WoS İndeksli Yayınlar Koleksiyonu

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End date: 2019Institution Author: search.filters.institutionAuthor.Hah, Dooyoung

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Now showing 1 - 7 of 7
  • Conference Object
    Citation - Scopus: 2
    Planar MEMS Variable Optical Attenuators (VOAs) With Linear Attenuation-Voltage Characteristics
    (IEEE, 2019-05) Hah, Dooyoung
    Variable optical attenuators (VOAs) are essential components in wavelength division multiplexing (WDM) networks, light waveform generators, and optical fiber test equipment. Among various types of planar MEMS VOAs, a shutter type and a reflective type have been most frequently studied so far. In a shutter type, a knife-edge-like beam blocker is inserted in between the butt-coupled input and output fibers, partially obstructing the coupling between the fibers. In a reflective type, a mirror that is placed in the optical path controls the alignment between the fibers to result in attenuation. The movement of the shutter or the mirror is controlled by MEMS actuators, such as comb-drive actuators. In most of the planar MEMS VOAs reported, the relationship between the attenuation and the control voltage has been highly nonlinear. This nonlinearity results in uneven resolution throughout the attenuation range. Although this nonlinearity can be addressed by employing a control system, a structure-based solution is preferred, which can curtail the requirement of power consumption, and prevent control instability issues. In this study, shaped-finger comb-drive actuators are used to obtain a linear relationship between the control voltage and the attenuation in planar MEMS VOAs. Two types, i.e. shutter-type and reflective-type, of VOAs are examined. First, the objective differential equation is established based on attenuation-displacement relationships, electrostatic/mechanical force balance equation, and the design objective (linearity) equation. Then, the differential equation (in terms of 2-D comb capacitance) is solved by using the Euler's method, and the finger gaps are calculated by using a conformal mapping method. When a single comb-drive actuator is used, an excluded zone needs to be introduced around the region of small displacement. Effects of the width of the excluded zone to the device characteristics are studied. The issue of zone exclusion can be addressed by adopting dual (control and bias) combs. The effects of design parameters to the VOA performances are studied. It is shown that the planar MEMS VOAs with linear attenuation-voltage relationships can be designed successfully by using the proposed method.
  • Conference Object
    Citation - Scopus: 2
    Design of Wide-Band Tunable Optical Filters With Cascaded Microring Resonators and Shaped-Finger Comb-Drive Actuators
    (Institute of Electrical and Electronics Engineers Inc., 2016-05) Hah, Dooyoung
    Utilizing the Vernier effect, series coupling of multiple microring resonators with different sizes is used to design a wide-band (free spectral range: 36 nm) tunable filter. For index modulators that shift the filter spectrum by changing effective indices through evanescent coupling, silicon waveguides are considered, which make the fabrication simpler. Effects of the index modulator width to the filter characteristics are studied. A narrower modulator (width: 50 nm) does not incur much loss to the resonator, but requires hopping among several bands since its tuning effect is moderate. On the other hand, a wider modulator (width: 100 nm) can cover the full free spectral range without band hopping, but induces severe loss when it is close to the resonator. The shaped-finger comb-drive actuator design method is applied to obtain linear drop channel control. © 2017 Elsevier B.V., All rights reserved.
  • Article
    Citation - WoS: 6
    Citation - Scopus: 6
    C-Band Optical Filters With Micromechanical Tuning
    (Springer, 2017-10-13) Hah, Dooyoung
    Tunable optical filters that cover the entire range of the C-band (1530-565 nm) are designed by utilizing the Vernier effect, i.e. series coupling of microring resonators of different sizes, and the micromechanical tuning method. The micromechanical tuning method employs lateral comb-drive actuators to control evanescent coupling between the resonators and index modulators. Single crystalline silicon is used as the material for all of the main components including bus waveguide cores, resonators, index modulators, and comb-drive actuators. A finite-difference time-domain method is used for optical analysis of the filter. The simulation results show good agreement with those by analytical methods, previously reported. The width of the index modulator is found to play an important role to the filter characteristics. A wider modulator (e.g., width: 100 nm) can cover the full tuning range of 35 nm without switching among different bands owing to stronger effective index change effect, but induces significant loss to the filter, especially when it is brought close to the resonator. While a narrow modulator (e.g., width: 50 nm), on the other hand, induces moderate loss to the filter, it requires hopping among multiple bands to cover the full range since the effective index change incurred is again moderate. In order to achieve linear tuning characteristics in the cascaded-resonator filters, the shaped-finger comb-drive actuator design method is applied. The design method based on two-dimensional slice approximation is further examined by three-dimensional finite element analysis for verification. It is shown that the design method can also work for the cascaded-resonator filters, even for the one that requires band hopping. Effects of fabrication imperfections to the designed device characteristics are studied as well.
