Lenses & Protective windows for fibre lasers
Fibre optics or laser applications using wavelengths between 1060-1080 nm.
The standard wavelength of fibre lasers is 1070nm and cannot use optics built for the CO2 laser wavelengths.
In the 1060 - 1080 nm wavelength range, BK7 or FS (Fused silica/quartz) is most commonly used to manufacture optics. This material is in fact similar to that which is used in regular camera lenses. The BK7 material is considered of slightly lower quality compared to FS or quartz. Therefore FS/quartz is more common in laser applications thanks to the higher thermal stability and subsequent low reaction to rapid temperature shifts.
Yet another step up in quality is Sapphire glass, manufactured from industrially developed Sapphire. It provides excellent properties and protects against surface scratches and is even more stable during temperature shifts. It is however significantly more expensive than its counterparts, but in sensitive applications this may be worthwile.