  Citation: Khorin, P.A.; Porfirev, A.P.; Khonina, S.N. Adaptive Detection of Wave Aberrations Based on the Multichannel Filter. Photonics 2022, 9, 204. https://doi.org/10.3390/ photonics9030204 Received: 30 January 2022 Accepted: 19 March 2022 Published: 21 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). photonics hv Article Adaptive Detection of Wave Aberrations Based on the Multichannel Filter Pavel A. Khorin 1 , Alexey P. Porfirev 2 and Svetlana N. Khonina 1,2, * 1 Samara National Research University, 443086 Samara, Russia; paul.95.de@gmail.com 2 Image Processing Systems Institute of RAS—Branch of the FSRC “Crystallography and Photonics” RAS, 443001 Samara, Russia; porfirev.alexey@ipsiras.ru * Correspondence: khonina@ipsiras.ru Abstract: An adaptive method for determining the type and magnitude of aberration in a wide range is proposed on the basis of an optical processing of the analyzed wavefront using a multichannel filter matched to the adjustable Zernike phase functions. The approach is based on an adaptive (or step-by-step) compensation of wavefront aberrations based on a dynamically tunable multichannel filter implemented on a spatial light modulator. For adaptive filter adjustment, a set of criteria is proposed that takes into account not only the magnitude of the correlation peak, but also the maximum intensity, compactness, and orientation of the distribution in each diffraction order. The experimental results have shown the efficiency of the proposed approach for detecting wavefront aberrations in a wide range (from 0.1λ to λ). Keywords: wavefront aberrations; adaptive method; Zernike functions; wavefront sensor; multichannel diffractive optical element 1. Introduction The problem of measuring and correcting wavefront aberrations is often encountered in optics, for example, in the design of ground-based telescopes, in optical communication systems, in industrial laser technology, and in medicine [1–12]. Usually, the measurement of wavefront distortions is performed in order to compensate them, in particular, with adaptive or active optics [13–18]. The major causes of wavefront aberrations are turbulence of the atmosphere, imperfect shapes of the optical elements of the system, errors in the alignment of the system, etc. It is known that weak wavefront aberrations (level ≤ 0.4λ) are well detected using spatial filters matched to the basis of Zernike functions [19–27] including multichannel diffractive optical elements (DOEs) [21,25,27]. However, with an increase in aberration level, the linear approximation of the wavefront by Zernike functions becomes unacceptable [27]. This is explained by the fact that the contribution of the second and subsequent nonlinear terms of the wavefront expansion to the Taylor series becomes more significant, which leads to the detection of false aberrations. With high aberrations (level > 0.4λ), when a significant blurring of the focal spot occurs, it makes sense to use methods focused on analyzing the intensity distribution pattern formed by an aberrated optical system in one or several planes. To determine the wavefront in this case, iterative [28–32] and optimization algorithms [10,33] are used, including those with the use of neural networks [34–39]. In turn, these approaches demonstrate significant errors for small aberrations, when the point spread function (PSF) is close to the Airy picture of an ideal system [27]. Thus, different methods work at different levels of aberrations, and in order to apply them, it is desirable to determine this level (or magnitude). One of the solutions is the use of additional optical and digital processing, for example, based on a dynamically tunable spatial light modulator (SLM). Previously, we studied the stability of the wavefront Photonics 2022, 9, 204. https://doi.org/10.3390/photonics9030204 https://www.mdpi.com/journal/photonics