Cladding-pumped fiber structure

Apparatus comprising a fiber including a core of size and transparency for transmitting electromagnetic wave energy of a first wavelength, the core being encompassed within a contacting pump cladding for receiving and transferring pump energy to the core, the pump cladding forming an interface with an enveloping second cladding, both core and pump cladding being composed of silica-rich glass, in which the interface is multiple-sided,

• CHARACTERIZED IN THAT
• the interface contains longitudinal, parallel troughs projecting into the pump cladding, a longitudinal pump trough being of a depth of at least 3%, as measured relative to the radial distance from the center of the fiber to a plane making glancing contact with the interface on either side of the trough.