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.  

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. Synthetic sapphire is water clear and pure, making it ideally suited for chemically resistant windows. 
 

Here are some of the different types of windows into which sapphire can be fabricated.

• Flat, transparent windows
• Plano convex and concave windows
• Brewster windows
• Sapphire wave plates

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

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

Matching the Grade of Sapphire to the Application

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.  Some applications, such as precision optical components, however, demand the highest level of quality while others such as mechanical applications permit use of less perfect material.

Why are there different grades?

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.

How sapphire is graded?

Sapphire quality is graded based on its optical and physical properties. There is no single globally accepted grading system that is used by all manufacturers of synthetic sapphire. 

In another commonly used grading system for sapphire, shown below, grades 1 through 4 are considered to be of optical quality while grades 5 and 6 are considered technical quality.

• Grade 1:Free of insertions, block boundaries, twins, micro-bubbles and scattering centers.
• Grade 2: Free of insertions, block boundaries and twins. Individual scattering centers (micro-bubbles < 10 µm located no closer to each other than 10 mm) are allowed.
• Grade 3: Free of insertions, block boundaries and twins.  Individual bubbles < 20 µm located no closer to each other than 10 mm are allowed.
• Grade 4:Free of insertions, block boundaries and twins. Bubbles < 20 µm located no closer to each other than 2 mm, well as bubble clusters (which may include individual bubbles to 50 µm) < 200 µm scattered no closer to each other than 10 mm within an effective volume of 20x20x20 mm are allowed.

Four Parameters to Consider when Specifying Sapphire Windows

Sapphire windows are available in a number of different grades, or quality levels, each designed to meet the needs of a particular application.  For example, the highest grades can be used for the most demanding optical applications, while lower grades are suitable for structural or mechanical applications.

The following are four important parameters to consider when specifying sapphire windows and wave plates.

1. Optical properties

2. Optical Transmission

3. Surface Quality

4. Surface Flatness

Optical properties: 

Both natural and synthetic sapphire have the same crystalline structure.  Each crystal has three axes of symmetry (a-axis, b-axis and c-axis).  The most desirable optical properties can be achieved when light is transmitted along the c-axis.  As a result, sapphire windows are usually cut and polished in such a way that the c-axis is aligned with the light source.

Optical Transmission: 

Sapphire windows have a transmission range from UV to Mid-IR (0.15-5.5µm).  They can be made extremely thin and still maintain tight optical specs, thus allowing for very high transmissions. In high power applications,transmission loss in a sapphire window is of less concern than in a standard glass or quartz window due to the high thermal stability and thermal conductivity of sapphire. A standard window, for example,will begin to deform at a much lower temperature than a sapphire window. 


Surface Quality: 

Surface quality specifications for optical components are typically defined by allowable scratches and digs on a polished surface, with the lowest numbers designating the highest quality. A scratch is a defect on a polished optical surface whose length is many times its width. 






Surface Flatness: 
Surface Flatness specifications for optical components are typically defined in terms of how accurately an optical surface conforms to its intended shape.  It is usually measured with a laser interference or reference test plate by forming an interference pattern,which is done by varying the optical path length across the surface.

SAPPHIRE OPTICAL WINDOWS PERFORM IN DEMANDING APPLICATIONS

Sapphire Windows manufactured from single crystal sapphire are ideal for demanding applications such as laser systems, where high pressures, high temperatures, high thermal loads, vacuum conditions, scratch/wear resistance, low friction and corrosive atmospheres are important considerations.  As a result, they are often used in research, medical, aerospace and military applications.  Here are some reasons why.

• Except for diamonds, sapphire is the hardest crystal known to man.  On the Mohs scale of hardness, which ranks materials from softest (1) to hardest (10), it is rated 9.  
• Because of its structural strength, sapphire optical windows can be made much thinner than other common dielectric optical windows with improved transmittance as a result.
• Sapphire opticalwindows have a very wide optical transmission band from UV to near-IR (0.15-5.5µm). Because of their high transmittance from 150 nm to 5 µm, they are much preferred in aerospace applications.

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