The Channel Waveguide (Correspondence)

A waveguide propagating the TE/sub 10/ mode can carry more power than the normal rectangular waveguide if it has a symmetrically placed channel in the E-plane as shown in fig. 1. The greater height of the channel in the center of the waveguide will allow a higher voltage to be applied before dielectric breakdown occurs. The TE/sub 10/ cutoff wavelength /spl lambda//sub c/ was investigated using the methods of Iashkin and Cohn to find out if the cutoff wavelength of the channel waveguide was equivalent to that of the rectangular waveguide /spl lambda//sub cr/.     

On the Mode and Square Correspondence Between Circular Multimode Tapered Waveguides

In an axially straight multimode circular waveguide taper excited with a pure TE/sup /spl circle// /sub 11/ dominant mode, the first and only converted mode at and near cutoff is the TM/sup/spl circle// /sub 11/ mode. It is shown that in an axially straight multimode square waveguide taper excited with a pure TE/sup/spl square// /sub 10/ dominant mode, the TM/sup/spl square// /sub 12/ mode corresponding to the TM/sup/spl circle// /sub 11/ mode in circular case is not the only first converted mode at and near cutoff. The overall behavior or coupling mechanism of waveguides is similar whether the waveguide is rectangular, square, circular, or elliptical: i.e., the overall coupling coefficient at cutoff of a converted mode or modes approaches an ininfinity of the order 0/sup -1/4/.     

15-GHz modulation performance of integrated DFB laser diode EA modulator with identical multiple-quantum-well double-stack active layer

Monolithically integrated InGaAsP 1.55-/spl mu/m ridge waveguide distributed feedback laser diodes with an electroabsorption modulator using an identical active multiquantum-well (MQW) layer structure with two different QW types exhibit low-threshold currents <18 mA. The 3-dBe cutoff frequency of 200-/spl mu/m-long modulators exceeds 15 GHz. 10-Gb/s transmission experiments with a voltage swing of 1.0 V/sub pp/ demonstrate the potential of this novel integration scheme.     

Measurements of V-band n-type InSb junction circulators

A V-band n-type indium antimonide (InSb) junction circulator supported in a three-port finline structure has been fabricated and measured. Broad-band operation for a semiconductor junction circulator over the frequency range 50-75 GHz at a temperature of 77 K has been demonstrated for the first time. With an applied magnetic flux density of 0.88 T, approximately 10 dB of differential isolation has been measured over the entire waveguide frequency band. The measured results also indicate that circulation is possible when the semiconductor material has /spl epsiv//sub eff/<0. In principle, broader bandwidths are predicted since frequency tracking can be achieved from /spl epsiv//sub eff/<0 to /spl epsiv//sub eff/>0, but the bandwidths of the circulators measured are restricted by the cutoff frequency of the V-band waveguide. Experimental evidence also showed that a disc or triangular-shaped semiconductor suspended in an E-plane junction without the finline circuit provides circulation. The experimental results clearly illustrate the broad-band behavior of semiconductor junction circulators for operation beyond 40 GHz, which is difficult to achieve with ferrite-based circulators.     

Sommerfeld Pre- and Postcursors in the Context of Waveguide Transients

For propagation in lossless waveguide, the rigorous impulse response function is given. It is shown that its instantaneous frequency is that which has reached the output at that time by propagating at the group velocity. For a square envelope pulse with a carrier frequency /spl omega/ above the cutoff frequency /spl omega//sub c/, the propagation of the envelope and of the phase are essentially described by the group and phase velocity, respectively. In addition, however, the bulk of the pulse is preceded by the so-called Sommerfeld precursors having an increasing amplitude and a frequency which decreases from a high value to /spl omega/. Similarly the bulk of the pulse k followed by the Sommerfeld postcursors in which both amplitude and frequency decrease, the latter from /spl omega/ to /spl omega//sub c/. The analytic results are illustrated by computed examples of waveguide transients.     

