Increasing the measurement accuracy for wave aberrations of an optical system using the intensity distribution of a focused light beam

Subject of study. A method for enhancing the measurement accuracy for wave aberrations of an optical system by using cross-section images of a focused light beam in the presence of the image's amplitude distortions is described. Method. The measurement procedure includes determining the wave aberrations of the optical system individually for each of the beam cross-section images recorded at different sides from the focal plane and subsequently averaging the obtained wave aberrations. Main results. The utilized method for obtaining the wave aberrations of an optical system involves analysis and processing of the aberration-distorted intensity distributions in the cross-section images of a focused light beam that are recorded in a plane before and after the focal plane. Noticeably, the presence of intensity nonuniformities in the light beam cross-sections that do not result from the wave aberrations inevitably causes measurement errors. The effect of such amplitude noise on the measurement accuracy can be suppressed manifold by recording and processing an additional image of the beam cross-section positioned at a particularly selected distance on the opposite side to the focusing plane. This is followed by averaging the wave aberrations obtained for each of these images. If the amplitude and aberrational distortions of the intensity distribution have similar profiles in one of the recording planes, they are added, and they increase the degree of nonuniformity of the total intensity distribution. However, the profiles of the amplitude and aberrational distortions of the intensity distribution in the other plane are mutually opposite, which reduces the degree of distribution nonuniformity. This effect allows the measurement errors caused by amplitude noise to be compensated by arithmetically averaging the wave aberrations obtained individually for each image. We demonstrate that the variation in the measured values of the root mean square deviation of the wave front of the beam of up to 20%-30% from the individual measurements of the wave aberrations in the presence of noise can be reduced to several percent using the proposed method. Practical significance. Amplitude distortions in the images of the cross-sections of a beam that are not caused by aberrations are ubiquitous because they are present in, e.g., high-aperture, apodized, or off-axis optical systems. Measurement of the wave aberrations of such optical systems by the standard method using one image of a focused beam does not provide accurate results. The modification of this measurement technique proposed in this paper enables the extension of its application area to the aforementioned systems while maintaining its characteristic measurement accuracy at lambda/20. (C) 2022 Optica Publishing Group.