  • Conference Object
    Citation - WoS: 3
    Citation - Scopus: 3
    Analytical Design of MEMS Variable Capacitors Based on Shaped-Finger Comb-Drives
    (Springer Heidelberg, 2019-02-23) Hah, Dooyoung
    A variable capacitor is one of the widely used components in radio frequency (RF) circuits. Variable capacitors can benefit from the microelectromechanical systems (MEMS) technology, to be equipped with attractive characteristics such as high quality factor and wide tuning range. One of the design goals for MEMS varactors has been to realize linear capacitance-voltage (C-V) characteristics, for which a design method is proposed in this paper, based on shaped-finger comb-drive actuators. The shaped-finger design method, originally developed for a tunable optical filter application by the author, is redeveloped in this work for a linear C-V varactor. Moreover, the conformal mapping method is employed in calculation of capacitances, making the whole design process more time-efficient, being almost all-analytical with the minimum usage of numerical analysis methods. Effects of sense capacitor finger shapes to the optimized drive capacitor finger shapes and the corresponding C-V characteristics are investigated as well. Variable capacitors with the shaped-finger design show linearity factor (LF)-defined as the maximum deviation from the perfect linear relationship-as good as 0.4%, enormously improved from that of the conventional constant-finger-gap devices (LF: 49.9%). Further probed by 3-D numerical analysis, the C-V characteristics of the designed variable capacitor show LF better than 2.62% in the case of constant-gap sense capacitors, and as good as 0.77% in the case of shaped-finger sense capacitors. Versatility of the design method is further demonstrated by presenting a varactor for linear resonant frequency-voltage (f-V) characteristics in voltage-controlled oscillator (VCO) applications. Finally, effects of etch bias, one of common fabrication imperfections, to the linearity of C-V characteristics are studied. The developed analytical design method with shaped fingers can find a wide range of applications where comb-drive actuators are used.
  • Conference Object
    Citation - WoS: 3
    Citation - Scopus: 4
    Analytical Design of Linear Variable Capacitors With Shaped-Finger Comb-Drive Actuators
    (Institute of Electrical and Electronics Engineers Inc., 2018-05) Hah, Dooyoung
    Variable capacitors have a broad usage in radio frequency (RF) circuits. Microelectromechanical systems (MEMS) technology can provide variable capacitors with high quality factor and wide tuning range characteristics. One of the design goals for MEMS varactors has been linear capacitance- voltage (C-V) characteristics. To design a linear C-V varactor, a shaped-finger comb-drive actuator is proposed in this paper. The shaped-finger design method, originally developed to obtain linear wavelength-voltage relationships in a tunable optical filter, is modified in this work for a linear C-V varactor, which involves development of a new governing equation. Moreover, conformal mapping is employed in calculation of capacitances, making the whole design process almost all-analytical with the minimum usage of numerical analysis methods. Variable capacitors with the shaped-finger design show linearity factor (LF) - defined as the maximum deviation from the perfect linear relationship - as low as 0.4%, tremendously improved from that of the conventional constant-finger-gap devices (LF: 49.9%). The characteristics of the designed variable capacitor are further investigated through 3-D numerical analysis, and show LF better than 11.5% for the finger thickness in the range between 1 and 10 micrometers. Versatility of the design method is further demonstrated by design of a varactor with linear resonant frequency-voltage (f-V) characteristics for voltage-controlled oscillator (VCO) applications. The developed analytical design method with shaped fingers can find a wide range of applications where comb-drive actuators are used. © 2018 Elsevier B.V., All rights reserved.
  • Article
    Citation - WoS: 4
    Citation - Scopus: 4
    All-Polymer Ultrasonic Transducer Design for an Intravascular Ultrasonography Application
    (Tubitak Scientific & Technological Research Council Turkey, 2019-07-26) Hah, Dooyoung
    Intravascular ultrasonography (IVUS), a medical imaging modality, is used to obtain cross-sectional views of blood vessels from inside. In IVUS, transducers are brought to the proximity of the imaging targets so that high-resolution images can be obtained at high frequency without much concern of signal attenuation. To eliminate mechanical rotation rendered in conventional IVUS, it is proposed to manufacture a transducer array on a flexible substrate and wrap it around a cylindrical frame. The transducer of consideration is a capacitive micromachined ultrasonic transducer (CMUT). The whole device needs to be made out of polymers to be able to endure a high degree of bending (radius: 1 mm) Bending of the devices leads to considerable changes in the device characteristics, including resonant frequency and pull-in voltage due to geometrical dimension changes and stress induced. The main purpose of this work is to understand the effect of bending on the device characteristics by means of finite element analysis. Another objective of the work is to understand the relationships between such an effect and the device geometries. It is learned that the bending-induced stress depends strongly on anchor width, membrane thickness, and substrate thickness. It is also learned that resonant frequency and pull-in voltage become lower in most cases because of using a flexible substrate in comparison to those of the device on a rigid substrate. Bending-induced stress increases the spring constant and hence increases resonant frequency and pull-in voltage, although this effect is relatively weaker. For most of the device geometries, pull-in voltage is too high for the polymer material to endure. This is the main drawback of the all-polymer CMUT. In order to meet the design goal of 20 MHz resonant frequency, the membrane radius has to be smaller than 7.7 mu m for a thickness of 3 mu m.
  • Article
    Citation - WoS: 9
    Citation - Scopus: 9
    A Design Method of Comb-Drive Actuators for Linear Tuning Characteristics in Mechanically Tunable Optical Filters
    (Springer, 2015-11-28) Hah, Dooyoung
    A new method is proposed in design of comb-drive actuators for specific voltage-displacement characteristics with finger gaps as the design parameters. The design method proposed by the author previously is further refined by adopting a more accurate model which considers fringe electric fields. The proposed method is applied to design comb-drive actuators with an aim to achieve linear tuning characteristics in mechanically tunable optical add-drop filters with microring resonators. To make an assessment of the accuracy of the proposed design method, three-dimensional electrostatic numerical analysis is conducted to obtain capacitances of the designed comb-drive actuators as functions of the moving finger displacement. Obtained capacitances are used to find the tuning characteristics (resonant wavelength vs. voltage) of the filter, in combination with the results from the author's other work where a relationship between the resonant wavelength and the displacement of an index modulator was studied. It is found that by employing the actuators designed by the proposed method, the maximum deviation from linearity (MDL) can be reduced by 17.2 % points (from 25.7 % of the conventional design to 8.5 % of the new design). MDL is further reduced to 4.4 % by making a few modifications in the design.