A composite optic fibre for laser includes a core surrounded with a pump guiding sheath in contact with the core, sheath being a photonic structure formed by a substantially regular matrix assembly of coaxial capillaries, spaced apart and arranged parallel to the core, the core being a material with doping elements which may be brought into at least one excited electronic state by absorbing the energy from a pump optical signal of a first determined wavelength running through the core and capable of giving-back the former by de-energization in the form of an optical signal of a second determined wavelength, the core having a determined diameter and the sheath having a determined diameter. The diameter of the guiding sheath 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.