Upward collapse process and apparatus for making glass preforms
An apparatus and related process for producing large glass preforms with minimal clad to-core waveguide distortion from a glass body having a weight, an outer surface, core rods, and a cladding surrounding and separated from the core rods by a gap. The apparatus includes collars affixed to the top and bottom of the cladding; a spacer upon which the core rods rest; a first unit holding and supporting both the bottom collar and the spacer; a second unit holding and supporting the top collar; and a frame defining a heating zone having a heating element to heat the glass body. The weight of the glass body above and below the molten glass in the heating zone is supported by the first and second units without contacting the outer surface of the glass body.
1. An upward collapse process for creating a glass preform having minimum clad-to-core waveguide distortion, the process comprising:
providing a glass body having a weight, an outer surface, and at least one free-standing core rod and a cladding separated by a gap;
affixing a top collar to a top of the cladding and a bottom collar to a bottom of the cladding;
supporting the bottom collar with a bottom collar holder and vacuum unit located below a heating zone positioned in a frame, wherein the heating zone comprises a heating element, wherein the heating element has a center portion;
supporting the at least one free-standing core rod inside the cladding on at least one spacer upon which the at least one free-standing core rod rests and which is supported by the bottom collar holder and vacuum unit, such that the bottom collar holder and vacuum unit supports up to the entire weight of the glass body without contacting the outer surface of the glass body;
supporting the top collar with a top collar holder and vacuum unit located above the heating zone;
positioning the glass body a predetermined distance from the center portion of the heating element located in the heating zone;
feeding the glass body upward through the heating zone to collapse the cladding onto the at least one free-standing core rod, thereby closing the gap, and to create the glass preform;
moving the bottom collar holder and vacuum unit at a controlled velocity V1 in a vertical (z) direction and moving the top collar holder and vacuum unit at a velocity V2 in the vertical (z) direction; and
applying a vacuum from both the bottom collar holder and vacuum unit and the top collar holder and vacuum unit such that the top collar collapses onto a glass disk located on top of the at least one free-standing core rod and the cladding and inside the top collar and a tip is pulled on top of the glass preform.
2. The process of claim 1 , further comprising the step of superimposing a constant oscillation perturbation onto the velocity V2 and using a load cell to measure a true viscosity of the glass body at the center portion of the heating element.
3. The process of claim 2 , further comprising the step of using the load cell measurement to control temperature and heating power of the heating zone.
4. The process of claim 1 , further comprising the step of positioning the glass body using a gripper or chuck system, the gripper or chuck system including clamping elements adapted to contact the bottom collar or the bottom collar holder and vacuum unit and mounting elements attaching the clamping elements to the frame and allowing a linear movement in the vertical direction of the clamping elements.
5. The process of claim 1 , further comprising the step of using a preform measurement device to determine one or more geometric or waveguide properties of the glass preform.
6. The process of claim 1 , further comprising the step of purging any gas used in the heating element of the heating zone so that no contaminants or deposits are formed on a surface of the preform.
7. The process of claim 1 , further comprising the step of managing a flow of data between and among at least the bottom collar holder and vacuum unit, the top collar holder and vacuum unit, and the heating element using a controller.