Mode control of photonic crystal fiber based broadband light sources

Disclosed is a mode control system and method for controlling an output mode of a broadband light source comprising a photonic crystal fiber (PCF). The mode control system comprises at least one detection unit configured to measure one or more parameters of radiation emitted from said broadband light source to generate measurement data; and a processing unit configured to evaluate mode purity of said radiation emitted from said broadband light source, from said measurement data. Based on the evaluation, the mode control system is configured to generate a control signal for optimization of one or more pump coupling conditions of said broadband light source. The pump coupling conditions relate to the coupling of a pump laser beam with respect to a fiber core of the photonic crystal fiber.