Nicolas Pavy

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

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.

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

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.

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

We report on Fabry-Pérot (FP) filters based on silicon cylindrical Bragg reflectors. Such filters exhibit unusual periodicity of the spectral response due to transverse resonance modes. The optical microcavities are also unusually long (L>210µm) while maintaining reasonably high quality factors (Q), above 1000. The presented device behaves as a mode-selective filter wherein the excited mode is controlled by varying the excitation wavelength. The device can also be used as a wavelength-selective MEMS switch, with extinction ratio of 7∶1. Cavities with different radii of curvature have been evaluated experimentally, their corresponding Q-factors and effective cavity lengths (Leff) have been determined.

Analysis of micromachined Fabry-Pérot cavities using phase-sensitive optical low coherence interferometry: Insight on dimensional measurements of dielectric layers

This paper details a low-power bi-axial miniaturized inclinometer based on a mobile mass (spherical ball or fluidic droplet) positioned on a precision curved surface that is generated using a novel MEMS process. The detection of the mobile mass was implemented through an external optical system, using a quadrant photodetector. Nanotopography and chemical treatment of the curved surface have been implemented to increase accuracy when using a fluidic mobile mass, by tailoring wetting properties and minimizing contact angle hysteresis. We achieve a range of ±1° with a true linear bi-axial measurement of precision better than 0.05°.

Mode-selective optical filtering and wavelength-selective switching through Fabry-Perot cavity with cylindrical reflectors

The measurement of thermal properties of solid materials at different temperatures above ambient is investigated using a set of microresistors. Samples consisted of suspended films with sets of long, parallel resistive wires deposited on their surfaces. One resistive wire was heated by an alternating current. Surface temperature changes in DC and AC regimes were then detected by measuring the change in electrical resistance of the other wires deposited on the surface. The length of wires was chosen so that they may be assumed isothermal and such that heat diffusion acts perpendicularly to their axes. By measuring the dependence of the surface alternating temperature oscillation on the modulation frequency f and on the separation between the heating wire and the probing wires, the thermal diffusivity of the sample was determined. Through adjustment of the alternating current amplitude in the source wire, the temperature at which the thermal diffusivity of the sample was evaluated was finely controlled. For the validation of the method, pure silicon samples were first studied. An experimental bench was set up and resistive source and probes were experimentally characterized. Results obtained from ambient temperature to 500 K for pure silicon are in accordance with reference data found in the scientific literature.