A Guide to Beamsplitters
Applications for beamsplitters range from research and development to automobiles and consumer electronics. The Laser Interferometer Gravitational-Wave Observatory (or LIGO) uses beamsplitters to detect gravitational waves, precision measurement systems depend on them, and high-end iPhones use them in FaceID. In short, beamsplitters have a host of uses in myriad applications. Beamsplitter selection is complicated by there being different types of splitters with different functionality and form factors. In this beamsplitter guide we aim to summarize the role of a beamsplitter in optical applications and address some key considerations when selecting one.
What is a Beamsplitter?
A beamsplitter is an optical device that divides an incident beam of light into two parts. One part is transmitted through the splitter, the other is reflected. Placing the splitter or reflecting surface at an angle relative to the incident light ensures reflected light goes off in a direction where it can be put to good use rather than going back to the source.
Beamsplitting can done on both coherent (laser) light and incoherent (normal) light. It’s also worth noting that a beamsplitter works both ways: it can combine two beams incident from different directions into a single output beam. But “beamcombiner” doesn’t roll off the tongue quite so easily!
Beamsplitting is done in one of three ways:
1. By Intensity
Splitting by intensity is done by giving the surface of a transparent material such as glass a thin dielectric (metallic) coating. The thickness of the optical coating determines the proportions of light reflected and transmitted, expressed as the reflection-to-transmission or R/T ratio. While a 50:50 ratio is most common there are plenty of applications that need R/T ratios of 30:70 or 40:60.
2. By Wavelength
Wavelength splitting is achieved with a dichroic mirror. A loose definition of “dichroic” is “two colors.” This is essentially a colored filter. Light is either reflected or transmitted based on wavelength. As with a dielectric coating, it is possible to change the R/T ratio by varying the coating.
Two common forms of dichroic beamsplitter are hot mirrors and cold mirrors. These have the transmission/reflection threshold in the IR part of the spectrum. A hot mirror diverts IR away from a sensor behind the splitter while a cold mirror reflects the visible portion.
3. By Polarization
A beamsplitter can also be used to separate unpolarized light into two different polarizations, (S and P). S-polarized light is reflected while P-polarized is transmitted. Polarization beamsplitters are a form of a wire grid polarizer. The alternative is the non-polarizing beamsplitter, which maintains the original polarization.
Form Factors of Beamsplitters
The most widely used shape is the plate beamsplitter. This is a flat piece of optical mirror with a coating (dichroic or dielectric) on the front face. Plate beamsplitters are usually mounted at 45⁰ to the incident beam but accept a wide range of input angles.
Limitations of plate beamsplitters:
- Refraction through the optical medium means the transmitted portion is offset slightly from the input direction
- The second surface produces a faint “ghost” reflection
The principal alternative to the plate is the cube beamsplitter. This is composed of two right-angled prims bonded on their hypotenuse surfaces. A portion of incident light is reflected at 90⁰ while the balance is transmitted straight through. Cube beamsplitters require incident light perpendicular to the first surface. In addition, they do not impose an offset on the transmitted light.
Less common types of beamsplitters include transmission grating and polka dot. In form factor these are very similar to plate beamsplitters.
Applications of Beam Splitters
One of the biggest application areas is interferometry. This is where a beam is split into two with one being reflected off a surface. Combining the returning light with the original beam results in interference patterns that can be used to measure distance.
Physics students will be familiar with the Michelson interferometer which relies on a beamsplitter to produce an interference pattern. Beamsplitters are also used extensively in R&D with quantum optics – a relatively new application area.
Fluorescence spectroscopy is another application. Here a dichroic beamsplitter filters by wavelength, sending only the emitted fluorescence to a detector.
Camera-based imaging systems, including those used in machine vision, often employ beamsplitters. Usually the plate-type, these make it possible to put incident light on the viewing axis, creating co-axial illumination. Another imaging system application is to split light by wavelength between several sensors using a dichroic filter.
Imaging systems may also use hot or cold mirrors to separate IR from visible light. One case would be to protect a sensor from potentially damaging thermal radiation, e.g., cold mirrors in Head-Up Displays (HUDs). In addition, some lighting systems use a dichroic filter to reduce the red content in white light for more blue illumination.
How to Select a Beamsplitter?
1. Application
The application will determine if the goal is simply to divide and/or combine a single beam of light, or whether the purpose is to filter by wavelength. For dividing or combining a light beam, choose a plate or cube type beamsplitter. Wavelength separation will need a dichroic filter with an appropriate coating. When choosing a dichroic beamsplitter consider the steepness of the transition: a steeper gradient provides sharper delineation between the wavelengths.
2. Light Source
The incident source of light also has a bearing on the beamsplitter selected. For white light a plate beamsplitter will produce less chromatic aberration than a cube. Cube beamsplitters perform best with monochromatic light sources. However, if that light source is a high-power laser, a plate beamsplitter may be a better choice as the laser light will generate less internal heat. Also, with reference to laser sources, cube beamsplitters perform best with collimated light while a plate beamsplitter will accept collimated and uncollimated light.
3. Packaging
Packaging is another consideration. In many devices, interferometers for example, there isn’t enough space to handle either the inclination of a plate-type splitter or the resulting offset. In such cases a cube beamsplitter is preferred.
Optometrics and EMF offer a variety of beamsplitters that you may purchase online including plate, dichroic (hot and cold mirrors), polka dot and transmission grating. Certain niche applications require custom beamsplitters designed to exacting specifications. Please contact us to discuss your specific requirements.
