Ultrafast Broadband Beamsplitters with Controlled Dispersion

Features

• Deterministic Group Delay Dispersion (GDD) in Reflection and Transmission
• Designed for P-Polarized Light Incident at 45°
• Split Ratios:
  • 20:80, 50:50, 80:20, or 90:10 (R:T) for 600 - 1500 nm
  • 50:50 for 1000 - 2000 nm
• Optimized for Femtosecond Ti:Sapphire and Yb Lasers
• 1.0 mm Thick Infrasil Windows to Balance UFBS5050 GDD

Thorlabs' Ultrafast Broadband Beamsplitters offer a 20:80, 50:50, 80:20, or 90:10 (R:T) beamsplitting ratio over the 600 - 1500 nm wavelength range, or a 50:50 (R:T) beamsplitting ratio over the 1000 - 2000 nm wavelength range. They are designed to be used with p-polarized light incident at 45° and are not intended to be used for s-polarized light. For s-polarized or circularly polarized light applications, please see our full selection of plate beamsplitters. As shown by the images at the top of the page, each ultrafast beamsplitter is marked with an arrow that points toward the beamsplitting coating; light being split should be incident on this surface to obtain the intended performance. The wide usable wavelength range of these beamsplitters makes them compatible with femtosecond pulsed Ti:Sapphire, Yb, and Er fiber lasers. The beamsplitters have an AR coating on the back surface that provides <0.5% absolute reflectance over the same wavelength range as the beamsplitter coating. The Infrasil substrate is a type of optical quartz that has nearly identical dispersion to fused silica but higher internal transmission around 1380 nm.

Mounting Options
Our ultrafast beamsplitters and Infrasil windows are too thin for standard setscrew-based mounts. Hence, we recommend mounting them in retaining-ring-based mounts, such as our Polaris^® SM1-threaded mounts or fixed optic mounts. Retaining rings apply less mechanical stress to the optic than setscrews, helping to prevent bending of the optical surfaces, which compromises performance. The 4.0 mm thick beamsplitters can also be mounted in our Polaris Low-Distortion Optic Mounts. Alternatively, the beamsplitters can be installed in a POLARIS-K1C4 glue-in kinematic mirror mount or a POLARIS-B1G glue-in fixed mount. The POLARIS-B1G features two cut-outs to maximize the transmissive clear aperture at 45° AOI, making it an ideal mount for a plate beamsplitter.

Balancing GDD in the Transmitted and Reflected Beam
When using the UFBS5050 ultrafast beamsplitter with pulses shorter than 50 fs, we recommend inserting one of our 1.0 mm thick Infrasil windows into the reflected beam at a 0° angle of incidence in order to balance the GDD. We offer 1.0 mm thick Infrasil windows in Ø1/2" (Item # UDP05) and Ø1" (Item # UDP10) sizes. The uncoated windows have flat transmission from 300 nm to 3 µm, as shown in the table below. Using one of these 1.0 mm thick windows reduces the GDD difference between the transmitted and reflected arms to <20 fs² at 800 nm. Longer pulses have a narrow spectral bandwidth that negates the need for additional dispersion compensation. See the Group Delay tab for additional details. For our other ultrafast beamsplitters, our 1.0 mm thick Infrasil windows are not thick enough to balance the dispersion exactly, but they will still improve dispersion matching between the beams. When using our 4.0 mm thick beamsplitters, the 3.0 mm and 5.0 mm thick Infrasil window (Item #s UDP053 and UDP105) may be used to provide some dispersion compensation, although it was not specifically designed to match those items. Alternatively, one of our uncoated UV fused silica windows may be used.

Thorlabs' selection of ultrafast optics also includes low-GDD mirrors for 700 - 930 nm, 950 - 1170 nm, 1400 - 1700 nm, and 1760 - 2250 nm. For our full selection, please see the Ultrafast Optics tab.

Recommended Configuration for Balanced Dispersion in Transmitted and Reflected Beams of 50:50 Ultrafast Beamsplitter (Only Necessary for Sub-50 fs Pulses)

Balanced Group Delay for 50:50 Ultrafast Beamsplitter

The UFBS5050 Ultrafast Beamsplitter provides a 50:50 beamsplitting ratio for femtosecond laser pulses with well-defined, nearly equal group delay dispersion in the transmitted and reflected beams. For the shortest pulses (<50 fs), additional dispersion compensation is recommended, as detailed in the text below. Please note for the following discussion that Infrasil^® and fused silica have nearly identical dispersion.

Transmitted Beam
When the UFBS5050 beamsplitter is oriented at a 45° incident angle, the transmitted pulse will be dispersed by traveling 1.7 mm through the Infrasil substrate (assuming the substrate is 1.5 mm thick). The plot below on the left compares the measured group delay through the UFBS5050 beamsplitter to the calculated group delay through 1.7 mm of Infrasil.

Reflected Beam
The beamsplitting coating is designed to impart on the reflected pulse the same amount of dispersion as traveling through 0.7 mm of fused silica (see the table to the right). The plot below on the right shows that the measured group delay of the reflected beam is similar to the calculated group delay through 0.7 mm of fused silica, as expected. The measurement stops at 1100 nm due to limitations of the white light interferometer used for this test.

In order to balance the dispersion of the reflected beam with that of the transmitted beam, either of our 1.0 mm thick Infrasil windows can be inserted into the reflected beam at a 0° incident angle. This geometry is shown in the figure to the right. The window allows the net dispersion of the reflected beam to be the same as traveling through 1.7 mm of Infrasil. For pulses with narrow spectral bandwidth (i.e., pulses longer than 50 fs), the group delay dispersion varies slowly enough that this additional window is not necessary.

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Group Delay in Transmission

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Group Delay in Reflection