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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 also be used to protect a light source from an outside environment.

These windows are designed to minimize both reflection and absorption while maximizing transmission over a target wavelength range. When choosing an optical window, you will have to keep in mind optical surface specifications, material transmission properties, and the mechanical properties required by your application.

 Optical Windows Available from Shanghai Optics

Our state of the art factory is able to manufacture both large optical windows and micro windows for nano-sized optical assemblies. A wide variety of optical substrates are available to choose from, and since production is carried on in-house our design team will be able to match your exact specifications when carrying out your order.

Substrate Properties

Substrate properties and optical surface specifications are two attributes you will want to keep in mind as you select custom optics 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. Fused silica has a density of 2.202g/cm and a index of refraction of which varies from 1.55 to 1.40.

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 light 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.

The refractive index of our sapphire windows is 1.76-1.77. 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. The Abbe number of sapphire is 72.24, and for BK7, 64.17.

MgF2 windows are hard and durable, with a refractive number of 1.378 and an Abbe number of 106.22. They have a high laser damage threshold, and very good broadband transmission from 120 nm to 8 μm. Our MgF2 windows are often used with UV radiation sources and receivers.

If your specifications 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, as specified by the United States Military Performance Specification MIL-PRF-13830B. The lower the number the better the surface quality.

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 these AR coatings allow the window to maximize transmission of the desired wavelength of light, it is wavelength specific. 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 high quality optical windows are suitable for a wide range of applications, from military defense to scientific experimentation, manufacturing, lasers, and high precision photography.

Optical Flats

Alongside our regular range of high quality optical windows, Shanghai Optics offers optical flats for measurement purposes or for use in applications where precision is at a premium.

These flat discs feature highly polished surfaces with flatness options of λ/4, λ/10, and λ/20. When these optical flats are placed in contact with a test surface, the presence of light and dark bands provide a straightforward visual determination of the surface quality of the test piece.

Our optical flats are carefully tested with precision metrology equipment to ensure they fully meet the flatness level specified.  Do be aware that optical flats do deteriorate over time and with use, and should be periodically recalibrated.

 

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