Electroforming low-scatter optical components

Minimizing optical scatter is crucial for achieving superior optical performance across various industries requiring precision optics. Whether it is for high-energy laser systems, imaging technologies, or advanced instrumentation, achieving optimal optical performance is critical. The choice of manufacturing method can significantly impact the quality and efficiency of a particular optical component as well as the overall system performance.

To meet the technical challenges associated with light scatter and surface roughness, electroforming stands as a superior replication process. The advantages of this type of additive manufacturing provide insight into creating low-scatter optical components with high precision, performance, and reliability. At Optiforms, for example, our manufacturing process created the off-axis parabolic mirrors used at Lawrence Livermore National Laboratory in December 2022 to confirm fusion ignition.

While more conventional manufacturing methods offer high degrees of precision, they have limitations that affect optical scatter. Diamond turning, for example, involves removing material with a diamond-tipped cutting tool. Despite the method’s high precision, the removal process can introduce surface imperfections, including tool marks, frequency-structured errors, and subsurface micro-cracks, significantly contributing to optical scatter. Electroformed optical components produce exceptionally smooth surfaces, free from the microgrooves and irregularities often associated with diamond turning, thus minimizing scattering effects.

Understanding optical scatter and surface roughness across applications

Surface roughness is characterized by deviations from an ideal smooth surface. The amount of light scattered from a surface is proportional to the square of the ratio of surface roughness to wavelength. Assuming wavelength is defined, depending on the particular application (although it is worth noting that the scatter intensity for longer wavelengths is significantly reduced compared to shorter wavelengths), it is furthermore critical to minimize surface roughness via optimal manufacturing techniques.

Optical or light scatter refers to the phenomenon in which light, upon interacting with a surface, is directed in various paths often due to surface roughness, microscopic defects, particulate matter, or material inconsistencies. Since scatter is characterized by its intensity, angular distribution, and spectral properties, its magnitude can also depend on the wavelength of light, angle of incidence, and material properties of the incident medium. The scattering of unwanted light can critically affect the functionality of optical systems. The following examples include relevant applications:

• Scatter can degrade image quality by reducing contrast, increasing background noise, and limiting resolution. Minimizing scatter helps ensure clear and sharp images with high fidelity. Affected applications would include various cameras, microscopes, and telescopes, particularly in defense and surveillance systems.
• Scatter contributes to background noise in optical systems, reducing the signal-to-noise ratio. High scatter levels can obscure weak signals or details of interest, making distinguishing between the expected signal and the noise difficult. Affected applications include laser systems, astronomy, and remote sensing.
• Scatter, by interfering with the intended propagation of light, can introduce errors in optical measurements and calculations. Affected applications include metrology, spectroscopy, and lithography, as well as various other laser applications utilizing high energy levels and requiring high precision.
• Minimizing scatter helps to achieve uniform illumination across a target area. Excessive scatter can lead to uneven brightness or intensity variations, affecting the overall performance of the lighting system. Affected applications include projections, displays, and optical lithography systems.
  
  

An advantageous manufacturing solution

Electroforming is a precise and versatile additive manufacturing process that replicates exact metal optical components with fine features. At Optiforms, our process relies on controlled electrodeposition of metal (mainly nickel or copper) onto a conductive template, referred to as the mandrel, which has been precisely shaped and polished so as to eliminate any tooling frequency patterns while maintaining accurate surface form and single-nanometer surface roughness specifications.

An electroformed coated reflector. Photo credit: Optiform

Through electrolysis and a controlled flow of electrical current, among other parameters, metal ions are deposited onto the mandrel surface, gradually building layers of metal until the desired thickness is achieved. The substrate is then separated from the mandrel, creating a standalone component. The resulting internal surface finish exactly mirrors the optimally crafted mandrel, with minimal surface roughness and defects.

Additional processes enhance low-scatter optics

Electroformed optical components generally exhibit minimized scattering, but additional surface enhancements can achieve even more significant results. For example, optical coatings and electroplated finishes can be applied to optical components. Smoother surfaces provide better contact and adhesion, resulting in more uniform and defect-free surfaces that enable improved light propagation. The final coating on the optical surface enhances the spectral properties and often protects the optical surface. This thin film coating can be either vacuum deposited or electroplated to achieve superior optical performance.

Additional benefits of electroformed optics

Beyond its capability to produce low scatter optics, electroforming offers several other advantages. For example, the method excels at producing parts with intricate and complex geometries. Precise control over the deposition method enables the creation of fine details, sharp corners, undercuts, and thin walls that may be difficult or impossible to achieve with other manufacturing processes, especially over larger areas.

Electroforming can also result in high dimensional accuracy, reproducing near-exact shapes and dimensional replicas of the mandrel while keeping tight tolerances. Finally, electroforming offers cost advantages compared to traditional manufacturing methods for specific applications. It can eliminate the need for extensive post-processing, easily incorporate holding or positional features, and be further enhanced with coatings and platings that improve low-scatter performance.

The choice of manufacturing method can significantly impact the quality and efficiency of optical components. Along with other significant advantages, the superior surface quality of electroformed optical components makes them particularly well-suited for applications that include directed energy systems, imaging systems, and laser applications. Electroforming is a valuable process that produces optics that meet demanding requirements for reduced light scatter and enhance the performance and reliability of advanced optical systems.

Natalie Pastuszka is an optical engineer and Dustin Lysiak is an electroforming polishing manager at Optiforms (www.optiforms.com).