Beam splitter, the beam splitter is some kind of window with a semitransparent mirror-like film, the beam can be splitted into two beams or more. Beam splitter will reflect part of the incident light, absorbing little part, and transmit the rest of the energy. The beam splitter is coated with multilayer dielectric film, no metal film absorption problem, the sum of the transmittance and reflectance ratio is close to 100%.
It is not used too much in laser machines. With the beam splitter, the beam is splitted into two, so the procession can be fulfilled in two different directions.
That might be used lots in laser research.