation theory for effective index values extending below the cladding index. Analytical results are derived in both geometries for the step-profile that are valid within the overall limitations of the theory. However, it is found that, for the fibre geometry only, the method breaks down at certain discrete wavelengths because of degeneracies that occur between the HE₁₂ (LP₀₂) and TE₀₁ (LP₁₁) mode propagation constants.     

Acousto-optic generation of orbital angular momentum states of light in a tapered optical fiber

We demonstrate an acousto-optic mode converter based on a tapered optical fiber to efficiently generate orbital angular momentum states of light. In our scheme an acoustic wave is deployed to the waist of tapered optical fiber where two degenerate HE₂₁ modes leading to +1 and −1 orbital angular momentum eigen-modes are resonantly excited. The excitation of TM₀₁ and TE₀₁ modes is suppressed by enlarging the intermodal index difference between near-degenerate spatial modes. Numerical calculation for optimization of the taper diameter is provided. The experimental characterization of generated states is performed by analyzing the output far-field pattern and the spatial interference fringes with a uniform reference beam.     

TE modes in parallel cylindrical surface waveguides

Dispersion equations for TE and TM modes in width-limited waveguides formed by parallel cylindrical surfaces are derived in adiabatic approximation. It is shown that the dominant TE₀₁ mode is strongly localized in the waveguide center.     

A dielectric rod waveguide coupled with metal rectangular waveguide

Measurements have been done in the millimeter wave region on a composite waveguide which comprises a dielectric rod waveguide connecting two metal rectangular waveguides. Such a waveguide has been used by us in a Josephson harmonic mixer installed in a small metal cryostat, to prevent the thermal invasion from outside environment and to transmit both signal and LO waves with small losses. The measured transmission loss, that is caused mainly by the coupling loss between metal rectangular waveguides (TE₁₀ mode) and a dielectric rod waveguide (HE₁₁ mode), has been less than 2dB in the frequency range of 52–104 GHz.     

Compact HE11 to surface wave converters for high power waveguide dummy loads

The HE₁₁ mode in corrugated circular waveguide can be converted to the EH₁₁ mode (surface wave) by a short, smooth-waveguide phase shift section followed by a short corrugation depth taper. Low-power measurements at 110 GHz in 1.25 in. aluminum waveguide demonstrated approximately 99% conversion with the proper phase shift length. As expected, the conversion efficiency versus length of the phase shifter varied periodically with the period of the TE₁₁ to TM₁₁ beat wavelength. Since the EH₁₁ surface wave is highly attenuated, this type of converter can be used effectively in a compact high-power dummy load.     

Higher order mode propagation in an optical nanofiber

The propagation of higher modes, such as the LP₁₁ mode, in optical nanofibers using the exponentially tapered optical fiber as a basic model is investigated. In order to preserve the LP₁₁ mode in the downtaper as far as the nanofiber waist, the effect of varying the cladding-core radius ratio on the LP₁₁ adiabatic criterion is modeled. A Laguerre-Gaussian beam is created in free space using a spatial light modulator (SLM) and coupled to a few-mode fiber. This device allows convenient switching between the fundamental and LP₁₁ fiber modes. By selecting a few-mode fiber with a relatively low cladding-core ratio, the propagation of the LP₁₁ mode down to a submicron waist has been maintained. Furthermore, by observing the transmission profile during tapering, it is possible to decisively terminate the pulling process in order to eliminate the two degenerate HE₂₁ modes of the LP₁₁ mode. As a result, a nanofiber can be fabricated through which only the TE₀₁ and TM₀₁ components of the LP₁₁ mode propagate. Such a nanofiber has promising applications in the area of mode interference for controlled particle trapping sites.     

双微波源共轴辐射馈线结构的设计

设计了一种将两个不同波段微波源输出微波馈向同一个馈源喇叭的双波段馈线结构。该结构不仅对两个微波源输出的TM01模进行了有效的模式转换,而且实现了两个波段的微波共用辐射天线。采用双弯曲圆波导模式转换器实现高频段微波源输出微波的模式转换和轴线的定量平移;采用中间模式为矩形波导TE10模的圆波导TM01模-同轴波导TE11模的模式转换器,实现低频段微波源输出微波的模式转换及与高频段微波源输出微波的共轴输出。数值仿真结果验证了设计思路的正确性。     

On‐chip silicon 8‐channel hybrid (de)multiplexer enabling simultaneous mode‐ and polarization‐division‐multiplexing

An 8‐channel hybrid (de)multiplexer to simultaneously achieve mode‐ and polarization‐division‐(de)multiplexing is proposed and demonstrated experimentally on a silicon‐on‐insulator platform to improve the link capacity of an on‐chip optical interconnect. The present hybrid (de)multiplexer has four channels for each polarization. A polarization beam splitter based on a three‐waveguide coupler is used to combine/separate the fundamental modes of TE‐ and TM‐polarizations (TE₀ and TM₀). Six asymmetric directional couplers are cascaded for (de)multiplexing the high‐order modes (TE₁, TE₂, TE₃, TM₁, TM₂, and TM₃). The experimental results show all eight channels have low loss and low crosstalk (<−10 dB) over a ∼ 30 nm wavelength range.     

