Propagation of infrared radiation in hollow microstructuralcylindrical waveguides

Theoretical and numerical results are presented on the propagation of infrared radiation in TE₀₁-, TM₀₁-, and TE₁₁-like modes in an empty cylindrical, gold-coated waveguide of micron dimensions. Low-loss propagation is possible for the TE₀₁-like mode, whose configuration is similar to that of the TE₀₁ mode in a guide with perfectly conducting walls. Propagation is more lossy for the TM₀₁- and TE₁₁-like modes. While for small guide radii these modes resemble the TM₀₁ and TE₁₁ modes in a guide with perfectly conducting walls, they transform into surface waves as the guide radius is increased, with losses that are independent of guide radius     

Behavior of the first higher-order modes of a circular core opticalfiber whose core cross-section changes into an ellipse

The vector finite element method is used for investigating the behavior of a set of the first higher-order, four-fold degenerate modes (TE₀₁, TM₀₁, and two-fold degenerate HE₂₁ modes) of a circular core optical fiber whose core cross-section changes into an ellipse. It is found that the TE₀₁ mode becomes the first higher-order mode polarized along the minor axis, E ₂₁^y mode, in a dual-mode region. The remaining three modes become, respectively, the first higher-order mode polarized along the major axis, E₂₁^x mode, and the second higher-order modes polarized along the major and minor axes, E₁₂ ^x and E₁₂^y modes, in the order of magnitude of the effective index. It is also found that the effective index of the E₂₁^x mode is smaller than that of the E₂₁^y mode in a small ellipticity region     

Axisymmetric modes of cylindrical resonators with cascadedinhomogeneous dielectrics

A generic numerical scheme is developed to calculate the resonant frequency of axisymmetric modes in an inhomogeneous cylindrical dielectric resonator. The resonator consists of sections of cylindrically stratified dielectrics within a cylindrical waveguide. In each section, the TM_0m and TE_0m waveguide modes are solved by expanding the H_φ and E_φ components in terms of the eigenmodes in an empty waveguide. The fields in each section are then expanded in terms of these TM_0m and TE_0m modes. The transverse resonance technique is then applied to obtain the resonant frequencies. Comparison with literatures validates the effectiveness of this approach. Results with continuous dielectric profiles are also obtained     

Design of short high-power TE11-HE11 modeconverters in highly overmoded corrugated waveguides

A theoretical parametric study of TE₁₁ to HE₁₁ mode conversion in highly oversized, circumferentially corrugated circular waveguides with different inner diameters is presented for various frequencies in the range of 28 to 140 GHz. The depth of the annular slots is tapered gradually from one half to one quarter wavelength. Computer-aided optimization of converter length, shape of corrugations, and nonlinear slot depth variation has been achieved with a scattering matrix code employing the modal field matching techniques (modular analysis concept). Relatively short mode transducers with matched converter lengths are feasible. The optimum conversion lengths are L₁≈0.5λ_B (short converter) and L₂≈1.2λ_B (long converter), where λ_B is the beat wavelength of the TE₁₁ and TM₁₁ modes in the corresponding smooth-walled circular waveguide. The predicted performances of short and long mode transducers are practically identical. Experimental results taken at 70 GHz (i.d.=27.79 mm) are in excellent agreement with the theoretically predicted performance data. In all cases the HE₁₁ output mode purity is 99% to 99.5%. The maximum cross-polarization and input-reflection levels are below -29 dB and -50 dB, respectively     

Dispersion characteristics of cylindrical metal-clad opticalwaveguides

A study of the dispersion characteristics of cylindrical metal-clad optical waveguides indicates that: (1) the propagation of electromagnetic energy in metal-clad optical waveguides differs in principal from that of dielectric step-index waveguides and ideal metallic waveguides; (2) hybrid modes exist in this structure, and the cutoff frequency of the HE₁₁ mode does not equal zero; (3) TM and HE(EH) modes have attenuation higher at least by an order of magnitude than that of TE modes; and (4) the surface plasma wave can propagate in this structure. Thus, the lowest order mode is TE₀₁      

Infrared surface waves in circular hollow waveguides with smallcore diameters

An asymptotic form of the characteristic equation that describes wave propagation at near-infrared wavelengths in small core hollow circular waveguides is developed. Analytic solutions for the transverse and axial propagation constants are obtained. These demonstrate the transition of the TE₁₁ and TM₀₁ modes to surface waves as the guide radius is increased to values much greater than at cutoff. Relative power density distributions illustrating these mode transitions are shown     

Resonant disk turnstile

In this paper, the resonant disk turnstile is introduced and analyzed. The disk turnstile differs from Dicke's full-waveguide turnstile in that four of the six ports are coaxial cable ports, yielding a much more compact turnstile, which is easier to construct. The disk turnstile operation is explained as a superposition of the disk TM₀₁ mode (nonradiating) and the disk TM₁₁ mode which couples directly to the circular waveguide (dominant) TE₁₁ mode. A useful operating fractional bandwidth of 3% is possible as compared to a Dicke turnstile bandwidth of about 6%     

