Equivalent Waveguide Model Research Articles

Mode Size Converter Based on Periodically Segmented Liquid Crystal Core Waveguide

We report the numerical and experimental study of a liquid crystal (LC) core periodically segmented waveguide (PSW) based mode size converter, fabricated on an ITO coated glass substrate. The effective index and mode size of the proposed device can be dynamically tuned by changing the applied voltage. The 5CB (4-cyano-4′-pentylbiphenyl) nematic LC has been used as the guiding layer, surrounded by the cladding of negative photoresist AZ15nXT. An equivalent waveguide model has been used to determine the waveguide characteristics, and we have shown that the effective index and mode size of the guided modes change with the duty cycle and the applied voltage. For a fixed duty cycle, we have experimentally demonstrated that the size of the fundamental mode can be tuned by the applied voltage. The advantage of using LC-PSW for mode size transformation is that the effective index and mode size in the fabricated waveguide can be varied by an externally applied electric field. It has also been experimentally demonstrated that the coupling efficiency between the LC-PSW and a single mode fiber can be maximized by appropriate choice of the applied voltage. The proposed mode size converter can be used in LC photonic lightwave circuits to couple the light between optical devices with maximum efficiency.

Multifunctional beam steering via switchable negative refraction, self-collimation, and zero refraction effects in conventional and annular photonic crystals.

Negative refraction (NR), self-collimation (SC), and zero refraction (ZR) effects of photonic crystals play an important role in beam steering. In this work, we report a multifunctional beam steering concept in photonic crystals, i.e., integrating two or three of the NR, SC, and ZR effects together at the same frequency. We find the square-lattice dielectric ring photonic crystal is an ideal candidate to realize the switchable function of ZR-SC while the square-lattice dielectric ring photonic crystal is more suitable for realizing the ZR-SC, ZR-NR, and ZR-SC-NR functions. The photonic band theory and an equivalent waveguide model are employed to explain these switchable functions in conventional and annular photonic crystals.

Selective absorption of a thermophotovoltaic cell using a thin semiconductor and a top fishnet-structured electrode

The electromagnetic field around a thermophotovoltaic (TPV) cell made of a metal–semiconductor–metal (MSM) multilayer with a square-grid electrode layer, referred to as a fishnet layer, was numerically calculated using a three-dimensional finite difference time domain method to obtain the absorptance of the MSM multilayer. In this study, gold and gallium antimony assumed to be materials of the metal and semiconductor layer, respectively. Even if there was no top metal layer, several peaks of absorptance were observed because of the optical interference inside the gallium antimony thin film. Here, the peak wavelength shifted to a shorter wavelength region by placing a fishnet layer on the gallium antimony layer. Moreover, the wavelength of the first peak of the optical interference could be adjusted at the active range of wavelength from 0.8 um to 1.8 um for the TPV cell made of gallium antimony for a practical power generation system. As a result, even in a several hundred nanometers thickness of semiconductor, a 60% of radiant energy absorbed could be concentrated in the active range of wavelength that is defined as a spectral efficiency. From the simulation results with an equivalent waveguide model, the shift of the first peak using the fishnet layer could be described using a dispersion relation of the waveguide, which was fulfilled with gallium antimony. Moreover, the superposition of different resonant modes originated from the waveguide-like structure and strip-wired structure affected the spectral absorption of the MSM multilayer when the wall of the fishnet structure was expanded.

Analytical Design Procedure for Forward Wave Couplers in RGW Technology Based on Hybrid PEC/PMC Waveguide Model

In this paper, a systematic design methodology of a 0dB and a 3dB forward couplers based on the ridge gap waveguide (RGW) technology is presented. This methodology is based on exact theoretical formulations rather than any approximate or empirical equations. The procedure of the proposed design methodology is mainly to build a virtual equivalent waveguide model. This waveguide has two horizontal upper and lower perfect electric conductor (PEC) walls, while the left and the right walls are made of perfect magnetic conductors (PMC). A detailed analysis for this hybrid PEC/PMC waveguide, a common waveguide for coupling, is introduced as the starting phase for designing the RGW couplers. The equivalent RGW coupler that assures the same operation of the hybrid PEC/PMC waveguide at a specific frequency range is deduced based on detailed theoretical aspects. Moreover, a simple analyzing of transitional bends and phase shifters with accurate calculations is presented in this paper, which are the fundamental building blocks of several mmW components such as the six-port junction and the butler matrix. The possibility of tuning the coupler center frequency is introduced without the need of using any nonlinear elements. The resulting RGW couplers are implemented through well-known full wave simulator (Ansoft HFSS), with verification through prototype measurements in order to confirm the validity of the proposed methodology. A good agreement is achieved between measurement and simulation results.

