Sapphire optical glass windows are ideal for applications that consider high pressure, high temperature, high thermal load, vacuum, scratch/wear resistance, low friction, and corrosive environments. Therefore, sapphire optical glass windows are often used in research, medical, space, and military applications due to their superior performance.
Synthetic Sapphire is manufactured in laboratories and then Sapphire wafer are cut out of them to be built as windows. Let’s see what makes Sapphire an ideal choice for optical windows.
Benefits of Sapphire Windows
The optical transmission band of Sapphire is from ultraviolet to near-infrared (0.15- 5.5 μm), which is stronger than other standard optical materials. The sapphire window has good transmission characteristics in the visible and near-infrared spectrum. Sapphire window has high mechanical strength, chemical resistance, thermal conductivity, and thermal stability. Therefore, sapphire windows are often used as window materials in specific fields, such as aerospace technologies that require scratch resistance or high temperature. Sapphire glass windows are best suited for scratch-resistant applications that require better transmission over a broad spectrum.
Types of Sapphire Windows
Sapphire optical glass windows are generally polished round clear glass windows, which are generally used to isolate the environment while allowing light to pass through relatively softly. Optical windows are generally used to observe ports or to seal and/or protect optical components, instruments, or other components in lasers.
Flat windows are usually parallel or slightly wedge-shaped, with the aim of making the incident light strike perpendicular to the window surface. The parallelism or wedge angle of the windows is also an important indicator to consider. For very parallel or wedge-shaped windows, they are generally specified in arc minutes. The problem with the windows being too parallel is that the second reflection from the surface can cause unnecessary back reflections from the system. However, the reflections on the surface of the wedge-shaped window are not parallel and are also slightly separated. One of the reasons for using sapphire is that compared to standard optical glass windows, the thickness of the window can be made thinner while maintaining the same strength.
The surface quality specifications of optical components are generally defined by the scratches and dents allowed on the polished surface. A scratch is a defect in a polished optical surface and its length is many times greater than its width. A dent is a defect in a polished optical surface, and its length and width are about the same, like a hole. The scoring and digging specifications are defined by the lowest number equal to the highest quality.
Since sapphire is a very stable material, it can be polished to any Scratch-Dig specification, including the extremely high free Scratch-Dig quality commonly known as “0-0”.
The surface flatness specification of an optical component is typically defined by measuring how accurately the optical surface conforms to its expected shape. Typically, a laser interferometer or reference test plate is used to measure the flatness of the surface by forming an interference pattern by changing the length of the optical path on the surface. The surface flatness specification is generally expressed as a fraction of the wavelength. Any stable wavelength reference can be used to measure surface flatness. Generally, the shorter the wavelength used for the measurement, the better the resolution.
These are the qualities that make Sapphire wafers and Sapphire windows great for use in optical instruments. Sapphire, apart from being extremely durable, also offers a high transmission range, which is a great advantage.