Polarization-dependent optical fibre amplifier

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.