Christophe Gorecki

Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques

We report a novel process technology of hemispherical shaped microlenses, using isotropic wet etching of silicon in an acid solution to produce the microlenses molds. Governed by process parameters such as temperature and etchant concentration, the isotropic wet etching is controlled to minimize various defects that appear during the molding creation. From the molds, microlenses are fabricated in polymer by conventional replication techniques such as hot embossing and UV-molding. The characterization of molds and measurements of replicated microlenses demonstrate high smoothness of the surfaces, excellent repeatability of mold fabrication and good optical properties. Using the proposed method, a wide range of lens geometries and lens arrays can be achieved.

Effects of Amplitude and Phase Mismatching Errors in the Generation of a Kinoform for Pattern Recognition

A model to determine the influence of the phase and amplitude mismatching on the diffraction orders is proposed. The coupling between amplitude and phase in liquid crystal televisions (LCTV) is also considered. The effect on the impulse response of the nonlinearities is studied from experimentally determined amplitude and phase modulations. It is shown that even with these nonlinearities one of the orders corresponds to the impulse response of the kinoform. The proper diffractional order may be separated by adding phase codes. Experimental results of impulse responses and correlations are given for different configurations of the spatial light modulator (SLM).

Towards micro-assembly of hybrid MOEMS components on a reconfigurable silicon free-space micro-optical bench

The 3D integration of hybrid chips is a viable approach for the micro-optical technologies to reduce the costs of assembly and packaging. In this paper a technology platform for the hybrid integration of MOEMS components on a reconfigurable silicon free-space micro-optical bench (FS-MOB) is presented. In this approach a desired optical component (e.g. micromirror, microlens) is integrated with a removable and adjustable silicon holder which can be manipulated, aligned and fixed in the precisely etched rail of the silicon baseplate by use of a robotic micro-assembly station. An active-based gripping system allows modification of the holder position on the baseplate with nanometre precision. The fabrication processes of the micromachined parts of the micro-optical bench, based on bulk micromachining of standard silicon wafer and SOI wafer, are described. The successful assembly of the holders, equipped with a micromirror and a refractive glass ball microlens, on the baseplate rail is demonstrated.

Coherent-population-trapping resonances in buffer-gas-filled Cs-vapor cells with push-pull optical pumping

We report on a theoretical study and experimental characterization of coherent-population-trapping (CPT) resonances in buffer-gas-filled vapor cells with push-pull optical pumping (PPOP) on Cs D-1 line. We point out that the push-pull interaction scheme is identical to the so-called lin perpendicular to lin polarization scheme. Expressions of the relevant dark states, as well as of absorption, are reported. The experimental setup is based on the combination of a distributed feedback diode laser, a pigtailed intensity Mach-Zehnder electro-optic modulator (MZ EOM) for optical sideband generation and a Michelson-like interferometer. A microwave technique to stabilize the transfer function operating point of the MZ EOM is implemented for proper operation. A CPT resonance contrast as high as 78% is reported in a cm-scale cell for the magnetic-field-insensitive clock transition. The impact of the laser intensity on the CPT-clock signal key parameters (linewidth, contrast, linewidth/contrast ratio) is reported for three different cells with various dimensions and buffer-gas contents. The potential of the PPOP technique for the development of high-performance atomic vapor-cell clocks is discussed. DOI: 10.1103/PhysRevA.87.013416

Micromachined array-type Mirau interferometer for parallel inspection of MEMS

We present the development of an array type of micromachined Mirau interferometers, operating in the regime of low coherence interferometry (LCI) and adapted for massively parallel inspection of MEMS. The system is a combination of free-space micro-optical technologies and silicon micromachining, based on the vertical assembly of two glass wafers. The probing wafer carries an array of refractive microlenses, diffractive gratings to correct chromatic and spherical aberrations and reference micro-mirrors. The semitransparent beam splitter plate is based on the deposition of a dielectric multilayer, sandwiched between two glass wafers. The interferometer matrix is the key element of a novel inspection system aimed to perform parallel inspection of MEMS. The fabricated demonstrator, including 5 × 5 channels, allows consequently decreasing the measurement time by a factor of 25. In the following, the details of fabrication processes of the micro-optical components and their assembly are described. The feasibility of the LCI is demonstrated for the measurement of a wafer of MEMS sensors.

