Furnace design to improve drawing of hollow-core fibers

A furnace assembly for manufacturing a hollow-core optical fiber from a hollow-core fiber preform comprising:

• a furnace including a body defining a body cavity extending along a longitudinal axis between a preform input port and a hollow-core fiber output port;
• the body cavity configured to locate the hollow-core optical fiber preform and a process gas, the hollow-core optical fiber preform comprising:
• a cladding tube having a cladding outer surface and a cladding inner surface, the
• cladding inner surface surrounding the longitudinal axis and defining a preform cavity,
• and
• a plurality of capillary elements attached to the cladding inner surface, each of the
• capillary elements having a capillary inner surface defining a capillary cavity,
• at least one primary heating element proximate a necking region of the hollow-core optical fiber preform and configured to maintain the necking region at a draw temperature, the draw temperature sufficient to soften the necking region;
• and
• wherein the process gas occupies a flow field surrounding the cladding outer surface, the flow field extending from the necking region to the preform input port, the process gas having a flow with an average Grashof number less than 1.6×104 in the flow field.