Beamsplitters separate incident light into two or more beams of the same wavelength. These exiting beams are differentiated by either their optical power (non-polarizing) or polarization states (polarizing). Non-polarizing beamsplitters are specified by their splitting ratio, i.e. the amount of light in the reflected arm versus the amount of light in the transmitted arm, while polarizing beamsplitters are specified by their extinction ratio, i.e. the ratio of P-polarized light to S-polarized light in the transmitted arm.
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Most beamsplitters are produced in two varieties: plate beamsplitters and beamsplitting cubes. Other beamsplitting solutions are available, including pellicle beamsplitters, crystal beamsplitters, Brewster Windows and wedged plates. Each variety is described below with some of their features and strengths. For detailed spec comparisons, see the BS Selection Guide tab.
The beams exiting a beamsplitter have the same wavelength as the incident light. This distinguishes beamsplitters from dichroic mirrors and hot and cold mirrors, which split an input beam into two wavelength bands. Conversely, some polarizing optics have two or more exiting beams whose polarization states are not distinct from each other; these are known as polarizers.
Plate beamsplitters offer a relatively lightweight solution in a small footprint, which is beneficial in space-constrained setups. They are usually placed in a beam path at a 45° angle of incidence (AOI). The plates are coated with a thin film that reflects a portion of the beam while the rest is transmitted. The transmitted beam is offset from the incident beam due to refraction. At the back surface of a plate beamsplitter, a second reflection is created, often referred to as a ghost reflection or ghosting. To mitigate ghosting, Thorlabs may apply an anti-reflective (AR) coating, 30 arcmin wedge, or a combination of the two to the back surface of some of our plate beamsplitters.
In high-sensitivity applications, the reflected beam may need to be put through a compensation plate to match its path length to that of the transmitted beam. A compensation plate is a window of the same material and thickness as the beamsplitter, which accounts for the path difference acquired traveling through the beamsplitter.
Cube beamsplitters are a more mechanically robust solution when compared to plates or pellicles. They are constructed from two right-angle prisms, joined at their hypotenuses, with a thin film coating at the interface which causes the beam to split. The two halves are connected either by cement or optical contacting. Optical contacting is a more difficult means of binding two glass surfaces together, but it removes the need for cement, which is often the limiting factor in calculating laser damage threshold.
These cubes are ordinarily placed in a beam path at a 0° angle of incidence (AOI) and cause one exiting beam to stay in line with the optical axis and one exiting beam to be deviated 90°. One of the benefits of a 0° AOI is that the transmitted beam is minimally offset by refraction. This and the AR coatings on the entrance and exit faces reduce ghosting. Conversely, their thickness introduces increases in optical path length and group delay dispersion (GDD) when compared to plate or pellicle beamsplitters. Cube beamsplitters also require more space to mount, which can be a disadvantage in space-constrained setups. See the BS Cube Mounting tab for our cube mounting options.
Pellicle beamsplitters are comprised of a nitrocellulose membrane mounted under tension in a metal housing. Since the membrane is only a few microns thick, the second surface reflection is superimposed on the first, effectively eliminating ghosting. Pellicle beamsplitters minimize chromatic dispersion and aberrations, making them ideal for use in applications requiring focused beams.
While pellicles are among the lightest beamsplitting solutions, their membrane is extremely delicate and flammable. These optics should be handled carefully and only used in low-power applications. A pellicle beamsplitter should be chosen carefully to match an experiment's operating conditions, as thin film interference causes sinusoidal fluctuations in the output intensity with wavelength. For details on interference effects, see the main presentation of our Pellicle Beamsplitters.
Some crystals, like calcite, MgF2, quartz, α-BBO, and YVO4, can induce polarization in transmitted and reflected beams via the light's interaction with the optical axes of the crystals. The main advantage of crystal beamsplitters over polarizing plate or cube beamsplitters is their relatively high laser damage threshold and extinction ratio, making them ideal for polarizing laser sources. These beamsplitters can be either single bulk crystals or multiple crystals bonded to each other through either cement or optical contacting. Care should be taken when changing the temperature of these crystals, as some may experience thermal shock if the temperature change is too rapid, leading to fractures or breakage.
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The different substrates of our crystal beamsplitters each offer an exceptional extinction ratio of 100 000:1 within their respective wavelength ranges. Our α-BBO beamsplitters are ideal for UV, calcite is best for visible to near-IR, and yttrium orthovanadate (YVO4) has the best performance from near-IR to mid-IR.
Brewster windows are windows of uncoated substrate (UV Fused Silica) that form a circular profile with the optical axis when positioned at Brewster's Angle. At this angle, the P-polarized component of incident light enters and exits the window without reflection losses, while the S-polarized component is partially reflected. Brewster windows can be used in series as polarizers or to improve a beam's polarization ratio.
A wedged plate beamsplitter splits a single input beam into multiple copies through successive reflections and refractions. This creates separate, progressively more attenuated copies of the incident beam at different exits angles. The angular deviation of each exit beam can easily be calculated. For details, see here.
The first class of beamsplitters we&#;ll discuss can be used to split the power of a light beam into two separate paths. This is common in interferometry, imaging, and for feedback loops in optical systems.
The beamsplitter acts to divide the light&#;s intensity in a given ratio over a range of wavelengths, generating two beams with the same spectral composition, if not the same intensity. The example below shows a standard AC300 beamsplitter designed for 450-650 nm operation, and reflects 50% of the light while the other 50% is transmitted. Common split ratios include 50/50, 70/30, and 80/20, though a beamsplitter can be designed to transmit or reflect as little as 5-10% of the light for monitoring purposes.
Many plate beamsplitters used for intensity splitting are designed for 45° AOI, though we routinely work at 30-45° AOI, and up to 60° or larger upon request. Not all beamsplitters are coated on plates or flat substrates, however. In augmented reality (AR) and mixed reality (MR) headsets, LCDs mounted to the left and right of the eye project the computer-generated image onto optics designed to route a portion of that light to the eye. Those optics have a beamsplitter coating designed to simultaneously reflect a portion of the projected image into the eye and transmit light from the environment, overlaying the two images to create the augmented reality scene. Both plate and curved beamsplitters are used in AR and MR headsets, each of which must be able to perform over a very large range of angles. The split ratio of beamsplitters used in these headsets range from 50/50 to 80R/20T to get the right balance of natural and projected light for the given external conditions, while minimizing the power needed to create the projected image.
This brings us to a challenge often seen in beamsplitter design and manufacture. Beamsplitters operating at large AOI and/or over a wide range of angles tend to exhibit polarization splitting, resulting in unequal distribution of s- and p-polarization in each beam and skewing the purity of the split. At AccuCoat, we mitigate that issue using a proprietary metal-dielectric hybrid coating that performs very effectively for 70/30 and 80/20 beamsplitters with a minimum of polarization splitting.
Beamsplitters can also be designed to operate as attenuation filters, as in the case of reflective neutral density (ND) filters. Inconel coatings in various thicknesses can be used to provide 5-90% transmission over a broad band of wavelengths very effectively simply by reflecting the unwanted portion of light.
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