We describe the design of a soldered sapphire optical viewport, which is useful for spectroscopic applications of samples at high temperatures and high pressures. The sapphire window is bonded via active soldering to a metal flange with a structure of two c-shaped rings made of different metallic materials in between, so as to mitigate thermally induced stress. A spectroscopic cell equipped with two of the optical viewports has been successfully operated with alkali metals in a noble gas environment at temperatures in the range of 20–450 °C at noble gas pressures from 10−6 mbar to 330 bars. In the upper pressure range, we observe a leakage rate smaller than our readout accuracy of 30 mbar per day.

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