Specifications
Item #	GC251B	GC252B
Path Lengtha	100.0 ± 3.0 mm
Cell Outer Diameter	25.0 ± 0.2 mm
Cell Clear Aperture	>Ø20.0 mm
Pressure (at 25 °C)	<10-7 Torr to 5 psi
Fill Tube Length	99.5 mm	30.0 mm
Number of Fill Tubes	One	Two
Fill Tube Outer Diameter (Typical)	6.4 mm (0.25")
Window Angle	55.5° ± 1.0°
Window Item #	BW2502
Transmitted Wavefront Error	λ/10 @ 633 nm
Window & Cell Substrate	UV Fused Silicab
UVFS Transmission (Click for Plot)
Diagram (Click to View)

• Length of the center of the cell excluding the windows.
• Click link for detailed specifications on the substrate.

Features

• Glass Cells with 100 mm Path Length for Spectroscopy Applications:
  • Both Ends Include Thorlabs' BW2502 Brewster Windows
  • Near-Zero Reflection Losses of P-Polarized Light
• Available with One Tube for Filling or Two Tubes for Flow
• Compatible with Our KF25C025 and KF16C025 Flange to Compression Fitting Adapters
• UV-Fused Silica Substrate for Windows Resists Chipping and Scratching
• High Transmission from 185 nm to 2.1 μm

Thorlabs' Empty Fused Silica Cells with Brewster Windows provide high transmission from 185 nm to 2.1 μm. The BW2502 Brewster windows at each end are hermetically sealed at the Brewster's angle of 55.5° at 633 nm, enabling near-zero reflection losses of the p-polarized component of light while suppressing transmission of s-polarized light. Please see the Graphs tab for more information on wavelength dependence and reflectance. 

Cells are available with one or two fill tubes. The GC251B glass cell is designed with a single fill tube which is useful when introducing a sample into the cell and sealing it off for measurements. The GC252B glass cell features two fill tubes to allow a sample to flow through the cell while under analysis. The cells do not come with integrated valves and can only isolate a sample by adding valves at some other points in the system.

The fill tubes have a Ø1/4" (6.4 mm) outer diameter and are compatible with our KF25C025 and KF16C025 flange to compression fitting adapters; for more information see the Vacuum Seal tab. As shown in the video to the right, custom reference cells can be manufactured upon request, including different cell lengths, cell diameters, or fill tube configurations. Please contact Tech Support with inquiries.

The cell windows at each end are made from UV-fused silica (UVFS), a rugged material that resists chipping and scratching and offers high transmission for wavelengths between 185 nm and 2.1 μm. All glass cells on this page are capable of withstanding vacuum environments as low as 10^-7 Torr and pressures as high as 5 psi at 25 °C. Post production, the cells undergo ultrasonic cleaning, a methanol rinse, and are coated with First Contact™* Polymer, a polymer coating that protects the the optics from scratching and chipping and can be easily peeled off before the cell is used. The cells are not vacuum sealed when shipped; before using the cells in a vacuum setup, we advise users to clean with a methanol rinse.

Mounting & Heating Options
The glass cells can be mounted in our V-Clamp Mounts, which are available in standard, kinematic, or Ø1.5" post-compatible versions. Please check that the cell diameter is supported by the mount. The glass cells can also be mounted and heated up to 50°C in our GCH25-75 Glass Cell Heater. 

*First Contact is a trademark of Photonic Cleaning Technologies, LLC.

Glass Cells Selection Guide
Empty Glass Cells	Empty Glass Cells with Angled Windows	Empty Glass Cells with Brewster Windows	Threaded Empty Glass Cells	Vapor Reference Cells	Acetylene Reference Cells	Glass Cell Heaters

This tab features information on the refraction and reflection of UV fused silica and UV fused silica windows at Brewster's angle.

UV Fused Silica Index of Refraction 

Click to Enlarge
This graph shows the calculated reflectance of uncoated UV fused silica for polarized light as a function of angle of incidence. The reflectance for P-polarized light goes to zero at Brewster's angle.

Click to Enlarge
This graph shows the calculated index of refraction of UV fused silica as a function of wavelength.

The index of refraction for UV fused silica is wavelength dependent, as shown in the graph above, and expressed by the following formula:

The image above shows the reflection of S-polarized light and transmission of P-polarized light through a Brewster window. Some S-polarized light will be transmitted through the window.

Click to Enlarge
This graph shows the calculated value of Brewster's angle as a function of wavelength when light is passing from air to UV fused silica.

Brewster's angle is given by

.

θ_B is Brewster's angle.
n_i is the index of refraction of the incident medium, which is 1.0003 for air.
n_t is the index of refraction of the transmitting medium, which is 1.45701 for fused silica at 633 nm.
Hence, Brewster's angle also displays a wavelength dependence, as shown in the plot above.

Absorption Spectroscopy

The reference substance pumped into the glass cell is associated with a unique absorption spectrum that serves as its fingerprint. The contents of a reference cell can be determined via a linear absorption measurement (as depicted by the simplified schematic below). By scanning a tunable diode laser over a wavelength range and detecting light absorption with a photodetector, a series of peaks will be recorded, which is characteristic of the vapor inside the cell. Please note that we also offer pre-filled and sealed acetylene reference cells, as well as vapor reference cells with quartz or borosilicate substrates.

A reference cell with Brewster windows minimizes back-reflections, etalon effects, and other stray reflections throughout a setup, which is critical for applications requiring a high power (linearly polarized) laser source. The Brewster angle changes little with wavelength as seen in the Graphs tab, and so retains its anti-reflectivity over a broader range of wavelengths than an AR coated cell.