There are some applications in optics where it is necessary to divide an input beam into many sub beams. The most common way of doing this beam splitting is to use a cube beam splitter or a plate beam splitter. In these cases, the spitting is performed simply by the laws of light refraction. For this same reason, this type of beam splitter is not very versatile. A beam can be divided into two, the original beam and then the reflected beam. But increasing the number of beams being generated is not straightforward. Furthermore, the quality of the beams will depend on the quality of the surfaces and the aberrations will tend to be different for the reflected and the transmitted beam.
For all these reasons described above, for many applications the diffractive beam splitter is a much better option. A diffractive beam splitter works on a different basis compared to the simple plate beam splitter. As suggested by its name, the diffractive beam splitter works by applying the principles of diffraction.
In a diffractive beam splitter an input beam is modulated by a periodic phase structure. Then, this modulation originates from multiple waves, or wavelets , interference. After some distance of propagation (or at the focus of a lens) , this interference effect among the many wavelets, which is in fact what is referred to as diffraction, gives rise to a new radiance distribution that contains several newly generated beams.
The remarkable difference between the plate beam splitter and the diffractive beam splitter is that in the latter the number of new beams is arbitrary, and determined by the design of the specific diffractive beam splitter . In addition, the new beams can be arranged in any geometrical shape and every beam in the array will have the same characteristics, or beam parameters, as the input beam. Hence the great versatility of the diffractive beam splitter.
The applications for diffractive beam splitter are numerous, including laser aesthetic skin treatment, material processing and many others . The simplest of diffractive beam splitters is the binary diffraction grating, and it has been used now for many years in optical disk media reproduction. In this case, the reading beam is divided into a main beam and two sub-beam with little power. The original beam can be used to read the data from the disc surface whilst the other beams can be used to monitor the power and beam characteristics. This element is also called a diffractive beam sampler.
Considering this beam sampler example above, diffractive beam splitters can be used in any application where beam monitoring needs to be carried out in real time and without disrupting the main functioning of the optical system.