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Versatility of Sapphire Permits Different Types of Windows

The physical, chemical and optical properties of sapphire make it one of the most versatile materials from which various types of windows can be fabricated.  Natural sapphire is a gemstone variety of the mineral corundum, or aluminum oxide (Al2O3), commonly referred to as alumina (α-alumina) or aloxide, one of nature's most abundant compounds.  In its natural state, aluminum oxide is a white powdery material used extensively as an industrial abrasive.  When heated to about 2050°C (almost 4000°F), however, the powder melts and can then be formed into a single crystal.


Because it is a single crystal, neither natural nor synthetic sapphire can be molded, drawn or cast.  Synthetic sapphire, however, can be "grown" into specific shapes to meet the requirements of different applications.  Unlike natural sapphire, which derives its brilliant colors from impurities such as iron and titanium, synthetic sapphire is water clear and extremely pure, making it ideally suited for strong, lightweight, chemically resistant windows.  Here are some of the different types of windows into which sapphire can be fabricated.


Flat, transparent windows

Because the manufacturing process used to grow synthetic sapphire results in long carrot-shaped boules up to 400 mm in diameter, windows fabricated from it are usually round rather than square or rectangular.  Because they are flat, they neither focus nor disperse light but simply transmit it without altering its wavelength or frequency.  As a result, they are often used in opto-mechanical applications to separate sensitive components of instruments such as lasers from corrosive or hostile environments.


Plano, convex and concave windows

A window can be convex on both sides (biconvex), concave on both sides (biconcave), perfectly flat on one side but either concave or convex on the other (plano), or convex on one side and concave on the other.  As a result, all of these types of windows act as lenses. 


When a collimated beam of light travelling parallel to the axis of a biconvex orplano-convex lens passes through the lens, for example, it will be focused to a spot at a certain distance, called the focal length, behind the lens.  When the same collimated beam of light passes through a biconcave orplano-concave lens, on the other hand, the light appears to be emanating from a particular point in front of the lens. The distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a biconvex or plano-convex lens.


A window or lens that is convex on one side and concave on the other, known as a meniscus, can be either positive or negative depending on the relative curvatures of the two surfaces. A negative meniscus lens, for example, has a steeper concave surface and is thinner at the center than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface and is thicker at the center than at the periphery.


All of these types of windows, when fabricated from sapphire, offer an outstanding combination of strength, light weight and transparency.


Brewster windows

When light encounters a boundary between two media with different refractive indices, such as air and sapphire, for example, some of it is transmitted while the rest is reflected. The fraction that is reflected is dependent upon the incoming light's polarization and angle of incidence.


Brewster's angle (also known as the polarization angle) is an angle of incidence (approximately 56 degrees) at which light with a particular polarization is perfectly transmitted through a transparent surface with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized. This special angle of incidence is named after the Scottish physicist Sir David Brewster (1781–1868).


Sapphire wave plates

Sapphire Wave plates have the same material properties as other sapphire components, including hardness second only to diamonds.  They can be made in sizes as thin as 0.4 mm and <25 mm in diameter with transmitted wave front error of less than of λ/10 @632.8 nm. 


Sapphire wave plates are very similar to sapphire windows and are highly resistant to scratching, chemicals, fluctuating temperatures and shock.  A sapphire wave plate can change the polarization state of a laser beam.  For example, a ¼-wave plate can transfer linear polarization to circular and vice versa. 


Because of their outstanding physical, chemical and optical properties, sapphire wave plates are often used in the most demanding military and aerospace applications.