A process and apparatus for producing an optical fiber preform by oxidizing a gaseous glass forming material with a flame from a burner and depositing the resulting soot in a rod form are described, wherein two signal beams, supplied by splitting a light beam, are projected in a direction normal to the direction in which the preform grows and are directed into the space between the burner and the deposition face of the preform, and they are positioned so that one beam partially contacts the deposition face of the preform, while the other beam is directed to pass between the burner and the deposition face, and the distance between the burner and the deposition face is controlled such that the ratio of the intensity of the two beams passing through the space between the burner and the deposition face is held constant.