Camille Graf Seminar (CRAL)

Abstract

The development of increasingly sophisticated optical systems now makes it possible to detect extremely faint astronomical signals, such as photons emitted by an exoplanet tens of light-years from our Sun or the gravitational wave resulting from the collision of two massive black holes hundreds of megaparsecs away. The high sensitivity of these instruments depends, among other factors, on their ability to detect minute phase variations in incident light or gravitational waves. Extreme adaptive optics (XAO) corrects the degradation of astronomical images obtained at the focal point of a telescope, caused by atmospheric turbulence. This correction is necessary to restore the ultimate limit of ground-based telescopes, whose angular resolution is determined by diffraction in the absence of turbulence. The analyzers used to measure deformations of the turbulent wavefront must be sensitive to phase variations ranging from several tens of micrometers to a few nanometers, with a very small number of photons available to perform the measurement. For example, the Shack-Hartmann wavefront analyzer (WFA), used on early adaptive optics (AO) systems, is a very robust analyzer that offers the highest dynamic range among the WFAs currently used in AO. However, this analyzer is limited in terms of sensitivity and noise propagation in the measurement. Today, the most popular analyzer is the pyramid WSA, which will be used in the next generation of AO systems for the Extremely Large Telescope (ELT). Although more sensitive than the Shack-Hartmann, the pyramid analyzer is limited in dynamic range and must be used with spatial modulation, which increases its linearity at the expense of its sensitivity.

During this thesis, I worked on the integrated Mach-Zehnder (iMZ) analyzer, an ASO developed for the XAO, capable of measuring wavefront distortions on the order of a nanometer with high precision. This analyzer is a Mach-Zehnder interferometer in which one of the two arms is spatially filtered to create a reference wave. This analyzer exhibits high sensitivity, comparable to that of the unmodulated pyramid analyzer. Its dynamic range, limited to one wavelength in a closed-loop configuration, can be extended via modulation strategies based on phase diversity and phase shifting. With its two complementary outputs, the iMZ analyzer offers the unique capability of measuring the amplitude of the incident wave in conjunction with its phase, enabling the correction of atmospheric scintillation effects. The calibration of the iMZ signal was developed during this thesis work by fitting an interferometric model to experimental data obtained on the XAO bench at the CRAL (Lyon Astrophysics Research Center). I also developed a numerical model to study the performance of this analyzer in XAO mode for different telescopes (Very Large Telescope (VLT) and ELT) under various turbulence conditions.

These results confirmed the excellent performance of this analyzer and the possibility of using it without a first-stage correction under low-turbulence conditions. I have also implemented new phase measurement methods on the experimental bench to perform phase corrections in XAO mode with the iMZ for the VLT and the ELT. The development of the iMZ ASO is also driven by the advent of 30-meter telescopes, which introduces new optical aberrations, such as differential pistons between the different segments of these telescopes’ apertures. The absolute measurement of the wavefront at each point in the pupil allows the iMZ ASO to measure this type of aberration, to which the previously mentioned analyzers are not very sensitive.