Beam Splitters
Author: the photonics expert Dr. Rüdiger Paschotta
Definition: devices for splitting a laser beam into two or more beams
Alternative terms: beamsplitters, power splitters
Opposite term: beam combiners
DOI: 10.61835/mjw Cite the article: BibTex plain textHTML Link to this page LinkedIn
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 sometimes more) beams, which may or may not have the same optical power (radiant flux).
Different types of beam splitters exist, as described in the following; the most important ones are plate and cube beam splitters. They are used for very different purposes. For example, beam splitters are required for various interferometers, autocorrelators, photo cameras, projectors and laser systems. The wide range of applications implies widely varying requirements, which can be fulfilled with different types of splitters.
Important Properties
Apart from the characteristics concerning the basic function of a beam splitter – the splitting ratio – other properties of beam splitters can be important in applications:
- Some beam splitters are polarizing, others are non-polarizing. There are also devices designed for use with only one polarization direction – for example, with a laser beam as the input, which is in most cases linearly polarized.
- While some devices work only in a narrow wavelength region (e.g. around a common laser line), others are designed for broadband operation, e.g. working throughout the whole visible wavelength region. Similarly, beam splitters may operate properly only with a finite range of incidence angles.
- 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.
- The spatial configuration can be important for applications. Some require the output ports to be at 0° and 90° relative to the input beam (possibly without any beam offset of the transmitted beam), while others require two parallel outputs or some other configuration.
- For bulk-optical devices, a large open aperture is sometimes needed.
Plate Beam Splitters Based on Dielectric Mirrors
Any partially reflecting mirror can be used for splitting light beams. In laser technology, dielectric mirrors are often used for such purposes, and they are called plate beam splitters to distinguish them from cube beam splitters (see below). The angle of incidence may be 45° (as in Figure 1), leading to a 90° deflection of one of the output beams, as is often convenient. However, one can design such beam splitters for other deflection angles; they will usually work only for a limited range of angles. A wide range of power splitting ratios can be achieved via different designs of the dielectric coating.
The transmitted beam always experiences an offset (spatial shift), the magnitude of which depends of the thickness and the refractive index of the substrate. This is a problem for some applications.
For infrared applications (e.g. infrared spectroscopy), the absorption of the substrate is often a limiting factor. One often uses beam splitters with calcium fluoride (CaF2) substrates for wavelengths up to 8 μm. KBr-based beam splitters with a germanium-based coating can be used up to 25 μm wavelength, but that material is hygroscopic and must therefore be carefully protected against moisture. For the far infrared, polymer films are available.
In general, the reflectance of a dichroic mirror depends substantially on the polarization state of the beam. Such a device can even 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 within a limited wavelength range. This is most easily achieved for near normal incidence.
Dielectric beam splitters usually have a strongly wavelength-dependent reflectance. 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.
A beam splitter as shown in Figure 1 will always lead to a transverse offset of the transmitted beam, which is proportional to the thickness of the used substrate. There are so-called pellicle beam splitters with a very thin substrate, minimizing that beam offset. Note, however, that parasitic reflections from the back side (which occur even if that side is anti-reflection coated) may lead to disturbing interferences, and therefore it is often better to use some larger thickness, so that the two reflections are spatially well separated.
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. One may also use some dielectric multilayer coating or a thin metal coating on one or both of the prisms to modify the optical properties, e.g. in terms of operation bandwidth or polarizing properties.
As the interface between the prisms is typically very thin, there is only a minimal transverse offset of the transmitted beam. For some applications, this is advantageous, possibly a reason not to use a partially transparent mirror at 45° as shown in Figure 1.
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.
Generally, cube beam splitters cannot tolerate a high optical powers as plate beam splitters, although optically contacted cubes can also exhibits substantial power handling capabilities. Concerning durability and handling, cube beam splitters are often preferred over plates.
Non-polarizing Beam Splitter Cubes
Non-polarizing beam splitter cubes can be made by refining the design, normally via a multilayer coating between the prisms. The substantial angle of incidence will naturally introduce a substantial polarization dependence, but there are certain design principles which can be used to minimize such effects at least within some limited optical bandwidth.
Note that “non-polarizing” usually does not imply that such a cube is polarization-preserving. For example, if an input beam is polarized at 45° against the axis, it can generally not be expected that the output beam is still linearly polarized, since the two polarization components will in general have different phase delays, apart from somewhat different amplitudes.
Polarizing Beam Splitter Cubes
Instead of glass, crystalline media can be used, which can be birefringent. This allows the construction of various types of polarizing beam splitter cubes (polarizers) 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).
