Waveguide Reflection Research Articles

Transient Chirp Suppression in Directly Modulated Microring Lasers

The effect of internal feedback through bus waveguide reflectivity has been numerically evaluated as a key parameter for the improvement of direct modulation properties of microring lasers. According to calculations, appropriate values of feedback power result in damped relaxation oscillations and consequent transient chirp reduction. An evaluation of the above technique at 10 Gb/s including fiber transmission over 10 and 20 km indicates that microring lasers could efficiently operate as low-cost directly modulated transmitters in metro/access networks.

Characterization of an elastic target in a shallow water waveguide by decomposition of the time-reversal operator

This paper reports the results of an investigation into extracting of the backscattered frequency signature of a target in a waveguide. Retrieving the target signature is difficult because it is blurred by waveguide reflections and modal interference. It is shown that the decomposition of the time-reversal operator method provides a solution to this problem. Using a modal theory, this paper shows that the first singular value associated with a target is proportional to the backscattering form function. It is linked to the waveguide geometry through a factor that weakly depends on frequency as long as the target is far from the boundaries. Using the same approach, the second singular value is shown to be proportional to the second derivative of the angular form function which is a relevant parameter for target identification. Within this framework the coupling between two targets is considered. Small scale experimental studies are performed in the 3.5 MHz frequency range for 3 mm spheres in a 28 mm deep and 570 mm long waveguide and confirm the theoretical results.

A modified field model of waveguide reflection dielectric resonator for microwave measurements of dielectric properties

A modified electromagnetic field model of a waveguide reflection dielectric resonator is suggested for measurements of dielectric properties of the homogeneous and isotropic medium in the microwave frequencies. Reflection signal is measured for the calculations of dielectric properties. A dielectric rod sample is put inside of a rectangular cavity made by a microwave waveguide. The sample’s dielectric constant and loss tangent are computed from the unloaded quality factor and the resonant frequency of the TE01δ mode as well as the structure dimensions. For first time, this waveguide reflection dielectric resonator is applied on dielectric constant measurement. A modified field model of the waveguide reflection resonator is developed from the Itoh-Rudokas model [IEEE Trans. Microwave Theory Tech. MTT-25, 52 (1977)] of the parallel-plate dielectric resonator. This modification is justified by the dramatic improvement in the accuracy of dielectric constant measurements. The main merit of this field model is that it provides very simple electromagnetic field expressions of this TE01δ field mode. In addition, accuracies of various methods for calculating the power factor and conducting loss, which have never been given before, will be investigated in this article.

Direct Modulation Properties of 1.55-$\mu\hbox{m}$ InGaAsP/InP Microring Lasers

The modulation properties of 1.55-mum InGaAsP/InP microring lasers are investigated using a multimode rate equation model. A detailed study, with respect to Q-factor and extinction ratio calculations, is carried out as a function of different microring's key design and operating parameters. The modulator's performance study shows the possibility for a successful operation at a bit rate of 2.4 Gb/s for radii between 30 and 50 mum and proper values for the bus waveguide reflectivity.

Investigation on the Multimode Dynamics of InGaAsPInP Microring Lasers

A multimode model based on the rate equation approximation is presented in order to simulate the mode dynamics of 1.55-mum InGaAsP-InP microring lasers. A detailed characterization is performed including calculated optical spectra, time traces and relative intensity noise spectra. Different operation regimes are observed, bidirectional multimode with/without alternate oscillations, unidirectional single mode, bidirectional single mode and mode hopping. The boundaries of the above operation regimes are investigated with respect to the current level, bus waveguide reflectivity and ring radius. Varying the current level a transition from multimode to single mode and eventually mode hopping operation is observed. Increasing the bus waveguide reflectivity a transition from unidirectional to bidirectional operation is revealed, while the use of nonequal reflectivities between the two facets, promotes unidirectional operation. Moreover, the ring radius is proved to be a critical parameter for the extent of each operation regime since it directly influences the modal wavelength separation

Measurements of coupling and reflection characteristics of butt-joints in passive waveguide integrated laser diodes

Amplified spontaneous emission (ASE) spectra of passive waveguide integrated lasers have been studied to estimate the coupling efficiency and the reflectivity at the butt-joints between active and passive waveguides. A new method has been proposed for the analysis of ASE to extract the coupling efficiency and reflectivity at the butt-joints. This method was applied experimentally to estimate the coupling and reflection of the passive waveguide integrated lasers with different amounts of vertical misalignments of active and passive waveguides.