Cutoff Phenomena for Guided Waves in Moving Media

This paper treats the propagation of electromagnetic waves in the interior of a waveguide that is filled with a moving medium. The medium is assumed to be homogeneous, isotropic, and lossless, and to move with a constant velocity along the axis of the waveguide. The Maxwell-Minkowski equations for the electromagnetic fields are solved by means of a pair of vector potential functions similar to those frequently used for stationary media. The fields inside the waveguide are derived for both rectangular and cylindrical waveguides. The well-known cutoff phenomenon for a waveguide is found to be modified in an interesting way when the medium inside the waveguide is moving The results show that for a slowly moving medium (a medium for which n/sp, Beta/<1, where n is the index of refraction and /spl Beta/ is the velocity of the medium divided by the velocity of light in vacuum/, there are two critical frequencies, separating three frequency ranges in each of which there is a different type of propogation. For a high-speed medium (n/spl Beta/<1), it is found that there is no cutoff phenomenon at all, although there is one critical frequency separating two frequency ranges in which the propagation is different.     

The Design of Evanescent Mode Waveguide Bandpass Filters for a Prescribed Insertion Loss Characteristic

A design theory for evanescent dominant mode waveguide filters is presented. The cutoff guide is represented by its equivalent /spl pi/-section network, the elements of which closely approximate lumped inductances. Resonators may be formed by introducing appropriate capacitive obstacles at suitable intervals along the guide. The filter that results is a microwave analog of lumped inductance filters with series inductance coupling, the magnitude of the series "inductance" being controlled by the separation between capacitive obstacles. The theory derives the equivalent ladder network from the low-pass prototype and is accurate up to at least 20 percent band-widths. Filters may be designed to couple into other cutoff waveguide components using the same basic principles, a propagating guide, and coaxial terminations. The filters are normally constructed in standard production waveguide and are simple and cheap to manufacture. A considerable size and weight reduction, compared with orthodox waveguide filters, is possible at the cost of a small increase in insertion loss.     

Excitation and Scattering of Modes on a Dielectric or Optical Fiber

A technique similar to that of Kirchhoff is used to obtain an analytic expression for mode Iaunching on a semi-infinite rod. The approximation is quasi-optical and only valid for small angles of incidence /spl theta/. When /spl theta/ = 0 only HE/sub 1M/ modes are excited; however, for /spl theta/<<1 many modes can be launched. The effect on HE/sub 11/ mode propagation of small imperfections in a dielectric waveguide is analyzed. At the frequency of interest for optical communication (cutoff for the TM/sub 01/ mode) the radiated power is 160 times larger than that scattered into the HE/sub 11/ mode.     

1.55-/spl mu/m spot-size converter integrated laser diode with conventional buried-heterostructure laser process

A 1.55-/spl mu/m spot-size converter integrated laser diode is demonstrated with conventional buried-heterostructure laser process. For the spot-size converter, we employed a double-waveguide structure in which a ridge-based passive waveguide was incorporated. The passive waveguide was optically combined with a laterally tapered active waveguide to control mode size. The threshold current was measured to be 5 mA together with high slope efficiency of 0.45 W/A. The beam divergence angles in the horizontal and vertical directions were as small as 9.0/spl deg/ and 7.8/spl deg/, respectively.     

High efficiency 90/spl deg/ silica waveguide bend using an air hole photonic crystal region

We propose the hybrid integration of an air hole photonic crystal (PhC) structure with a high /spl Delta/ (0.75%) single-mode silica waveguide to achieve an ultracompact high efficiency 90/spl deg/ bend for transverse-magnetic polarized light. Diffraction from the periodic boundary between the PhC and silica waveguide regions is shown to seriously degrade the optical efficiency of the bend. A microgenetic algorithm (/spl mu/GA) combined with a two-dimensional finite-difference time-domain method is used to modify the PhC and its boundary layer to suppress this diffraction which in turn maximizes bend efficiency. The final optimized structure has a 99.4% bend efficiency at a wavelength of 1.55 /spl mu/m and occupies an area of only 27 /spl times/ 27 /spl mu/m.     

