What Is a Beamsplitter?
A beamsplitter is an optical element used to separate a single beam (the speed of light) into two beams.
Beamsplitters are sometimes abbreviated as BS or B/S in optical path diagrams. When light passes through a beamsplitter, it is divided into transmitted and reflected beams at a ratio designed into the Beamsplitter. The ratio can vary from 1:1 to 2:8.
In particular, a 1:1 ratio is sometimes referred to as a half-mirror. Usually, the ratio is fixed, but it is possible to make a device that can arbitrarily divide light by combining it with a wavelength plate. Also, separated beams can be recombined by passing them through a beamsplitter again.
Uses of Beamsplitters
Beamsplitters are mainly used in optical instruments, such as cameras and microscopes. Beamsplitters are available in two types: cube-type beamsplitters, which consist of two right-angle prisms laminated together, and plate-type beamsplitters, which consist of thin glass with a special coating.
Beamsplitters of the plate type are often used in fluorescence optical microscopes. The cube type is used when the optical system needs to be compact or when the amount of transmitted and reflected light paths needs to be aligned. In general, cube-type beamsplitters tend to be more expensive, while plate-type beamsplitters tend to be less expensive.
Principle of Beamsplitters
Beamsplitters can separate two beams by reflecting some of the light through a dielectric multilayer film. Beamsplitters come in cube and plate shapes, and the principle differs depending on the shape.
1. Cube Type
Cube-type beamsplitters consist of two right-angled prisms bonded together with an optical film called a dielectric multilayer. By adjusting the thickness of the dielectric multilayer film, the ratio of reflected light to transmitted light can be changed. The characteristic feature of the cube type is that the angle of incidence of light into the Beamsplitter is 0 degrees. Therefore, reflection may occur on the same axis of the incident light and return as stray light in the direction of the light source.
2. Plate Type
In the plate type, a dielectric multilayer film is deposited on a flat glass plate. Since light enters the plate at a 45-degree angle, stray light is less likely to occur as in the cube type, but since transmitted light is refracted out, there is an optical path difference with reflected light. Therefore, alignment in the optical system is important, and care must be taken because frequent removal and insertion can quickly cause axis misalignment, etc.
Types of Beamsplitters
There are two types of beamsplitters, depending on the polarization characteristics of the reflected beam
1. Non-polarizing Beamsplitter (NPBS)
Non-polarizing beamsplitters simply split the beam and have no polarity. They are used in a variety of optical applications and are indispensable in microscopes and interference optics.
Usually, the reflective side has a chromium-based coding and the transmissive side has none. Therefore, it is important to note that, if the incident path is wrong, the intensity of the two split beams will differ greatly. As mentioned above, when the ratio of transmitted light to reflected light is 1:1, it is called a half mirror and is also utilized in coaxial epi-illumination, one of the inspection lighting applications. By using coaxial epi-illumination, the positive reflected light from the object can be efficiently picked up and a clean image can be acquired.
2. Polarizing Beamsplitter (PBS)
Beamsplitters are used to separate beams into S and P polarization. They are used in semiconductor and liquid crystal lithography equipment, interference optics, and various measuring instruments, and are utilized as elements to create a polarization state from an unpolarized state by utilizing the characteristics that P-polarized light is transmitted and S-polarized light is reflected. Their extinction ratio is high, often around 1,000:1, depending on the product.
By combining a polarizing beamsplitter and a waveplate, it is possible to split light at any desired ratio; linearly polarized light passing through a quarter-waveplate can have its polarization angle changed according to the angle of the waveplate. If a polarizing beamsplitter is placed behind the optical path of the waveplate, it is possible to create a device in which the beamsplitter can split the beam at an arbitrary intensity ratio by adjusting the angle of the waveplate.
Other Information on Beamsplitters
Difference Between a Beamsplitter and a Prism
The difference between a beamsplitter and a prism is the application. A cube-type beamsplitter consists of two right-angle prisms. On the other hand, a cube-shaped prism is created by forming an optical thin film on the slope of one prism to function as a beamsplitter and joining it with the other prism. Since two prisms are utilized, the optical thin film is not in direct contact with the air, and no degradation of the thin film occurs.
Prisms are used in applications that actively use refraction to change the optical path or spectroscopy, but when utilized as beamsplitters, there is no refraction of transmitted light, and the optical element utilizes reflection and transmission.