Yves-Alain Peter

Flatness-Based Control of Electrostatically Actuated MEMS With Application to Adaptive Optics: A Simulation Study

Typical adaptive optics (AO) applications require continual measurement and correction of aberrated light and form closed-loop control systems. One of the key components in microelectromechanical system (MEMS) based AO systems is the parallel-plate microactuator. Being electrostatically actuated, this type of devices is inherently instable beyond the pull-in position when they are controlled by a constant voltage. Therefore extending the stable travelling range of such devices forms one of the central topics in the control of MEMS. In addition, though certain control schemes, such as charge control and capacitive feedback, can extend the travelling range to the full gap, the transient behavior of actuators is dominated by their mechanical dynamics. Thus, the performance may be poor if the natural damping of the devices is too low or too high. This paper presents an alternative for the control of parallel-plate electrostatic actuators, which is based on an essential property of nonlinear systems, namely differential flatness, and combines the techniques of trajectory planning and robust nonlinear control. It is, therefore, capable of stabilizing the system at any point in the gap while ensuring desired performances. The proposed control scheme is applied to an AO system and simulation results demonstrate its advantage over constant voltage control

Optical MEMS: From Micromirrors to Complex Systems

Microelectromechanical system (MEMS) technology, and surface micromachining in particular, have led to the development of miniaturized optical devices with a substantial impact in a large number of application areas. The reason is the unique MEMS characteristics that are advantageous in fabrication, systems integration, and operation of micro-optical systems. The precision mechanics of MEMS, microfabrication techniques, and optical functionality all make possible a wide variety of movable and tunable mirrors, lenses, filters, and other optical structures. In these systems, electrostatic, magnetic, thermal, and pneumatic actuators provide mechanical precision and control. The large number of electromagnetic modes that can be accommodated by beam-steering micromirrors and diffractive optical MEMS, combined with the precision of these types of elements, is utilized in fiber-optical switches and filters, including dispersion compensators. The potential to integrate optics with electronics and mechanics is a great advantage in biomedical instrumentation, where the integration of miniaturized optical detection systems with microfluidics enables smaller, faster, more-functional, and cheaper systems. The precise dimensions and alignment of MEMS devices, combined with the mechanical stability that comes with miniaturization, make optical MEMS sensors well suited to a variety of challenging measurements. Micro-optical systems also benefit from the addition of nanostructures to the MEMS toolbox. Photonic crystals and microcavities, which represent the ultimate in miniaturized optical components, enable further scaling of optical MEMS.

All-silicon integrated Fabry–Pérot cavity for volume refractive index measurement in microfluidic systems

We report a refractive index (RI) sensor based on the use of vertically etched silicon Bragg reflectors. The device is robust and performs measurements through tens of micrometers of liquid. A sensitivity of 907 nm/RIU (RI units) and a resolution of 1.7×10−5 RIU are obtained and are in good agreement with optical simulations. This resolution is the highest reported for a volume RI sensor integrated with a microfluidic system. Expected applications for the sensor in the fields of single cell characterization and chip based liquid chromatography are discussed.

Photonic crystal slabs demonstrating strong broadband suppression of transmission in the presence of disorders.

We characterize the transmission spectra of out-of-plane, normal-incidence light of two-dimensional silicon photonic crystal slabs and observe excellent agreement between the measured data and finite-difference time-domain simulations over the 1050-1600-nm wavelength range. Crystals that are 340 nm thick and have holes of 330-nm radius on a square lattice of 998-nm pitch show 20-dB extinction in transmission from 1220 to 1255 nm. Increasing the hole radius to 450 nm broadens the extinction band further, and we obtain >85% extinction from 1310 to 1550 nm. Discrepancies between simulation and measurement are ascribed to disorder in the photonic lattice, which is measured through image processing on high-resolution scanning electron micrographs. Analysis of crystal imperfections indicates that they tend to average out narrowband spectral features, while having relatively small effects on broadband features.

Tunable Fiber Laser Using a MEMS-Based in Plane Fabry-Pérot Filter

We propose a tunable erbium doped fiber laser based on a Fabry-Pérot (F-P) cavity tuned by an electrostatic actuator. The device is made of single crystalline silicon. The F-P cavity consists of two Bragg mirrors, one being displaced by a comb-drives actuator. The F-P cavity, grooves for optical fibers and electro-mechanical structure are fabricated by deep reactive ion etching on a 70 μm silicon on insulator wafer and are integrated in a ring fiber laser. The resulting tunable fiber laser has a tuning range of 35 nm in the C-band and a spectral width of less than 0.06 nm. The maximum applied voltage for full tuning of the laser is 37 V. The mechanical resonance frequency of the actuated mirror is 14.4 kHz allowing fast tuning of the laser. The maximum output power is 1.8 mW.