Achievement of pencil-beam radiation from a TM01 modecircular waveguide source

A method employing appropriately placed parallel-metallic plates in the aperture of a TM₀₁ mode circular waveguide source so as to have it produce pencil-beam radiation is described and analyzed. Programmed computations, with experimental verification, are given for the case of a C-Band TM₀₁ source flared out to a conical horn so as to produce moderate (22 dBi) gain. Means of extending the method to achieve higher gains (40 dBi to 60 dBi) and to modify the plate arrangement so as to produce the same effect with a TE₀₁ mode are briefly outlined     

Theoretical Investigation of Metal Cladding for Nanowire and Cylindrical Micropost Lasers

We investigate the transverse modal properties of cylindrical subwavelength metal-clad nanowire and micropost lasers via rigorous theoretical waveguide analysis, including the effects of finite thickness metal cladding and gain in the core. The results of this analysis show that air-metal surface guided TM₀₁ and some hybrid surface guided modes suffer less loss but are less confined to the core, while core-metal surface guided modes are better confined to the core but suffer greater loss. An increase in the thickness of the metal cladding reduces the loss of the core-metal surface guided modes. The modal gain and confinement of the metal-clad cavity are compared to an unclad cavity.     

Study of TE0 and TM0 modes in dielectricresonators by a finite difference time-domain method coupled with thediscrete Fourier transform

An application of a numerical method of finite differences in the time domain (FDTD) coupled with the discrete Fourier transform is presented to determine the resonant frequencies of the TE₀ and TM₀ modes of axially symmetric dielectric resonators closed in a cavity. The technique is conceptually and computationally simple, and it allows access at once to information on the entire modal spectrum by means of the fast Fourier transform (FFT) applied to the time series. The cylindrical cavity dielectrically loaded at the base and the resonant frequency of the TE_01δ mode are analyzed in two systems: a cylindrical cavity with a cylindrical dielectric resonator of variable radius, and the shielded dielectric resonator on a microstrip substrate. The results obtained are compared with the rigorous (exact) theoretical solutions and with experimental results     

Full-wave analysis of the image hybrid dielectric/HTS resonator

An analysis of the image hybrid dielectric/high-temperature superconductor (HTS) resonator is carried out. A full-wave radial mode-matching method is used to obtain the electromagnetic fields inside the resonator for single TE₀₁ and dual HE₁₁ modes. Measured resonant frequencies and quality factors of these modes are compared with numerical results of the analysis. The resonator power-handling capability is estimated from the field at the surface of the HTS film, assuming a certain value for the critical field of the HTS film     

The input impedance of a coaxial line fed probe in a cylindricalwaveguide

Dyadic Green's functions for determining the electric and magnetic fields in a cylindrical waveguide due to a radially directed infinitesimally short electric dipole are derived. The waveguide is shorted at one end and terminated at a perfectly matched load at the other. Both TE and TM modes are considered. Based on these dyadic Green's functions, the input impedance of a coaxial line fed probe in front of the plunger is derived. The formula is expressed in a closed form. Excellent agreement between theoretical results and experimental data for exciting the TE₁₁ mode in the X band for various probe positions is observed     

A composite optical waveguide-based polarimetric interferometer forchemical and biological sensing applications

A new polarimetric interferometer has been developed on the basis of the phase difference between transverse electric (TE)₀ and transverse magnetic (TM)₀ modes in a composite optical waveguide (OWG). The composite OWG consists of a single-mode potassium ion-exchanged planar waveguide overlaid with a high-index thin film that has two tapered ends and supports only the TE₀ mode. Applying tapered velocity coupling theory, we found that the TE₀ and TM₀ modes coexisting in the potassium ion-exchanged layer were separated in the thin film region of the composite OWG: the TE₀ mode was coupled into the thin film while the TM₀ mode was confined in the potassium ion-exchanged layer. Interference occurs between TE- and TM-polarized output components when a single output beam is passed through a 45°-polarized analyzer. The phase difference φ between both orthogonal output components is very sensitive to the superstrate index n_c in the thin film region. Our experimental results indicate that a slight change of Δn_c=3.71×10^-6 results in the phase-difference variation of Δφ=1° for a 5-mm-long TiO ₂/K⁺ composite OWG with a 34-nm-thick TiO₂ film. Such a simple polarimetric interferometer can be applied to chemical or biological sensors by modifying the upper film surface of the composite OWG with a chemically or biologically active substance     

A method of analysis of TE11-to-HE11 modeconverters

An efficient method for the determination of the scattering matrix of TE₁₁-to-HE₁₁ corrugated cylindrical waveguide mode converters has been developed, based on the representation of the fields inside the corrugations by a small number of radial waveguide modes. Numerical results show that the method, when compared to the usual mode-matching techniques, reduces the computation time without loss of accuracy     

