M. de Labachelerie

Mode-hop suppression of Littrow grating-tuned lasers

Mode-hop suppression in a tunable laser employing a Littrow grating can be obtained through a simultaneous sweep of the Littrow grating angle and cavity length. The simplest way to obtain such coupled movements is to rotate the Littrow grating about a particular axis: We show theoretically and experimentally that it is necessary to improve the formalism used in previous calculations to treat the problem correctly. The current model explains the different choices made in previous studies and gives the optimal rotation point and mechanical tolerances to obtain a maximal continuous tuning range. It has been successfully tested experimentally.

A micromachined connector for the coupling of optical waveguides and ribbon optical fibers

This paper describes in details the fabrication and tests of a micromechanical connector, which is used for the precise optical self-alignment of multi-waveguide optical integrated circuits (OIC) to ribbon optical fibers, without injecting light in the fiber. Nickel alignment pins are electrodeposited on the OIC using a photolithographic process, and these pins are inserted into suitable openings made on a silicon micromachined platform, on which optical fibers are accurately positioned using V-grooves. A simultaneous fabrication of several microstructures which are used as an assistance for the assembly of the fibers and the waveguides is presented for the first time. Design and fabrication issues are reported, as well as preliminary experimental results which show that excess optical losses on the order of 3 dB per coupling facet can be obtained.

Mode-hop suppression of Littrow grating-tuned lasers: erratum.

A recent paper [Appl. Opt. 32, 269-274 (1993)) contained an analysis of the conditions required to suppress mode hops when one is tuning a Littrow grating-loaded laser cavity. Although the model was correct, an algebraic mistake led to a wrong conclusion. The corrected results are given and tend to recommend a previously investigated configuration.

Laser diode optically pumped cesium beam frequency standard investigations at LHA

Summary form only given. Investigations of the laser-diode optically pumped cesium beam resonator are discussed. The basic atomic resonator operates with a switched-capacitor laser diode tuned to either the D/sub 1/ or D/sub 2/ cesium line at 894 and 852 nm, respectively. The optical transitions that give the highest clock signal amplitude are found to be sensitive to the Hanle effect. The optical pumping efficiency as well as the fluorescence yield decrease when the static magnetic field is too low in the light atom interaction regions. An efficient optically pumped cesium beam resonator having three magnetic field regions has been designed. When it operates with a commercially available laser diode tuned on the 3 to 3 sigma transition at 852 nm, it exhibits an S/N ratio=2*10/sup 4/ in 1-Hz bandwidth, leading to a measured short-term frequency stability of 2*10/sup -12// square root tau . The influence of the anisotropic character of the fluorescence light on the collection efficiency of the fluorescence light in the detection region has also been investigated. Preliminary investigations of the statistical properties of the fluorescence signal and their dependence on the different noise sources have been carried out. >

An electrostatically actuated valve for turbulent boundary layer control

A large displacement electrostatic valve has been designed and realized for a realistic turbulent boundary layer control. This actuator consists of a pair of rigid electrodes and a flexible film having a S-shape providing local high electrostatic forces required for controling rather large pressure differences. The aim is the reattachment of the boundary layer near an aircraft flap trailing edge by transferring momentum energy via high speed pulsated micro air jets. The valve controls the frequency and the speed of the micro jets. An array of fifteen actuators has been manufactured and characterized by particles images velocimetry (PIV) and hot wire anemometry (HWA). The devices are able to control pressure differences up to 27 kPa with a supply voltage of 400 V. For this maximum pressure, the corresponding jet velocity downstream a 45 degree skew micro orifice (0.4 mm diameter) is close to 100 ms-1

Local Heat Transfer and flow Patterns During Condensation in a Single Silicon Microchannel

Condensation of steam in a single silicon microchannel was investigated using a simultaneous condensate flow visualization and heat transfer measurement along the flow direction. A silicon microchannel of a rectangular cross section covered with a transparent Pyrex glass was used. Different condensate flow patterns were identified, such as mist flow, churn flow, upstream elongated bubble flow followed by a bubble sequence, and slug flow. Both surface temperature measurements and video images obtained for different flows showed that the condensate flows were periodic or stable during time. The local surface temperature, local heat flux, and local heat transfer coefficient related to different condensate flow patterns were of particular interest. The local heat transfer coefficient was deduced from the local heat flux and the local surface temperature was measured through micro-instrumentation in a microchannel. The local thermal performance of condensation flow in a microchannel was better for mist flow and upstream elongated bubble flow relative to slug and bubbly flows.

New optical technology for cold atom experiments

In this proceeding we present a set of studies which are in progress in different labs and industrials. The aim of this project is to study the possibilities to design a very compact and reliable laser cooling bench for space and inboard applications.

Thermal actuators used for a micro-optical bench: application for a tunable Fabry-Perot filter

We report on a thermal actuator based on asymmetrical thermal expansion of two beams caused by ohmic heating. Because of the intervention of the different heat transfer mode (such as conduction, convection and radiation), the motion simulation can be complexed. This work demonstrates a good comprehension of the phenomena during a Joule heating process for micro systems and the development of an analytical model and a finite element analysis. Comparisons between theory and experimental results validate the steady-state conditions. Therefore, this actuator has been successfully integrated in an optical bench and allows the displacement of a micro-lens supported by a movable silicon-frame.

The design, fabrication and characterization of fluidic membranes for micro-engines with the aim of frequency lowering

This paper describes the design, microfabrication and linear dynamic characterization of low frequency thick membranes as a potential technological solution for resonant micro-engines, for which classical pistons cannot be used. The proposed structure is called a hybrid fluid–membrane and consists of two thin flexible membranes that encapsulate an incompressible fluid. Lower frequency structures, compared to geometrically equivalent single layer membranes, are thus obtained. Each flexible membrane is based on a composite structure which comprises a silicon planar logarithmic spiral spring embedded in a room temperature vulcanization silicone polymer. Thus, the stiffness and sealing features are dissociated for a better design control. The developed realization and assembly process is demonstrated at the wafer level. The process involves the anodic bonding of multiple stacks of silicon/glass structures, fluid filling and sealing. Various dimensions of hybrid fluid–membranes are successfully fabricated. Their dynamic characterization underlines the agreement between experimental and theoretical results. The results provide the opportunity for the design and fabrication of low frequency membranes to match the dynamics requirements of micro-engines.