Philip Dowd

Tilted folded-beam suspension for extending the stable travel range of comb-drive actuators

We investigate the electromechanical side instability and the stable travel range of comb-drive actuators. The stable travel range depends on the finger gap spacing, the initial finger overlap, and the spring stiffness ratio of the compliant suspension. Proper design of the suspension structure is the most effective way to stabilize the actuator and therefore to achieve a large deflection. In this paper, we propose an improved suspension design, the so-called tilted folded-beam suspension. Using such suspension, the stability of the comb-drive actuator is improved and the stable travel range is enhanced. We give the expressions for the spring constants of the proposed suspension, both in the stroke direction and perpendicular to it. The suspension designs are also studied numerically using the finite element method (FEM), in which the geometric nonlinearities, such as large deflections and stress stiffening, are considered. Analytical calculations and FEM simulations are compared. The results demonstrate that an enhanced stable travel range, compared to that of a comb-drive actuator with the most commonly used folded-beam flexure, can be achieved by using the proposed suspension design. Comb-drive actuators with various tilted folded-beam suspensions have also been fabricated using the standard surface micromachining technology and their operational performances have been characterized. The experimental results are in good agreement with the theoretical predictions.

Design of diffractive phase elements for beam shaping: hybrid approach

Hybrid approaches that combine genetic algorithms (GA’s) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive phase elements (DPE’s) for laser beam shaping. These hybrid methods exploit the global nature of the GA’s as well as the local improvement capabilities of the gradient-based local search techniques and will perform a more improved search in comparison with each of the individual approaches. The incorporated local search technique that we used here is the Davidon–Fletcher–Powell method. A cost function that can directly control the performance of the final solutions is also used. By performing the DPE design with different desired diffraction efficiencies, we obtain a set of results that approximately reflect the trade-off between the design objectives, namely, signal-to-noise ratio (SNR) and diffraction efficiency. Reasonable solutions can be chosen on the basis of the knowledge of the problem. Simulation computations are detailed for two rotationally symmetric beam-shaping systems, in which an incident Gaussian profile laser beam is converted into a uniform beam and a zero-order Bessel beam. Numerical results demonstrate that the proposed algorithm is highly efficient and robust. DPE’s that have high diffraction efficiency and excellent SNR can be achieved by using the algorithm that we propose.

Influence of strain on annealing effects of In(Ga)As quantum dots

Post-growth rapid thermal annealing has been performed with In(Ga)As quantum dots (QDs) at different strain statuses. It is confirmed that the strain-enhanced interdiffusion decreases the inhomogeneous size distribution. The preferential lateral interdiffusion of QDs during annealing was observed. we attribute it to the naturally anisotropic strain distribution in/around the dots and the saturation of strain difference between the base boundary and the top of the dots. There exist strain-enhanced mechanism and vacancy diffusion enhanced mechanism during the annealing. As to which one dominates the QD interdiffusion depends on the thickness of capping layer and the annealing temperature

A method to include micromechanical components into the system level simulation

In this paper, a modeling approach is introduced that includes micromechanical components into the system level simulation. In our approach, a complicated microsystem is first decomposed into a set of much simpler components. Each component is treated as an n-pole functional block. The behavioral model of a micromechanical component is generated from the finite element (FE) analysis results of its corresponding 2D/3D FE model by using the system identification methods. The behavioral model is then described by an analog hardware description language (AHDL). The global performance of the system is simulated in an analog behavioral simulator by connecting the subdivided behavioral models together. The proposed approach can be implemented by combining the commercially available software ANSYS (for finite element method (FEM) analysis), MATLAB (for system identification) and an analog behavioral simulator such as SPECTRE (for system level simulation) together. As examples of the proposed methodology, simulations and results of a linear lateral resonator, a Q-controlled resonator and a series two-resonator micromechanical bandpass filter are presented.

Analysis and modeling of distributed feedback and distributed Bragg reflector lasers using regrowth-free index-coupled surface grating technology

The modeling of single-mode distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers based on an index-coupled sur- face grating on the InGaAs/InGaAsP multiple-quantum-well (MQW) structures is carried out. Such lasers require relatively simpler grating fabrication processes without multistep-epitaxial growth or regrowth, as required in conventional devices. A key concern for surface grating is designing structure that can provide sufficient feedback to achieve single-mode operation and high laser performance. Bragg wavelength operation of the surface grating (SG) DFB and SG DBR lasers can be satisfied by etching deep and fine gratings on the surface of both side portions and along the top of the ridge stripe, respectively. The numerical results obtained enable optimization of grating and laser geometries to provide the desired feedback effect. The simulated laser design shows that single-frequency operation at a wavelength of 1.55 mm with high a side-mode suppression ratio (SMSR) is attainable.

