Adaptive optics for high-resolution imaging

Abstract

Adaptive optics (AO), a technique that corrects for optical aberrations, was originally proposed to correct for the blurring effect of atmospheric turbulence on images in ground-based telescopes; indeed, the technique was instrumental in the work that resulted in the discovery of a supermassive compact object at the centre of our galaxy, which was awarded the 2020 Nobel Prize in physics. When AO is used to correct for the eye's imperfect optics, retinal changes at the cellular level can be detected, allowing us to study the operation of the visual system and to assess ocular health in the microscopic domain. By correcting for sample-induced blur in microscopy, it has pushed the boundaries of imaging in thick tissue specimens, such as when observing neuronal processes in the brain. The focus of this primer is the application of AO for high resolution imaging in astronomy, vision science, and microscopy. It begins with an overview of the general principles of AO and its main components, which include methods to measure the aberrations and devices for their correction. These components are linked in operation via a control system. Results and applications from each field are presented, along with reproducibility considerations and limitations. Finally, future directions are discussed. [H1] IntroductionHigh resolution optical imaging relies upon the high fidelity focussing of light. Light can be described in terms of optical fields[G], and thus its properties are parametrized for a given wavelength at each point in space and time in terms of amplitude, phase and polarization. However, these fields can be perturbed (in amplitude, phase and polarization) as they propagate through optical systems and other media, and the performance of the imaging systems can be highly sensitive to those perturbations. For instance, astronomical image quality is limited by atmospheric turbulence; microscopes produce blurred images when samples have a non-uniform refractive index distribution; and ophthalmoscopes that image the back of the eye are detrimentally affected by the eye's imperfect optics. Adaptive optics (AO) is an ensemble of electro-optical and computational methods that aim at recovering the optimal performance of an optical system 1-6 . This has brought benefits to a range of applications. For example, by integrating AO in their telescopes, astronomers have been able to expand the observation of celestial bodies 7 . Implemented into microscopes, AO has enabled neuroscientists to monitor the activity of neurons embedded deep inside the living mammalian brain 8,9 . And integrated into ophthalmoscopes, vision scientists and ophthalmologists are able to visualize, quantify, and track in situ the many different types of cells that compose the retina, offering significant clinical potential 10-14 .There are many imaging applications in which AO has a significant impact, and the above list is far from exhaustive.Compensation [G] through modulation of the optical field is the fundamental working principle through which AO allevia...