Effects on the OTF of MSF structures with random variations

Fast Zernike fitting of freeform surfaces using the Gauss-Legendre quadrature

Zernike polynomial orthogonality, an established mathematical principle, is leveraged with the Gauss-Legendre quadrature rule in a rapid novel approach to fitting data over a circular domain. This approach provides significantly faster fitting speeds, in the order of thousands of times, while maintaining comparable error rates achieved with conventional least-square fitting techniques. We demonstrate the technique for fitting mid-spatial-frequencies (MSF) prevalent in small-tool-manufacturing typical of aspheric and freeform optics that are poised to soon permeate a wide range of optical technologies.

Workflow for modeling of generalized mid-spatial frequency errors in optical systems

We propose a workflow for modeling generalized mid-spatial frequency (MSF) errors in optical imaging systems. This workflow enables the classification of MSF distributions, filtering of bandlimited signatures, propagation of MSF errors to the exit pupil, and performance predictions that differentiate performance impacts due to the MSF distributions. We demonstrate the workflow by modeling the performance impacts of MSF errors for both transmissive and reflective imaging systems with near-diffraction-limited performance.

Use of pupil-difference moments for predicting optical performance impacts of generalized mid-spatial frequency surface errors

In this work, we present a methodology for predicting the optical performance impacts of random and structured MSF surface errors using pupil-difference probability distribution (PDPD) moments. In addition, we show that, for random mid-spatial frequency (MSF) surface errors, performance estimates from the PDPD moments converge to performance estimates that assume random statistics. Finally, we apply these methods to several MSF surface errors with different distributions and compare estimated optical performance values to predictions based on earlier methods assuming random error distributions.

Pupil-difference moments for estimating relative modulation from general mid-spatial frequency surface errors

Standard surface specifications for mid-spatial frequency (MSF) errors do not capture complex surface topography and often lose critical information by making simplifying assumptions about surface distribution and statistics. As a result, it is challenging to link surface specifications with optical performance. In this work, we present use of the pupil-difference probability distribution (PDPD) moments to assess general MSF surface errors and show how the PDPD moments relate to the relative modulation.

Topography stitching in the spatial frequency domain for the representation of mid-spatial frequency errors

Sub-aperture fabrication techniques such as diamond turning, ion beam figuring, and bonnet polishing are indispensable tools in today's optical fabrication chain. Each of these tools addresses different figure and roughness imperfections corresponding to a broad spatial frequency range. Their individual effects, however, cannot be regarded as completely independent from each other due to the concurrent formation of form and finish errors, particularly in the mid-spatial frequency (MSF) region. Deterministic Zernike polynomials and statistical power spectral density (PSD) functions are often used to represent form and finish errors, respectively. Typically, both types of surface errors are treated separately when their impact on optical performance is considered: (i) wave aberrations caused by figure errors and (ii) stray light resulting from surface roughness. To fill the gap between deterministic and statistical descriptions, a generalized surface description is of great importance for bringing versatility to the entire optical fabrication chain by enabling easy and quick exchange of surface topography data between three disciplines: optical design, manufacturing, and characterization. In this work, we present a surface description by stitching the amplitude and unwrapped phase spectra of several surface topography measurements at different magnifications. An alternative representation of surface errors at different regimes is proposed, allowing us to bridge the gap between figure and finish as well as to describe the well-known MSF errors.

Parametric Mid-Spatial Frequency Surface Error Synthesis

Standard mid-spatial frequency tooling mark errors were parameterized into a series of characteristic features and systematically investigated. Diffraction encircled and ensquared energy radii at the 90% levels from an unpowered optical surface were determined as a function of the root-mean-square surface irregularity, characteristic tooling mark parameters, fold mirror rotation angle, and incident beam f-number. Tooling mark frequencies on the order of 20 cycles per aperture or less were considered. This subset encompasses small footprints on single-point diamond turned optics or large footprints on sub-aperture tool polished optics. Of the characteristic features, off-axis fabrication distance held the highest impact to encircled and ensquared energy radii. The transverse oscillation of a tooling path was found to be the second highest contributor. Both impacts increased with radial tooling mark frequency.

Wigner function-based modeling and propagation of partially coherent light in optical systems with scattering surfaces

Light scattering from residual manufacturing errors of optical surfaces has a large impact on the image quality of optical systems. Classical ray-based methods to simulate surface scattering in optical systems depend on statistical models of surface errors and neglect the wave properties of light, which prohibit the integration of statistical surface error models with beam propagation methods. Additionally, the impact of multiple scattering from different frequency components of surface errors cannot be easily modelled by existing methods. Here we analyze the impact of different frequency components of surface errors induced by diamond-turned surface grinding on image quality, and we propose a Wigner function-based approach in which light is modelled as partially coherent. In this unified model, by selecting the proper definition of light coherence, we can combine the statistical and deterministic models of surface errors, enabling efficient, simultaneous simulation of multiple scattering from high- and mid-spatial frequency (HSF and MSF, respectively) surface errors, as well as the interference and edge diffraction of light.

Validity of the perturbation model for the propagation of MSF structures in 3D

Mid-spatial frequency (MSF) structures on optical surfaces degrade system performance and a perturbation model is typically used to simplify the assessment of their effects. In this simple model, MSF phase structures are dragged along the nominal rays of a system to yield estimates of wavefronts in the exit pupil that may be used for further analysis. However, the validity of the perturbation model remains an open area of study. We extend our previous assessment of the validity of this model [K. Liang, Opt. Express 27, 3390-3408 (2019)] that was focused on the analysis of single-frequency MSF structures in two dimensions to now include error estimates for broad-spectra MSF structures in three dimensions.