Optimizing Optical Performance with Power Spectral Density

Power Spectral Density (PSD) is highly beneficial for specifying multiple varieties of optics and is a useful metric in extreme application spaces, including laser fusion.

This specification approach could be applied to demanding applications and high-energy laser optics as it helps to comprehensively understand and manage optical surface quality, which traditional approaches have not previously recorded. This article explores PSD’s value.

Understanding Surfaces at Diverse Scales

When examining the surface of a mirror or lens utilized in a high-energy laser system, it may seem smooth to the naked eye. However, at a microscopic level, it may have multiple minute imperfections or roughness. These imperfections may scatter light, harming use cases requiring meticulous laser beam control.

Optimizing Optical Performance with Power Spectral Density

Image Credit: Zygo Corporation

Common optical specifications, including peak-to-valley (PV), root mean square (RMS) roughness, power, and irregularity, are typically utilized to detail the general smoothness and figure of optical surfaces.

These metrics are useful for typical commercial use cases, with PV pointing towards the maximum height difference between the surface’s highest and lowest points, RMS providing a statistical average of surface deviations, and irregularity assessing the deviation from the best fit sphere.

These measures’ perspectives are limited, giving an overall view without comprehensive insight into how surface characteristics of different sizes contribute to the general surface texture. Specific aberrations resulting from a surface figure error may impact a beam’s energy distribution, which is particularly important in HEL use cases (Figure 1).

Examples of low order figure errors and their effect on energy distribution of a focused beam

Figure 1. Examples of low order figure errors and their effect on energy distribution of a focused beam. Image Credit: Zygo Corporation

PSD, which analyzes the surface characteristics of an optical component by transforming the surface topography data from the spatial domain into the frequency domain, provides a significantly more comprehensive analysis by breaking down the surface texture into the spatial frequencies that make it up.

PSD can provide information on how much each different surface feature size contributes to the general roughness, which is important for high-precision use cases such as high-energy lasers where system efficiency is a crucial driver of energy demand and cost.

Unlike traditional metrics such as PV or RMS, PSD identifies and controls specific surface imperfection types that can critically affect a laser beam’s propagation and quality. This provides a more powerful framework for predicting and improving optical performance in sophisticated use cases.

Figure 2 demonstrates a PSD plot for a polished surface assessed via several interferometers and atomic force microscopes. The plot covers a broad spectrum of spatial periods from 10 mm to 10 nm, or six orders of magnitude, providing data about various scales of surface artifacts that impact scatter and haze.

Example of composite 1D PSD information measured using a combination of interferometers and atomic force microscopes

Figure 2. Example of composite 1D PSD information measured using a combination of interferometers and atomic force microscopes. Image Credit: Zygo Corporation

By utilizing PSD, it is possible to identify how smooth or rough the surface of an optical component should be to perform optimally in a high-energy laser system. This is crucial because:

  • High-energy lasers should focus very intense light beams with high precision. Even small quantities of scattered light may result in inefficiencies, diminished performance, or even harm to other optical system elements. PSD enables engineers to comprehend and limit this scattering by controlling the varieties of surface imperfections.
  • Utilizing PSD as a specification instrument helps ensure each optical component produced meets the same elevated standards. This consistency is important in systems where high performance is crucial and can additionally aid in troubleshooting or fine-tuning the production process.
  • Contemporary manufacturing approaches that enhance the surface quality of laser optics depend on precise measurements. PSD offers a quantitative way to assess modifications and guide manufacturing processes, including polishing and coating, to reach desired results.
  • PSD represents a detailed map that displays the mountains and valleys on an optical surface alongside fine terrain details. This map enables better design, manufacturing, and use of high-energy laser optics, ensuring these systems are effective in their roles.

Image

This information has been sourced, reviewed, and adapted from materials provided by Zygo Corporation.

For more information on this source, please visit Zygo Corporation.

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