Optical Communications: A Hollow Future Ahead?

While the idea of guiding light in an air or vacuum filled core predates the development of ultra-low loss glass in the 1970s, HCFs as we know them today stem from seminal work at the University of Bath in the mid-1990s. Initial fibers were made of silica and air, with a 2D photonic crystal extending indefinitely along the fiber axis to guide light in a core defect (i.e., filled with air or vacuum) by means of a photonic bandgap [1]. The very first reported HCF could only guide light in air through 40 mm of fiber [2], but the breakthrough nonetheless stimulated a first wave of industrial and academic excitement that in the space of only a few years managed to bring the loss of such photonic bandgap guiding HCFs down 1.7 dB/km [3]. Despite predictions that lower than 0.2 dB/km might soon be achievable, loss in such fibers has not been improved substantially since those heroic works. Subsequent research demonstrated that surface scattering fundamentally limits the loss that can be achieved in such HCF type, and large values of intermodal interference (IMI) put an upper bound to its maximum reach.