Beamsplitters are such an integral part of optical systems that it’s difficult to overstate their importance to the field of optics. As the name suggests, beamsplitters split an incident beam of light into two separate beams—a transmitted beam and a reflected beam—according to a designated ratio. They are so ubiquitous in optics that various beamsplitter configurations have been developed to meet different specifications, including geometric constraints and optical performance criteria such as polarization and bandwidth/spectral range. Beamsplitters can also be used in a reverse configuration where two beams are combined into a single beam.
Types of Beam Splitters
Beamsplitters are classified as either a cube or plate configuration based on how they are manufactured.
Cube Beamsplitters are typically made of two right-angle, triangular prisms bonded across their hypotenuse. When a light beam enters one face, it is split at the interface between the two prisms. One beam passes straight through the cube while the other is reflected. Cube beamsplitters are often configured with the interface at 45°, so the reflection exits an orthogonal face of the cube.
A transparent polyester, epoxy, or urethane-based adhesive is used for bonding and the thickness of the bonding layer dictates the power-splitting ratio for a given wavelength. Dielectric or metallic coatings may also be used at the interface surface to enhance optical performance.
Since the bonding line is very thin, there is not much offset (spatial beam shift) between the reflected and transmitted beams. In some applications, such as quantum measurements, this minimal offset is an advantage for cube beamsplitters. However, due to their solid construction and geometry, cube beam splitters are heavy and take up more space than plate beamsplitters.
Plate Beamsplitters are thin, flat optical substrates coated on the first surface with a partially reflective coating. The coating might be metallic or dielectric.
For example, a half-silvered mirror may have a coating of aluminum thin enough to allow half the incident light to be transmitted while the remainder is reflected. However, the partial reflection might be achieved by combining dielectric coatings to form a dichroic mirror that reflects and transmits preferred wavelengths of light. Finally, the coating might be preferentially deposited in a polka dot pattern or as a transmission gratings to split the incident light.
Unlike cube beamsplitters, plate beamsplitters exhibit an offset (spatial beam shift) between the transmitted and reflected beams due to the substrate’s thickness and refractive index. This offset may be problematic for some applications.
In addition, to offset, there may be a Fresnel reflection of the transmitted beam off the second surface of plate beamsplitters. The second surface is often coated with an anti-reflective coating to prevent this.
What Does a Beamsplitter Split?
As mentioned, the function of a beamsplitter is to divide a beam of light into reflected and transmitted beams of light. This division can happen according to power/intensity, wavelength, or polarization. Beamsplitters are used to combine multiple beams into one beam function similarly.
Half-silvered mirrors and polka-dot beamsplitters divide the intensity of an incoming beam reflecting a portion of the light with a mirrored coating. For polka dot beamsplitters, the ratio of the split is determined percentage of the surface coated, while the thickness of the layer on half-silvered mirror dictates the ratio.
Cube beamsplitters also are intensity beamsplitters. Due to the thinness of the bonding layer, there are limits to the amount of power cube beamsplitters can handle. If a dielectric layer is used at the interface surface, a cube beamsplitter may also divide the beam according to wavelength based on the dielectric material and thickness.
Dichroic plate beamsplitters or dichroic mirrors are primarily wavelength beamsplitters. These are really just filters that use a dielectric coating to dictate which wavelengths are transmitted and which are reflected. As such, they have particular bandwidths they operate. For example, a cold mirror is a short-pass beamsplitter while a hot mirror is a long-pass beamsplitter.
Transmission grating beamsplitters also split light into wavelengths using a finely ruled surface to differentially diffract transmitted light into multiple orders. In this case, the incoming beam may be divided into more than one transmitted beam. These beamsplitters are used in Helium-Neon (He-Ne) laser systems.
Finally, polarizing beamsplitters transmit p-polarized light and reflect s-polarized light. They are used for polarization separation in optical isolation applications. Wollaston prisms are cube beamsplitters that use birefringent material for the prisms for polarization separation. A wire grid polarizer is also a type of polarizing beamsplitter.
Most beamsplitters cannot be strictly categorized by intensity, wavelength, or polarizing. Beamsplitters tend to function across all three. For example, a non-polarizing beamsplitter doesn’t imply that it preserves the incoming polarization.
Applications of Beamsplitters
Although beamsplitters are simple devices, they are at the heart of precision instruments that use electromagnetic waves for precision measurements. Interferometry uses a coherent light source coupled with a beamsplitter to measure the difference between two optical paths. In the Michelson interferometer shown in Figure 4, the half-silvered mirror splits the beam from the light source then recombines it. The difference between the two paths can be determined from the resulting interference pattern.
Interferometry techniques are capable of measuring over vast distances as well as on atomic scales. As such, they are widely used in applications across various industries such as astronomy, oceanography, geology, spectrometry and metrology.
In addition to precision measurements, beamsplitters are essential to photography and projection. SLR cameras incorporate thin, half-silvered pellicle mirrors for light exposure measurement, light management, and high-speed photography.
In projectors, beamsplitters are used to combine RGB images to provide full-color projections. Hot and cold mirrors help direct infrared light to manage heat in projection systems. In augmented reality (AR) and mixed reality (MR) headsets, beamsplitters are required for routing images differentially to the left and right eye and overlaying images for a fully immersive experience.
Broadly, beamsplitters are geometrically either cube-style or plate-style. Within those categories, there are various ways to split a beam of light, whether by intensity, wavelength, or polarization. Using different substrates, adhesives, dielectric and metallic coatings alongside manufacturing techniques, a beamsplitter can be designed to meet specific application needs. While the function of beamsplitters—to split a beam of light—may seem straightforward, beamsplitters are a pretty diverse breed of optics.