Alessandro Polo

Identification of a dynamical model of a thermally actuated deformable mirror

Using the subspace identification technique, we identify a finite dimensional, dynamical model of a recently developed prototype of a thermally actuated deformable mirror (TADM). The main advantage of the identified model over the models described by partial differential equations is its low complexity and low dimensionality. Consequently, the identified model can be easily used for high-performance feedback or feed-forward control. The experimental results show good agreement between the dynamical response predicted by the model and the measured response of the TADM.

Iterative learning control of a membrane deformable mirror for optimal wavefront correction

We present an iterative learning control (ILC) algorithm for controlling the shape of a membrane deformable mirror (DM). We furthermore give a physical interpretation of the design parameters of the ILC algorithm. On the basis of this insight, we derive a simple tuning procedure for the ILC algorithm that, in practice, guarantees stable and fast convergence of the membrane to the desired shape. In order to demonstrate the performance of the algorithm, we have built an experimental setup that consists of a commercial membrane DM, a wavefront sensor, and a real-time controller. The experimental results show that, by using the ILC algorithm, we are able to achieve a relatively small error between the real and desired shape of the DM while at the same time we are able to control the saturation of the actuators. Moreover, we show that the ILC algorithm outperforms other control algorithms available in the literature.

An innovative and efficient method to control the shape of push-pull membrane deformable mirror

We carry out performance characterisation of a commercial push and pull deformable mirror with 48 actuators (Adaptica Srl). We present a detailed description of the system as well as a statistical approach on the identification of the mirror influence function. A new efficient control algorithm to induce the desired wavefront shape is also developed and comparison with other control algorithms present in literature has been made to prove the efficiency of the new approach.

Theoretical analysis for best defocus measurement plane for robust phase retrieval

We study the phase retrieval (PR) technique using through-focus intensity measurements and explain the dependence of PR on the defocus distance. An optimal measurement plane in the out-of-focus region is identified where the intensity distribution on the optical axis drops to the first minimum after focus. Experimental results confirm the theoretical predictions and are in good agreement with an independent phase measurement.

Wavefront reconstruction using intensity measurements for real-time adaptive optics

The crucial step in adaptive optics feedback control is the reconstruction of the wavefront from wavefront sensor measurements. One of the sensors commonly used is the (Shack-)Hartmann sensor, which makes the wavefront reconstruction problem linear, in a least-squares sense. In this paper, a new methodology is proposed to reconstruct the wavefront for real-time adaptive optics control using the complete intensity measurements provided by the sensor, instead of the classical centroid algorithm which approximates the local wavefront slopes. In addition to an outline of the new wavefront reconstruction method, its performance is illustrated via a numerical simulation study. The advantages of the new method are highlighted by comparing it, in both open- and closed-loop, with a modal reconstruction algorithm that uses local wavefront slopes.

Experimental demonstration of phase retrieval from a single defocused intensity measurement in the approximation of small aberrations

We analytically demonstrate that Phase Retrieval (PR) from defocused intensity measurements is achievable using a single measurement plane. Following this approach, the predicted plane is found at a defocus distance of 4? optical unit. Experimental results confirm the theoretical predictions

High resolution Hartmann Wavefront sensor for EUV Lithography system

We discuss the use of a Hartmann Wavefront Sensor as an instrument to measure the aberration in an Extreme Ultraviolet Lithography system. Simulations demonstrate the feasibility and advantages in terms of dynamic range and accuracy.