The Numerical Calculation and Analysis of the Spectrum of Lower-Hybrid Waves Launched by Grill-Type Antenna on HL-2A

The LHCD antenna that will be used in LH-2A is the original one used in LH-1M, which will be modified. We measured the feature of the antenna, including the reflection of main waveguide, VSWR, the phase shift between the adjacent sub-waveguides and so on. The paper describes the measured results and spectrum calculation based on the measured parameters. From the works we can assess our antennas correctly, which will be useful in LHCD experiment on HL-2A in the years to come.

Calculation of coupling to the electron Bernstein wave with a phased waveguide array

Conventional electron cyclotron heating using the O- and X-modes to carry energy from the plasma edge to the cyclotron resonance layer is not possible for high density, low magnetic field devices (RFPs and STs, for example), since these modes are evanescent in all but the very edge of the plasma. As an alternative, we consider coupling to the electron Bernstein wave (EBW) with a phased waveguide array, the mouth of which is inserted to the vicinity of the upper hybrid resonance at the edge of the plasma. The calculation of the waveguide reflection coefficient is similar to the lower hybrid coupling problem solved by Brambilla, but the character of the plasma surface admittance is quite different for the EBW from that for the lower hybrid wave. Two models for the surface admittance are compared. In the first, the lowest-order EBW is included in the calculation, while in the second, the cold plasma model (which does not have any mode corresponding to the EBW) with weak collisions is used. The surface admittances obtained in those two models for parameters relevant to coupling experiments performed in the Madison symmetric torus (MST) are compared, and found to agree closely, despite the very different physics in the models. A significant asymmetry with respect to the direction mutually perpendicular to the static magnetic field and the radial direction (the poloidal direction in tokamak geometry, the toroidal direction in the edge of the RFP) is found. This phenomenon is related to the well-known up/down asymmetry in fast wave launch in the ion cyclotron radio frequency and to the up/down asymmetry previously reported for the fast wave in the lower hybrid radio frequency. The effect is very strong in this situation due to the density gradient scale length being much shorter than the local wavelength (violation of WKB condition) in the coupling region.

A broadband waveguide for protein crystallography under intense microwave fields

We present a slab-line waveguide whose geometry is optimized for wide-angle x-ray diffraction (XRD) experiments on protein crystals during irradiation with intense microwave fields. Characterization of the waveguide transmission and reflectivity (using time-domain reflectometry) and of the electric field distribution inside the waveguide (using finite-difference time-domain calculations) shows that the present device has a broad bandwidth from below 0.5 to 18 GHz, allowing one to perform frequency-dependent XRD studies with a well-defined transverse mode structure and negligible reflection losses. As shown with a specific example, our device provides a simple way to couple microwave irradiation experiments with high-resolution x-ray diffraction measurements from millimeter-size crystalline samples. The present design might prove useful for systematic studies of microwave effects on protein structure and dynamics.

Incident angle-dependent effective surface impedance (Z/sub S/) of Electromagnetic Crystal (EMXT)

Electromagnetic crystal (EMXT) surfaces are modeled with effective surface impedance method. Effects of oblique incidence are studied using a technique similar to the waveguide simulator method for analyzing infinite phased array. Numerical results are compared with an analytical model. EMXT samples are characterized by open-ended waveguide reflection measurements using different aperture sizes (WR28 and WR22). The measurements confirm the angular dependence of the EMXT surface impedance. A rectangular waveguide with EMXT sidewalls is modeled by using the angle-dependent effective surface impedance as the sidewall boundary condition. Excellent agreement between this model and full-wave finite-element simulation of the entire waveguide structure is obtained.

