AN ADAPTIVE OPTICS SYSTEM FOR THE AUTOMATIC CONTROL OF LASER BEAM JITTERS IN AIR

Abstract

Due to their weakness, the direct detection of Gravitational Waves (GW) has never been demonstrated up to date and therefore it is one of the most investigated fields of research in the recent years. Long baseline interferometric antennas are very promising GW detectors as they perform wideband and low noise measurements based on the fine movement of optics properly suspended for seismic isolation. Among the others, the Virgo antenna is a large scale ground based interferometer designed for GW detection in the band 10 Hz ÷10 kHz with sensitivity of Hzh 110 23 =

It is installed on Cascina site (Pisa, Italy) and represents one of the most performing systems currently operating in the world. Nonetheless, it has not yet measured any GW event and therefore new efforts are being carried out in order to further suppress the noise and gain an improved sensitivity level that allows several GW measurements per year. The present Doctoral Thesis focuses on the development of a novel Adaptive Optics (AO) system proposed by the author for the active suppression of laser beam jitters at the input of the Virgo antenna. In fact such laser perturbations couple with the interferometer asymmetries and originate additional phase noise that limits the antenna sensitivity. From a short description of the Virgo Project, it is stated that the requirements for the control of input laser beam jitters correspond to reduction of 40 dB in the low frequency band below 1 Hz and of 20 dB in the region of some tens of Hz. These conditions cannot be fulfilled by standard AO systems based on the Shack Hartmann sensor because of high noise and limited speed. On the other hand, starting from the theoretical analysis of the aberrated wavefront, it is possible to demonstrate that laser beam jitters can be alternatively represented in terms of higher order Hermite Gauss modes perturbing the fundamental Gaussian beam or in terms of Zernike polynomials expanding the distorted phase profile. This in turn allows the author to design an innovative AO system that uses interferometric techniques for the extraction of error signals in terms of Hermite Gauss coefficients and automatically corrects the laser beam with an adaptive mirror deformed on Zernike modes. The AO system performs simultaneous correction of 6 degrees of freedom corresponding to first and second order Hermite Gauss modes. In particular its dynamics is well represented by elegant 6x6 matrix equations that are connected to the block diagram of the closed loop automatic control. Using an experimental prototype properly implemented in laboratory, the proposed AO system is completely characterized in terms of effectiveness and stability from the Facoltà di Ingegneria Dipartimento di Ingegneria Elettronica measurement of its frequency response functions, that exhibit robust operation on all the 6 degrees of freedom. Direct measurements of spectral residual noise show the suppression of 60 dB up to 1 Hz and of 20 dB over 200 Hz, even of astigmatism and defocus modes, which at the best of the present technology fulfils the Virgo requirements previously stated. Finally, the quality control of the laser beam cleaned up from jitters is carried out by measuring its transverse intensity and fitting the experimental data with the expected Gaussian profile, that is matched with 96% reliability using the 2 parameter. The AO system developed in this Doctoral Work performs laser beam jitters reduction in good accordance with the theoretical prediction and therefore can be seriously proposed for application to interferometric Gravitational Wave antennas.