Vincent Coude du Foresto

Using single-mode infrared fibers for interferometry: progress report

With the recent availability of low-loss single-mode fluoride glass fibers, it becomes possible to consider interferometric beam recombination with fibers at infrared wavelengths. After a brief description of their present performances, specific aspects of the use of single-mode fibers in the infrared are studied here. In an astronomical interferometer thermal emission from the fibers will limit the sensitivity if they are not cooled. It is interesting to use the fibers at large X/? ratios: the beam is wider inside the fiber, most of the energy being routed by the cladding, and the coupling to starlight is eased because it can be done at slower f-ratios. This is true as long as X/X<2; beyond, evanescent field leaks through the fiber coating at the bends become too strong and prevent any practical use of the fiber. Far field beam profiles have been measured in the K and L bands. A project for a laboratory demonstration interferometer is presented: fringes were obtained in the visible and an extension to longer wavelengths is under way.

Proposal for the Implementation of the ALOHA Up-Conversion Interferometer on the CHARA Telescope Array

In this paper, we present a new concept of instrument for high resolution imaging in astronomy, involving the sum frequency generation in non-linear wavegui des. The aim is to convert the infrared radiation emitted by an astronomical source to the visible spectral domain where the optical components are mature and efficient. We present the main experimental results obtained in laboratory, and propose a new design for this instrument for its implementation on the Center for High Angular Resolution Astronomy (CHARA) telescope array. Preliminary stability and photometric results obtained at CHARA are pre- sented. Using these last measurements, we estimate the limiting magnitudes which could be reached by this interferometer in the H spectral band.