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. 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, of light incident at a 45
degree angle, one half is transmitted and one half is reflected. Instead of a
metallic coating, a dielectric
optical
coating may be used. Such mirrors are commonly used as output
couplers in laser
construction. Similarly, a very thin pellicle
film may also be used as a beam splitter.

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.[1]

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.