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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.

In order to optimize the cost/performance ratio of the window, it is necessary to match the grade of sapphire to the application.  Failure to do so can lead to over-specification, increasing the cost of the window unnecessarily, or under-specification, resulting in unsatisfactory performance.

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

1. 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.

The reason for this is a key property of sapphire known as birefringence, or ability to shift and redirect polarized light.  Optimum birefringence is achieved when the c-axis serves as the optic axis of the sapphire crystal.  Components of light with linear polarizations parallel and perpendicular to the optic axis have unequal indices of refraction, and will therefore be displaced by different amounts.

A Sapphire window has an optical transmission range from 0.15 - 5.5µm. Sapphire is considered to have a weak birefringence of 0.008 at right angles to the C-axis, or optic axis. The birefringence is eliminated along the C-axis, so for certain applications C-axis (C-plane) sapphire should be specified.

2. 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,willbegin to deform at a much lower temperature than a sapphire window.

UV quality sapphire windows also will not darken when exposed to high intensity UV light, unlike many standard windows that absorb UV rays and are then rendered useless and need to be replaced, Sapphire windows are therefore suitable for applications where UV radiation is present.

3. 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. A dig is a defect on a polished optical surface that is nearly equal in terms of its length and width, such as a pit. Sapphire windows can be polished to any Scratch-Dig Specification, including the extreme Scratch-Dig free quality often called “0-0”.

New buyers tend to over-specify the Scratch-Dig parameter, which results in paying a significant amount of money for a specification that they do not really need. The method used to measure the Scratch-Dig severity of a component needs to be specified in accordance with the conditions encountered by theintended application. By using the wrong illumination, for example,Scratches and Digs can look worse than they really are

4. 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 interferometer or reference test plate by forming an interference pattern,which is done by varying the optical path length across the surface.

Surface Flatness specifications are usually presented in fractions of a wavelength, typically at the helium/neon laser line of 632.8 nm, even though it is measured at a different reference wavelength. The surface flatness can be measured using any stable wavelength reference. In general, the shorter the wavelength used for the measurement, the better the resolution that can be achieved.

In the event that you do not have the tools/equipment to image the fringesat the chosen wavelength, the next best choice is to perform the measurement where the human eye has its best sensitivity, i.e. in the green spectrum. Therefore, a common wavelength for measuring flatness is the Hg line of 5421Å due to its excellent green contrast, which can be easily perceivedby the human eye.