AbstractOptical communication fibers are pulled at high temperatures from solid quartz rods with radially varying indices of refraction. These solid quartz rods are called preforms and their manufacturere presents the largest portion of the price of the finished fiber. This paper deals with the theoretical and experimental identification of the single most time consuming (and thus expensive) stepsin preform fabrication: the collapse of a quartz tube to a solid rod. It is the result of a coope rative effort with the Nordiske Kabel- og Traadfabrikker A/S (NKT).An optical fiber preform is manufactured from a quartz tube in which a glass dust is deposited. This glass dust has impurities which give the core of the preform the requiredrefractive index profile.Finally the tube is collapsed to a solid rod by slowly passing a high temperature flame a long the tube. The total collapse can consurne as much as 24 hours for some tubes. Moreover for low loss fiber sit is extremely important that the core of the preform is exactly circular. This implies an automatic control which can only be achieved if the dynamics of the process are known.The identification of the collapse process has been carried out using direct physical modelling of a system which involves the solution of the Navier Stokes differential equation in two dimensions. This complicated system has been reduced to an analytical nonlinear state space model. Experiments show that this model describs the collapse process quite weIl. Automatic contra1 will be attempted in the near furture.