Methods of and apparatus for collapsing a preform tube into a preform from which lightguide fiber is drawn

A method of making a preform having a generally circular cross-section from a glass substrate tube having a generally circular crosssection, said method comprising the steps of:

• supporting the substrate tube at its ends for rotation about a longitudinal axis thereof;
• exposing each successive increment of length of the outer surface of the substrate tube to a zone of heat having a temperature profile and a maximum temperature within a first range by causing relative motion between the zone of heat and the substrate tube in a direction from one end of the tube to the other;
• while
• causing rotation of the substrate tube by turning its supported ends;
• and
• depositing doped reactants into the substrate tube to form a preform tube having a predetermined profile;
• then
• exposing each successive increment of length of the outer surface of the preform tube to a torch which provides a zone of heat having a temperature profile and a maximum temperature within a second range, that is higher than the first range, by moving the torch in a plurality of passes from one end of the preform tube to the other;
• moving a contact device into engagement with the preform tube and incrementally closer to the longitudinal axis of the preform tube at the beginning of predetermined ones of the plurality of passes;
• and
• causing the preform tube to become collapsed into a solid preform about the longitudinal axis which is a substantially straight line by causing relative motion between the contact device and successive increments of length of the preform tube with the contact device being spaced from the torch a distance as measured along the longitudinal axis to cause the contact device to apply forces to each successive increment after it has been exposed to at least an initial portion of the zone of heat of the second range.

An apparatus for collapsing an elongated glass tube into a solid preform, said apparatus including:

• means for supporting each end of the tube for rotation about a longitudinal axis thereof;
• heating means for producing a zone of heat having a temperature profile;
• means for causing relative motion between the tube and said heating means in a plurality of passes and in a direction along the longitudinal axis of the tube to cause each successive increment of length of the outer surface of the tube to be exposed to the zone of heat;
• means for causing rotation of the tube by turning its ends;
• contact means spaced from said heating means as measured along the length of the tube for causing the tube to be collapsed into a solid preform about the longitudinal axis, said contact means adapted to be in continuous engagement with the tube during each of a plurality of passes and capable of applying forces in a reproducible manner to each successive increment of length after each successive increment has been exposed to at least an initial portion of the zone of heat;
• and
• moving means rendered effective at the beginning of predetermined passes for causing said contact means to be moved toward the longitudinal axis of the tube to cause the point of application of said forces to become incrementally closer to the longitudinal axis.

An apparatus for making a solid preform having a generally circular cross-section from a glass substrate tube having a generally circular cross-section, said apparatus including:

• supporting means for holding ends of the substrate tube to allow the substrate tube to be turned about a longitudinal axis thereof through a rotary path of travel;
• torch means for heating the substrate tube, said means including means for directing a flow of combustible gases toward the tube to provide a zone of heat having a temperature profile and including means for providing confinement of the heat to a portion of the length of the tube and to a portion of the rotary path of travel to control the temperature along the portion of the length;
• means for moving said torch means in a plurality of passes along the substrate tube to cause the zone of heat to traverse the substrate tube and to heat successive increments of the length thereof;
• means for rotating the substrate tube about the longitudinal axis;
• means for depositing doped reactants into the substrate tube to form a preform tube having a predetermined profile;
• contact means in tandem with and spaced along the longitudinal axis from said torch means and moved therewith relative to the preform tube for collapsing the preform tube into a solid preform, said contact means being adapted to engage along the portion of the rotary path of travel each successively heated increment of the preform tube after it has been passed by a centerline of said torch means to apply forces to each increment and being adjustably mounted to permit movement inwardly and outwardly with respect to the longitudinal axis;
• and
• means responsive to the completion of a pass of said torch along the length of the preform tube for causing said torch to be returned to one end of the preform tube and rendered effective at the beginning of predetermined passes for causing said contact means to be moved incrementally toward the longitudinal axis of the preform tube.