Integrated TE-TM mode converter on Y-cutZ-propagating LiNbO3 with an electrooptic phasematching for coherence multiplexing

The use of an integrated TE-TM mode converter as a coherence multiplexer is described. The component is based on a buried waveguide obtained by proton exchange and postannealing. The electrode structure consists of three electrodes. Two are used to achieve TE-TM conversion, and the third is used to perform phase matching to ensure a high conversion efficiency     

Proton-exchanged optical waveguides in Z-cut LiNbO3 using phosphoric acid

Proton-exchanged Z-cut LiNbO₃ planar waveguides formed using phosphoric acid were characterized optically. The refractive index profile and the diffusion parameters were studied systematically. These waveguides have propagation losses of less than 1 dB/cm and exhibit properties that are different from those obtained using benzoic acid. The index profile is not a simple step function and can be modeled accurately by a polynomial expression. A maximum surface index increase of 0.145 was measured at a 0.633-μm wavelength. The diffusion constant D₀ and the activation energy Q for the proton-exchange process using this acid were found to be 6.43×10⁸ μm²/h and 82.91 kJ/mol, respectively. The annealing properties of these waveguides were also established, and the effects of annealing on surface index change and waveguide depth increase were found to follow a power-law relationship     

Frequency-dependent FDTD methods using Z transforms

The frequency-dependent finite-difference time-domain method (FD) ²TD method has been shown to be capable of correctly calculating electromagnetic propagation through media whose dielectric properties are frequency dependent. However, as researchers search for more elaborate applications, the formulation of the (FD)²TD methods becomes more complex. In this work, the mathematics of the (FD) ²TD method is developed using Z transform theory. This has the advantages of presenting a clearer formulation, and allowing researchers to draw on the literature of systems analysis and signal processing disciplines     

Analysis of Z-invariant and Z-variantsemiconductor rib waveguides by explicit finite difference beampropagation method with nonuniform mesh configuration

An efficient and simple explicit finite difference beam propagation method (EFD-BPM) incorporating nonuniform mesh is described. The criteria for stability are developed, and it is shown that this algorithm is power conserving when the stability criteria are met. EFD-BPM is applied to the analysis of single and coupled semiconductor rib waveguides and its accuracy is confirmed by comparing the results with the reported results. Nonuniform mesh is found to improve the efficiency of the method significantly for the analysis of weakly guiding waveguide structures. Several coupled rib waveguide structures with curved input and output branching sections are analyzed using both three-dimensional EFD-BPM and two-dimensional finite difference BPM combined with effective index approximation     

Nonlinear analysis of Y-junction laser arrays

An analysis of Y-junction arrays above threshold is presented. The analysis incorporates nonlinear gain saturation and shows that the gain becomes nonuniform above threshold even if the array is otherwise ideal. The nonuniform, nonlinear gain saturation causes the near field and the radiation patterns to deteriorate with increasing output power. Although the far-field lobe width does not necessarily increase greatly above threshold, the Strehl ratio may decrease significantly depending on the device design. Empirically, it is found that, when the Strehl ratio decreases to approximately 0.7, the second-order array mode attains threshold and both the first and second array modes lase simultaneously. An improved design, whose far-field Strehl ratio remains 0.85 even well in excess of 1 W output power and in which the second-order array mode never attains threshold, is discussed     

An algorithm for computing the inverse Z transform

The authors determine an infinite impulse response of a causal system via a sampling algorithm applied to the transform on the unit circle     

Direct hardware solution to quadratic equation Z2⊕Z⊕β=0 in Galois fields based on normalbasis representation

Normal basis representation is considered to represent the elements of Galois fields. The quadratic equation, Z²⊕Z⊕β=0, is solved directly and a new, simple, regular and expandable hardware structure is introduced to solve this equation. The main advantage of this structure over the structures in non-normal basis representations is its independence from generating polynomial of the field     

Confidence bounds for Pr{X>Y} in 1-way ANOVArandom model

Approximate confidence bounds for reliability, R=Pr{X >Y|X,Y}, are obtained, where X and Y are independent normal (Gaussian) random variables, and X and Y are vectors of measurements for X and Y, respectively. Balanced 1-way ANOVA (analysis of variants) random effect models are assumed for the populations of X and Y. Confidence bounds are derived for R under three cases for standard deviations, σ_x and σ_y. An example shows how the results are used     

Acoustooptic interaction at 520 MHz in proton-exchangedY-lithium niobate waveguides

Experimental results are reported on acoustooptic interaction at 520 MHz in proton-exchange waveguides in Y-lithium niobate. The acoustooptic diffraction efficiency for an electrical input of 300 mW is 39% in proton-exchanged devices as compared to 22% in titanium-indiffused ones. The dynamic range observed in proton-exchanged and Ti-indiffused acoustooptic devices is 22 and 25 dB, respectively     

Theoretical modeling and characterization of annealedproton-exchanged planar waveguides in z-cut LiNbO3

Planar optical waveguides in z-cut LiNbO₃ fabricated by the annealed proton-exchange technique using pure pyrophosphoric acid as the initial proton source have been investigated. A generalized Gaussian function was used to accurately model the refractive-index profile, resulting in considerable improvements over previous work. The nonlinear dependence of the index on the proton concentration was verified by directly calculating the change in the area under the index versus depth curves. Annealing was found to allow for flexibility in the tailoring of the waveguide parameters. An empirical correlation of the changes in these parameters with the fabrication conditions was achieved through the use of a general power law     

