Beyond Interferometers Based on Silicon-Air Bragg Reflectors: Toward On-Chip Optical Microinstruments-A Review

In this paper, we review recent advances in optical microinstruments based on distributed Bragg reflectors (DBR) focusing mainly on two application areas: spectrometry and profilometry. For spectrometry applications, we consider state-of-the-art architectures. Among these are novel architectures of high-Q Fabry-Perot resonators based on cylindrical surfaces. In addition, we consider a microelectromechanical Michelson interferometer based on a robust (silicon-air) beam splitting interface. Its measurement principle relies on recording the spatial interferogram using movable mirror followed by inverse Fast Fourier transform (iFFT) to deduce the spectral content of the tested sample. For profilometry applications instead, an elaborated Michelson interferometer based on planar DBR is deployed for profile measurements using the technique of wavelength sweep and FFT to extract the dimensional characteristics. In the last section, we revisit the applications of DBR structures in optofluidics wherein a curved FP cavity has been deployed for optical trapping and binding of microparticles. Besides, a Michelson-based optofluidic probe has been deployed for off-chip measurements of fluid refractive index and absorbance. Globally, DBR based microinstruments demonstrate a strong potential in the targeted application areas

A contribution to photonic MEMS Contribution aux MEMS photoniques : étude de résonateurs et interféromètres optiques basés sur des réflecteurs de Bragg tout silicium

This research work has been conducted to introduce a novel class of Fabry-Perot (FP) resonators : curved FP cavity based on coating-free Bragg mirrors of cylindrical shape, obtained by silicon micromachining. Another specificity is the rather large cavity lengths (L>200 µm) combined with high quality factor Q (up to 104), for the purpose of applications requiring cavity enhanced absorption spectroscopy, in which the product Q.L is a figure of merit. In this contest, the basic architecture has been modeled analytically to know the high order transverse modes supported by such cavities. Hence, the experimental conditions which lead to preferential excitation (or rejection) of these modes have been tested experimentally leading to the validation of our theoretical model and to a better understanding of the cavity behaviour. A second architecture, based on the curved FP together with a fiber rod lens has been developed for the purpose of providing stable designs. It was also modeled, fabricated and characterized leading to the expected performance improvements. On another side, a highlight on one of the potential applications that we identified for the curved cavities is presented by inserting the cavity into an electro-mechanical system. It consists of exciting and measuring tiny vibration through opto-mechanical coupling in a MEMS mechanical resonator embedding an FP cavity.Finally, as a complement to our study on resonators, we started exploring applications of optical interferometers based on similar micromachined silicon Bragg mirrors. For this purpose, an optical measurement microsystem was designed, fabricated and characterized ; it consists of an optical probe for surface profilometry in confined environments, based on an all-silicon Michelson interferometerCe travail de recherche a été mené afin d'introduire une nouvelle classe de résonateurs Fabry-Pérot (FP) : les cavités FP incurvées basées sur des miroirs de Bragg sans revêtement, de forme cylindrique sont obtenues par micro-usinage du silicium. Une autre spécificité est la longueur de la cavité relativement grande (L> 200 µm) combinée à un haut facteur de qualité Q (jusqu'à 10^4 ), pour répondre aux applications de type spectroscopie d'absorption améliorée par résonance optique, dans lesquelles le produit Q.L est une figure de mérite. Dans ce contexte, l'architecture de base a été modélisée analytiquement pour déterminer les modes transverses d'ordre élevé supportés par de telles cavités. Par conséquent, les conditions expérimentales qui conduisent à une excitation préférentielle (ou rejet) de ces modes ont été testées menant à la validation de notre modèle théorique et à une meilleure compréhension du comportement de la cavité. Une seconde architecture,basée sur la cavité FP incurvée avec une lentille cylindrique a été développée dans le but de fournir une architecture plus stable. Cette dernière a été également modélisée, fabriquée et caractérisée, menant à l'amélioration attendue en termes de performances. D'un autre côté, un point surlignant l'une des applications potentielles que nous avons identifiées pour les cavités incurvées est présentée en insérant la cavité dans un système électromécanique. Ceci consiste à exciter et mesurer les vibrations d'amplitude nanométrique par couplage opto-mécanique dans un résonateur mécanique MEMS intégrant une cavité optique FP. Enfin, comme complément à notre étude sur les résonateurs, nous avons commencé à explorer les applications des interféromètres optiques à base de miroirs de Bragg en silicium. À cette fin, un microsystème de mesure optique a été conçu, fabriqué et caractérisé, il consiste en une sonde optique pour la profilométrie de surface dans des milieux confinés, basé sur un interféromètre de Michelson monolithique en siliciu

