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Zygo Advocates the Use Of Power Spectral Density For Specifying High Energy Laser Optics

As a world-leading optical component and optical assembly innovator, Zygo — a business unit of AMETEK, Inc. (NYSE: AME) — prioritizes bestin-class technologies to manage and certify the quality of optical surfaces. As such, the company uses the Power Spectral Density (PSD) concept for specifying many types of optics and has proved it to be a valuable metric in extreme application spaces such as high energy laser (HEL) optics. This is because it helps in understanding and managing the quality of optical surfaces in a detailed way not captured by conventional methods.

Kevin Quist, National Sales Manager - Optics at Zygo says, “Imagine you're examining the surface of a mirror or a lens used in a high-energy laser system. This surface might look smooth to the naked eye, but at a microscopic level, it can have all sorts of tiny imperfections or roughness. These imperfections can scatter light, which can be detrimental in applications where precise control of the laser beam is critical.”

Typical optical specifications such as Peak-to-Valley (PV), Root Mean Square (RMS) roughness, power, and irregularity are widely used to describe the overall smoothness and “figure” of optical surfaces. These metrics are quite effective for general commercial applications, with PV, or preferably PVr indicating the maximum height difference between the highest and lowest points on the surface, RMS providing a statistical average of surface deviations, and irregularity measuring the deviation from the best fit sphere. However, these measures provide only a limited perspective — they give a summary view without detailed insight into how surface features of various sizes contribute to the overall surface texture. Certain aberrations caused by surface figure error can affect the energy distribution of a beam, which is especially critical in HEL applications.

On the other hand, PSD — which analyses the surface features of an optical component by transforming the surface topography data from the spatial domain into the frequency domain — offers a much more detailed analysis by breaking down the surface texture into its constituent spatial frequencies.

Quist continues, “PSD can quantify the rms over a specific spatial band, which is vital for high-precision applications like high-energy lasers where the efficiency of a system is a critical driver of energy demand, and therefore cost. Unlike traditional metrics like PV or RMS, PSD allows for the identification and control of specific types of surface imperfections that can critically impact the propagation and quality of a laser beam, thus providing a more robust framework for predicting and enhancing optical performance in advanced applications.”

By using PSD, optical engineers can specify how smooth or rough the surface of an optical component needs to be to perform optimally in a high-energy laser system. This is important because high-energy lasers need to focus very intense light beams very precisely. Even small amounts of scattered light can lead to inefficiencies, reduced performance, or even damage to other parts of the optical system. PSD allows engineers to understand and limit this scattering by controlling the types of surface imperfections.

Also, using PSD as a specification tool Zygo can ensure that every optical component produced meets the same high standards. This consistency is key in systems where high performance is critical and can also help in troubleshooting or refining the production process. PSD provides a quantitative way to assess improvements and guide post-manufacturing processes, such as polishing and coating, to achieve desired outcomes.

Quist concludes, “At Zygo, when specifying optics, we see PSD as like having a detailed map that shows not just the mountains and valleys on an optical surface, but also the fine details of the terrain. This map allows for better design, manufacturing, and application of highenergy laser optics, ensuring that these systems are both efficient and effective in their end use applications.”


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