Numerical modeling of coherent coupling and radiation fields inplanar Y-branch interferometers

The beam-propagation method is used to study intermodal power transfer and radiation losses in slab waveguide splitters and combiners. Numerical computations demonstrate how modal power at an interferometer's recombination point exhibits an oscillatory dependence on the length of each distinct section of the structure. Recoupling of guided and unguided modes can be anticipated and adjusted so as to maximize output modal power and minimize sensitivity to transition lengths. Examples are given for systems formed from both angle bends and S bends     

An asymmetric Y-branching circuit using total reflectionfor semiconductor integrated optical circuits

A new structure for a compact optical branching circuit is proposed. An asymmetric Y-branching circuit using total reflection is fabricated on an AlGaAs/GaAs wafer. Good branching operation is observed in a single-mode branching circuit with a branching angle of 45°. The exact origins of loss in the branching circuit are also clarified     

Analysis of single-mode optical Y-junction by the boundedstep and bend approximation

An approximate method for analyzing planar optical Y-junctions is presented. The junction structure is replaced by a discrete succession of bounded waveguide segments, and the presence of waveguide bends is assumed. Using this approximation, single-mode symmetrical Y-junctions operated as power dividers are investigated numerically. It is shown that strong mode conversion occurs between the guided and radiation modes of the local normal mode. A perturbation analysis shows that reflected fields are negligible. Reliable values of the transmitted power are obtained     

1/f noise in MODFETs at low drain bias

The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S_I/I² versus the effective gate voltage V_G=V_GS-V_off shows three regions which are explained. The observed dependencies are S_I/I²∝V_G ^m with the exponents m=-1, -3, 0 with increasing values of V_G. The model explains m =-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V_G or V_GS≅0 is due to the dominant contribution of the series resistance. In the region at intermediate V_G , m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance     

DC I-V characteristics of field emitter triodes

A simple model that is applicable to Spindt-type emitter triodes is presented. Experimentally, it has been observed that the gate current at zero collector voltage follows the same Fowler-Nordheim law as the collector current at high collector voltage, and that for low emission current densities, the sum of gate and collector currents is constant for any collector voltage and is given by the Fowler-Nordheim current I_FN. Based on these observations, a simple model has been developed to calculate the I-V characteristics of a triode. By measuring the Fowler-Nordheim emission, emission area and field enhancement can be obtained assuming a value for the barrier height. Incorporating the gate current, the collector current can be calculated from I_c=I_FN-I_g as a function of collector voltage. The model's accuracy is best at low current density. At higher emission currents, deviations occur at low collector voltages because the constancy of gate and collector currents is violated     

Gain measurements on the KrF(B→X), XeF(B→X), and XeF(C→A) lasertransitions in an XeF(C→A) laser gas mixture

Optimum energy extraction from an electron-beam-pumped XeF(C →A) laser is achieved with a five-component rare gas halide mixture. The characterization and modeling of laser action in such a gas mixture requires a knowledge of small-signal gain and absorption coefficients not only on the blue-green XeF(C→ A) transition, but also in the ultraviolet (UV) region for the competing XeF(B→X) and KrF(B→X ) transitions. The authors report gain measurements on the XeF(C→A) transition and small-signal gain and absorption coefficients at or near both the XeF(B→X ) (351 and 353 nm) and KrF(B→X) (248 nm) transitions. A study of the gain for the UV and visible transitions as a function of Kr and Xe partial pressure is reported, and its impact on the XeF(C→A) kinetics is discussed     

Public-key cryptosystem over ring Z[√-3]/(2m)

The author improves the public-key cryptosystem presented recently (see IEE Proc. E, vol.134, no.5, p.254-6, 1987) and designs a new method of compressing the data expansion     

Binary images of cyclic codes over Z4

We determine all linear cyclic codes over Z₄ of odd length whose Gray images are linear codes (or, equivalently, whose Nechaev-Gray (1989) image are linear cyclic codes or are linear cyclic codes)     

High-performance Ka-band and V-band HEMTlow-noise amplifiers

Quarter-micron-gate-length high-electron-mobility transistors (HEMTs) have exhibited state-of-the-art low-noise performance at millimeter-wave frequencies, with minimum noise figures of 1.2 dB and 32 GHz and 1.8 dB at 60 GHz. At Ka-band, two-stage and three-stage HEMT low-noise amplifiers have demonstrated noise figures of 1.7 and 1.9 dB, respectively, with associated gains of 17.0 and 24.0 dB at 32 GHz. At V-band, two stage and three-stage HEMT amplifiers yielded noise figures of 3.2 and 3.6 dB, respectively, with associated gains of 12.7 and 20.0 dB and 60 GHz. The 1-dB-gain compression point of all the amplifiers is greater than +6 dBm. The results clearly show the potential of short-gate-length HEMTs for high-performance millimeter-wave receiver application     

Modeling temperature effects in the DC I-Vcharacteristics of GaAs MESFET's

A simple model is presented to account for the main temperature effects influencing the DC performance of GaAs MESFETs. The model is based on a consistent solution of heat flow and current equations that accounts for nonuniform power dissipation within the device. The simulation results are satisfactorily compared with experimental data obtained with pulsed and DC measurements performed on conventional devices as well as on suitable test structures