Aberrations in Optical Imaging Systems

The main functionality of an imaging system is to gather as much of the light emanating from each of the object points as possible, and to make those cones of light converge again at the image plane, so that each object point is mapped univocally to its counterpart in the image plane. The performance of such systems is generally judged on the basis of how well this correspondence between object and image points is maintained, with the well-known theoretical limit imposed by the phenomenon of diffraction: even in an optical system that, according to the laws of geometrical optics, will map all the rays coming from one object point exactly to a single, mathematical, image point, diffraction will cause that image point to smear into a small, but finite-sized, spot. This diffraction-limited situation is what the design of an imaging system typically aims for, with the diffraction-limited field having a spherical wavefront. Geometrical deviations from this spherical wavefront are known as “aberrations”, and are characterized using different polynomial bases that help quantify their strength and shape. The presence of aberrations will increase the smearing of the image spot and consequently decrease the quality of the imaging system.

With the fast physical optics software VirtualLab Fusion aberration effects can be studied well. For this week’s newsletter, we have selected two examples related to aberrations: the first, of how the typical wavefront aberrations affect the pattern in focus of a spherical wave, and the second, of how the astigmatism of a high-power laser diode influences performance in the focal region. Using the free space propagation field solver and the Local Plane Interface Approximation (LPIA), diffraction, polarization and vectorial effects that can potentially degrade an image can all be included in the investigation.

Aberration Effects of a Spherical Wave on the Focal Spot

Different types of typical wavefront aberrations are selected exemplarily, and the resulting focal spots are investigated for varying strength of the aberrations.

Focus Investigation behind Aspherical Lens

A laser diode with asymmetric divergence and astigmatism is first collimated and then focused. Evolution of the field in the focal region is investigated in detail.

OASIS - We are coming!

OASIS - We are coming!