Index of Refraction
for glass range from 1.4 - 4.0; visible glasses have lower ranges than those optimized for the infrared. For example, N-BK7 (a popular visible glass) has an index of 1.517, whereas, germanium (a popular IR glass) has an index of 4.003.
The index of refraction of an optical glass is an important property because the power of an optical surface is derived from both the radius of curvature of the surface and the difference in the index of refraction of the media on either side of the surface. Inhomogeneity, specified by the glass manufacturer, describes the variation of index of refraction in a glass. It is specified according to different classes, where class and inhomogeneity are inversely related – as class increases, inhomogeneity decreases.
|Inhomogeneity Class||Maximum Permissible Variation of Index of Refraction|
|0||+/- 50 x 10-6|
|1||+/- 20 x 10-6|
|2||+/- 5 x 10-6|
|3||+/- 2 x 10-6|
|4||+/- 1 x 10-6|
|5||+/- 0.5 x 10-6|
Another material property of glasses is the Abbe number, which quantifies the amount of dispersion that a glass exhibits. It is a function of the refractive index of a material at the f (486.1nm), d (587.6nm), and c (656.3nm) wavelengths:
vd = nd - 1 / nf - nc
Typical values of Abbe number range from 25 – 65. Glasses with an Abbe number greater than 55 (less dispersive) are considered crown glasses and those with an Abbe number less than 50 (more dispersive) are considered flint glasses. Due to dispersion, the index of refraction of a glass varies with wavelength. The most notable consequence of this is the fact that a system will have slightly different focal lengths for different wavelengths of light.
Laser Damage Threshold
Laser damage threshold indicates the maximum amount of laser power per area that a surface can withstand before it is damaged. Values are provided for pulsed lasers and continuous wave (CW) lasers. Laser damage threshold is a very important material specification for mirrors since they are used in conjunction with laser products more than any other optic; however, any laser-grade optic will provide a threshold. For example, consider a Ti: Sapphire Laser Mirror with damage threshold ratings of 0.5 J/cm2 @ 150 femtosecond pulses and 100kW/cm2 CW. This means that the mirror can withstand energy densities of 0.5J per square centimeter from a high repetition femtosecond pulsed laser or 100kW per square centimeter from a high power CW laser. If the laser is concentrated on a smaller region, then the proper consideration must be taken to ensure the overall threshold does not exceed the specified values.
Though a host of additional manufacturing, surface, and material specifications exist, understanding the most common optical specifications can greatly alleviate confusion. Lenses, mirrors, windows, filters, polarizers, prisms, beamsplitters, gratings, and fiber optics share a variety of attributes, therefore, knowledge of how they relate to each other and can affect overall system performance helps to choose the best components for integration into optics, imaging, or photonics