Preferably in this embodiment a first end of the doped fibre is provided with a broadband reflector, for example supply by mirroring an end of the doped fibre, or alternatively by connecting the doped fibre to a fibre loop mirror, while a second end of the doped fibre has low reflectivity, for example by being anti-reflection coated or angled and is connectorised i.e. is attached to one component