A short introduction to light beam splitting with emphasis on a diffractive beam splitter, it’s unique qualities and advantages
Introduction to beam splitting
Beam splitters are optical devices who, when used in an optical system, split the incident input beam into two or more beams at the output.
Different types of beam splitters are used to control different parameters of the output beams (such as optical power ratio, polarization, number of spots and separation angle between them) to achieve various desired outputs.
Types of beam splitting optics
The most common beamsplitter design is perhaps the cube beam splitter, constructed of two prisms attached together to form a cube structure, this beam splitter splits the input light into two beams with 90 degrees angle between them and no beam shift. This type of beam splitter typically provides a 50/50 Reflection/Transmission (R/T) ratio (but not limited to it) and can come in either polarizing or non-polarizing designs.
Another common beamsplitter option for splitting the input light beam into two output beams, is the plate beamsplitter. This device consists of a flat optical window, one side coated with partial reflective coating and the other side typically coated with anti reflective coating to avoid undesired Fresnel reflections. The angle between the transmitted and reflected beams is 90 degree, and the R/T ratio can be controlled by the deployed coating. In this case the transmitted portion of the beam does suffer from a slight beam shift.
The third type of beamsplitter, used in completely different applications, is the diffractive beam splitter, also known as a “multispot” (MS) or “dot generator”. Unlike the former types of beamsplitters, diffractive beam splitters are not limited in their functionality to generating only 2 output beams. They can be designed to generate multiple output beams in a desired configuration from a single input beam. Diffractive beam splitters are non polarizing optics, that operate at the discrete wavelength for whom they were designed (and the narrow bandwidth around it +-1.5%).
Diffractive beam splitter principle of operation
A diffractive beam splitter is a periodic, grating-like, diffractive optical element (DOE). It splits a single input laser beam into any number of output beams in any desired predefined arrangement and separations. The output beams are sometimes referred to as the diffraction orders.
These split beams, or diffraction orders, have the exact same characteristics as the original input beam:
- Beam size,
- Beam divergence,
- Intensity distribution,
- Polarizarion, and
- Beam quality (M2)
The DOE beam splitter uses microstructures fabricated on a flat optical window to create a phase delay pattern on the beam propagating through it and thus utilizes the wave nature of light to split the output as desired. These structures are designed to a specific wavelength, and if used with a different one will cause a reduction in efficiency and change the diffraction angles.
Typical applications of diffractive beam splitters
The greatest advantage of a diffractive beam splitter laser system is its ability to massively increase an optical system’s throughput by a factor equal to the number of diffracted orders.
Some of the classic applications who have managed to make the most of these unique components use them to generate a 2-dimensional matrix of spots to cover an entire large area work surface. These include:
Another application using a 2-D multispot configuration is structured light laser projection for 3D sensing or flash LiDAR.
Other applications use a 1-dimensional array of spots typically with stage movement or a scanner to enable multiple processing. Some of the more common applications in this sector includes:
- Laser scribing
- Laser dicing
- Laser cutting
- Laser welding (for example in double spot welding)
Another application using a 1-D multispot configuration is the unique diffractive beam sampler which offers real time sampling of the laser beam along the optical path.
Diffractive beam splitters, like all DOE, are planar and thin, window like optics, thus are easy to integrate into any system.