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

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 microoptical device for analytical applications

Polymer based single mode optical waveguide for spectroscopy applications

In this work, we present for the first time, two dimensional low loss single mode optical waveguides fabricated with EPO materials. The waveguide geometry has a rectangular shape and it is buried between two EPOCLAD layers. Its refractive index distribution has a step-index profile with an index difference of 8.6x10(-3). Realized by polymer deposition, it consists mainly, of a stack of EPOCLAD/EPOCORE/EPOCLAD layers sandwiched between two thick glass substrates. The waveguide dimensions are: 3 x 2.2 mu m(2). A cleaved single mode optical fiber, dedicated to the visible range (658 nm), is used to perform all the optical characterizations. The structure demonstrates an overall insertion loss of 2.6 dB and a numerical aperture of 0.19. Such waveguides will be used to create standing wave interference pattern using a high reflectance mirror embedded at the waveguide exit. Sampling of the pattern using an array of nano-antennas deposited on top of the waveguide yields information about the spatial distribution of the electrical field inside the waveguide. Hence, the spectral content of the signal can be deduced and the developed architecture demonstrates a strong potential for spectroscopic applications

On-chip near-infrared optical spectrometer based on single-mode Epo waveguide and gold nanoantennas

We report about optical spectrometry using gold nanostructures printed on top of an integrated optical waveguide. The optical waveguide is a single-mode buried waveguide made from a combination of photo-polymerizable materials and is fabricated by photolithography on a glass substrate. To detect the electric field inside the waveguide, a gold nanocoupler array of thin lines (50 nm thick and 8 mu m in length) is embedded on top of the aforementioned waveguide. They are produced by e-beam lithography. Both waveguide ports are polished, and the output port, in particular, is coated with a thin gold layer to assimilate a mirror and hence, it enables the creation of stationary waves inside the structure. Stationary waves generated inside the guide constitute a spatial interferogram. Locally, light is out-coupled using the nanocouplers and allows measuring the interferogram structure. The resulting pattern is imaged by a vision system involving an optical microscope with the objective lenses of different magnifications and a digital camera mounted on top of the microscope. The 5x objective lens demonstrates a superior performance in retrieving the investigated spectrum compared to 20x and 100x objectives. Fast Fourier transform is performed on the captured signal to extract the spectral content of the measured signal. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE

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

International audienceWe 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

Design, modeling and characterization of stable, high Q-factor curved Fabry-P,rot cavities

International audienceIn this paper, we introduce a novel design for high performance silicon-based Fabry-P,rot cavities and their corresponding design model. According to the design model, the new design shows higher stability, lower insertion loss and higher quality factor Q. Our methodology was based, on one hand, on taking advantage of light reflection and refraction over curved surfaces with curvatures along 2 orthogonal directions, in order to confine the Gaussian beam inside the cavity, thus reducing loss due to beam divergence. Such design enables approaching new limits, where Q-factor is mainly governed by the mirrors reflectance. On the other hand, the use of Bragg reflectors, obtained by DRIE etching enables reaching reflectances above 99%, thus enabling very high Q-factors

Design, modeling and characterization of stable, high Q-factor curved Fabry-Pérot cavities

International audienceIn this paper, we introduce a novel design for high performance silicon-based Fabry-Pérot cavities and their corresponding design model. According to the design model, the new design shows higher stability, lower insertion loss and higher quality factor Q. Our methodology was based, on one hand, on taking advantage of light reflection and refraction over curved surfaces with curvatures along 2 orthogonal directions, in order to confine the Gaussian beam inside the cavity, thus reducing loss due to beam divergence. Such design enables approaching new limits, where Q-factor is mainly governed by the mirrors reflectance. On the other hand, the use of Bragg reflectors, obtained by DRIE etching enables reaching reflectances above 99%, thus enabling very high Q-factors