Adaptive Optics - Principle and applications

he concept of adaptive optics is an ancient one: a legend dating back to Antiquity credits Archimedes with setting fire to enemy galleys during the siege of Syracuse, using large mirrors oriented to focus the Sun's rays. Under Louis XIV, this experiment was reproduced by Buffon: the faceted mirror created for this purpose can still be seen at the Musée des Arts et Métiers in Paris. Finally, in 1953, HW. Babcock took into account the wave-like nature of light and set out the principle of "modern" adaptive optics.

However, without the right technology, the most promising concepts could not succeed. It was not until the 1970s that the first deformable mirrors, high-speed computers and digital cameras appeared, followed by the development of adaptive optics systems. The effort to focus very high-power laser beams in the atmosphere, in the context of Star Wars, was spurred on by the American defense establishment. The astronomical community then seized on this technology to access the ultimate performance of large ground-based telescopes. Although it is theoretically possible to achieve the resolution imposed by diffraction through image processing, adaptive optics is essential to concentrate the signal, improve detection and achieve the ultimate sensitivity of these instruments. So, in the 1980s, European industrialists and researchers joined forces to demonstrate adaptive optics for astronomy for the first time, at the end of the decade.

Since then, the evolution of adaptive optics has been a constant back-and-forth between technological and conceptual developments. In the first phase, astronomers were very active and continued on the path of their success, constantly working to develop new concepts to push back the limits of adaptive optics. Today, almost all large telescopes are equipped with ever more ambitious, but ever more expensive, adaptive optics. At the same time, other scientific communities have adopted this technology. The first was large-scale equipment, such as high-power lasers, for which the investment associated with the development of dedicated components, notably deformable mirrors, was not an obstacle. Ophthalmologists and microscopists have in turn implemented this technology, to improve the resolution of their imaging devices and the focusing of lasers in biological media. The biomedical community's interest in adaptive optics has led to a real democratization of components (deformable mirrors, real-time computers, high-speed cameras). This democratization has in turn aroused the interest of many engineers and researchers, who are using it for an ever wider range of applications. Adaptive optics is set to be a key element in future free-space optical links.

The aim of this article is to lay the foundations of adaptive...