弱导阶跃折射率光纤中的模式简并问题

模式是光纤光学中的基本概念,也是光纤激光器研究中最关注的问题之一。在一般的教材、文献中对模式的简并问题涉及较少,没有给出清晰、直观的物理图像。文中采用经典的电磁场理论,阐明了矢量模的简并度、标量模的简并度以及矢量模和标量模之间的简并关系。在弱导阶跃折射率光纤中,矢量模和标量模都是描述光场的正交完备基,矢量模[HE1n(o)、HE1n(e)]与标量模[LP0n(ye)、LP0n(xe)]描述的光场空间是2维的,矢量模[TE0n、 HE2n(o)、 HE2n(e)、 TM0n]与标量模[LP1n(yo)、 LP1n(ye)、 LP1n(xo)、LP1n(xe)]描述的光场空间是4维的,矢量模[EHm-1,n(o)、 EHm-1,n(e)、 HEm+1,n(o)、 HEm+1, n(e)]与标量模[LPmn(yo)、LPmn(ye)、LPmn(xo)、LPmn(xe)]描述的光场空间也是4维的。     

Broadband linearly polarized beat-wave TEm1/TE11 mode converters

Broadband linearly polarized waveguide mode converters have been developed to transform the high-order cylindrical TE_m1 output wave from harmonic gyrotron amplifiers into the more useful TE₁₁ fundamental waveguide mode. The converter's corrugation period is equal to the beat between the two waves and the bandwidth is predicted to be inversely proportional to the number of periods. Four-period TE ₃₁/TE₁₁ and TE₄₁/TE₁₁ converters with an azimuthal perturbation of m_c=4 and m_c =5, respectively, have yielded a peak conversion efficiency of 98% with a bandwidth greater than 3% and a one-period beat-wave converter has been designed to yield 12% bandwidth. However, it has been observed in measurements that the strong coupling in a short converter can lead to a shift of the center-frequency with an accompanying reduction of the efficiency and bandwidth. A two-period TE₄₁/TE₁₁ converter with 5% bandwidth displayed a 5% frequency shift and a conversion efficiency of only 86%     

A low-loss bandpass filter using electrically coupled high-Q TM01δ dielectric rod resonators

For a TM_01δ mode dielectric rod resonator placed coaxially in a TM₀₁ cutoff circular waveguide, characteristics such as the resonant frequency, its temperature coefficient, the unloaded Q, and the other resonances are discussed on the bases of accurate calculations using the mode-matching method. The results show that this resonator compares favorably with a conventional TE_01δ mode dielectric resonator, particularly for realization of a high unloaded Q. Analytical results also verify that interresonator coupling between these two resonators can be expressed equivalently by a capacitively coupled LC resonant circuit. A four-stage Chebyshev filter having a ripple of 0.035 dB and an equiripple bandwidth of 27 MHz at a center frequency of 11.958 GHz was fabricated using these resonators. Its insertion loss is 0.5 dB, which corresponds to an unloaded Q of 17000, and no spurious response appears in the frequency range below 17 GHz     

Compact quasi-periodic and aperiodic TE0n modeconverters in overmoded circular waveguides for use with gyrotrons

Designs of compact quasi-periodic and aperiodic TE_0n-TE _0n-1 circular waveguide converters for use with gyrotrons in an electron cyclotron heating (ECH) system are developed by analytically and numerically solving the coupled-mode differential equations. Quasi-periodic mode transducer designs which convert the TE₀₂ mode to the TE₀₁ mode, and in some cases also include a taper (waveguide radius reduction), are developed. A 60-GHz aperiodic mode converter-taper combines a 6.35-cm-2.779-cm waveguide diameter taper and a TE₀₂-TE₀₁ mode converter. A 140-GHz aperiodic mode converter-taper combines a 6.35-cm-2.779-cm waveguide diameter taper and a TE₀₃-TE₀₂-TE₀₁ mode converter. The resulting designs are highly efficient (conversion efficiencies ⩾99.4%), are shorter, have a broader bandwidth than previous designs, and have a waveguide radius greater than or equal to 1.389 cm over the entire length of the transducer to allow for high-power transmission. Experimental results consistent with theoretical calculations are presented     

A millimeter-wave omnidirectional dielectric rod metallic gratingantenna

A new millimeter-wave omnidirectional dielectric rod metallic grating antenna is investigated theoretically and experimentally, in this paper. The radiation characteristics of the antenna are carefully investigated by a rigorous formulation for the TE₀₁-mode and TM₀₁-mode excitation in the Ka-band frequency range. Based on the analysis, an omnidirectional antenna excited by the TE₀₁ mode is designed and measured. The measured results are compared with theoretical predictions and good agreement is found. Extensive numerical results are given to establish some useful guidelines for the design of the omnidirectional antenna     

The design of serpentine-mode converters for high-power microwaveapplications

In this paper, we report the design methodology and numerical results for two mode converters that are suitable for high-power microwave applications. Both converters are designed to operate at 11.424 GHz and utilize periodic serpentine structures to convert between modes with different azimuthal-mode indexes. The first converts about 98.5% of an incident linearly polarized TE₁₂ mode to the TE ₀₁ mode when oriented as an H-plane bend, but has just 1% mode conversion to all modes when oriented as an E-plane bend. The second device converts a linearly polarized TE₁₁ mode to a TM ₀₁ mode with over 99.99% effectiveness. The performance of both devices with respect to parametric variations is detailed. Experimental measurements of the radiation patterns from the TE₁₂ -to-TE₀₁-mode converter are consistent with the theoretical predictions