A beam splitter has several functions in splitting ports

A beamsplitter is a common optical component that partially transmits and partially reflects an incident light beam, usually in unequal proportions. a laser beam) into two (or sometimes more) beams, which may or may not have the same optical power (radiant flux). For our purposes it can simply be viewed as a device that has two input and two output ports, which we label with ∣ 0 ⟩ ∣0⟩ and ∣ 1 ⟩ ∣1⟩. The beam splitter has played numerous roles in many aspects of optics. [pdf]