Beam Splitters with Geometric Splitting
It is also possible to split beams geometrically (aperture splitting), e.g. by inserting a highly reflecting mirror only partially into a light beam, so that some part of the light can pass. One may also use other means, such as pattern of reflecting stripes or dots on a glass surface. A common design with dots is the Polka dot plate beamsplitter.
Ad advantage over dichroic beam splitters is the small wavelength dependence of the splitting ratio. The resulting modification of the intensity profile can be tolerated in some applications (but generally not for imaging).
Beam Splitters with Multiple Outputs
While most beam splitters have only two output ports, there are also beam splitters with multiple outputs. They may be realized, for example, based on diffractive optics. Another option is to use multiple cascaded beam splitters.
There are devices which produce some number of output beams of quite similar optical powers with a certain spatial pattern (e.g. all in one row, four at the edges of a square, etc.).
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
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
Beam Splitters in Quantum Optics
In quantum optics, a beam splitter cannot be regarded as a device where the optical amplitudes at the outputs are simply given by constant factors times the input amplitude. This is essentially because there is always a second input port; even if that port remains unused, it must be considered as an input for vacuum fluctuations of the optical field. In a semi-classical picture, one can consider those vacuum fluctuations to influence the output beams, adding intensity noise and phase noise to the outputs. In a photon picture, one can see the amplitude noise in the form of partition noise – noise which results from the random “decisions” of the device to send an input photon to one output or the other one. This is also related to the fact that the shot noise level of the outputs, measured relative to the average powers (→ relative intensity noise), is increased. Similar effects occur for other types of linear attenuation of optical beams, e.g. by partial absorption.
Combining Beams
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.
See the article on beam combining for more details.
More to Learn
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Suppliers
The RP Photonics Buyer's Guide contains 184 suppliers for beam splitters. Among them:
TOPTICA Photonics
TOPTICA Photonics AG offers a wide range of optical fibers ideally suited for use with TOPTICAs lasers and FiberDock. These economically priced fibers cover a wide range of wavelengths. TOPTICA recommends to always purchase a fiber along with a laser and fiber coupler, as this ensures maximum fiber coupling efficiency. Also specialty fibers for power monitoring, beam splitting or combining are available with various ratios and also polarization maintaining.
Laserton
Laserton offers various types of beam splitters, including lateral displacement polarization beam splitters, polarization and non-polarizing beamsplitter cubes, beamsplitter plates, pellicle beamsplitters and variable beamsplitters/attenuators.
Avantier
A beam splitter is an optical component used for splitting light into two separate beams, usually by wavelength or polarity. It can also be used, in reverse, as a beam combiner, to join two light beams into one. These specialty mirrors have applications in many fields, including laser, semiconductor and photonics instrumentation.
Artifex Engineering
Artifex Engineering offers high quality custom beamsplitters tailored to your requirements. Coatings for single wavelengths or broadband are possible in the UV-NIR range. We offer beam splitters as plates, cubes and pentaprisms. Artifex offers unpolarized, non polarizing and polarizing versions for the three types. Visit our product page for more information. We look forward to your inquiry.
DayOptics
DayOptics produces polarizing cube beamsplitters with epoxy interface. Coatings are available for a wide range of operation wavelengths.
Gentec Electro-Optics
Gentec Electro-Optics offers beam splitters used as optical attenuators for measurements on high-power laser beams.
DataRay
DataRay offers two unique beam splitters: the Polarization Preserving Beam Sampler (PPBS) and the Compact Beam Sampler (CBS) for various applications.
Sinoptix
We offer on-demand fabrication for beam splitters. Cube beams splitters (CBS) or plate beam splitters. Custom coatings to reach customer needs.
Perkins Precision Developments
Perkins Precision Developments (PPD) manufactures polarizing and non-polarizing beam splitters, beam splitter cubes, dichroic laser mirrors, prism polarizers, partial reflectors and output couplers for both R & D and OEM applications. Because we utilize Ion Beam Sputtering (IBS) coating technology, our beam splitters and beam splitter assemblies are environmentally stable, thus there is no spectral shift caused by either time, moisture or temperature.
As with all of our precision laser optics and optical assemblies, PPD's laser-line and broadband beam splitters and output couplers exhibit both low absorption and high damage thresholds (>20 J/cm2!), making them ideal for use with high energy Nd:YAG and fibers lasers as well as other high-power pulsed and CW laser systems.
