Jean-Thomas Gomes

Demonstration of a frequency spectral compression effect through an up-conversion interferometer

This paper reports on the experimental implementation of an interferometer featuring sum frequency generation (SFG) processes powered by a pump spectral doublet. The aim of this configuration is to allow the use of the SFG process over an enlarged spectral domain. By analyzing the converted signal, we experimentally demonstrate a frequency spectral compression effect from the infrared input signal to the visible one converted through the SFG process. Recently, such a compression effect has been numerically demonstrated by Wabnitz et al. We also verify experimentally that we fully retrieve the temporal coherence properties of the infrared input signal in the visible field. The experimental setup permits to demonstrate an experimental frequency spectral compression factor greater than 4. This study takes place in the general field of coherence analysis through second order non-linear processes.

ALOHA 1.55 μm Implementation on the CHARA Telescope Array : On-sky sensitivity tests

We intend to implement the ALOHA at 1.55μm up-conversion interferometer on the CHARA Array. After a full laboratory investigation, a sensitivity evaluation is conducted on several stars using a single interferometric arm in a photometric mode. The on-sky photometric results allows us to calibrate a numerical simulation of the interferometric configuration, and to predict the future performance of ALOHA at 1.55μm as a function of the seeing conditions.

Co-phasing of a diluted aperture synthesis instrument for direct imaging - II. Experimental demonstration in the photon-counting regime with a temporal hypertelescope

Context. Amongst the new techniques currently developed for high-resolution and high-dynamics imaging, the hypertelescope architecture is very promising for direct imaging of objects such as exoplanets. The performance of this instrument strongly depends on the co-phasing process accuracy. In a previous high-flux experimental study with an eight-telescope array, we successfully implemented a co-phasing system based on the joint use of a genetic algorithm and a sub-aperture piston phase diversity using the object itself as a source for metrology. Aims. To fit the astronomical context, we investigate the impact of photon noise on the co-phasing performance operating our laboratory prototype at low flux. This study provides experimental results on the sensitivity and the dynamics that could be reached for real astrophysical observations. Methods. Simulations were carried out to optimize the critical parameters to be applied in the co-phasing system running in the photon-counting regime. We used these parameters experimentally to acquire images with our temporal hypertelescope test bench for di erent photon flux levels. A data reduction method allows highly contrasted images to be extracted. Results. The optical path di erences have been servo-controlled over one hour with an accuracy of 22.0 nm and 15.7 nm for 200 and 500 photons/frame, respectively. The data reduction greatly improves the signal-to-noise ratio and allows us to experimentally obtain highly contrasted images. The related normalized point spread function is characterized by a 1:1 10 4 and 5:4 10 5 intensity standard deviation over the dark field (for 15 000 snapshots with 200 and 500 photons/frame, respectively). Conclusions. This laboratory experiment demonstrates the potential of our hypertelescope concept, which could be directly transposed to a space-based telescope array. Assuming eight telescopes with a 30 cm diameter, the I-band limiting magnitude of the main star would be 7.3, allowing imaging of a companion with a 17.3 mag.

ALOHA project: how nonlinear optics can boost interferometry to propose a new generation of instrument for high-resolution imaging

The ALOHA research program aims to propose a breakthrough generation of instrument for high resolution imaging in astronomy. This fully innovative concept results from our unique skills with a simultaneous competence in nonlinear optics and high resolution imaging with telescope arrays. Acting like a mixer in a radio receiver, the nonlinear process (sum frequency generation) shifts the infrared radiations emitted by the observed astrophysical source to a visible spectral domain. This way, the light beam is more easily processed by mature optical devices and detectors. The compatibility of the nonlinear process with the spatial coherence analysis has been successfully tested through preliminary in lab experiments. Now it’s time to apply this technique in a real astronomical environment. First on-sky results have been observed during the last missions at the CHARA Array.

Imaging astronomical sources at high resolution with novel aperture synthesis devices

Aperture synthesis devices are commonly used in highresolution astronomical imaging applications. At present, these instruments work efficiently in visible and near-IR (NIR) wavelengths. Such high-resolution imaging is now also expanding into midand far-IR domains, which are particularly relevant to the study of exoplanets. It is more difficult, however, to design these devices for use at the longer wavelengths because the required components (e.g., fibers, integrated optical components, and detectors) are not yet available or they currently have poor performance levels. Aperture synthesis instruments are interferometers that combine signals from several telescopes. The resultant angular resolution of the images is the same as that of an instrument equal in size to the whole set of telescopes. The Astronomical Multi-Beam Combiner (AMBER)1 and the Michigan IR Combiner (MIRC)2 are examples of aperture synthesis devices that are currently in use as part of the European Southern Observatory and the Center for High Angular Resolution Astronomy (CHARA) at Georgia State University, respectively. For more than ten years, we have been developing reliable aperture synthesis devices for mid-IR and far-IR wavelength ranges. In our novel approach, we implement a sum-frequency generation (SFG) process3 on each arm of a stellar interferometer. This shifts part of the long-wavelength spectrum of the astronomical source to visible wavelengths and simultaneously preserves the coherence properties of the spectrum (see Figure 1). After the light is collected by each telescope, the optical waves are therefore shifted to a spectral domain where an efficient optical chain of components (e.g., optical fibers, couplers, and detectors) can be used. Figure 1. Global scheme of the stellar up-conversion interferometer dedicated to spatial coherence analysis of an astronomical source. PPLN: Periodically poled lithium niobate. s : Wavelength of obtained signal. C : Converted wavelength.