Measurements of a semiconductor waveguide using a low-coherence interferometric reflectometer

A low-coherence interferometric reflectometer is studied as a tool to measure both semiconductor waveguide loss and reflectivity. Accurate estimation is given by using the integral values of the interference envelopes observed as reflected power from the sample. This estimation is free of influence from coupling between the sample and reflectometer or dispersion in the semiconductor materials. The experimental results are in good agreement with those obtained by the Fabry-Perot method. The facet reflectivity measured in 0.5-mm-long very short samples shows waveguide thickness dependence agrees with theoretical results. This approach shows good repeatability and accuracy for short samples.

First experiments of pulse compression radar reflectometry for density measurements on JET plasmas

The different reflectometry methods currently used for the determination of electron density profiles of fusion plasmas suffer serious limitations due mainly to plasma fluctuations and to parasitic reflections in the transmission waveguides. An advanced method, based on pulse compression radar techniques, can be applied to overcome these limitations. This paper reports two series of results obtained at JET, which for the first time validate the pulse compression method for reflectometry on thermonuclear plasmas. The first results are for a reflector positioned a short distance beyond the end of a long, complex and oversized waveguide. The same waveguide is used for transmission and reception, inducing many parasitic reflections. The second series of results are from measurements on JET plasmas, using different waveguide configurations. Results both with the reflector and the plasma show that multiple reflections within the transmission line can be eliminated by a correction technique in the signal analysis procedure. A precision of typically 1 cm can be obtained for the measured position of the reflector, or the plasma cutoff layer, with only 20-50 frequency steps. With the fast frequency synthesizers currently available, the complete measurement could be made in 2-5 mu s, thereby freezing plasma fluctuations due to MHD and most of the microturbulence.

Time resolved one-dimensional optical waveguide microscopic imaging of the water drainage on vertical [formula omitted] interface

One-dimensional optical waveguide microscopy is presented as a novel technique that allows for time and spatially-resolved recording of attenuated total internal optical waveguide reflectivity curves at a 20 ms time and 20 μm resolution. We demonstrate its usefulness by measuring the water-glycerol drainage on a vertically mounted Ag SiO 2 - substrate .

Selective acoustic plate mode DNA sensor

Acoustic wave sensors have been proposed for (bio)sensing applications for the past 20 years. One of the most attractive acoustic wave sensor geometries for these applications is the acoustic plate mode (APM) delay line. However, the use of APM delay lines has been hampered by the relative immaturity of the associated design techniques. The principle issue in the design of APM delay lines is to excite and detect electrically a single acoustic mode within the plate with distortion from intermode interference or multiple waveguide reflections. The use of single-phase unidirectional transducers (SPUDT) enables the excitation and detection of a single acoustic mode, reducing the distortions that occur in conventional transducer designs. The current work examines the sensing properties of the resulting APM device for the selective detection of chemically denatured, double-stranded deoxyribonucleic acid (ds-DNA) obtained through polymerase chain reaction (PCR) amplification of genomic viral DNA from cytomegalovirus (CMV).

Microwave evaluation of the conductive filler particles of carbon black-rubber composites

Samples of carbon black loaded nitrile rubber are measured at microwave frequencies (waveguide transmission and reflection). The observed dielectric relaxation is interpreted using a two phase effective medium model (Bottcher formula). The two phases are identified with free rubber as insulating matrix and a conductive filler of a priori unknown parameters (volume fraction, permittivity, conductivity). Supported by SEM and de measurements the results allow to associate the filler phase with adsorbate covered carbon black. In this way the bulk conductivity of carbon black loaded rubber can be explained by (statistical) percolation, which has been not possible taking pure carbon black particles as the percolating elements. Because the effective conductivity of the filler particles deduced from the microwave measurements characterizes mainly the particles' surface it is supposed that the adsorbate may become conductive by the chemical and physical interactions of polymer and carbon black during the preparation of the composite. Reference is made to the doping of rubber resulting in conductivities of the same order of magnitude.