New Leaky Surface Waves in Anisotropic Metal-Diffused Optical Waveguides

Propagation of guided waves in anisotropic metal-diffused optical waveguide is investigated. Two-dimensional guide-mode dispersion curves are computed and classified for a metal-diffused waveguide with arbitrary optic-axis orientation in various diffusion depth. It is found that a new leaky surface wave exists in the region where the refractive index is above the cutoff value, not below it. Typical values of decay constant are about 5 dB/cm for the wave propagating along X axis on 128/spl deg/ rotated Y-cut LiNbO/sub 3/, and 35 dB/cm for the wave propagating along the direction making an angle of 70/spl deg/ to X axis on Y-cut plane LiNbO/sub 3/. We were able to observe the leaky surface waves experimentally.     

Large-core single-mode waveguides with cross-sectional antiresonant confinement

This paper presents a single-mode waveguide based in cross-sectional antiresonant confinement. The confinement in the vertical direction is obtained placing a Fabry-Pe/spl Sigma/rot tuned at its antiresonant condition underneath the core [defining the antiresonant reflecting optical waveguides (ARROW) structure]. Small weightings are placed on both sides of the core to achieve a local increase in the effective refractive index, obtaining the so-called lateral antiresonant structures (LASs). They assure both the cross-sectional confinement and the single-mode behavior of the global structure. Simulations predict the transition of the symmetrical mode of the LAS to a bidimensional ARROW mode below the cutoff condition of the former, while at the cutoff condition of the asymmetrical LAS mode power is directly transferred to a radiative mode. Experimental results have shown that losses decrease as the lateral core width increases, which is in agreement with a minor confinement in the structure. Near-field images from a 3-mm-thick, 16 mm-wide ARROW-two-dimensional structure have shown that when a misalignment between the input optical fiber and the waveguide is produced, no higher order modes are excited, confirming the single-mode behavior of these structures.     

AlGaN/GaN/InGaN/GaN DH-HEMTs with an InGaN notch for enhanced carrier confinement

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin In/sub x/Ga/sub 1-x/N(x=0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm/sup 2//V/spl middot/s and a sheet resistance of 420 /spl Omega//sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 14.5 GHz, and a peak power gain cutoff frequency (f/sub max/) of 45.4 GHz were achieved on a 1/spl times/100 /spl mu/m device. The off-state source-drain leakage current is as low as /spl sim/5 /spl mu/ A/mm at V/sub DS/=10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz.     

Spot-size converter using vertical ridge taper for low fibre-coupling loss in 2.5%-/spl Delta/ silica waveguides

A novel spot-size converter based on a vertical ridge waveguide taper for super-high-/spl Delta/ silica waveguides is demonstrated. This structure can be formed with a simple fabrication process. The coupling loss between a singlemode fibre and a 2.5%-/spl Delta/ silica waveguide was reduced to 0.31 dB/point compared to 2.7 dB/point for conventional straight waveguides.     

A Simple Numerical Method for Studying the Propagation Characteristics of Single-Mode Graded-Index Planar Optical Waveguides

A simple numerical method based on the Runge-Kutta method is presented to compute the propagation constant, the modal field, and the cutoff wavelength corresponding to the fundamental TE/sub 0/ and TM/sub 0/ modes of a planar optical waveguide with an arbitrary refractive index profile. The method is much simpler and requires less computational effort than the earlier reported numerical methods. We have also used the technique for an estimation of the effect of the /spl nabla/epsilon term in TM modes.     

Compact polarization-independent Mach-Zehnder space switch combining carrier depletion and the quantum confined Stark effect

