Finite element method analysis of LiNbO3fiber type modulator

High-speed electro-optic switch based on nonlinear polymer-clad waveguide incorporated with quasi-in-plane coplanar waveguide electrodes

Molecular orientation states in the liquid crystal (LC) cell can easily be deformed and change their properties by applying low driving voltage with low power consumption. Those features are also attractive for the various functional devices in the wider frequency regions such as a millimeter wave (MMW), although only the optics applications are well investigated as the display application is put the head of them. Waveguide systems are well known as an excellent way to make a low loss transmission circuit in the ultrahigh frequency region such as microwaves and MMWs, and then the LC devices utilizing the waveguide have been proposed so far. However, the planar waveguide has gathered many attractions for the wider functionality by integration. We adopt here a coplanar waveguide (CPW) substrate to prepare the planar type of LC MMW devices combined with an ITO glass substrate. Since the ITO film acts as a floating electrode to simplify the electrode structure, LC molecules can be driven by applying the voltage only to the CPW substrate. Unfortunately, since the ITO electrode close to the CPW decreases the transmission of the MMW, the molecular orientation effects related with the electrode structure are very important to minimize the electrode area. We prepare the transparent LC cell which has exactly the same electrode structure with the CPW LC cell, and observe the molecular orientation states in detail by using a polarization microscope. It becomes clear that only the narrow area above the signal electrode of the CPW affects strongly to the phase change of the MMW propagation in the CPW type LC cell.

High speed semiconductor optical modulators are very important components for optical communication systems [1]. Characteristics such as low drive voltage, wide bandwidth, low insertion loss, low wavelength and temperature sensitivity, and low cost are essential for these optical modulators. Recently 40 GHz InP modulators with a drive voltage of only 2.3 V have been reported which use an n-i-n waveguide structure instead of the conventional p-i-n structure to reduce both the microwave and optical losses [2]. However InP-based modulators have the drawback of the wavelength dependence of the drive voltage. GaAs based modulators do not have this problem due to its larger bandgap energy. To date GaAs modulator has been demonstrated with a 35 GHz optical bandwidth and 5 Vcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</inf> [3]. In this paper, we report a new design for GaAs modulators based on an n-i-p-n structure. The travelling-wave coplanar waveguide (CPW) electrodes are employed to realize high speed operation. By optimization, an electrical 3-dB bandwidth of nearly 40 GHz (optical 3-dB bandwidth of 80GHz) and a V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</inf> around 6.6 V are predicted for a 5 mm long phase modulator.

A DC contact series MEMS switch for reconfigurable antenna system application is designed, fabricated, and its RF characteristics are measured. Variable pivot seesaw concept is applied because this design minimize the driving voltage and prevent stiction of membrane. The proposed switch structure is fabricated on the Si wafer, a coplanar waveguide(CPW) signal lines and electrodes are fabricated on the glass wafer. Both wafer are bonded by anodic bonding method. The designed chip size is within 2 mmx2 mm and it is actuated by electrostatic force. Low actuation voltage has been achieved by means of small distance between signal line and membrane using the design scheme, switching is executed in the pull-in range. Minimum actuation voltage is about 10-12 V, isolation is around 50 dB and insertion loss is about 0.25 dB at 2 GHz.

To find the optimal driving voltage and bandwidth for electro-optic polymer-waveguide-based directional couplers, four electrode structures are investigated. They are the microstrip line (MSL), the asymmetric coplanar waveguide with ground (ACWG), the coplanar waveguide with ground (CPWG), and the edge-coupled microstrip line (CMSL). UV15:Polymethyl methacrylate/dispersed-red 1:UV11-3-based directional couplers are evaluated. The CPWG and the ACWG have almost the same driving voltages, which are about twice the driving voltage of the CMSL. The MSL has the largest driving voltage, which is 20 to 25% higher than the diving voltages of the CPWG and ACWG. Simulation results further conclude that the MSL has the largest bandwidth, and its bandwidth is about 1.6 times that of the bandwidth of the CMSL, which has the narrowest bandwidth. The bandwidths of the ACWG and CPWG are 1.4 and 1.2 times that of the CMSL, respectively. The driving voltages for the four different devices are experimentally confirmed.

