  We describe the design of a soldered sapphire optical viewport, 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, 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 $20\,${\deg}C to
$450\,${\deg}C at noble gas pressures from $10^{-6}\,$mbar to $330\,$bar. At
the upper pressure range, we observe a leakage rate smaller than our readout
accuracy of $30\,$mbar per day.
