particles are produced by means of flame hydrolysis of sicl 4 and are deposited in layers on a rotating mandrel the germanium doped inner layer 22 is obtained in that during the deposition of the inner layer 22 gecl 4 is added to sicl 4 after a ge doped soot material layer has been deposited which layer in its thickness corresponds to the inner layer 22 the supply of gecl 4 is stopped and undoped material continues to be built up in this way a porous sio 2 body is obtained after the removal of the carrier the soot body thus produced is subjected to chlorine treatment at increased temperature in order to remove hydroxyl groups to a level of under 30 ppb by weight thereupon the porous sio 2 dehydrated soot body is vitrified under formation of the substrate tube 21 the inner and outer surfaces of the substrate tube 21 are then mechanically smoothed and chemically etched in order to produce the preform for the optical fiber with the refractive index profile represented in fig2 a the inner walls 24 of the substrate tube 21 as shown in fig2 b are first coated by means of the mcvd process with an undoped sio 2 layer to a thickness of about 0 88 mm and at the same time directly vitrified then a ge doped layer with a thickness of 0.