Photonic crystal fibers are typically fabricated by stacking into a pre-form a set of glass canes and/or capillaries which may also be doped with rare earth materials to increase or decrease the refractive index of the glass. The pre-form is then packed into an outer tube or sleeve and drawn into an optical fiber though an optical fiber draw tower furnace. The complexity of designs that can be achieved with this method depends on the number of canes that can be plausibly stacked. The full size of the stack is limited by the diameter of the largest outer tube that can fit into a given fiber draw tower furnace. For this reason, smaller canes are desired so that a greater number of features may be included. Smaller canes, however, are more susceptible to bending out of place and crossing over other canes during stacking. Hence, there is a practical limit to how many canes, and therefore features, that can be included in a single draw for a given furnace.
A possible method to increase the number of features in a photonic crystal fiber is to draw photonic crystal canes from the original pre-form which would then be stacked into a new, more complex, photonic crystal pre-form. This results in reduced fill factor and a loss in control of the features of the final photonic crystal fiber. Drawing sleeveless photonic crystal canes to stack into new photonic crystal pre-forms is a major step toward very high complexity photonic crystal fibers.
If a photonic crystal stack is lowered into a furnace without fastening, the surface tensions between the individual canes and capillaries will prevent them from dropping into a unified photonic crystal cane; instead they will drop out as individual canes.
LLNL researchers have developed a method in which a sleeveless photonic crystal optical fiber cane can be fabricated. A set of glass canes and capillaries, doped or un-doped, are stacked into a hexagonal pre-form. A piece of outer tube which is much shorter than the pre-form, but longer than the "hot zone" of a draw tower furnace, is placed around the pre-form on either end, and crimped to the preform near the outer edge. A photonic crystal fiber pre-form now exists in which the two ends of the pre-form have outer tubes holding the shape of the photonic crystal stack, while the central region of the preform is sleeveless, and takes the shape of the photonic crystal stack which need not be hexagonal and may be arbitrary.
The photonic crystal pre-form is then lowered into a draw tower furnace where a portion of the lower tube is melted and dropped off. The remainder of the lower tube initially forces the preform to keep its shape through the hot zone of the furnace while canes are pulled out of the furnace from below. The lower outer tube which is much shorter than the full-length pre-form is slowly lowered through the furnace, and only the sleeveless photonic crystal structure continues into the furnace. The result is that the final canes pulled out of the furnace maintain the shape of the photonic crystal structure but are simply reduced in size without the need for a tube. This allows the resulting canes to be re-stacked into a new photonic crystal structure with full fill factor and no tube barrier between each cane.
LLNL's method allows fabrication of sleeveless photonic crystal canes with an arbitrary shape.
Sleeveless photonic crystal canes can be used for re-stacking into a new photonic crystal pre-form which could have significantly greater complexity than a pre-form built from featureless round canes.
Sleeveless photonic crystal canes can be used as parts to stack a full fill factor photonic crystal fiber pre-form with greatly increased complexity over what would otherwise be possible. The outer shape of the photonic crystal cane can be arbitrary because there is no need for an outer tube or sleeve to hold the photonic crystal structure.
Fabrication of highly complex photonic crystal fibers with full fill factors.
LLNL has filed for patent protection on this technology; LLNL internal case number (IL-12757).