Optimization of plasma-deposited silicon oxinitride films for optical channel waveguides

Abstract In view of applications of SiO x N y thin films in MOEMS technology, a study of the optomechanical characteristics of the PECVD deposited material are investigated. To optimize the quality of SiO x N y layers we establish the relationship between the chemical properties, optical performances, micromechanical stress, and growth parameters of deposited films. To use the SiO x N y thin film for the core layer of a strip-loaded waveguide, we propose preparation conditions of SiO x N y that offer a low-loss optical waveguide with well-controlled refractive index, based on a low-internal stress multilayer structure.

Coherent population trapping resonances in Cs–Ne vapor microcells for miniature clocks applications

We report the characterization of dark line resonances observed in Cs vapor microcells filled with a unique neon (Ne) buffer gas. The impact on the coherent population trapping (CPT) resonance of some critical external parameters such as laser intensity, cell temperature, and microwave power is studied. We show the suppression of the first-order light shift by proper choice of the microwave power. The temperature dependence of the Cs ground state hyperfine resonance frequency is shown to be canceled in the 77–80 °C range for various Ne buffer gas pressures. The necessity to adjust the Ne buffer gas pressure or the cell dimensions to optimize the CPT signal height at the frequency inversion temperature is pointed out. Based on such Cs–Ne microcells, we preliminary demonstrate a 852 nm vertical cavity surface emitted laser (VCSEL)-modulated based CPT atomic clock exhibiting a short term fractional frequency instability σy(τ)=1.5×10−10τ−1/2 until 30 s. These results, similar to those published in the literat...

Silicon-based integrated interferometer with phase modulation driven by surface acoustic waves.

As a novel application of silicon-based integrated optics, results from a proposed compact Mach-Zehnder interferometer are presented. The deposition of a ZnO thin-film transducer upon the reference arm of the interferometer transforms this optically passive device into a device with active sinusoidal phase modulation.

Active microelement testing by interferometry using time-average and quasi-stroboscopic techniques

Increasing technological capabilities to produce active microelements (incl. microbeams, micromembranes and micromirrors) and their expanding areas of application introduce unprecedented requirements concerning their design and testing. The paper presents a concept of an optical measurement system and methodology for out-of-plane displacement testing of such active microelements. The system is based on Twyman-Green microinterferometer. It gives the possibility to combine the capabilities of time average and quasi-stroboscopic interferometry methods to find dynamic behavior of active microelements (e.g., resonance frequencies and amplitude distributions in vibration modes). For mapping the zero-order Bessel function modulating the contrast of two-beam interference fringes the four-frame technique is applied. The calibration of the contrast variation in time-averaged interferograms enables quantitative evaluation of the vibration amplitude encoded in the argument of the Bessel function. For qualitative estimation of the vibration amplitude sign a simple quasi-stroboscopic technique is proposed. In this technique, laser pulses have the same frequency as the signal activating the microelement under test. This self-synchronous system enables to visualize the shape of the tested element at maximum deflection. Exemplary results of measurements performed with active micromembranes are presented.

Guided-wave acoustooptic interaction with phase modulation in a ZnO thin-film transducer on an Si-based integrated Mach-Zehnder interferometer

This paper presents the design and the realization of a novel integrated Mach-Zehnder interferometer (MZI) with heterodyne scheme. Well-controlled machining as well as ZnO thin-film transducer integration on the same Si substrate permits to transform an optically passive device to an active device with sinusoidal phase modulation. Theoretical and experimental results are presented, demonstrating that an efficient sensor can be designed and fabricated.