High-resolution focal-plane wavefront sensing for time-varying aberrations

Abstract

Phase aberrations in optical systems,which occur in various applications such as astronomy, microscopy and ophthalmology, degrade the quality of obtained images. The exact cause and nature of the aberrations depends on the application. In astronomy, turbulence within the Earth’s atmosphere creates fluctuations of the refractive index, leading to phase aberrations. In order to compensate for the distorting effect of aberrations, adaptive optics (AO) systems are used to correct for phase aberrations in real-time. A deformable mirror (DM) is often used to apply the necessary corrections to improve the image quality. Due to the temporally dynamic nature of atmospheric turbulence and the corresponding phase aberrations, estimation errors caused by delays within the AO control loop are a significant part of the total estimation error. Accurate prediction of the phase aberrations is therefore an important aspect when aiming to improve the performance of AO systems. Reconstructing the phase aberrations from focal plane images only is known as focal plane wavefront sensing. Many focal plane sensing methods are based on solving the phase retrieval problem, which is the problem of reconstructing the phase aberrations from the point spread function (PSF). Due to the non-linear optimization problem that underlies phase retrieval, developing real-time solvers is very challenging and a wavefront sensor (WFS) is often included to avoid the phase retrieval problem. Due to the linear relation between the phase aberration (i.e. wavefront) and WFS signal, WFSbased AO is often preferred over the wavefront sensorless (WFSless) AO systems that use focal plane sensing. There are, however, also a number of disadvantages. First, the addition of extra hardware components, including the WFS and a beam splitter, makes the system more complex and expensive than WFSless systems. Second, splitting the light between the focal plane camera and WFS results in non-common path aberrations (NCPAs), which can be a limiting factor in high-resolution imaging systems.