High-power kilowatt-level fiber lasers (wavelength in spectral range 1.0 – 1.1µm) are used in important applications in macro material processing, particularly for cutting and welding metal sheets. In these applications, optics (lenses, protective windows) are needed to work properly at correspondingly high laser power and power density levels. For these optics, absorption losses are of crucial importance: absorption of laser radiation leads to increased temperature and a correspondingly higher refractive index of the optics. This changes the properties of the transmitted laser beam and can therefore impair the performance of the manufacturing process. In addition, temperature increase can cause internal mechanical stress and eventually damage the optics. To avoid this, optics with the lowest possible absorption losses must be used.
The most common raw material for these optics is fused silica, which is available with sufficiently low absorption. The quality of the optics is therefore crucially dependent on the quality of the surface treatment (polishing) and especially on the absorption of the AR coatings.
In addition, the AR coatings must have the lowest possible residual reflectance because reflected radiation hitting the inner wall of the processing head can heat it up, and this can impair the performance of the manufacturing process. Sometimes, low reflectance is also needed for an additional laser pointer.
Finally, a long service life is important for the optics user in order to minimize downtime of the laser machine. This can be achieved only if absorption of the AR coatings is very low.
In this paper, we review the materials, coating technologies, and measurement techniques used for high-performance low absorption fiber laser optics manufacturing.