A source arrangement for providing an electromagnetic radiation comprises at least one laser gain medium (2) providing the electromagnetic radiation. The arrangement further comprises at least one spectral separating first arrangement (4, 16; 4') configured to physically separate one or more components of the electromagnetic radiation based on a frequency of the electromagnetic radiation. The arrangement further comprises a second arrangement (20, 22) configured to receive at least some of the one or more components, and modify at least one characteristic of the received one or more components to forward at least one signal which is associated with one or more further components of the electromagnetic radiation. The arrangement further comprises at least one continuously rotating optical third arrangement (24) configured to receive the at least one signal. The arrangement further comprises at least one beam selecting fourth arrangement (10) configured to receive the at least one signal, wherein the first, second, third and fourth arrangements form a configuration that varies a mean frequency of the at least one signal over time.