Structure inside the beam splitter

A beam splitter is an optical device that divides an incident light beam into transmitted and reflected beams, typically constructed as a cube of two glued prisms or a coated glass plate.

Cube Beam Splitters

Cube beam splitters are made by cementing two right-angle prisms together along their hypotenuse faces, forming a cubic shape. One hypotenuse is coated with a thin-film dielectric or metallic layer to control the reflection and transmission ratio, often 50/50 for standard laboratory use. The prisms are glued using adhesives such as epoxy, polyester, or urethane, with the layer thickness adjusted to achieve the desired splitting ratio.

This design ensures that the reflected and transmitted beams exit at a precise 90-degree angle, simplifying alignment and protecting the coating from environmental damage.

Plate Beam Splitters

Plate beam splitters consist of a thin, flat glass plate coated on one surface with a reflective layer. They are usually positioned at a 45° angle of incidence, allowing part of the light to reflect and the remainder to transmit. Plate splitters are lightweight, compact, and do not require cement, making them suitable for setups where space is limited or high-power lasers are used.

Anti-reflection coatings on the second surface reduce unwanted reflections and improve optical performance.

Coatings and Materials

Beam splitters use dielectric coatings (alternating layers of high and low refractive index materials) to achieve precise splitting ratios with minimal absorption, or metallic coatings (aluminum or silver) for broader wavelength coverage but slightly higher energy loss. Some specialized designs, like pellicle beam splitters, use ultra-thin membranes to minimize ghosting effects, while polarizing beam splitters employ birefringent materials to separate light based on polarization rather than intensity.

Other Variants

Additional structures include Wollaston prisms for polarization splitting, Brewster windows, and wedged plates. Some beam splitters use dichroic coatings to split light by wavelength rather than intensity, enabling selective reflection and transmission for multi-wavelength applications. In summary, the structure of a beam splitter depends on its type and application, with cube and plate designs being the most common, and coatings or materials tailored to control the light splitting ratio, polarization, or wavelength characteristics.