Patent ID: 7120339

Claim:
Apparatus comprising an optical fibre having a waveguide and at least one stress applying region, wherein: the waveguide is defined by a numerical aperture; the stress applying region is defined by a depressed refractive index; the optical fibre is configured such that the waveguide supports at an operating wavelength at least two polarised fundamental modes, two polarised first second-order modes, and two polarised second second-order modes; the waveguide comprises a gain medium; the optical fiber is disposed in a bend; and the stress applying region, the waveguide, the disposition of the gain medium, and the bend of the optical fiber are such: as to provide preferential guidance to at least one of the modes at the operating wavelength; the two polarised first second-order modes and the two polarised second second-order modes are leaky at the operating wavelength; and the optical fibre operates as a single polarisation optical fibre at the operating wavelength.