Single-Crystal-Silicon Continuous Membrane Deformable Mirror Array for Adaptive Optics in Space-Based Telescopes

In this paper, we present a single-crystal-silicon (SCS) continuous membrane deformable mirror (DM) as a corrective adaptive-optics (AO) element for space-based telescopes. In order to correct the polishing errors in large aperture (~8 m) primary mirrors, a separate high-quality surface DM array must be used. Up to 400000 elements and a mirror stroke of ~100 nm are required for the correction of these polishing errors. A continuous membrane mirror formed by the the SCS device layer of a silicon-on-insulator (SOI) wafer is used to achieve a high-quality optical surface and to minimize the additional diffractive effects in the optical system. To achieve substantial local deformation needed to correct high-order errors, we use a highly deformable silicon membrane of 300-nm thickness. This thin membrane is able to deform locally by 125 nm at an operating voltage of 100 V with a pixel pitch of 200 mum. The resonance frequency of a pixel is 25 kHz with a low Q-factor of 1.7 due to squeeze-film damping. The device is fabricated by processing the microelectromechanical system (MEMS) and electronic chips separately and then combining them by flip-chip bonding. This allows optimization of the MEMS and electronics separately and also allows the use of an SOI layer for the mirror by building the MEMS bottom up. A small prototype array of 5times5 pixels with 200-mum pitch is fabricated, and we demonstrate single pixel and multiple pixel actuation

Simultaneous measurement of quality factor and wavelength shift by phase shift microcavity ring down spectroscopy

Optical resonant microcavities with ultra high quality factors are widely used for biosensing. Until now, the primary method of detection has been based upon tracking the resonant wavelength shift as a function of biodetection events. One of the sources of noise in all resonant-wavelength shift measurements is the noise due to intensity fluctuations of the laser source. An alternative approach is to track the change in the quality factor of the optical cavity by using phase shift cavity ring down spectroscopy, a technique which is insensitive to the intensity fluctuations of the laser source. Here, using biotinylated microtoroid resonant cavities, we show simultaneous measurement of the quality factor and the wavelength shift by using phase shift cavity ring down spectroscopy. These measurements were performed for disassociation phase of biotin-streptavidin reaction. We found that the disassociation curves are in good agreement with the previously published results. Hence, we demonstrate not only the application of phase shift cavity ring down spectroscopy to microcavities in the liquid phase but also simultaneous measurement of the quality factor and the wavelength shift for the microcavity biosensors in the application of kinetics measurements.

Guided-mode resonance photonic crystal slab sensors based on bead monolayer geometry

Using finite-difference time-domain method, we investigate photonic crystal slabs consisting of spherical voids or silica beads embedded into a dielectric slab as bio-chemical sensors. We study the dependence of the spectral position of guided-mode resonances on the refractive index of a slab material. The most sensitive design is based on voids filled with analyte. We also study the effects of the slab and analyte thicknesses on guided-mode resonance properties. We eventually demonstrate an aqueous analyte sensor with high sensitivity at visible wavelength as electro-magnetic energy distribution in some guided-mode resonances can be strongly localized in the analyte region.

Design and Demonstration of an In-Plane Silicon-on-Insulator Optical MEMS Fabry–Pérot-Based Accelerometer Integrated With Channel Waveguides

In this paper, we present a novel optical microelectromechanical systems (MEMS) accelerometer sensor dedicated to space applications. An in-plane Fabry-Pérot (FP) microcavity (FPM) with two distributed Bragg reflectors (DBRs) is used to detect the acceleration. One of the DBR mirrors is attached to two suspended proof masses, allowing the FP gap to change while proof masses experience acceleration. Acceleration is then detected by measuring the spectral shift of the FPM. The optical accelerometer presented here uses silicon strip waveguides integrated with MEMS on a single silicon-on-insulator wafer, making it compact and robust. All of the device components are fabricated using one single fabrication step. Immunity to electromagnetic interference, high sensitivity and resolution capability, integrability, reliability, low cross-sensitivity, simple fabrication, and possibility of having two- and three-axis sensitivities are numerous advantages of our sensor compared to the conventional ones. The sensor performance demonstrated a 90-nm/g sensitivity and 111-μg resolution and better than 250-mg dynamic range.

Advances in Modeling, Design, and Fabrication of Deep-Etched Multilayer Resonators

We present recent advances in modeling, design, and fabrication of in-plane multilayer optical resonators fabricated by high aspect ratio etching of silicon. We first revisit the model of Gaussian beam divergence proposed by A. Lipson to correct a mistake that leads to an underestimation of the losses affecting this type of resonator. Secondly, we discuss the influence of surface roughness at the silicon-air interfaces of multilayered structures. Roughness profiles-measured by white light interferometry on the sidewalls of silicon trenches etched by deep reactive ion etching (DRIE)-are presented. The single absorbing layer model of Carniglia is used to predict the influence of the measured roughness ( nm RMS). This model is combined with the corrected model for Gaussian beam divergence and is compared with recent experimental results obtained for a new generation of deep-etched Fabry-Perot refractive index sensors. These sensors are fabricated using the contour lithography method, which is demonstrated to greatly improve the predictability of their optical characteristics. The combined model for roughness and divergence is found to correspond remarkably well with the experimental results, with predictions of loss and finesse of the resonances within an average error of 1.3 dB and 25%, respectively. We therefore expect the models and the simulations presented in this article to become a useful tool for the design of devices based on deep-etched multilayer resonators.