Number of layers
Anti-Reflection coatings (AR)
Number of layers
A simple single layer (EAR) decreases the natural reflection of an uncoated surface, best if the refractive index of the layer is near to the square root of the index of the substrate. For Fused Silica (FS) no coating material can be found which fits perfectly. The best matching material is MgF2.
By using two layers of different coating materials the reflection can be minimized for one single wavelength (VAR).
Anti-reflection coatings even for broadband areas (BBAR) can be realized through an increased number of layers. The larger the bandwidth the higher the losses in transmission due to the increased reflection.
Partial Reflectors (PR)
Number of layers
Most PR coatings are produced by alternatively applying layers of two distinct coating materials with different refractive indices.
By increasing the number of layers the transmission decreases and the reflection increases.
You can achieve any transmission value by changing the thickness of some of the layers.
High Reflectors (HR)
Number of layers
To reach high reflection over a broad wavelength area you have to increase the number of layers. An easy way to realize broadband HR (BBHR) is to stack single HR systems on each other. The HR system for the most critical wavelength (with the highest laser power and highest reflection) has to be on top.