Beam splitter

Schematic representation of a beam splitter cube

A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.

In its most common form, a cube, it is made from two triangular glass prisms which are glued together at their base using Canada balsam. The thickness of the resin layer is adjusted such that (for a certain wavelength) half of the light incident through one "port" (i.e. face of the cube) is reflected and the other half is transmitted due to Frustrated Total Internal Reflection. Polarizing beam splitters, such as the Wollaston prism, use birefringent materials, splitting light into beams of differing polarization.

Another design is the use of a half-silvered mirror. This is a plate of glass with a thin coating of aluminum (usually deposited from aluminum vapor) with the thickness of the aluminum coating such that part, typically half, of light incident at a 45 degree angle is transmitted, and the remainder reflected. Instead of a metallic coating, a dielectric optical coating may be used. Such mirrors are commonly used as output couplers in laser construction. A half-silvered mirror used in photography is often called a pellicle mirror. Depending on the coating that is being used, reflection/transmission ratios may differ in function of the wavelength.

A third version of the beam splitter is a dichroic mirrored prism assembly which uses dichroic optical coatings to split the incoming light into three beams, one each of red, green, and blue. Such a device was used in multi-tube color television cameras and also in the three-film Technicolor movie cameras. It is also used in the 3 LCD projectors to separate colors and in ellipsoidal reflector spotlights to eliminate heat radiation.

Beam splitters are also used in stereo photography to shoot stereo photos using a single shot with a non-stereo camera. The device attaches in place of the lens of the camera. Some argue that "image splitter" is a more proper name for this device. Beam splitters with single mode fiber for PON networks use the single mode behavior to split the beam. The splitter is done by physically splicing two fibers "together" as an X.