New insight into quasi leaky mode approximations for unified coupled-mode analysis

To facilitate the analysis of radiation mode couplings, quasi leaky mode approximations were utilized in coupled-mode analysis. The key to effectively and accurately apply this approach is how to well approximate radiation modes by the quasi leaky modes in an equivalent closed waveguide model. In this paper, the principle, applicability and accuracy of the approximations are demonstrated, and the detailed implementation is also suggested by applying a unified coupled-mode analysis to fiber gratings. First of all, based on a thorough study on the characteristics of the complex modes, for the first time, quasi leaky modes are classified into guided-mode-like inner-cladding and radiation-mode-like outer-cladding leaky modes so as to explicitly establish equivalence relationships between the discrete leaky modes and the continuous radiation modes. With this new insight, the whole analysis process especially for some of the practically tricky issues such as the criteria for developing the proper equivalent waveguide model and the subsequent mode expansion basis are better understood and easier to be dealt with for different problems where radiation modes come into play. Moreover, as essential preconditions to extend the conventional coupled mode analysis to the present unified one, the couplings between the guided core mode and a leaky mode are studied in a systematic and consistent manner. An intuitive and then a deep understanding on the roles of complex modes on mode coupling and power exchanging are thus gained for further simulations. Lastly, the transmission spectra of fiber gratings with different surrounding indices are simulated. The simulated results agree well with those obtained theoretically and experimentally in the literatures, which strongly validate the principle of quasi leaky mode approximations and its implementation on the unified coupled-mode analysis expounded in this paper.

Propagation characteristics of a novel complementary-structure planar segmented waveguide

A new waveguide structure with periodically segmented cover and substrate regions and a uniform film region is proposed. This complementary segmented waveguide is analysed in terms of the effective index and the spot size of the guided mode, by expressing the propagating field as a linear combination of the local normal modes of the corresponding uniform slab waveguides in different propagation regions. The results match very well with those obtained using the equivalent waveguide model. The propagation loss of the mode of the proposed structure is calculated using a coupled mode approach, and the results are similar to those of a normal segmented waveguide. This waveguide structure should prove useful in the realization of novel segmented waveguide devices.

An analysis of the folded waveguide: a compact waveguide launcher for ICRF heating

Theoretical and experimental investigations of the folded waveguide launcher as proposed for use in the ion-cyclotron range of frequencies (ICRF) heating of tokamak plasmas are presented. Theoretical results include an analysis of the folded waveguide using Ritz's method to obtain various field quantities. The cutoff wavenumbers from this analysis are within 10% of those calculated using an unfolded equivalent rectangular waveguide model. Measurements of cavity resonances for a scale-model ICRF-folded waveguide with a cutoff frequency of 2.4 GHz also agree closely with those calculated using an unfolded equivalent model. An unfolded equivalent waveguide model is used to evaluate the properties of an inductive strip placed in the folded waveguide. Applying variational analyses provides upper and lower bounds to the reflection magnitude mod S/sub 11/ mod that are in good agreement with measured values. Polarizing plates (which have been proposed to increase the on-axis directivity of the radiated power) are modeled using an unfolded equivalent waveguide. A variational analysis is used to determine an upper-bound value of mod S/sub 11/ mod in air due to a polarizing plate inside a matched folded waveguide. Measured values are found to be within 3% of the calculated values, with mod S/sub 11/ mod >0.95 for frequencies between 2.5 and 4.5 GHz. >

Generalized Scattering Matrix Method for Analysis of Cascaded and Offset Microstrip Step Discontinuities

Detailed algorithms are presented for characterizations of cascaded microstrip step discontinuities, symmetric stubs, and offset step. The analysis is based on the generalized scattering matrix techniques after the equivalent waveguide model is introduced for the microstrip line.

Dielectric and permeability effects in HE111 open demagnetised ferrite resonators

The half-wave HE111 open ferrite or dielectric resonator, open circuited or loaded by image walls at each end, is a common building block in the construction of waveguide circulators and other devices. Its resonant frequency is usually evaluated by catering for the image wall effect but neglecting that of the open lateral wall. The latter omission is largely reconciled in this paper by employing an equivalent waveguide model for the resonator which has the same radius as the original but with an effective instead of the constitutive dielectric constant. The import of the diagonal elementμd of the demagnetised resonator is separately catered for.

Perturbation for Open Analysis and Design Equations and Closed-Ring Microstrip Resonators

Simple closed-form expressions for the resonant frequency and electromagnetic field distribution for various modes of the open- and closed-ring microstrip resonators are derived by utilizing the perturbation analysis of the equivalent curved waveguide model. These results are shown to be in good agreement with the exactly computed values obtained by the solution of the eigenvalue equation for the equivalent waveguide model and the experimental data. The effect of gap capacitance on the eigenvalues of the open-ring resonator is also examined.