A hollow-core photonic crystal fiber (HC-PCF) assembly for converting input radiation to broadband radiation, the hollow core fiber assembly including: a micro-structured fiber with a hollow core extending along a length of the fiber from an input end configured to receive input radiation to an output end configured to output broadband radiation, wherein the hollow core of the fiber is configured to include a medium; and a density control system configured to control a density profile of the medium along at least a part of the length of the fiber to establish a desired zero dispersion wavelength profile along at least a part of the length of the fiber.