A beam splitter is an optical element that splits incident light into two beams of the same wavelength or two beams of different wavelengths.
It is also possible to combine the separated beams.
Table of Contents
1. Types of Beam Splitters
・Cube-type Beam Splitters
・Plate-type Beam Splitters
・Others
2. Characteristics of Beam Splitters
・Unpolarizing Beam Splitters
・Polarizing Beam Splitters
・Non-polarizing Beam Splitters
・Dichroic Beam Splitters
3. Differences between P-polarization and S-polarization
There are two main types of beam splitters: cube-type and plate-type.
The cube-type beam splitter is a stable beam splitter that utilises mechanical characteristics. It is made by joining the inclined surfaces of two right-angle prisms, and a thin film coating is applied to the boundary surface to separate the light.
It is typically positioned in the optical path with an incident angle of 0°. One output beam is emitted along the optical axis, while the other is emitted at a 90° angle. The advantage of having an incident angle of 0° is that the angular difference between the incident angle and the exit angle caused by the refraction of transmitted light is minimised. Additionally, applying AR coating to both the input and output surfaces reduces ghosting. However, compared to plate-type beam splitters, it has greater thickness, resulting in a longer optical path length and increased group delay dispersion (GDD).
Plate-type splitters are lightweight, have a small installation area, and can be used in limited spaces. Typical plate-type beam splitters are positioned in the optical path so that the incident angle is 45°. A thin film is coated on one or both sides of the plate to reflect a portion of the light and transmit the rest. The transmitted light is refracted, causing a shift between the incident light and the transmitted light. On the back side of the plate-type beam splitter, a reflection known as a ghost image occurs.
Here are some other types.
Pellicle beam splitters consist of a nitrocellulose membrane mounted inside a metal housing. Since the membrane is only a few micrometres thick, the reflected light from two surfaces overlaps with the reflected light from one surface, eliminating ghosting. Additionally, the Pericle-type beam splitter minimises chromatic dispersion and chromatic aberration, making it ideal for applications requiring beam focusing. Although it is extremely lightweight, the thin film is highly delicate and requires careful handling. Due to its flammability, it is only suitable for use with low-power light.
Crystals such as magnesium fluoride (MgF2), calcite, quartz, α-BBO, and YVO4 generate polarisation in transmitted and reflected light due to the interaction between light and the optical axis of the crystal. The advantage of crystal beam splitters is that they have relatively high damage thresholds and extinction ratios compared to plate-type or cube-type polarising beam splitters. Therefore, they are suitable for use with polarised laser light sources. Crystal-type beam splitters come in two types: bulk single-crystal types and types where multiple crystals are bonded using adhesive or optical contact. Since crystals can be damaged by thermal shock, care must be taken in handling and environmental conditions.
The Brewster window is an uncoated substrate (UV-grade fused silica) that, when positioned at the Brewster angle, has a circular profile relative to its optical axis. At the Brewster angle, the P-polarised component is transmitted without reflection loss, while the S-polarised component is partially reflected. Brewster windows can be stacked and used as polarizers or to improve the polarisation ratio of a beam.
A wedge-shaped beam splitter separates a single incident light into multiple lights through reflection and refraction. The incident light gradually attenuates and becomes multiple exit lights with various exit angles. The deflection angles of these exit lights can be calculated.
Non-polarised beam splitters are designed based solely on the average component, or composite wave, of the transmitted and reflected light, rather than on the p-component or s-component polarization components. Therefore, they are designed assuming that the incident light is non-polarized light, such as natural light. Non-polarized beam splitters are used for multicoloured light and applications where the p-component does not need to be considered.
Polarization beam splitters split randomly polarized light into s-polarized and p-polarized light. They are also called PS separation beam splitters. They are used to split randomly polarized laser beams or to obtain linearly polarized light with high extinction.
Unpolarizing beam splitters split transmitted and reflected light in a specified ratio while maintaining the polarization state of the incident light. A non-polarizing beam splitter with a 1:1 splitting ratio divides the p-component (Tp) and s-component (Ts) of the transmitted light, as well as the p-component (Rp) and s-component (Rs) of the reflected light, all at the same ratio. Therefore, the 1:1 beam splitting of polarized light, such as laser light, is a typical application. Additionally, the polarization states of the transmitted and reflected light remain the same as those of the incident light.
Dichroic beam splitters separate incident light into different wavelength bands. There are various products available, such as beam combiners for specific laser wavelengths, and hot mirrors and cold mirrors with a broad-band design that separate visible light and infrared light. Dichroic beam splitters with very high extinction ratios are also used in fluorescence applications.
S-polarization is linear polarization that is oriented in the same direction as the axis of rotation when the angle of the reflecting surface is adjusted, while P-polarization is linear polarization that is perpendicular to S-polarization.
Reflected light must be considered separately because the laws of reflection differ between P-polarization and S-polarization.