Method of making a large MCVD single mode fiber preform by varying internal pressure to control preform straightness

A method for fabricating singlemode optical fiber comprising the steps:

• selecting a first cylindrical tube which has length, a central axis, and is made from glass having a hydroxyl ion (OH.sup.-) level which is less than 0.5 parts per million (ppm) by weight;
• disposing the first cylindrical tube in a generally horizontal position, and rotating said tube about its central axis;
• depositing successive layers of cladding material onto the inside surface of the first tube along its length;
• depositing successive layers of core material onto the cladding material along its length;
• monitoring the outside diameter of the first tube;
• varying gas pressure within the first tube to control its outside diameter during the deposition of said core and cladding materials;
• monitoring the straightness of the first tube;
• varying the rate of rotation the first tube according to its angular position to control its straightness during the deposition of said core and cladding materials;
• exposing the first tube to a heat source that moves along the length of the tube, wherein the heat from the source causes the tube to collapse inwardly to form a core rod, wherein the deposited core material has a diameter that is greater than about 5 millimeters and the deposited cladding material has a diameter that is less than about 15 millimeters;
• selecting a second cylindrical tube having an inside diameter that is slightly larger than the outside diameter of the core rod;
• placing a substantial portion of the core rod into the second tube;
• and
• exposing the second tube to a heat source that causes it to collapse inwardly upon the core rod to create a first preform.