a core (2), the core (2) having a determined diameter and comprising a core material with additional doping elements which may be brought into at least one excited electronic state by absorbing energy from a pump optical signal of a first determined wavelength running through said core and capable of giving-back the former by de-energization in the form of an optical signal of a second determined wavelength;

a pump guiding sheath (4) having a determined diameter surrounding and in contact with the core (2), the pump guiding sheath (4) being a photonic structure formed by a substantially regular matrix assembly of spaced capillaries arranged parallel to the core (2) in a pump guiding sheath material;

a confinement sheath (5) around the pump guiding sheath (4), said confinement sheath (5) having an optical index smaller than an optical index of a material forming the pump guiding sheath (4); and

a mechanical rigidification sheath (6) arranged around the confinement sheath (5), a material of the mechanical rigidification sheath (6) being glass or silica, the mechanical rigidification sheath (6) having a thickness of at least 2 mm,

wherein the diameter of pump guiding sheath (4) is greater than the core diameter and smaller than or equal to four times the core diameter, the core diameter being greater than or equal to 35 micrometers, the fibre having a length less than 50 cm, and the fibre is straight and rigid.