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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.

This paper demonstrates the use of a nonlinear upconversion process to observe an infrared source through a telescope array detecting the interferometric signal in the visible domain. We experimentally demonstrate the possibility to retrieve information on the phase of the object spectrum of an infrared source by using a three-arm upconversion interferometer. We focus our study on the acquisition of phase information of the complex visibility by means of the phase closure technique. In our experimental demonstration, a laboratory binary star with an adjustable photometric ratio is used as a test source. A real time comparison between a standard three-arm interferometer and our new concept using upconversion by sum-frequency generation demonstrates the preservation of phase information which is essential for image reconstruction.

Astronomical Light Optical Hybrid Analysis (ALOHA@3.39 μm) is a new instrumental concept designed for high angular resolution astronomical imaging in the L band. The originality of our experimental device is to include a frequency conversion process in each arm of the interferometer that frequency shifts the mid-infrared signal to the near-infrared domain. This method relaxes the stringent limitations due to the thermal noise background radiated by the instrument itself. The aim of this paper is to estimate the current sensitivity limit of the instrument and validate the ability to provide calibrated contrasts with a quasi-monochromatic signal. We demonstrate the possibility to measure fringe contrasts greater than 94.5 per cent with a signal-to-noise ratio close to 7. The flux level at the input of each arm of the interferometer is then equal to 100 fw. This would correspond to a zero magnitude star in the L band for a 1 m2 telescope surface and a 3 nm converted spectral bandwidth. [ABSTRACT FROM AUTHOR]

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 different photon flux levels. A data reduction method allows highly contrasted images to be extracted. Results. The optical path differences 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. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the sensitivity of frequency conversion of starlight using a non-linear optical sum frequency process. This study is being carried out in the context of future applications of optical interferometry dedicated to high-resolution imaging. We have implemented a complete experimental chain from telescope to detector. The starlight frequency is shifted from the infrared to the visible using an optically non-linear crystal. To fulfil the requirements of interferometry, our experimental setup uses spatially single-mode and polarization maintaining components. Due to the small size of the collecting aperture (8 inches Celestron C8) with a 3 nm spectral bandwidth, on-sky tests were performed on bright stars in the H band. The detection was achieved in a true photon counting operation, using synchronous detection. Betelgeuse (HMag =−3.9), Antares (HMag =−3.6) and Pollux (HMag =−1) were successfully converted and detected in visible light. Despite the low transmission of our experiment, our results prove that the efficiency of frequency conversion offers sufficient sensitivity for future interferometric applications. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports on numerical simulations of spatial filtering efficiency using optical fibers. The filtering capability of the waveguide is evaluated by launching a non-guided optical field and measuring the power rejection along the propagation, over a given integration area. The Beam Propagation Method (BPM) allows to compute the transverse field distribution all along the guide, whose refractive index profile has been defined beforehand. This work is restricted to the study of straight waveguides. Scaling rules are proposed as functions of operating wavelength and numerical apertures. [Copyright &y& Elsevier]

Abstract: We analyze the temporal coherence of an optical infrared radiation in the visible domain by using a Mach–Zehnder interferometer and a wavelength conversion stage in each arm. We exploit a sum frequency generation process in bulk PPLN crystal to convert the infrared radiation at 1.55μm into 0.63μm before the interferometric mixing. The applicability of the Wiener–Kintchine theorem through up-conversion processes is here demonstrated by direct comparisons among visible and infrared measurements. [Copyright &y& Elsevier]

Phase closure is a well-known technique able to eliminate phase biases in a three-arm interferometer. This property is very useful to avoid the phase effect of the atmosphere in stellar interferometry. In this paper we theoretically investigate the effect of differential dispersion in a three-arm interferometer. We demonstrate that phase closure can be corrupted by differential dispersion between the interferometric arms. We begin by a general demonstration of such effect and propose a few examples where symmetry properties avoid phase closure errors. An analytical model for gaussian shaped spectrum and second order dispersion development is proposed. This analytical solution is illustrated by experimental results on a fibre version of a stellar interferometer. [Copyright &y& Elsevier]

The article focuses on the interferometric coupling of the Keck Telescopes using single-mode fibers. Increasing the resolving power of telescopes to image compact astronomical sources requires large apertures; however, 100 meter is probably a technological limit for single apertures. Creating a larger aperture will require combining telescopes into interferometric arrays and using aperture synthesis techniques. The goal of Optical Hawaiian Array for Nanoradian Astronomy is to investigate the value of single-mode fibers for kilometric baselines and to combine the large optical telescopes of the Mauna Kea Observatory into an 800-m interferometer to image faint objects at resolutions below 1 milli-arc second and at near-infrared wavelengths. The article reports the first results obtained using 2 x 300-m-long fluoride glass fiber cables to combine the Keck telescopes in the near infrared between 2 and 2.3 µm. Single-mode fibers were proposed to propagate coherent light for astronomical interferometry. The long distances and large numbers of mirrors required in optical interferometers are a cause of low throughput. Fibers have attenuations of 3 to 1 dB/km in the near infrared, leading to potential transmissions superior to those of optical trains in the J, H, and K bands. Twisting nonbirefringent fibers to match polarization planes solved polarization issues.