Custom dielectric beam splitter coatings and low-loss anti-reflection (AR) coatings can also be deposited on customer supplied substrates, including flats, curves and prisms.
Shanghai Optics
Shanghai Optics manufactures custom cubic beamsplitters, plate beamsplitters, and lateral displacement beamsplitters. All our beamsplitters are made of high quality glass, with high surface quality to allow tight tolerance on all parameters.
EKSMA OPTICS
Our Femtoline beam splitters are designed for use in femtosecond laser applications with fundamental wavelengths of Ti:sapphire and Yb:KGW/KYW lasers and their harmonics. Nd:YAG LaserLine beam splitters are designed for the Nd:YAG laser fundamental wavelength and its harmonics.
Shalom EO
Shalom EO offers the stocked and custom high power narrow band laser line and broad band Polarizing Cube Beamsplitters (PBS), with typical wavelengths of 355 nm, 405 nm, 532 nm, 633 nm, 780-808 nm and 1064 nm, damage threshold of 10 J/cm2 at 1064 nm, 10 ns, 10Hz pulses. The extinction ratio is 1000:1. The interface of these beamsplitter cubes is based on an epoxy-free optical contact bonding, which minimizes absorption and scattering loss. It is thermally stable with high transmission and minimal beam displacement. Besides the high power PBS, Shalom EO also offers the general power low cost PBS and non-polarizing beamsplitters.
Edmund Optics
Edmund Optics offers plate, cube, pellicle, polka dot, and specialty prism beamsplitters in a variety of anti-reflection coatings or substrates. Standard beamsplitters, which split incident light by a specified ratio that is independent of wavelength or polarization state, are ideal for illumination subassemblies or as one way mirrors. Dichroic beamsplitters, which split light by wavelength, are often used as laser beam combiners or as broadband hot or cold mirrors. Non-polarizing beamsplitters, ideal for laser beam manipulation, split light by overall intensity. Polarizing beamsplitters, often used in photonics instrumentation, split light by polarization state. Edmund Optics’ anti-reflection coatings are designed for the ultraviolet (UV), visible, or infrared (IR).
Cailabs
Cailabs offers beam splitting modules with excellent splitting quality and homogeneity, combined with the ability to electronically choose between several patterns. The reflective design and the high transmission enables parallel processing with ultrashort pulse lasers.
Vortex Optical Coatings
Custom designed beam splitters for visible and IR applications are a particular specialty. We feedback preliminary performance data in Excel format as part of the quoting process, the link above gives specific examples. Contact us today for expert advice on your needs.
OPTOMAN
OPTOMAN offers laser beam splitters which are optimized to split or combine high-power laser beams operating in the visible through infrared wavelengths. OPTOMAN design coatings with high accuracy splitting ratios and low GDD behavior for an optimal result in ultrafast applications. Non-polarizing beamsplitter coatings with S and P components matched to within 1% are also available.
UltraFast Innovations
UltraFast Innovations (UFI®) manufactures various beamsplitters suitable for broadband ultrashort pulses: they provide consistent performance over a broad bandwidth and low group delay dispersion (GDD). Versions for different wavelengths, splitting ratios, and angles of incidence are available.
Knight Optical
Knight Optical provides plate beamsplitters in economy, standard and precision ranges. With 30/70, 50/50 and 60/40 ratio available in the visible, NIR and Telecom wavelength regions. Custom beamsplitters are also available including polarising cube beamsplitters.
Frankfurt Laser Company
Frankfurt Laser Company offers beam splitters based on diffractive optical elements. An input beam is replicated precisely in the pattern determined by the beam split up. The input beam can be any collimated laser beam of white light source with a beam diameter larger than 100 μm and smaller than the aperture of the element.
Schäfter + Kirchhoff
Schäfter+Kirchhoff offers compact, rugged and highly efficient and fully fiber-coupled opto-mechanical units for splitting fiber coupled radiation for the configurations 1 ⇾ 2 and 2 ⇾ 2.
LASEROPTIK
LASEROPTIK can produce beam splitters for a wide range of wavelengths from the mid-IR to the ultraviolet region.
Bibliography
[1] | M. Gilo, “Design of a nonpolarizing beam splitter inside a glass cube”, Appl. Opt. 31 (25), 5345 (1992); https://doi.org/10.1364/AO.31.005345 |
[2] | M. D. Turner et al., “Miniature chiral beamsplitter based on gyroid photonic crystals”, Nature Photon. 7, 801 (2013); https://doi.org/10.1038/nphoton.2013.233 |
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