These optics are used for the real-time measurement of laser power in applications in which the polarisation of the laser beam can change. For example, |Rs-Rp|<0.2% applies to the reflectance values, thus the ratio of s and p polarisation does not change during beam splitting.
The manufacture of polarisation-independent beam splitters is a complex process in which the coating process must be carefully monitored. Usually, the coatings are deposited in an ion-assisted deposition (IAD) or ion beam sputtering (IBS) system. This produces coatings with a very high packing density, which in turn allows the optics to retain their properties even in a vacuum or under severe temperature fluctuations.
The real-time monitoring of laser power is used in applications that require permanently stable performance. If irregularities are detected, the pump power can be adjusted to compensate for them. The measurement is performed on a decoupled part of the laser beam. Even slight irregularities in the polarisation of the laser would falsify the measurement results of a polarising beam splitter.