The performance of traveling-wave type guided-wave light modulators with phase reversed electrodes is studied theoretically. Asymmetric stripline and coplanar waveguide are applied to the phase-reversed structure. modulators with these two types of electrodes are compared by using three figures of merit to characterize the qualities of modulators. They are the bandwidth-length product, the voltage-length product, and the bandwidth-voltage ratio. Design examples of modulators are also given. for high-speed and low-drive-voltage light modulation, the advantages of the use of asymmetric stripline are shown. >

Several new nonperiodic spatial codes for phase-reversal coplanar waveguides (CPWs) used in the framework of the quasi-phase match technique (QPM) for broadening the optical response of z-cut LiNbO/sub 3/ electrooptic modulators (EOMs) are presented. The peculiarity of the codes is that they open the possibility for time-domain applications of EOMs based on nonperiodic QPM structures. They were obtained by using a specialized version of the genetic algorithm that generates an optimal length and number of the phase-reversal sections. The codes provide both flat amplitude and nearly linear phase frequency responses of the EOM within several frequency ranges important for fiber-optical/radio systems: from dc to 25, 40, and 90 GHz. The phase-reversal structures described here were optimized taking into account the CPW electrode losses and under the matching condition with 50 /spl Omega/-sources. They possess as low value of the drive voltage (V/sub /spl pi//, required for realization of 100% EO modulation efficiency) as possible for EOMs based on QPM, under the above conditions.

In this paper, the optical and electrical pulse responses of an optimized high-speed organic light-emitting diode (OLED) is investigated as a function of the pulse duration. The original design is based on the coplanar waveguide electrodes that are implemented on a transparent and conductive oxide thin film. The designed electrodes with reduced serial resistances (13 $\Omega $ ) allow to maximize the pulse energy delivery and to feed the organic device with short electrical pulses of few nanoseconds for the first time. This is achieved by designing the electrodes to have 50- $\Omega $ characteristic impedance, thanks to microwave techniques, and by implementing small values of capacitance (4.5 pF). The experimental results demonstrate $\boldsymbol \mu $ -OLED electrical and optical responses to electrical excitation pulses as short as 2.5 ns, for the first time, and state-of-the-art current densities above 1.7 kA/cm2. Moreover, and rather unexpectedly, the developed characterization procedure to extract the reported results demonstrated the first direct measurement of the electroluminescence lifetime of organic materials in the nanosecond time scale.

A numerical method has been developed for the analysis of microwave devices made with superconducting materials or/and normal (perfect or imperfect) metal conductors and lossy dielectrics. This method, based on 2D and 3D Finite Element Method (F.E.M.) is applied here to evaluate the propagation characteristics of microstrip lines, coplanar waveguides, and the scattering parameters of planar circuits (resonators,. . .). The results given by F.E.M. are compared with those obtained by using other theoretical analysis or/and with experimental data.

A slow-wave electrode structure for integrated optic traveling-wave modulators in which the microwave's effective refractive index is matched to the optical wave's effective refractive index is described. The electrode structure is a capacitively loaded coplanar strip waveguide which can be formed in a single layer of metallization. Fabrication can be accomplished by a single photo-resist patterning, followed by an etching and a standard lift-off technique. Based on the use of gallium arsenide substrates and modern lithographic techniques allowing fabrication with micron scale resolution, slow-wave electrodes having a microwave effective refractive index of 3.5, as well as 50 and 75 Omega characteristic impedances are proposed. The theory of slow-wave electrodes is developed, and slow-wave electrodes have been designed, fabricated, and tested to verify the theory. Measurement results are found to agree well with the theory. >

The handling and control of towed cable and body systems onboard surface ships and submarines presents a significant technical challenge to design engineers in the defense and ocean industries. The current approaches rely heavily on the empirical methods and the time-consuming and costly prototype testing. Computer simulation provides a cost effective way to reduce the high risks associated with the towed cable/body system. However, the current dynamic analysis of towed cables is mostly done by the finite difference (FD) method in stead of the finite element (FE) method that is widely used in almost all engineering fields. This paper presents an alternative FE method to simulate the dynamics of towed cable and body system, in which the large rigid body motion is coupled with small elastic deformation. The newly derived FE method is formulated in terms of element nodal positions, which is different from the existing FE methods that use displacements. The alternative FE method solves for the cable position directly in order to eliminate accumulated numerical errors arising from existing FE methods that solve for displacements first in order to obtain the cable position over very long period of time. The alternative FE formulation is implemented and applied to real applications to demonstrate its robustness by comparing simulation results with the experimental and sea trial data.

