In order to provide the end user with a diffraction limited collimated beam, adaptive optics phase correction systems are now a standard feature of ultra intense laser facilities. Generally speaking, these systems are based on a deformable mirror controlled in closed loop configuration in order to correct the aberrations of the beam measured by the wavefront sensor. Such implementation corrects for most of the aberrations of the laser. However, the aberrations of the optical elements located downstream of the wavefront sensor are not measured and therefore not corrected by the adaptive optics loop while they are degrading the final focal spot. We present an improved correction strategy and results based on a combination of both usual closed loop and phase retrieval in order to reach the diffraction limit at the focal spot inside the interaction chamber. The off axis parabola alignment camera located at the focal spot is used in combination of the deformable mirror and wavefront sensor to get images of the focal spot. The residual aberrations of the focal spot are measured by a Phase Retrieval algorithm using the acquired focal spot images. Then the adaptive optics loop is run in order to precompensate for these aberrations, which leads to diffraction limited focal spot in the interaction chamber.
An active reflective component can change its focal length by physically deforming its reflecting surface. Such elements exist at small apertures, but have yet to be fully realized at larger apertures. This paper presents the design and initial results of a large-aperture active mirror constructed of a composite material called carbon fiber reinforced polymer (CFRP). The active CFRP mirror uses a novel actuation method to change radius of curvature, where actuators press against two annular rings placed on the mirror’s back. This method enables the radius of curvature to increase from 2000mm to 2010mm. Closed-loop control maintains good optical performance of 1.05 waves peak-to-valley (with respect to a HeNe laser) when the active CFRP mirror is used in conjunction with a commercial deformable mirror.
Adaptive optics (AO) is used to correct wavefront aberrations in light in real-time. An AO system is principally made up
of three parts; a wavefront measuring device, a correction device, and a control algorithm to compute the residuals between the measured and a reference wavefront. Deformable mirrors (DM) are commonly used as the correction devices in such a system. This paper presents a method to improve a DM's temporal performance by attenuating parasite oscillations of its reflective membrane when applying high-frequency signals to the mirror actuators. The method consists of implementing low-pass filtering into the software driving the mirror. Different filtering functions were studied both when stimulating one single actuator, and when applying voltages to the complete array of actuators. A linear decomposition in 41 substeps showed the best performance for all considered configurations. The obtained results represented an important
reduction of the settling time as well as the overshoot of the signal response.
In this paper we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-Petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.