Consumers are often unaware of the complexities of modern technology. The vast majority of smartphone users are unaware that a diffractive beamsplitter, makes up a key component of front-facing cameras in advanced smartphone models, such as the iPhone XS and XR. As we explained in an earlier blog post, this component plays a major role in the devices’ Dot Projector assembly, as well as the True Depth Camera system. Together, they enable the iPhone’s FaceID feature. Apple uses a variation of a Philips prism-cube beamsplitter.
Beamsplitters represent one of the most important members of the optical-component family and aid numerous operations in spectroscopy, fluorescence, photography, interferometry, biology, and security.
Anatomy of a Beamsplitter
The primary function of a beamsplitter is to separate a single beam of light into two parts, one reflected and one transmitted. The incident light is split at the surface which is usually set at some angle so the reflected and transmitted beams are separated. Any metal layer that forms a partially reflective mirror can serve as a beamsplitter, and many under the “plate” category are no more sophisticated than that.
The two beams that split from the original incident light will normally have constant values over a specified wavelength range. These are called neutral beamsplitters. Other types separate colors with the transmitted and reflected beams displaying different parts of the visible spectrum or split the visible region from the infrared as is done with hot and cold mirrors. Beamsplitters also affect the polarization of the incident light and this property can be used to make effective polarizers.
Plate and Cube Beamsplitters
The most common type of beamsplitter consists of a single layer or multiple layers of dielectric materials on a suitable flat plate substrate. Normally used at a 45º angle of incidence, they are known by the ratio of reflected to transmitted light. Standard ratios include 30%R/70%T, 40%R/60%T and of course, 50%R/50%T.
Cube beamsplitters are formed by coating one or both hypotenuses of two prisms and joining them together using optical cement or epoxy resin, or by the optical contact method. If the prisms are of the right-angle type then the transmitted beam will be colinear with the incident beam, an advantage over the plate beamsplitter which will exhibit an offset equal to the thickness of the substrate. However, cube beamsplitters generally do not perform well over wide angles of incidence.
As with the plate beamsplitter, different splitting ratios are achievable. Further, spectral separation can also be produced. A common example can be found in TV cameras where specially designed coatings separate the red, green and blue color components of visible light into individual beams that are then focused onto the image sensors.
So named because it is formed by depositing tiny dots of aluminum on glass or silica using a photolithography process. Polka-dot beamsplitters divide incident beams by reflecting some off of the aluminum dots and transmitting the remaining light through the non-aluminized portion. This results in a multiplicity of tiny light beams with a near-constant reflection to transmission ratio and minimal angle sensitivity.
In many scientific and electronics-industry applications, it is necessary to split an incident beam into two spectral bands – one transmitted, the other reflected – and maintain an extremely sharp slope of transition between the bands. These are called dichroic beamsplitters. Since they utilize dielectric materials, they carry the advantage of negligible optical loss compared to absorptive types of color separators. EMF, a leader in optical coating solutions, supplies cold mirrors, aka dichroic beamsplitters, used in Head-Up Display (HUD) assemblies to a global automotive components manufacturer based in Japan.
Transmission Grating Beamsplitter
Transmission-grating beamsplitters separate laser beams into multiple orders, all in visible wavelengths. When paired with the appropriate diffraction grating, a transmission grating beamsplitter will allow different power distributions and dispersions. While the splitter won’t operate at peak capacity if it’s not placed at the right diffraction angle for the beam’s specific wavelength, scientists or engineers can determine this using the standard grating equation for normal incident light: 1) sinα + sinβ = 10-6 knλ.
Importance of Quality Assurance
For beamsplitters to meet the challenging application requirements in aerospace, defense, and healthcare they must undergo a number of critical testing procedures ranging from atomic force and optical microscopy to diffraction grating efficiency measurements and accelerated environmental testing.