Approximate analytic theory of the multijunction grill

A general approximate analytic theory of the multijunction grill is developed. Omitting the evanescent modes in the subsidiary waveguides both at the junction and at the grill mouth and neglecting multiple wave reflection between the same places we derive simple formulae for the reflection coefficient, the amplitudes of the incident and reflected waves and the spectral power density. All these quantities are expressed through the basic grill parameters (the electric length of the structure and the phase shift between adjacent waveguides) and two sets of the reflection coefficients describing wave reflections in the subsidiary waveguides at the junction and at the mouth. The approximate expressions for these coefficients are also given. The results are compared with numerical solution of two specific examples and they prove to be useful for the optimization and design of the multijunction grills. For the JET structure, which is composed of multijunction sections, it is shown that, in the case of the dense plasma, many results can be obtained from the simple formulae for two-waveguide multijunction grill.

Analysis of an ICRF waveguide launcher for CIT

We study the effects of an L-to H-mode transition in a tokamak on the loading of fast wave launchers in the Ion Cyclotron Range of Frequencies. For analytical simplicity, a dielectric-filled waveguide is chosen for analysis. However, other apertures or coils can readily be accommodated within the formalism. The H-mode density at the edge is modeled as a pedestal followed by a region of Gaussian variation. We calculate the waveguide reflection coefficient, the equivalent surface plasma impedance and the electric field profile at the aperture, for conditions appropriate to CIT (Compact Ignition Torus). It is concluded that an L- to H-mode transition can impose substantial loading variations on the launcher system. These variations can be matched by appropriate fast response tuner systems, similar to those used for coil antennas.

Tunable active chirped-corrugation waveguide filters

A novel tunable semiconductor waveguide reflection filter is proposed and analyzed. The filter is based on spatially selective gain pumping of a chirped-corrugation waveguide. This active chirped-corrugation waveguide filter (ACF) is considered for monolithic broadband tuning of semiconductor lasers.

H-polarization diffraction by a thick metal-dielectric join

An analysis is presented of the H-polarization diffraction due to a material discontinuity formed by the junction of a thick dielectric half-plane with a metallic half-plane having the same thickness. This is accomplished by first considering the solution of several subproblems. These include the direct diffraction and coupling due to a plane wave incident on a loaded open-ended parallel plate waveguide and radiation and reflection by a waveguide mode. The final solution for diffraction by the metal-dielectric join is obtained by introducing a perfectly conducting stub within the loaded guide and subsequently using the generalized scattering matrix formulation with the stub brought to the waveguide opening. All the analysis relating to the subproblems is done by the dual integral equation approach. As expected, the final expressions involve several Wiener-Hopf split functions which are evaluated numerically or analytically.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

界面有機薄膜の赤外ラマンスペクトル

Recent studies on infrared and Raman spectra of monolayers, bilayers, and Langmuir-Blodgett (LB) films are reviewed. Regarding infrared measurements of these films, the transmission, attenuated total reflection (ATR), reflection-absorption (R-A), and surface electromagnetic wave (SEW) techniques are outlined. For Raman measurements, on the other hand, the external reflection, optical waveguide, and total internal reflection techniques are briefly described. The advancement of these techniques and the improvement in sensitivity of infrared and Raman spectrophotometers allowed us to study the structure and molecular orientation even in single monolayers at various interfaces.The infrared ATR spectroscopy of stearic acid LB films indicates that there exists the structural and orientational heterogenity among the first few monolayers. The R-A spectroscopy of cadmium arachidate LB films suggests the almost perpendicular orientation of the molecular axis on the silver substrate as well as significant disruption of molecular conformation and lateral packing during heating and melting processes. Application, of the total internal reflection spectroscopy to the study on resonance Raman spectra of spread monolayers on water and of adsorbed monolayers at an oil-water interface indicates changes in molecular orientation and interaction with changes in the amount of molecules in the monolayers.