Beam Splitters

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.

While most beam splitters have a fixed splitting ratio, variable beam splitters allow for the continuous adjustment of the ratio between reflected and transmitted power. These are often realized as rotating disks with a gradient dielectric or metallic coating, where the local reflectance changes with the angular position on the disk. Another common approach, particularly for linearly polarized laser beams, involves the combination of a rotatable half-wave plate and a polarizing beam splitter. Rotating the waveplate changes the polarization direction of the input beam relative to the axes of the beam splitter, thereby continuously tuning the power distribution between the two output ports according to Malus' law.

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.

• The surface flatness and surface quality (e.g. scratch–dig specification) of a beam splitter affect wavefront distortion and scattering, which can be critical for imaging and interferometric applications.

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 on 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 substrate. There are so-called pellicle beam splitters with a very thin substrate, minimizing that beam offset. Note, however, that parasitic reflections from the backside (which occur to some extent 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.

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 two prisms are typically right-angle prisms joined at their hypotenuse faces, forming a 45-degree internal interface for the coating. The thickness of the joining resin or cement 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 exhibit substantial power handling capabilities. Concerning durability and handling, cube beam splitters are often preferred over plates.

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.

An 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).

Metallic plate beam splitters (e.g. with Inconel coatings) provide an alternative broadband and largely polarization-insensitive splitting, but at the cost of substantial absorption losses on the order of 30%.

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.).

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

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.

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.

For professional purchasing, our buyer's guide for beam splitters explains additional buyer-oriented technical background and suggests selection criteria: Splitting ratio, Wavelength range, Device type, Polarization properties, Clear aperture, Damage threshold, Surface flatness, Beam deviation, Surface quality. It also lists 190 suppliers of beam splitters. Some of these show their product descriptions as sponsored content:

⚙ hardware🧩 accessories and parts🧴 consumables🔧 maintenance, repair📏 metrology, calibration, testing💡 consulting🧰 development

Hangzhou Shalom EO offers stocked and custom beamsplitter cubes including high power polarizing beamsplitter cubes, polarizing beamsplitter cubes, and non-polarizing beamsplitter cubes.

With superb production craft, Shalom EO is capable of delivering beamsplitter cubes with an extinction ratio of an extensive range between 300:1 to 10000:1 and precise dimensional tolerance. Low loss AR coatings of various laser line wavelengths and broadband spectra can be deposited. Shalom EO's beamsplitter cubes are available in cemented or optical-contacted structures. For the high-power laser polarizing beam splitter cubes, Shalom EO utilizes the adhesive-free, optical-contacted method to assemble the constituent prisms, the cubes are high-precision laser-grade polished to ensure minute wavefront distortion and allow optical contact, the beamsplitters exhibit a high laser induced damage threshold LIDT of > 15 J/cm² @ 1064 nm, 20 ns, 20 Hz.

In addition, Hangzhou Shalom EO also supplies a wide selection of dichroic filters including longpass dichroic filters, shortpass dichroic filters, dichroic filters for RGB light shows, laser line dichroic filters (various HR or HT coatings at specific wavelengths), and other categories encompassing dichroic bandpass filters, dichroic band stop (notch) filters.

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Perkins Precision Developments (PPD) offers a range of precision laser optics and optical assemblies, including:

• Polarizing beam splitters
• Beam splitter cubes
• Dichroic laser mirrors
• Prism polarizers
• Partial reflectors
• Output couplers

These products are suitable for both R&D and OEM applications. Utilizing ion beam sputtering (IBS) coating technology, PPD ensures that their beam splitters and assemblies are environmentally stable, with no spectral shift due to time, moisture, or temperature.

Key features of PPD's products include:

• Low absorption and high damage thresholds (> 20 J/cm²)
• Ideal for high energy Nd:YAG and fiber lasers, as well as other high-power pulsed and CW laser systems
• Space-flight qualified

PPD also offers custom services such as dielectric beam splitter coatings and low-loss anti-reflection (AR) coatings on customer-supplied substrates, including flats, curves, and prisms.

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DataRay offers two unique beam splitters: the Polarization Preserving Beam Sampler (PPBS) and the Compact Beam Sampler (CBS) for various applications.

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LASEROPTIK can produce beam splitters for a wide range of wavelengths from the mid-IR to the ultraviolet region.

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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.

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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.

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Gentec Electro-Optics offers beam splitters used as optical attenuators for measurements on high-power laser beams.

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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).

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Torrent Photonics offers different kinds of beam splitters:

• Plate and cube beamsplitters
• Pellicle beamsplitters with superior wavefront transmission qualities while preventing ghosting and beam offset, polarizing and non-polarizing types; premounted versions in cubes for use with our lens tube and cage systems
• Dichroic beamsplitters (harmonic separators)

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Avantier designs and manufactures a comprehensive range of plate and cube beamsplitters, including polarizing, non-polarizing, and dichroic configurations, for laser systems, semiconductor manufacturing, biomedical imaging, and photonics applications. Each is fabricated from high-quality optical glass with advanced anti-reflection coatings to minimize stray light and ensure high transmission and reflection efficiency, with fully custom solutions available for unique system requirements.

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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.

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At Vortex we can design and manufacture any custom beamsplitters, beam-combiners, and dichroic filters between 300 nm to 6000 nm whether they be cubes, plates or polarising filters. We are able to provide comprehensive spectral reports for any angle of incidence for all enquiries.

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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.

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We offer on-demand fabrication for beam splitters. Cube beams splitters (CBS) or plate beam splitters. Custom coatings to reach customer needs.

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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.

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Knight Optical offers stock and custom plate beamsplitters and beamsplitter cubes.

The plate beamsplitter quality range is coated on N-BK7 or equivalent substrates, our standard range is coated on white float glass and our economy range is coated on a soda-lime float glass substrate.

The polarising cube beamsplitter types will split light of s- and p-polarization states differently, while the non-polarizing cube beamsplitters are designed to split light by a specified ratio, at a specific wavelength range, irrespective of it's polarisation state.

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OPTOMAN offers laser beamsplitters 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.

In-stock non-polarizing beam splitters are available on OPTOSHOP.

Related buyer's guides: mirrors.

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