Features

• 50/50 Beamsplitting Over a Broad Spectral Range
• Aluminum Coating is Relatively Insensitive to Incident Angle
• Two Substrate Options: B270 Glass or UV Fused Silica
• Available in Ø1", Ø2", or 1" Square Versions

Thorlabs' 50/50 Polka Dot Beamsplitters offer a nearly constant beamsplitting ratio over their entire specified spectral range. They consist of a vacuum-deposited aluminum coating on either B270 glass or UV fused silica, which provide high transmission over the 350 nm - 2.0 µm spectral range or 250 nm - 2.0 µm spectral range, respectively. Due to their metal coatings, they can be used through a wide range of incident angles with only negligible changes to the reflected and transmitted intensity. To view wavelength- and angle-dependent reflection and transmission data, please see the Graphs tab.

Detail of Aluminum Coating Pattern

The aluminum coating is applied in a regularly repeating array, which lends the beamsplitter its "polka dot" appearance, as shown to the left. Light is reflected by the aluminum-coated portion of the beamsplitter and transmitted through the uncoated portion of the beamsplitter. To maximize the reflected intensity, light should be incident on the coated side of the beamsplitter. The square dots have 0.0040" (100 µm) long sides, and the spacing between the dots is 0.0022" (56 µm) in all directions.

The Ø1" and Ø2" polka dot beamsplitters can be mounted into any of our SM1 and SM2 lens tubes, respectively. The 1" square beamsplitters can be held in a fixed filter mount like our DH1 Compact Dual Filter Holder.

Thorlabs also offers a family of plate beamsplitters with dielectric coatings. For more information on selecting a beamsplitter, please see the BS Selection Guide tab.

Below are measured transmission and reflection curves for our aluminum-coated, 50:50 polka dot beamsplitters on B270 glass or UV fused silica substrates. The shaded regions in these plots denote the spectral ranges over which we recommend using these optics. All data shown is for unpolarized light.

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Excel Spreadsheet Containing Raw Data

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Click to Enlarge

Excel Spreadsheet Containing Raw Data

Polka dot beamsplitters are typically used at a 45° angle relative to the incident beam as shown in the diagram to the left. Our polka dot beamsplitters transmit 50% ± 5% when a beam is larger than 2 mm in diameter. These beamsplitters have a 1.5 mm nominal thickness, which allows them to be mounted in many of our threaded mounts.

Beamsplitter Selection Guide

Thorlabs offers five main types of beamsplitters: Pellicle, Cube, Plate, Economy, and Polka Dot. Each type has distinct advantages and disadvantages.

Pellicle Beamsplitters - Pellicle beamsplitters are the best choice when dispersion must be kept to a minimum. They virtually eliminate multiple reflections commonly associated with thicker glass beamsplitters, thus preventing ghosting. In addition, unlike plate beamsplitters, there is a negligible effect on the propagation axis of the transmitted beam with respect to the incident beam.

Pellicle beamsplitters have two disadvantages: They exhibit sinusodial oscillations in the splitting ratio as a function of wavelength, due to thin film interference effects. Click Here for more details. They are also extremely delicate. Since they are fabricated by stretching a nitrocellulose membrane over a flat metal frame, the beamsplitter cannot be touched without destroying the optic. Thorlabs offers pellicle beamsplitters mounted in metal rings for use in kinematic mounts as well as cage cube mounted pellicles.

Beamsplitting Cubes
Thorlabs’ beamsplitter cubes are composed of two right-angled prisms. A dielectric coating, which is capable of reflecting and transmitting a portion of the incident beam, is applied to the hypotenuse surface. Since there is only one reflecting surface, this design inherently avoids ghost images, which sometimes occur with plate-type beamsplitters. Antireflection coatings are available on the entrance and exit faces of certain models to minimize back reflections. As well as providing a cost-effective solution, another advantage of the beamsplitting cube is the minimal shift it causes to the path of the transmitted beam. Thorlabs offers both polarizing and nonpolarizing beamsplitting cubes, in mounted and unmounted configurations, the former being compatible with our 30 mm cage systems.

Polarizing Beamsplitting Cubes - Thorlabs’ polarizing beamsplitter cubes split randomly polarized beams into two orthogonal, linearly polarized components (S and P), as shown in the diagram to the right. S-polarized light is reflected at a 90° angle with respect to the incident beam while P-polarized light is transmitted. Polarizing beamsplitting cubes are useful in applications where the two polarization components are to be analyzed or used simultaneously. Thorlabs offers mounted and unmounted polarizing beamsplitter cubes.

Nonpolarizing Beamsplitting Cubes - These cubes provide a 50:50 splitting ratio that is nearly independent of the polarization of the incident light. The low polarization dependence of the metallic-dielectric coating allows the transmission and reflection for S- and P-polarization states to be within 10% of each other. These beamsplitters are particularly useful with randomly polarized lasers and are specifically designed for applications in which polarization effects must be minimized. Thorlabs offers mounted and unmounted beamsplitter cubes.

Plate Beamsplitters - Thorlabs' plate beamsplitters are optimized for an incidence angle of 45° and feature a dielectric coating on the front surface for long-term stability. To help reduce unwanted interference effects (e.g., ghost images) caused by the interaction of light reflected from the front and back surfaces of the optic, a wedge has been added to these beamsplitters. Dispersion, ghosting, and shifting of the beam may all be potential problems, however. These are the best choice for a general-purpose beamsplitter. Thorlabs offers both polarizing and nonpolarizing plate beamsplitters.

Economy Beamsplitters - These are the most cost effective of all the beamsplitter types. Thorlabs' economy beamsplitters, which have an exposed oxide coating on one side and are uncoated on the other side, are designed to have either a 50:50 or 30:70 splitting ratio throughout the visible spectrum (450 - 650 nm) when used with unpolarized light incident at 45°.

Please note that the Fresnel reflections off of the uncoated back surface of these economy beamsplitters can lead to interference effects in the reflected beam. For applications sensitive to these effects, consider using a beamsplitting cube or a pellicle beamsplitter.

Polka Dot Beamsplitters - This type of beamsplitter consists of a glass substrate with a vacuum-deposited reflective coating that is applied over an array of apertures, giving the beamsplitter a "polka dot" appearance. Half of the incident beam is reflected from the coating, and half of the beam is transmitted through the uncoated portion of the substrate.

Polka dot beamsplitters are useful over a wide wavelength range and are negligibly angle sensitive, which makes them ideal for splitting the energy emitted from a radiant source. These are not recommended for imaging applications, as the polka dot pattern will affect the image.