Beamsplitter Guide

Beamsplitter Overview

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.

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

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.

Plate Beamsplitter Features

Beamsplitters	Polarization	Available Wedged	Backside AR Coating	AOI	Bandwidth	Mounted	Applications and Features	Diagram (Click to Enlarge)
Economy Beamsplitters	Non-Polarizing	No	No	45°	Broadband	Unmounted	Preliminary Set-Ups; Single-Use Applications
Standard Non-Polarizing Plate Beamsplitters		Yesa	Most	45°	Broadband	Unmounted	Low Aberrations Compared to Cubes
Polka Dot Beamsplitter		No	No	0 - 45°	Broadband	Unmounted	Splitting without Polarization Artifacts; Controlling Lamp Intensity
Ultrafast Beamsplitters with Low Dispersion	Non-Polarizingb	No	Yes	45°	Broadband	Unmounted	Ultrafast Laser Experiments
Standard Polarizing Plate Beamsplitters	Polarizing	Yesa	No	45°	Narrowband	Unmounted	Small Footprint and Low Weight Compared to Cubes

• The 30 arcmin wedged back surface on these beamsplitters minimizes ghosting.
• Designed for Use with P-Polarized Light

Cube Beamsplitters

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.

Cube Beamsplitter Features

Beamsplitters	Polarization	Cemented	AR Coating	AOI	Bandwidth	Mounted	Applications and Features	Diagram (Click to Enlarge)
Standard Non-Polarizing Beamsplitter Cubes	Non-Polarizing	Yes	Yes	0°	Broadband	Mounted and Unmounted	Low Influence from Mechanical Stress Compared to Plate Beamsplitters
Wire Grid Beamsplitters (Mounted and Unmounted)	Polarizing	Yes	Yes	0° - 25°	Broadband	Mounted and Unmounted	High Imaging Contrast; Wide Acceptance Angle
Standard Polarizing Beamsplitter Cubes		Yes	Yes	0°	Broadband	Mounted and Unmounted	Transmits P-Polarized Light with Extinction Ratio TP:TS > 1000:1
Laser Line Beamsplitter Cubes (Mounted and Unmounted)		Yes	Yes	0°	Narrowband	Mounted and Unmounted	Beam Sampling
High-Power, Laser Line Beamsplitter Cubes		No	Yes	0°	Narrowband	Mounted and Unmounted	High-Power Durability
Variable Beamsplitter		Yes	Yes	0°	Narrowband or Broadband	Mounted	Beam Polarization Analysis
Circular Polarizing Beamsplitter Cubes		Yes	Yes	0°	Narrowband or Broadband	Mounted	Generate Circularly, Elliptically, or Linearly Polarized Light

Pellicle Beamsplitters

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.

Pellicle Beamsplitter Features

Beamsplitters	Polarization	Cemented	AR Coating	AOI	Bandwidth	Mounted	Applications and Features	Diagram (Click to Enlarge)
Pellicle Beamsplitters	Non-Polarizing	No	No	45°	Broadband	Mounted and Unmounted	Minimizes Chromatic Dispersion; Eliminates Ghosting; Low Power Applications

Crystal 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.

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.

