Method of detecting when an optical preform MCVD tube is completely closed

1. Field of the Invention A method of detecting a critical state of closing of a preform during a phase of closing of the preform following a phase of deposition of layers of material that reduce the radial size of an opening formed in the preform smaller in the radial direction comprises the steps of injecting a gas into the opening, measuring a pressure in a pipe element through which the injected gas flows and detecting the critical state of closing of the optical preform on the basis of a measured pressure.