A fiber gain medium provided by a rare-earth doped fiber (10) is contained in a first resonant cavity by end reflectors (12, 18). The reflector (12) is wavelength selective to limit the frequency band of the first resonant cavity. The first resonant cavity also contains a second resonant enhancement cavity (16) with multiple transmission bands lying within the first resonant cavity's frequency band. Multiple standing wave modes of the first resonant cavity lie within both the frequency band of the first resonant cavity and the transmission bands of the second resonant cavity, and it is these standing wave modes that support laser action when the rare-earth doped fiber is suitably pumped by pump lasers (40).

Provided is a tunable laser source including a plurality of optical waveguides, at least three optical resonators provided between the plurality of optical waveguides and optically coupled to the plurality of optical waveguides, the at least three optical resonators having different lengths, and at least one optical amplifier provided on at least one of the plurality of optical waveguides, wherein a ratio of a first length of a first optical resonator of the at least three optical resonators to a second length of a second optical resonator of the at least three optical resonators is not an integer.

A radiation field generating system comprising an optical unit with an optical assembly which defines an optical path is provided, wherein the optical unit is operable in several different operation conditions and the optical assembly comprises at least one optical switching component with which switching between at least two different operation conditions of the several operation conditions can be performed.