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10 Things to Consider When Buying fused silica windows

Author: Daisy

May. 06, 2024

Top Considerations When Purchasing Fused Silica Windows

Understanding Optical Windows

Optical windows serve to separate two different environments for chemical, environmental, mechanical, or thermal purposes. These windows come in a variety of sizes, from a few millimeters to meter-class optics. Key specifications for optical windows include transmitted wavefront, cosmetics, and dimensional requirements.

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Especially for larger optical windows used in external environments, exposure to elements and contaminants can affect their surface. Fortunately, repolishing and recoating these windows can often restore their quality, offering significant cost savings compared to purchasing new ones.

Weight is a crucial factor as it influences material procurement cost, handling, machining, polishing, coating, and ultimately the mounting of the window in its application.

For this article, we'll consider windows over 300mm in diameter/diagonal, extending up to over a meter. These optical windows can be circular, square, rectangular, elliptical, or even custom shapes.

Applications of Large Format Windows

Large optical windows are used across various critical applications:

Aerospace: Space-based aircraft and habitats
Astronomy: Instrumentation and sensors
Defense: Aerial reconnaissance and laser-based defenses
Free Space Optical Communication: Laser communication
High Laser Energy Research: Vacuum windows and debris shields
LIDAR: Atmospheric testing

Choosing the Right Material for Optical Windows

The choice of material is essential, involving factors like wavelength transmission, homogeneity, and annealing for stability. Popular materials include BK7, Borofloat®/Supremax®, and Fused Silica for the visible spectrum; and germanium, silicon, zinc selenide, and zinc sulfide for the IR spectrum.

Borofloat®/Supermax® is an economical option with low expansion, while N-BK7/SBL-7 offers good performance in stable thermal conditions. However, fused silica windows are among the best due to their low expansion and superior homogeneity, particularly for large sizes needed in Internal Confinement Fusion (ICF) applications.

The Importance of Polishing Technique

Polishing technique significantly affects the cost and performance of the optical window. For single pieces, single-side polishing is common, although it's time-consuming and costly. For bulk manufacturing, double-side polishing is more efficient and cost-effective, ensuring optimal transmitted wavefront quality.

Transmitted Wavefront Error (TWE) Specifications

TWE specifications are critical and should align with the component's application. Accurate definition of the clear aperture, including sub-apertures, is necessary, especially if the beam size varies. Coupling TWE with slope specifications might add to the cost due to the need for advanced surface figuring technology.

Phase-shifting large aperture interferometers are best for testing TWE on large windows, capturing full-aperture interferograms for comprehensive surface characterization.

Coating and Shipping Considerations

Large windows often have anti-reflective (AR) coatings to enhance performance, with optional oleophobic or hydrophobic coatings. Coating uniformity is crucial, as deposition techniques, chamber size, part geometry, and other factors impact the filter coating's wavelength space and specifications.

Large windows are generally shipped in sturdy cases. Packaging can range from simple tissue and bubble wrap to more protective cleanroom-sealed containers.

Though often the final optical component in a system, windows must be carefully specified to ensure proper functionality and environmental separation.