N-BK7 and special glasses
is a frequently used optical glass, a chemically stable borosilicate crown glass, molten in ceramic vessels to be free of Platinum impurities. Normally N-BK7® has a more instable polishing and is softer than FS, so the surface can be damaged easier. Unfortunately often defects will only become visible after the cleaning process.
Additionally N-BK7® has higher losses through higher absorption and scattering and therefore cannot be used for high power applications.
These are the reasons why LASEROPTIK recommends using Fused Silica instead and offers N-BK7® or equivalent substrates only on request.
Low expansion glasses
If applications require low thermal expansion and FS will be too expensive, commonly the following materials are used: Borofloat®, Duran®, Pyrex® (Corning 7740). Glass materials with a thermal expansion coefficient lower than FS and close to zero are for example ULE® (Corning 7972) or Zerodur®. But all glasses have a limited transmission range (see table "Characteristics of the most common substrates").
Glasses with special refractive index
LASEROPTIK can supply all sizes of nearly all common substrate materials, like all types of flint glass or crown glass. For these normally we use optical glasses from Schott, Ohara or Hoya. On customer request we can of course also work with the material of other manufacturers. We can for example provide a refractive index between nd = 1.487 (S-FSL5) and nd = 2.00 (S-LAH 79).
Optical filter glasses
We can also supply and/or coat on optical filter glass (mainly from Schott), which works as natural filter, for example as band-, long- or short-pass filter. A number of different colorants and concentrations can be provided on request.