Real-world groundwater modeling deals with heterogeneous and anisotropic aquifer systems. Numerical models, particularly the finite difference and finite element methods, are extensively used for modeling such a complex aquifer system. The finite difference method is an age-old straightforward approach, whereas the finite element method is more idealistic, relatively new, and has emerged as a popular approach. The unique advantages with the finite element method are (i) an irregular-shaped domain can be discretized into elements with a matching boundary, (ii) a node can be connected with the neighboring nodes in any direction. Thus, the finite element analysis allows water/solute to flow more freely toward the surrounding nodes along nodal links. On the other hand, the mathematical formulation of the finite element technique follows a set of rules step by step. Conventionally, this idealistic approach uses shape functions and integration techniques to develop element matrices, which, in turn, are assembled to form the global matrix. This conventional global matrix was found to be inconsistent with the basic partial differential equation representing the mass balance in a nodal domain as a control volume. As a result, the matrix solution, particularly for nonlinear problems (e.g., vadose zone flow, density dependent flow, and so forth) becomes unstable and computationally burdensome. To overcome the limitations of the conventional finite element method for real-world groundwater modeling, a new approach, called the direct-formulation finite element (DFFE) method, has been developed. In this approach, a two-dimensional model domain is discretized into triangular elements. Element matrices are formed directly based on flow principles and material properties of elements without using shape functions and integration process. The DFFE method is robust. The global matrix is consistent, and the solution is stable and efficient. The results obtained from this approach were found in a good agreement with that obtained from analytical and other widely used models.

We investigated the propagation losses and the characteristic impedances Z/sub L/ of coplanar waveguides (CPWs) and microstrip lines (MSLs) on a planar lightwave circuit (PLC)-platform formed on a silica/silicon substrate. The loss of the CPWs was 2.7 dB/cm at 10 GHz on the PLC-platform with 30 /spl mu/m thick silica layer. Thus, a cm-order circuit of this CPW is difficult to fabricate in the 10 Gb/s module. This is because the silicon substrate has a large loss tangent (tan /spl delta/). On the other hand, the loss of the MSLs, where a ground plane shielded the high loss silicon substrate, could be improved to 0.9 dB/cm at 10 GHz with 30 /spl mu/m thick polyimide. These lower loss MSLs on a PLC-platform can be applied to module operation at 10 Gb/s. Furthermore they have the advantage that they are suitable for application to array device circuits or circuits in a module where several devices are integrated because unlike CPWs the ground planes are not divided by signal lines or DC bias lines. The structure of CPWs and MSLs on a PLC-platform with a Z/sub L/ of 50 /spl Omega/ was also studied in detail.

Chapter 11 - Mixed finite element–computational fluid dynamics method for nonlinear fluid–solid interactions

It is common knowledge that conventional finite element method (FEM) has intrinsic limitations in analyzing large deformation problems like high-velocity impact, explosion, etc. because of mesh distortion and tangling; while these problems can be easily avoided by the meshfree method (MM), the latter involves greater computation time. Therefore, in this article, in order to simultaneously utilize the respective advantages of the two methods, a coupled simulation method between both FEM and MM was employed to analyze the high-velocity impact on ductile metal pipe with polymer coating. The impacted area with large deformation was discretized by SPH (smoothed particle hydrodynamics) particles, a classic meshfree model, and the remaining section was modeled by FEM meshes. By this method, the interfacial shear stresses between the coating and the substrate and the residual stresses beneath the contact points were studied, which would have referenced values in analyzing failure modes of components with similar composite structure. Then, the results were compared with sole FEM and MM too.