Thierry Lépine

Imaging in diffuse media with ultrafast degenerate optical parametric amplification

We demonstrate the application of a subpicosecond optical parametric amplifier working at degeneracy to imaging in diffuse media. This optical parametric amplifier exhibits small-signal gains greater than 10(4), thereby acting as a high-gain ultrafast amplifying gate. We have used it to construct the image of a grid pattern hidden behind 20 mean free paths of a highly diffusing solution of latex microspheres with a spatial resolution of 200 microm.

Video rate depth-resolved two-dimensional imaging through turbid media by femtosecond parametric amplification

We report two-dimensional imaging through a liquid scattering medium by noncollinear femtosecond parametric amplification in a reflection configuration. The experiment presented permits direct observation at video rate of two-dimensional images with 24-mum depth resolution and 90-mum transverse resolution for an area with a 2.5-mm diameter on the object. These resolutions are achievable through a turbid phantom with a depth near 12 scattering mean free paths in double pass.

Femtosecond switching of the solid state phase transition in the smart-system material VO2

We have measured the optical response speed of the semiconductor to metal phase transition in VO2 films for excitation with femtosecond laser pulses at 780 nm wavelength. By probing at a wavelength of 780 nm on a time scale from 0 to 0.5 ns and at 633 nm for longer times, we have determined the dynamic response of the complex refractive index and the complex permittivity as determined from transmission and reflection measurements. The phase transition was found to be largely prompt with the final high-temperature metallic state reached in less than 5 ps.

Nulling interferometry for the Darwin Mission: polychromatic laboratory test bench

The Darwin mission is a project of the European Space Agency that should allow around 2015 the search for extrasolar planets and a spectral analysis of their potential atmospheres in order to detect gases and particularly tracers of life. The basic concept of the instrument is a Bracewell nulling interferometer. It allows high angular resolution and high dynamic range. However, this concept, proposed 25 years ago, is very difficult to implement with high rejection factor and has to be demonstrated in laboratory before being applied in space. Theoretical and numerical approaches of the question highlight strong difficulties : - The need for very clean and homogeneous wavefronts, in terms of intensity, phase and polarisation distribution ; - The need for achromatic optical devices working in a wide spectral range (typically 6 to 18 microns for the space mission). A solution to the first point is the optical filtering which has been successfully experimentally demonstrated at 10 microns using a single mode laser. We focus now on the second point and operate a test bench working in the near infrared, where the background constraints are reduced. We present this test bench and the first encouraging results in the 2-4 microns spectral range. We particularly focus on recent optical developments concerning achromatic component, and particularly the beam combiners and the phase shifter, which are keypoints of the nulling interferometry principle. Finally, we present the future of this experimental demonstration, in the thermal infrared, covering the real and whole spectral range of Darwin.

Opto-thermo-mechanical numerical simulations of three different concepts of infrared achromatic phase shifters

The Darwin/TPF mission aims at detecting directly extra solar planets. It is based on the nulling interferometry, concept proposed by Bracewell in 1978, and developed since 1995 in several European and American laboratories. One of the key optical devices for this technique is the achromatic phase shifter (APS). This optical component is designed to produce a π phase shift over the whole Darwin spectral range (i.e. 6-18 μm), and will be experimentally tested on the NULLTIMATE consortium nulling test bench (Labèque et al). Three different concepts of APS are being simulated: dispersive plates focus crossing and field reversal. In this paper, we show how thermal, mechanical and optical models are merged into a single robust model, allowing a global numerical simulation of the optical component performances. We show how these simulations help us to optimizing the design and present results of the numerical model.

OPO radiance optimization using a numerical model

We report on numerical simulations of Optical Parametric Oscillators (OPO’s) which take into account the effects of diffraction. Different OPO cavities (stable or instable) are optimized theoretically in order to improve the OPO radiance. Variable-reflectivity mirrors are also investigated.

Optical design options for hypertelescopes and prototype testing

Hypertelescopes are large optical interferometric arrays, employing many small mirrors and a miniature pupildensifier before the focal camera, expected to produce direct images of celestial sources at high resolution. Their peculiar imaging properties, initially explored through analytical derivations, had been verified with simulations before testing a full-size testbed instrument. We describe several architectures and optical design solutions and present recent progress made on the Ubaye hypertelescope experiment. Arecibo-like versions with a fixed spherical primary meta-mirror, or an active aspheric one, have a suspended focal beam combiner equipped for pupil-drift accommodation, with a field-mosaic arrangement for observing multiple sources such as exoplanetary systems, globular clusters or active galactic nuclei. We have developed a cable suspension and drive system with tracking accuracy reaching a millimeter at 100m above ground.

Activity status and future plans for the Optical Laboratory of the National Astronomical Research Institute of Thailand

The National Astronomical Research Institute of Thailand (NARIT) has developed since June 2014 an optical laboratory that comprises all the activities and facilities related to the research and development of new instruments in the following areas: telescope design, high dynamic and high resolution imaging systems and spectrographs. The facilities include ZEMAX and Solidwork software for design and simulation activities as well as an optical room with all the equipment required to develop optical setup with cutting-edge performance. The current projects include: i) the development of a focal reducer for the 2.3 m Thai National Telescope (TNT), ii) the development of the Evanescent Wave Coronagraph dedicated to the high contrast observations of star close environment and iii) the development of low resolution spectrographs for the Thai National Telescope and for the 0.7 m telescopes of NARIT regional observatories. In each project, our activities start from the instrument optical and mechanical design to the simulation of the performance, the development of the prototype and finally to the final system integration, alignment and tests. Most of the mechanical parts are manufactured by using the facilities of NARIT precision mechanical workshop that includes a 3-axis Computer Numerical Control (CNC) to machine the mechanical structures and a Coordinate Measuring Machine (CMM) to verify the dimensions. In this paper, we give an overview of the optical laboratory activities and of the associated facilities. We also describe the objective of the current projects, present the specifications and the design of the instruments and establish the status of development and we present our future plans.

Spectral narrowing of a pulsed optical parametric oscillator using an intracavity photorefractive crystal pulsed optical parametric oscillator using an intracavity photorefractive crystal

We report on the first use of a photorefractive crystal inside a pulsed OPO cavity, which acts as a longitudinal mode selector, providing an efficient spectral narrowing.

Imagerie 2D résolue en profondeur à cadence vidéo à travers un milieu diffusant par amplification paramétrique optique en régime femtoseconde

Nous decrivons une experience damplification parametrique optique en regime femtoseconde permettant lacquisition dimages resolues en profondeur a cadence video a travers un milieu diffusant liquide de 12 libres parcours moyen de diffusion en aller et retour. Les resolutions spatiales obtenues avec une mire placee dans lair derriere ce milieu diffusant sont de =70 μm lateralement et de 30 μm longitudinalement.