Beamsplitters: Cube or plate?

All you need to split a beam into two beams is a single partially reflective plane surface, and something to hold it up. That surface can be the first surface of a plate, or the interior diagonal surface of a cube formed by mating two right-angle prisms. In this installment we’ll examine the advantages of each.

Cube type

A beam splitter cube is robust. It can be glued on its broad square base if desired, thus requiring no surrounding structure. Its exterior surfaces are anti-reflection (AR) coated, and those coatings are durable and easy to clean. Its beamsplitter coating is applied to the hypotenuse of only one of the two prisms, then glued to the other prism. Thus buried, it is protected from dust, stains, and scratches; it is also supported stiffly by the surrounding prisms and thus retains a very accurate surface figure regardless of vibration, thermal stress or mounting stress. And because of its cubic form with faces square to the beams it is easy to mount and align, and doesn’t displace the transmitted beam.

Plate type

A single platebeamsplitter isof course less expensive than a beamsplittercube. It is also lighter in weight. These factors alone can be sufficient to offset the advantages of a beamsplitter cube. Another factor may make the plate beamsplitter your best choice: It has no optical cement layer. The optical cement that is used to mate the prisms together in most beamsplitter cubes has a lower threshold for high power laser damage and degradation in ultraviolet light than the coatings or the glass.

But a plate does require more elaborate mounting and its reflected wavefront shape varies more than the cube. And unlike the cube, the back side does create a (very weak) ghost reflection despite being AR coated.

Orientation

Whichever configuration you use, there is a preferred orientation.

• In the plate beamsplitter, the BS coating should generally be the first surface encountered by the reflecting beam; this minimizes ghost reflections and wavefront distortion.
• In a cube beamsplitter, it is important to note that the beam portion reflected from the hypotenuse coating can either go through the glue twice, or not at all, depending on its orientation. (The beam portion transmitted straight through goes through the glue once in any case.) Cube beamsplitters have a mark on one of the ground surfaces indicating the preferred (glueless) side.

The right solution

Whether you call them beam splitters, beamsplitters, or beam-splitters, Tower Optical has the expertise to guide you to the best solution for your situation, and the facilities and experience to produce them to your specification.Please contact us.