A fiber optic feedthrough includes a housing defining a cylindrical bore, the housing fitting hermetically into an aperture of a wall. A first collimator sealed within the bore receives output from an optical fiber located on a first side of the wall, and a second collimator also sealed within the bore, optically aligned with the first collimator, receives the signal and provides it to a second optical fiber located on a second side of the wall. The polarization extinction ratio of an optical signal traveling through the feedthrough is maintained at a value of at least about 12 dB, and the hermeticity of the feedthrough across the wall is better than about 10.sup.-6 atm.cc.sec.sup.-1.