Hollow core optical fibers and methods of making

A method of manufacturing a hollow core optical fiber, the method comprising:

• positioning at least one glass tube in a glass outer cladding to form a preform precursor, the glass tube comprising an inner capillary, a first open end, and a second open end;
• forming a preform from the preform precursor;
• drawing the preform into a hollow core optical fiber;
• and
• while drawing the preform, thermally treating the preform by heating at least a portion of the preform to increase gas pressure within the glass tube,
• wherein heating the glass tube increases an outer diameter of the inner capillary and decreases a wall thickness of the glass tube and
• wherein the thermally treating comprises heating the portion of the preform at a heating rate of about 100° C. per second to about 350° C. per second.