Method to achieve large displacements using comb drive actuators

The electromechanical side-instability and the stable travel range of comb-drive actuators are investigated. The stable travel range depends on the finger gap spacing, the initial finger overlap, and the spring stiffness ratio of the compliant suspension. Proper design of the suspension structure is the most effective way to stabilize the actuator and therefore to achieve a large deflection. In this paper, an improved suspension design, so called tilted folded-beam suspension, is proposed. The expressions for the spring constants of the proposed suspension both in and perpendicular to the stroke direction are given. Using such suspension, the stability of the comb-drive actuator is improved and the stable travel range is enhanced. Comb drive actuators with various tilted folded-beam suspensions were fabricated using the standard surface micromachining technology and their operational performances were characterized. The experimental results are in good agreement with the theoretical predictions.

Efficient method for evaluation of the diffraction efficiency upper bound of diffractive phase elements

The theoretical diffraction efficiency upper limit of diffractive phase elements (DPEs) with finite apertures is investigated. A successful numerical method of evaluating the efficiency upper bound of DPEs is proposed. The method includes a hybrid optimization procedure that combines a genetic algorithm with the conjugate gradient method. This efficient global optimization technique can also be used to design DPEs. Simulation computations are detailed for rotationally symmetric beam shaping in which a Gaussian profile laser beam is converted into a uniform beam. Numerical results demonstrate that the estimated diffraction efficiency upper bound is consistent with the design results.

Diffractive optical elements designed by hybrid genetic algorithm for the generation of nondiffracting beams

The concept of nondiffracting beams was first introduced by Durnin. The beam spot of nondiffracting beam undergoes diffraction-free spreading over a long propagating distance. Therefore, nondiffracting beams could have potential applications in precision alignment, optical interconnections, and power transport. In this paper, hybrid genetic algorithms that combine genetic algorithms (GAs) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive optical elements (DOEs) for the generation of nondiffracting beams. In the hybrid genetic algorithms, an offspring obtained by genetic operators, such as crossover and mutation, is not included in the next generation directly but used as a seed for the sequent local search. The local search method searches the neighborhood of each offspring, and selects a better point, which is included in the next generation. In such a manner, the efficient exploitation of local information is provided by the incorporated local search procedure and the reliable locating of the global minimum is provided by the use of mechanisms of nature selection. The proposed hybrid methods exploit the global nature of the GAs as well as the local improvement capabilities of the gradient-based local search techniques, and will perform a more improved search while comparing with both of the single ones. The incorporated local search technique we used here is the Davidon-Fletcher-Powell (DFP) method, which is well known for its good convergence property. Numerical results demonstrate that the designed DOEs can successfully produce both zero-order and high-order nondiffracting beams.

Modeling and Simulation of Microelectromechanical Systems with an Analog Hardware Description Language

In this paper, a modeling approach is introduced that includes the micromechanical components into the system level simulation. The proposed approach can be implemented by combining the commercially available software ANSYS (for FEM analysis), MATLAB (for system identification), and analog behavioral simulator (for system level simulation) together. Modeling and simulation results based on our methodology are presented.

Analysis and structure design of distributed-feedback laser (DFB) and distributed Bragg reflector (DBR) laser using regrowth-free surface grating technology

The realization of single-mode Distributed Feedback (DFB) and Distributed Bragg Reflector (DBR) lasers, based on surface grating structures is of considerable interest. Such devices offer a relatively simple grating fabrication process without complicated multistep-epitaxial growth or regrowth, as required in more conventional devices. This simplified processing could potentially reduce the fabrication cost for these lasers. A key concern for the surface grating lasers is designing the structure to provide sufficient feedback to achieve single mode operation with high yield and high quality as compared to conventional buried-grating DFB and DBR lasers. This paper reports numerical modelling of surface grating DFB and DBR lasers based on 1.55 micrometers wavelength InGaAs/InGaAsP/InP with graded-index separate confinement heterostructure multiple quantum well (GRINSCH-MQW) structures. Bragg wavelength operation of the DFB and DBR lasers may be satisfied by deeply etching fine surface gratings on both side portions and along top of the ridge stripe respectively. Sufficiently strong optical coupling between the corrugated structure and evanescent field can be achieved by controlling various parameters such as ridge width, etch depth, and grating width. The numerical results obtained allow optimum grating geometries to be developed to provide the desired feedback effect. Furthermore, the model has been used to investigate the influence on the device performance of fabrication errors and processing effects on the grating structures, prior to fabrication.