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Wavelength References specializes in fiber coupled gas cells but also offers bare gas absorption tubes. These tubes are used where the device will be in a free space optical application or used at a wavelength where optical fiber is not applicable. We can fill a gas absorption tube with almost any gas or gas mixture. Some materials that we have done for customers includes: HCN both carbon 12 and carbon12, C2H2 both carbon 12 and carbon 13, CO both carbon 12 and carbon 13, H2O vapor, CH4, HCl, HF, C2H4, CO2, H2S, NH3, and others. Often customers want to fill a tube with a gas and a background gas to mimic atmospheric conditions, for example the cell may be filled with 1 torr of ammonia and 750 torr of nitrogen to broaden and shift the line to mimic normal atmospheric conditions. We can offer as a background gas either nitrogen or argon.
Tubes are offered either in a glass envelope or a copper housing. For glass tubes the windows are glass with a slight wedge to eliminate the etalon effect of reflections from the two sides of the window. The windows may also be tilted with respect to the tube axis. We generally apply broadband AR coatings for the wavelength range of interest to the windows as well. The glass used for the windows (Schott B270) has useful transmission for 0.35 micron to 2.8 microns wavelength. For the copper housing we offer windows of sapphire or zinc selenide. These widows allow operation to much longer wavelength then glass windows and are also more resistant to corrosive gases. An example is the use of hydrogen fluoride, which would attack glass, which has absorption lines in the 1300nm region as well as the 2.5 micron region.
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