Maurine Malak

Free-Space Tunable and Drop Optical Filters Using Vertical Bragg Mirrors on Silicon

Vertical Bragg grating mirrors are realized by the anisotropic etching of Si using deep reactive ion etching (DRIE), thus producing multiple vertical interfaces between Si and air. The Bragg mirrors are used to realize two optical filter configurations. The first is a tunable Fabry-Peacuterot cavity composed of two mirrors, where tuning is achieved by moving one of the mirrors using silicon-on-insulator (SOI) electrostatic microelectromechanical system (MEMS) actuation. The second is a drop filter, where a tilted Bragg mirror acts as a wavelength selective reflector. The enhanced etching process involving a mix of cryogenic and Bosch DRIE processes is presented. The realized structures, fabrication process, as well as measured performance are also presented

Cylindrical Surfaces Enable Wavelength-Selective Extinction and Sub-0.2 nm Linewidth in 250

In this paper, we propose two different designs of micromachined Fabry-Pérot optical cavities, with first motivation of improving the quality factor (Q -factor) and in the same time allowing increased cavity length L. Our approach consists of providing a solution to the main loss mechanism in conventional FP cavities related to the expansion of the Gaussian light beam after multiple reflections inside the cavity. The first design is based on all-silicon cylindrical Bragg mirrors, which provide 1-D confinement of light. In addition to wavelength selectivity, the first design also demonstrates its potential for a new class of applications, including wavelength selective extinction through mode-selective excitation, where the fiber-to-cavity distance is used as the control parameter. The second design is based on cylindrical Bragg mirrors combined with a fiber rod lens to provide a complete solution for 2-D confinement of light. This approach outperforms the first design in terms of Q-factor, of nearly 9000 for around 250 μm-long cavity, which suggests its potential use for biochemical sensing and analysis as well as cavity enhancement applications requiring high Q.L values.

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We demonstrate experimentally optical quality factor of nearly 9000 in a micromachined Fabry–Perot resonator based on free space propagation of light and direct coupling to optical fibers. This result is obtained on long cavity resonators (L>250 μm), a usually difficult case in terms of power loss, but very useful configuration for experiments requiring either long optical path or enough space for manipulation. The resonator architecture includes two multilayered silicon-air Bragg mirrors of cylindrical shape, combined with a fiber rod lens. The specific stability criteria are derived for the proposed resonator architecture. Dimensions of the fabricated devices are chosen accordingly.

-Gap Silicon Fabry–Pérot Cavities

We study the behavior of Fabry-Perot micro-optical resonators based on cylindrical reflectors, optionally combined with cylindrical lenses. The core of the resonator architecture incorporates coating-free, all-silicon, Bragg reflectors of cylindrical shape. The combined effect of high reflectance and light confinement produced by the reflectors curvature allows substantial reduction of the energy loss. The proposed resonator uses curved Bragg reflectors consisting of a stack of silicon-air wall pairs constructed by micromachining. Quality factor Q ~1000 was achieved on rather large cavity length L = 210 microns, which is mainly intended to lab-on-chip analytical experiments, where enough space is required to introduce the analyte inside the resonator. We report on the behavioral analysis of such resonators through analytical modeling along with numerical simulations supported by experimental results. We demonstrate selective excitation of pure longitudinal modes, taking advantage of a proper control of mode matching involved in the process of coupling light from an optical fiber to the resonator. For the sake of comparison, insight on the behavior of Fabry-Perot cavity incorporating a Fiber-Rod-Lens is confirmed by similar numerical simulations.

Micromachined Fabry-Perot resonator combining submillimeter cavity length and high quality factor

A tunable Fabry-Perot cavity composed of two Bragg mirrors is realized using enhanced deep reactive ion etching (DRIE) process. Filter tuning is achieved by moving one of the mirrors using a high resolution electrostatic actuator over SOI. Measured filter response in C&L bands is presented

Analysis of Fabry-Perot optical micro-cavities based on coating-free all-Silicon cylindrical Bragg reflectors

For the first time, we demonstrate experimentally high optical quality factor Q ∼9000 in MEMS-compatible silicon Fabry-Pérot (FP) resonators based on free space propagation of light and direct coupling to optical fiber. This result is obtained on long cavity resonators (L > 250 µm), a usually difficult case in terms of power loss. The resonator design includes two multilayered silicon-air Bragg mirrors of cylindrical shape, combined with a Fiber Rod Lens (FRL). Dimensions are chosen according to stability criteria imposed on the optical resonator. The core of the presented device is entirely made of single-crystal silicon. It is obtained by DRIE using an optimized single step process.

Electrostatically-tuned Optical Filter Based on Silicon Bragg Reflectors

We present a novel optical technique for simultaneously measuring the absorbance and the refractive index of a thin film using an infrared optofluidic probe. Experiments were carried on two different liquids and the results agree with the bibliographical data. The ultimate goal is to achieve a multi-functional micro-optical device for analytical applications.

Stable, high-Q fabry-perot resonators with long cavity based on curved, all-silicon, high reflectance mirrors

A miniature Michelson interferometer is analyzed theoretically and experimentally. The fabricated micro-interferometer is incorporated at the tip of a monolithic silicon probe to achieve contactless distance measurements and surface profilometry. For infrared operation, two approaches are studied, based on the use of monochromatic light and wavelength sweep, respectively. A theoretical model is devised to depict the system characteristics taking into account Gaussian beam divergence and light spot size. Furthermore, preliminary results using visible light demonstrate operation of the probe as a visible light spectrometer, despite silicon absorbance, thanks to the micrometer thickness involved in the beam splitter.

Simultaneous measurement of liquid absorbance and refractive index using a compact optofluidic probe

We developed an interferometric optical micro-probe, for the purpose of non-contact measurement of distance-to-surface for samples in confined environments, such as holes and trenches whose lateral dimensions are in the order of hundreds of microns (typically fuel injection nozzles). A Michelson interferometer was integrated at the end of a 4mm-long, 390μm-thick, 550μm-wide cantilever, which incorporates grooves for input-output optical fibers. The whole system is a monolithic silicon block obtained by DRIE. The fabricated probe demonstrates simultaneous measurements of distance and thickness for silicon wafer used as a test surface. Data are obtained from Fast Fourier Transform of the spectral interferogram.

All-silicon Michelson instrument on chip: Distance and surface profile measurement and prospects for visible light spectrometry

Herein, we highlight a behavior underlying the physics of Fabry-Perot micro-cavities with distributed reflectors as there is a need to discriminate between effective and physical cavity lengths. Hence, Phase-Sensitive Optical Low Coherence Interferometry has been implemented to characterize micro-cavities with planar or curved reflectors. Beside the retrieved physical length, we obtain valuable information about the reflector thickness and number of layers. The accuracy of the technique has been estimated. Results suggest that this technique might be suitable to retrieve dimensional characteristics of any device constructed from multiple dielectric layers, whose thickness ranges from 2 micrometers up to hundreds of micrometers.