Conventional optical fibers have a core with a refractive index that is higher than that of the cladding. However, new types of optical fibers have a core with a refractive index that is lower than that of the cladding (i.e., “a low-index core”) and so are referred to herein as low-index fibers. The use of low-index fibers in fiber-based applications is of increasing interest because they can be made to have ultra-low loss and ultra-low nonlinearity. They can be used, for example, in optical communications systems for long-haul transmission or in laser-based optical systems that require the delivery of high optical power from the laser to a device or other optical fiber.
One type of low-index optical fiber is a hollow-core optical fiber. Two types of hollow-core fibers have been demonstrated to date. The first has a photonic bandgap structure that uses a periodic array of longitudinally running air holes in a solid material. The second has a solid cladding formed from multiple layers that surround the hollow core, wherein the multiple layers are configured in a Bragg-grating structure so that light is confined to the core.
Photonic bandgap fibers require a very high air-fill fraction (˜95%) to confine the light to the hollow core. Because of the high air-fill fraction, the bridges supporting the periodic air-hole structure are very thin (e.g., about 40-50 nm). Consequently, such bridges are difficult to make and are delicate.
On the other hand, Bragg-grating-based fibers need a very high index contrast to confine the light to the core, which is difficult to achieve using glass material. Bragg fibers have been demonstrated with the high index layer being tellurium (refractive index 4.6) and the low index layer being polymer (refractive index 1.59). Because the tellurium and polymer layers are made by two different processes, the fiber fabrication requires many process steps and is not suitable for large-scale manufacturing.