Crystal Beamsplitter Features

Beamsplitters	Polarization	Cemented	AR Coating	AOI	Bandwidth	Mounted	Applications and Features	Diagram (Click to Enlarge)
Calcite Beam Displacers	Polarizing	No	No	0°	Broadband	Unmounted	Parallel Exit Beams; Orthogonally Polarized Outputs; High Extinction Ratio in Both Beams
Yttrium Orthovanadate Beam Displacers			Yes			Mounted and Unmounted
Wollaston Prisms		Mosta	Yes	0°	Broadband	Mounted and Unmounted	Non-Orthogonal Beam Exit Angles, Symmetric Around Entrance Beam; Orthogonally Polarized Outputs; High Extinction Ratio in Both Beams
Rochon Prisms		No	No	0°	Broadband	Mounted	Non-Orthogonal Beam Exit Angles; Orthogonally Polarized Outputs; High Extinction Ratio in Both Beams; Ordinary Ray is Not Deflected
Glan-Taylor Polarizers		No	Yes	0°	Broadband	Mounted	Low Scatter; Air-Spaced Birefringent Crystal Prisms
Glan-Laser Polarizers (α-BBO and Calcite)		No	Yes	0°	Broadband	Mounted	Specialized Glan-Taylor Polarizer Designed for High-Power Applications
Double Glan-Taylor Polarizers		No	No	0°	Broadband	Mounted	Ideal for High-Quality Polarization with Extinction Ratios Greater than 100 000:1 in the 0.35 - 2.3 µm Wavelength Range
Glan-Thompson Polarizers (Mounted and Unmounted)		Yes	Yes	0°	Broadband	Mounted and Unmounted	Offers the Wide Field of View of Calcite Polarizers While Maintaining a High Extinction Ratio

• Thorlabs' quartz, calcite, and YVO₄ Wollaston prisms sold here are made of two prisms cemented together. In contrast, our magnesium fluoride and α-BBO Wollaston prisms are made of two prisms that are optically contacted, allowing for a higher damage threshold compared to cemented prisms.

Brewster Windows

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.

Brewster Window Features

Beamsplitters	Polarization	Cemented	AR Coating	AOI	Bandwidth	Mounted	Applications and Features	Diagram (Click to Enlarge)
Brewster Windows	Polarizing	No	No	55° 32' @ 633 nm	Broadband	Unmounted	Improving Polarization Ratio

Wedged Plate Beamsplitter

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.

Wedged Plate Beamsplitter Features

Beamsplitters	Polarization	Cemented	AR Coating	AOI	Bandwidth	Mounted	Applications and Features	Diagram (Click to Enlarge)
Wedged Plate Beamsplitter	Non-Polarizing	No	No	Variable	Broadband	Unmounted	Duplicating a Source Beam

Thorlabs Lab Fact: Beamsplitter Package Matters

We present laboratory measurements of the polarization angle, split ratio, and total throughput power of a beam transmitted through Thorlabs plate, cube, and pellicle beamsplitters. While all non-polarizing beamsplitters function similarly, the exact performance is different for different types of beamsplitter. Each type of beamsplitter contains its own advantages and disadvantages compared to other types of beamsplitters. Appropriate choice of beamsplitter is essential to sensitive experimental systems. We present a complete analysis and comparison of optical parameters for three common types of non-polarizing beamsplitters.

For our experiment we used the previous-genration HRS015 stabilized HeNe as the light source for our investigation. A linear polarizer is used to set the laser beam's polarization axis to 45° in order to provide equal S- and P-polarized light incident on the beamsplitter. The beamsplitter under investigation was then placed in the beampath, and its split beams directed to appropriate detectors. The total power though the optic, polarization states, split ratios, and angle of incidence effects were investigated under this configuration.

Click for full Lab Facts summary

The plots below summarize the measured results for all three types of beamsplitters. From these graphs the performance of each optic can be easily compared to one another. The bottom left plot summarizes the results for the total power throughput for each optic. The total power throughput is measured as the fraction of input power. While the plate and pellicle beamsplitters perform rather similarly, the cube shows signs of absorption inside the optic. Additionally, this plot shows the relative insensitivity of throughput power to angle of incidence. The bottom middle graph summarizes the results for the output polarization angle for each optic. The cube shows the most similar polarization angles between the reflected and transmitted beams, with the plate producing the largest difference in polarization between beams. The bottom right plot summarizes the results for the split ratio, as a fraction of input power, for the beamsplitters. Here it can be shown that the plate beamsplitter demonstrates the most ideal for 50/50 power splitting. For details on the experimental setup employed and the results summarized here, please click here.

Click to Enlarge

Click to Enlarge

Click to Enlarge