Since the advent of the laser, a huge e ort has been underway to guide this light in a con ned manner over distances that are much larger than the Rayleigh range. Guiding light within air or other gases in a con ned way over length scales of many kilometers has proven to be particularly challenging until recently. is is because total internal re ection (TIR), which enabled con ned optical guidance to be realized in bers comprising solid materials (mainly glass materials), cannot be used to con ne light in a gas or vacuum. Although several methods have been attempted to guide light in air, it is the advent of the hollow-core photonic crystal ber (HC-PCF) that has nally made the con ned low-loss guidance of light in air possible.