Generation of radially polarized mode using an active cylindrically symmetric birefringence fiber

A class of active fiber is presented as gain mediums of fiber lasers, where a silica layer with spoke-like air slits is introduced between the active core and cladding to obtain birefringence. With an appropriate choice of the design parameters, the fiber can support only the HE₁₁ and TM₀₁ modes. Furthermore, the numerical results show that, corresponding to the optimal rare earth doping profile, the fiber lasers can output a pure TM₀₁ mode with about 79% slope efficiency without using other mode-selective elements.     

Coupling external cavity mid-IR quantum cascade lasers with low loss hollow metallic/dielectric waveguides

We report on the optical coupling between hollow core waveguides and external cavity mid-IR quantum cascade lasers (QCLs). Waveguides with 1000???m bore size and lengths ranging from 2 to 14?cm, with metallic (Ag)/dielectric (AgI or polystyrene) circular cross-section internal coatings, have been employed. Our results show that the QCL mode is perfectly matched to the hybrid HE₁₁ waveguide mode, demonstrating that the internal dielectric coating thickness is effective to suppress the higher losses TE-like modes. Optical losses down to 0.44?dB/m at 5.27???m were measured in Ag/polystyrene-coated waveguide with an almost unitary coupling efficiency.     

Analysis of the coupling of two non-parallel dielectric rod waveguides

In this paper the coupling between two closely spaced single mode dielectric rod waveguides is investigated when the corresponding propagation axes are not in the same plane. An approximate analysis is presented by assuming only first order interaction between the guided waves. For a given incident HE₁₁ mode (in one of the rod waveguides) the coupled wave amplitudes are computed in the other waveguide for both propagation directions. Numerical results are presented for several coupling geometries.     

A partially reflecting metal cladding: its effect on the modes of dielectric optical waveguides

If the metal cladding of a dielectric optical waveguide is sufficiently thin to be only partially reflecting, then the waveguide modes differ from those of the conventional metal-clad waveguide. The TE modes are little affected by a variation in the metal thickness but the TM modes change considerably due to a coupling between the waveguide modes and the surface plasma waves supported by both the metal: dielectric interfaces and the metal film. It is the refractive index of the dielectric cladding which is remote from the guiding core that determines whether the lowest order TM mode is the TM₀ or TM₁ mode. This dielectric cladding also strongly influences the attenuation of the TM modes and, if the guide supports a TM₀ mode, then the attenuation of the TM modes far from cut-off are an order of magnitude higher than that of the corresponding modes when the guide cannot support a TM₀ mode. If the guide can support a TM₀ mode then its dimensions can be chosen such that it will support the TE_N and TM_N modes with equal phase velocities. A lossless approximation is used to develop an expression which will specify the required guide dimensions directly and parametric plots of these dimensions are discussed.     

Dual-core leaky optical waveguide as an integrated-optic polarizer

We present a design of a dual-core leaky waveguide that can be used as an integrated-optic polarizer. The proposed polarizer works on the principle of mode filtering. The structure is characterized by two cores, namely core-1 and core-2 and a high index layer in the upper-most region, such a structure supports leaky modes. The leakage losses of the modes have been calculated by using the transfer matrix method (TMM). Single polarization operation is ensured by high differential leakage loss between fundamental TE and TM modes. We show TE-pass operation with 0.5 dB/mm loss in TE₀ mode and 13 dB/mm loss in TM₀ mode, and TM-pass operation with 0.36 dB/mm loss in TM₀ mode and 7.45 dB/mm loss in TE₀ mode. Besides single-polarization operation, single-mode operation of the structure is ensured by high leakage loss of all the other higher-order modes.     

矩形波导与圆柱波导或圆柱谐振腔间的小孔耦合

本文讨论了三种不同系统的小孔耦合的近似解法:矩形波导与圆柱波导的同轴耦合,矩形波导中的TE₁₀型波与圆柱谐振腔中的TM₁₂₀型波间的耦合和矩形波导中的TE₁₀型波与圆柱波导中的TM_mn型波间的耦合。通过这几个问题的解法,我们希望读者可以看到处理复杂微波元件问题时所需的从物理概念出发的灵活应用已有方法的必要性。这里提供的几个公式有着广泛的应用范围而是在现有文献中探讨得不够的。这里介绍的方法可以应用到其他新的微波网络问题中去。