How can you split a laser beam?

Beam splitters are optical components that partially reflect a laser beam, so the initial laser beam is split in two: a reflected beam and a transmitted beam. 

Beam splitters are also commonly used in interferometry, the most famous example being the famous Michelson-Morley experiment, which proved that light propagates at the same speed no matter the direction.

Some beam splitters are also called "beam samplers" or "optical pick-offs" when the reflected beam is a very small fraction of the transmitted beam. They sample a laser beam while it is in use, allowing for real‑time beam diagnostics.

Because industrial and scientific applications have varied requirements, many different beam splitter designs have been invented over the years. Let us go through the main ones, taking note of their main advantages and disadvantages.

Conventional beam splitters

Conventional beam splitters have a partially reflective surface. Depending on the angle, wavelength and polarization of the incident light, the transmission and reflection ratios will vary.

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Plate beam splitters

Plate beam splitters are thin windows coated with a semi-reflective coating. They are optimized for specific angles of incidence (usually 45 degrees), and can be manufactured with a variety of transmission and reflection ratios. They are usually not too wavelength-dependent, which limits the amount of chromatic aberration they induce.

Plate beam splitters are thin, but not negligibly so. Because of this, the transmitted and reflected beams have different optical path lengths, and there can be some lateral displacement. Also, a “ghost” reflection is sometimes created on the second surface of the plate. Many manufacturers add a small angle in the glass, making them as wedges instead of perfectly parallel windows so the ghost reflections get deflected out of the main beam's path.

Cube beam splitters

To create a cube beam splitter, a cube of transparent material is split in two diagonally. A specially selected semi-reflective coating is applied at the juncture between the two halves, which are then glued back together. This allows for a semi-reflective mirror of negligible thickness, thereby eliminating ghost images and image displacement.

In addition, the fact that the reflective surface is safely enclosed in the middle of a cube means that cube beam splitters are typically quite resistant, making them well-suited to more rugged environments.

However, they tend to be less resistant than plate beam splitters for high optical powers and are usually not suggested when non-collimated beams are at play.

Geometric beam splitters

Geometric beam splitters split incident light using a very simple concept.

Many small mirrors are placed on a transparent surface, such that only half of the surface is covered. Therefore, half of the light goes through, while the other half is reflected. By putting more or less mirrors, other splitting ratios can be achieved.

Because the small mirrors are often round in shape, these types of beam-splitters are often nicknamed “polka‑dot” beam splitters.

Thanks to their simple design, polka-dot beam splitters are compatible with a very broad range of wavelengths and function well with any reasonable angle of incidence.

Diffraction gratings

Beam splitters usually have just two output ports. One for the transmitted beam, and one for the reflected beam. The beam splitter industry’s focus has therefore understandably gone to this use case.

However, it is possible to “split” the beam into more exotic configurations using diffraction gratings. One surprising example is the diffraction gratings included as a security feature in Canadian money. If light is shone through them, the resulting pattern will write the denomination of the bill.

High-power beam samplers

At Gentec-EO, we designed beam samplers for high-power laser beams, based on the plate beam splitter principle. Using two uncoated glass wedges to achieve very high damage thresholds, the BA samplers divert a very small fraction of the laser beam to the output port, while maintaining the laser beam's original polarization and profile. This is the only beam splitter type we manufacture, but our expert staff uses many different beam splitting technologies routinely for internal purposes and will be happy to help should you have any questions.