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Definition: devices for splitting a laser beam into two or more beams
A beam splitter (or beamsplitter, power splitter) is an optical device which can split an incident light beam (e.g. a laser beam) into two or more beams, which may or may not have the same optical power. Different types of beam splitters exist, as described in the following, and are used for very different purposes. For example, beam splitters are required for interferometers, autocorrelators, cameras, projectors and laser systems.
Types of Beam Splitters
Any partially reflecting mirror can be used for splitting light beams. In laser technology, dielectric mirrors are often used for such purposes. The angle of incidence, also determining the angular separation of the output beams, may be 45° (as in Figure 1), which is often convenient, but it can also have other values, and influences the characteristics of the beam splitter. A wide range of power splitting ratios can be achieved via different designs of the dielectric coating.
In general, the reflectivity of a dichroic mirror depends on the polarization state of the beam. Such a device can be optimized to function as a thin-film polarizer, where in some wavelength range a beam with a certain polarization can be nearly totally reflected, while a beam with different polarization is largely transmitted. On the other hand, it is also possible to optimize for a minimized polarization dependence to obtain a non-polarizing beam splitter. This is most easily achieved for near normal incidence.
Dielectric beam splitters can also have a strongly wavelength-dependent reflectivity. This can be used for dichroic beam splitters (→ dichroic mirrors), which can separate spectral components of a beam. For example, such a device may be used after a frequency doubler for separating the harmonic beam from residual pump light. The separation may occur based on the difference in wavelength or polarization.
Beam Splitter Cubes
Many beam splitters have the form of a cube, where the beam separation occurs at an interface within the cube (Figure 2). Such a cube is often made of two triangular glass prisms which are glued together with some transparent resin or cement. The thickness of that layer can be used to adjust the power splitting ratio for a given wavelength.
Instead of glass, crystalline media can be used, which can be birefringent. This allows the construction of various types of polarizing beam splitter cubes such as Wollaston prisms and Nomarski prisms, where the two output beams emerge from the same face, and the angle between these beams is typically between 15° and 45°, i.e., much smaller than shown in Figure 2. Other types are the Glan–Thompson prism, and the Nicol prism, the latter having a rhombohedral form (i.e., not that of a cube).
It is also possible to use a multilayer coating within a cube. This further expands the possible device characteristics, e.g. in terms of operation bandwidth or polarizing properties.
Beam splitter cubes can be used not only for simple light beams, but also for beams carrying images, e.g. in various types of cameras and projectors.
Fiber-optic Beam Splitters
Various types of fiber couplers can be used as fiber-optic beam splitters. Such a device can be made by fusion-combining fibers, and may have two or more output ports. As for bulk devices, the splitting ratio may or may not strongly depend on the wavelength and polarization of the input.
Fiber-optic splitters are required for fiber-optic interferometers, as used e.g. for optical coherence tomography. Splitters with many outputs are required for the distribution of data from a single source to many subscribers in a fiber-optic network, e.g. for cable-TV.
Other types of beam splitters are:
- metal-coated mirrors (e.g. half-silvered mirrors), where the metallic coating is made thin enough to obtain partial reflectance
- pellicles, which are thin membranes, sometimes used in cameras
- micro-optic beam splitters, often used for generating multiple output beams
- waveguide beam splitters, used in photonic integrated circuits
Apart from the characteristics concerning the basic function of a beam splitter – the splitting ratio as a function of wavelength and polarization – other properties of beam splitters can be important in applications:
- The optical losses vary significantly between different types of devices. For example, beam splitters with metallic coatings exhibit relatively high losses, whereas devices with dichroic coatings may have negligible losses: the total output power nearly equals the input power.
- The losses may also be related to the damage threshold, which can be important particularly for use with Q-switched lasers.
- For bulk-optical devices, a large open aperture is sometimes needed, and it can be convenient in optical engineering to have output beams only in the direction of the input beam and in a perpendicular direction. Finally, beam splitters may operate properly only with a finite range of incidence angles.
Any beam splitter may in principle also be used for combining beams to a single beam. This can be considered as operation with the reversed direction of time. However, the output power is then not necessarily the sum of input powers, and may strongly depend on details like tiny path length differences, since interference occurs. Such effects can of course not occur e.g. when the different beams have different wavelengths or polarization.