A beam splitter consists of several ports

A standard beam splitter consists of four ports: two input ports and two output ports. A typical lossless beam splitter has two input ports where light enters and two output ports where the split beams exit, resulting in a total of four ports (input 1, input 2, output 3, output 4) in classical and quantum optics contexts. The input light can be split into transmitted and reflected components according to the beam splitter's reflection/transmission ratio, and the outputs carry the resulting beams.

Types of beam splitters include cube, plate, pellicle, and polka dot designs, but regardless of type, the fundamental port structure remains the same for standard two-way splitting. Cube beam splitters, for example, are made from two right-angle prisms cemented together with a semi-reflective coating at the interface, while plate beam splitters use a coated flat glass plate to achieve the splitting effect. In quantum optics, the four-port model is essential for describing photon behavior, where a photon entering one input port can emerge in a superposition across the two output ports, and the transformation is represented by a unitary matrix.

This four-port configuration allows the beam splitter to function both as a splitter and, in reverse, as a combiner of beams. Thus, whether in classical or quantum applications, a beam splitter fundamentally consists of four ports, enabling controlled division and recombination of light.