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.