Application Notes: C-cut and Random-Cut Sapphire Windows:

The orientation of a crystal is a vector describing a random line connecting two nodes on the lattice. Due to the anisotropic nature of crystals, the distribution and the arrangement manner of the atoms change along different directions or upon different lattice planes. The result of this is that the properties and behaviors, even of the same crystals, but with different orientations will differ to a significant extent. This is the reason of choosing the proper orientations and cutting planes of crystals is critical when using crystals to produce various components and elements. 

The Sapphire crystals utilized to produce sapphire windows of different functions will be grown and cut/sliced with an engineered orientation, and the orientation is often determined because it will optimize the crystal’s performance to achieve the intentions of interest.

The lattices within the sapphire are arranged in a hexagonal structure. When a sapphire element is produced, the direction of its inner architecture affects the functionalities of the element.

Figure 4.The Lattice Structure of Sapphire and Common Sapphire Crystal Orientations

C-Cut Sapphire Windows has an orientation index of (0001). The sapphire is cut in a direction perpendicular to the c-axis. The c-axis is the optical axis of sapphire. light projected along the direction of the optical axis will not encounter birefringence. In practice, the light will be incident with a perpendicular angle to the apertures of the sapphire windows, which means the light will travel inside the sapphire windows parallel to the optical axis, eliminating the birefringence effect. C-cut sapphire windows are often chosen for critical optical missions (e.g. laser windows). C-cut sapphire is sometimes called zero-degree sapphire, or z-cut sapphire.

There are also Random-cut Sapphire Windows. Random-cut implies the element is cut or sliced with no specific regard to directions, it might contain any orientations. This orientation is common because of lower costs and is acceptable if there are no stringent requirements about optical or mechanical qualities. However, as mentioned above, because the behavior of sapphire varies depending on orientations, random orientation might be subject to spontaneous variations of strength and other properties in the final product.

Before the cutting or slicing procedure, sometimes the manufacturer would grow the bulk sapphire crystal of a specific orientation. For example, the bulk sapphire designated to produce a c-cut sapphire window will be grown with an orientation that maximizes the utilization efficiencies of the c-planes.