Method and burner for increasing the deposition rate of porous glass deposit in the manufacture of optical preforms, by modifying the cross-sectional shape of the flow of glass particles impacting onto a target preform. In particular, the cross-section of the flow of glass particles is modified by increasing the dimension of the flow from a circular cross-section to one having a major and minor axis in a direction substantially perpendicular to the longitudinal axis of the target preform.