Beam splitters: applications and principles
A beam splitter is an optical element that separates one beam (speed of light) into two.
When light is passed through the beam splitter, it can be split into transmitted light and reflected light at the ratio designed for the beam splitter. The ratio varies from 1: 1 to 2: 8, but the 1: 1 one is sometimes called a half mirror.
The separated light can also be recombined through the beam splitter.
Beam splitters are sometimes abbreviated as BS or B / S in optical path diagrams.
Usage of beam splitter
There are two types of beam splitters: a cube type with two right-angle prisms bonded together, and a plate type with a special coating on thin glass.
It is mainly used for optical equipment such as cameras and microscopes.
Plate-type beam splitters are often used for fluorescence optical microscopes, and cube-type beams are used when you want to make the optical system compact or when you want to align the optical path amounts of transmitted light and reflected light.
In general, the cube type tends to be more expensive, and the plate type tends to be cheaper.
Beam splitter principle
A beam splitter divides one beam into reflected light and transmitted light, and there are cube type and plate type.
Two right-angle prisms are attached to an optical film called a dielectric multilayer film on a cube-type adhesive surface. The ratio of reflected light to transmitted light can be changed by adjusting the thickness of the dielectric multilayer film. The feature of the cube type is that the angle of incidence of light on the beam splitter is 0 degrees. Therefore, reflection occurs on the same axis as the incident light, and it may return as stray light in the direction of the light source.
- Plate type The
plate type has a mirror-like shape at first glance, with a dielectric multilayer film deposited on a flat glass plate. Since the plate type emits light at 45 degrees, stray light like the cube type is unlikely to occur, but the transmitted light is refracted and comes out, so there is an optical path difference from the reflected light. Therefore, alignment in the optical system becomes important, and it is necessary to be careful because frequent insertion and removal will cause the axis to shift immediately.
Beam splitter type
There are two types of beam splitters.
- Nonpolar Beam Splitter (NPBS)
Used to split the beam into two. Each output is a simple split of the input and has no polarity. It is used in various applications of optical equipment and is indispensable for microscopes and interferometric optical systems.
Normally, the reflective surface has a chrome-based coating, and there is nothing on the transmissive side. Therefore, it should be noted that if the incident path is wrong, the intensities of the two divided beams will increase. As mentioned above, when the ratio of transmitted light to reflected light is 1: 1, it is called a half mirror and is also used for coaxial epi-illumination, which is one of the inspection lighting. By using coaxial drop-off lighting, it is possible to efficiently pick up specularly reflected light from an object and obtain a beautiful image.
Polarized Beam Splitter (PBS) A beam splitter for splitting a beam into S-polarized light and P-polarized light. It is used in semiconductor / liquid crystal exposure equipment, interference optics, and various measuring instruments. P-polarized light is transmitted, and S-polarized light is reflected, which is used as an element to create a polarized state from an unpolarized state. Its extinction ratio is high, and although it depends on the product, it is often about 1000: 1.
Difference between beam splitter and prism
It is a difference in usage.
The cube beam splitter consists of two right angle prisms. An optical thin film is formed on the slope of one prism to function as a beam splitter, and a cube shape is created by joining with the other prism.
Since two prisms are used, the optical thin film does not come into direct contact with air, and the thin film does not deteriorate.
Prism is used for applications such as changing the optical path by actively using refraction and splitting, but when used as a beam splitter, there is no refraction of transmitted light, and it becomes an optical element that uses reflection and transmission. It transforms.
It can be said to be an interesting example where the physical meaning changes just by changing the usage.
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