Method of and apparatus for overcladding a glass rod

A method of providing an overclad optical preform in which an overcladding is disposed substantially concentrically about an optical preform rod, said method including the steps of:

• providing a substantially straight optical preform rod which has an outer diameter and a longitudinal axis;
• providing an overcladding tube which is made of an optical material having suitable optical and geometrical characteristics and which has an inner diameter with the difference between the inner diameter of the tube and the outer diameter of the rod being relatively small, said overcladding tube having a longitudinal axis;
• causing the preform rod to become disposed within the tube and then supporting individually the preform rod and the tube in positions substantially fixed relative to each other in directions along the longitudinal axes thereof;
• and
• subjecting successive increments of length of the tube with the rod disposed therein to a relatively narrow zone of heat having a controlled temperature gradient;
• while
• causing a pressure gradient between the outside and the inside of the tube to be esttablished and to be maintained with the pressure outside being substantially greater than that inside, the cooperation of the pressure gradient with said relatively narrow zone of heat and said relatively small difference between the outer diameter of the rod and the inner diameter of said tube being effective to collapse the tube substantially into engagement with the preform rod and with the overcladding tube being disposed substantially concentrically about the preform rod, with the difference between the inner diameter of the tube and the ouer diameter of the rod being sufficiently small to prevent substantially any buckling of the tube as the tube is caused to collapse onto the rod.

An apparatus for overcladding an optical preform rod, said apparatus including:

• first supporting means for holding a preform rod which is substantially straight along a longitudinal axis thereof so that the preform rod may be turned rotatably about its longitudinal axis;
• second supporting means for holding a tube which is made of an optical material having predetermined characteristics and which has a longitudinal axis so that the tube may be turned rotatably about its longitudinal axis, said first and second supporting means allowing the preform rod to be held with a substantial portion of its length disposed in the tube, the tube having an inner diameter that is greater than an outer diameter of the rod to provide a relatively small clearance therebetween;
• means for causing the preform rod and the tube to be turned rotatably each about its longitudinal axis;
• heating means for causing successive increments of length of the tube with the rod therein to be subjected to a relatively narrow zone of heat while the tube and the rod are being turned rotatably;
• and
• means rendered effective by said heating means causing successive increments of length of the tube and rod therein to be subjected to a relatively narrow zone of heat for establishing and for maintaining a pressure gradient between the inside and outside of the tube such that the pressure outside the tube is substantially greater than that inside, the cooperation of said pressure gradient, said relatively narrow zone of heat and said relatively small clearance between the tube an the rod being effective to cause said tube to be collapsed onto the rod, said clearance between the tube and the rod being sufficiently small to prevent any significant buckling of the tube as it is collapsed onto the rod.

A method of making an optical fiber, said method including steps of:

• providing a substantially straight optical preform rod which has an outer diameter and a longitudinal axis;
• providing an overcladding tube which is made of an optical material having suitable optical and geometrical characteristics and which has an inner diameter with the difference between the inner diameter of the tube and the outer diameter of the rod being relatively small, said overcladding tube having a longitudinal axis;
• causing the preform rod to become disposed within the tube and then supporting individually the preform rod and the tube in positions substantially fixed relative to each other in directions along the longitudinal axes thereof;
• subjecting successive increments of length of the tube with the rod disposed therein to a relatively narrow zone of heat having a controlled temperature gradient;
• while
• causing a pressure gradient between the outside and the inside of the tube to be established and to be maintained with the pressure outside being substantially greater than that inside and to cooperate with said relatively narrow zone of heat and the relatively small difference between the outer diameter of the rod and the inner diameter of the tube to cause the tube to be collapsed into engagement with the preform rod to provide an overclad preform in which the overcladding tube is disposed substantially concentircally about the preform rod, the difference between the inner diameter of the tube and the outer diameter of the rod being sufficiently small to prevent substantially buckling of the tube as the tube is caused to collapse onto the rod;
• and
• drawing optical fiber from the overclad optical preform.