An optical fibre including a waveguide and at least one stress applying region is described. The waveguide is defined by a numerical aperture, and the stress applying region is defined by a depressed refractive index. The optical fibre is configured such that the waveguide supports at least two polarised fundamental modes, two polarized first second-order modes, and two polarised second second-order modes. The waveguide includes comprises a gain medium. The stress applying region, the waveguide and the disposition of the gain medium are such as to provide preferential guidance to at least one of the modes at an operating wavelength.