Surface characterization techniques for determining the root-mean-square roughness and power spectral densities of optical components

TLDR

The study measured surface topography and light scattering on 15 samples—including polished and ground fused silica, silicon carbide, sapphire, electroplated gold, and diamond‑turned brass—using AFM, mechanical and optical profilers, confocal laser scanning microscopy, angle‑resolved scattering, and total scattering. Two‑dimensional PSDs were derived from the data, allowing direct comparison of rms roughness across instruments, and remaining discrepancies were largely attributed to isolated surface particles that affected the measurement techniques differently.

Abstract

Surface topography and light scattering were measured on 15 samples ranging from those having smooth surfaces to others with ground surfaces. The measurement techniques included an atomic force microscope, mechanical and optical profilers, confocal laser scanning microscope, angle-resolved scattering, and total scattering. The samples included polished and ground fused silica, silicon carbide, sapphire, electroplated gold, and diamond-turned brass. The measurement instruments and techniques had different surface spatial wavelength band limits, so the measured roughnesses were not directly comparable. Two-dimensional power spectral density (PSD) functions were calculated from the digitized measurement data, and we obtained rms roughnesses by integrating areas under the PSD curves between fixed upper and lower band limits. In this way, roughnesses measured with different instruments and techniques could be directly compared. Although smaller differences between measurement techniques remained in the calculated roughnesses, these could be explained mostly by surface topographical features such as isolated particles that affected the instruments in different ways.