Optical Windows

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Optical windows are flat plates made of optically transparent material, designed to allow light into an optical instrument. They can be manufactured to minimize both reflection and absorption while maximizing transmission over a target wavelength range. Shanghai Optics manufactures a wide variety of the highest quality optical windows, and we are able to customize as needed to meet your requirements.

Substrate Properties

Substrate properties and optical surface specifications are two attributes you will want to keep in mind as you select optical windows for your application.

The material properties of the substrate used will determine transmission, refractive index, and hardness. Our Potassium Bromide substrates, for instance, will transmit UV, visible, and infrared light. They have a density of 2.75 g/cm3 and an index of refraction of 1.527. Zinc Selenide, on the other hand, blocks UV as well as some visible light and transmits higher wavelength visible light and infrared. It has an index of refraction of 2.631.

The index of refraction quantifies how much the light is slowed down as it is transmitted through the substrate. It is calculated as the ratio of the speed of life in a vacuum to the speed of light through the substrate. For instance, the index of refraction of Zinc Selenide, 2.631, means that light travels through a vacuum 2.631 times faster than it does through ZnSe.  For optical windows, the refractive index is typically specified at 587.6 nm, the Helium d-line wavelength.  Optical glasses with a high index of refraction are sometimes called ‘flints’, while windows with low index of refraction are called “crowns”.

Another important specification, the Abbe Number, describes how the refractive index varies with wavelength.  The lower the Abbe number, the higher the dispersion.

If your application is weight-sensitive, you may need to pay attention to substrate density. Although the refractive index of optical materials tends to increase as density increases, this is not always the case, and the relationship is not always linear.

Optical Surface Specifications

Surface flatness is described in terms of the deviation from a completely flat surface, and is often measured with a precise reference piece called an optical flat. Deviations from perfect flatness can be quantified and are given in what is called waves, abbreviated λ.  Lower λ implies higher flatness. While flatness of 1λ is sufficient for most applications, precision optics such as high power lasers may require surface values of as low as λ/20.

Surface quality refers to the presence or absence of surface imperfections: scratches and digs. It is quantified with a two-part scratch dig number, and the lower the number the better the surface quality. While a scratch-dig number of 40-40 or even 80-50 is appropriate for most optical windows—especially those used for imaging systems—some precision applications may require 40-20. High power laser systems require high surface quality, perhaps 20-10 or 10-5. The lower the scratch-dig number, the higher the manufacturing cost, and there is a high cost premium associated with 20-10 or 10-5 windows.

Most of our windows are coated with an anti-reflective coating that increases durability and efficiency. Since this AR coating allows the window to maximize transmission of the desired wavelength of light, it is wavelength specific, and the full spectral range of your system must be considered before a selection is made.

Shanghai Optics offers wedged windows in addition to our range of parallel window offerings. These optical components, which feature a controlled wedge in the optical path, are ideal for laser systems because they prevent common problems that a parallel window setup might entail. For instance, power spikes through unwanted reflections, interference effects, and mode-hopping can all be eliminated or control through the use of wedged windows.

We can also customize optical filters for your device, as requested. These are optical windows with dyes injected into the substrate in manufacturing, or with special coatings, and they transmit selective bands or colors of light.

Our optical windows are suitable for a wide range of applications, from military defense to scientific experimentation, manufacturing, lasers, and high precision photography.




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