We have developed an n-doped InGaAs-InAsP quantum well between InP, which is suited for a polarization-independent Mach-Zender interferometric (MZI) space switch operating at 1.55 /spl mu/m. The InAsP is compressively strained and the InGaAs is tensile strained for polarization independence and for strain balancing. The important boundary condition for the design of this structure is the waveguide loss, which we limit to 0.6 dB/cm, and the crosstalk due to imbalance in the MZI, which we limit to <-30 dB. To reduce the size of the phase shifting region, while imposing this boundary condition, we combine the quantum confined Stark effect (QCSE) effect and the carrier-depletion effect by using an n-doped quantum well. The QCSE was first optimized for an undoped InGaAs-InAsP quantum well. A polarization independent /spl Delta/n of 7.8/spl middot/10/sup -4/ at 100 kV/cm was obtained at the expense of 0.2-dB/cm excess waveguide loss and 0.1-dB/mm electroabsorption loss. The carrier-depletion effect in a 2/spl middot/10/sup 11/cm/sup -2/-doped QW increases /spl Delta/n with a factor 2.6 to 2/spl middot/10/sup -3/, at the expense of 0.4-dB/cm free-carrier absorption-induced waveguide loss. The combination of the QCSE and carrier depletion results in a phase-shifter length of 0.46 mm for an MZI in push-pull configuration.     

A fully integratable 1.55-/spl mu/m wavelength continuously tunable asymmetric twin-waveguide distributed Bragg reflector laser

We demonstrate a single-frequency continuously tunable three-section distributed Bragg reflector laser operating at a center wavelength of /spl lambda//sub 0/=1.548 /spl mu/m using a fully integratable asymmetric twin-waveguide structure. A low-loss tapered mode transformer couples the light between the active waveguide, or gain region, and the passive ridge waveguide where the phase and grating tuning sections are located. The device has a threshold current of 50 mA and output power of nearly 13 mW, with a slope efficiency of 0.12 W/A and a tuning range of 4.8 nm under pulsed operation. An independent phase section is used to continuously tune the wavelength, thus avoiding mode hops. Using a delayed self-heterodyne technique, we determine the linewidth to be (146/spl plusmn/2) kHz.     

Analysis of a Narrow Capacitive Strip in Waveguide

A theoretical determination is made for the susceptance of a narrow conducting strip inserted vertically into a waveguide. The theory is based upon a variational form for the susceptance. A suitable current distribution along the strip is obtained for the variational equation, and is found to be similar to that determined from analysis of backscattering by a cylindrical obstacle irradiated from an incident plane wave. Accurate theoretical results may be obtained using a sinusoidal current distribution having a phase constant of /spl pi//2d, where d is the strip depth. Experimental results agree closely with the theory in the dominant-mode range and also at frequencies below cutoff.     

Modal characteristic equation and dispersion curves for an elliptical step-index fiber with a conducting helical winding on the core-cladding boundary - an analytical study

A special type of elliptical step-index fiber with a conducting helical winding on the core-cladding boundary is proposed and investigated theoretically. Using elliptic cylindrical coordinates, boundary conditions are derived and longitudinal field components for the even and odd modes are obtained. The characteristic equation for the waveguide to be studied is determined by solving the fourth-order determinantal equation, which also contains complex quantities. In order to study the fundamental mode, the modal index /spl nu/ is put as /spl nu/=1. The equations obtained involve modified Mathieu functions of the first order and their derivatives. Applying approximations, cutoff conditions are obtained. Numerical computations are made. Dispersion curves are obtained for different pitch angles of the helical winding.     

An experimental study of harmonic SIS mixing at 205-235 GHz

A 230-GHz subharmonically pumped waveguide mixer employing superconducting tunnel junctions has been developed. We present, in this paper, an experimental study of harmonic superconductor-insulator-superconductor (SIS) mixing at 230 GHz, focusing mainly on its noise behavior. The mixer has a double-tuned waveguide structure and employs an array of four 1.7-/spl mu/m/sup 2/ Nb-AlOx-Nb SIS junctions in series, with /spl omega/R/sub n/C/sub j//spl sim/3 at 230 GHz. Harmonic quantum mixing occurred over an experimental frequency range of 205-235 GHz (local oscillator: 112.5-117.5 GHz), exhibiting corresponding double sideband noise temperatures of lower than 150 K, with a lowest value of 75 K at /spl sim/230 GHz. The measured mixer noise is believed to be the lowest yet reported for a mixer using subharmonic-pump configuration at this frequency. A phenomenon that we attribute to the third harmonic SIS mixing has also been observed.