Patent ID: 7289709

Claim:
A photonic crystal fibre comprising a core region for propagating light in a longitudinal direction of said photonic crystal fibre, a cladding region surrounding said core region, said cladding region comprising micro-structural elements extending in said longitudinal direction and having a first coefficient of thermal expansion, wherein said cladding region further comprises at least one stress element having a second coefficient of thermal expansion different from the first coefficient of thermal expansion and extending in said longitudinal direction of said photonic crystal fibre, said at least one stress element being located in a cladding background material having a third coefficient of thermal expansion different from the second coefficient of thermal expansion, wherein said core region has a centre when viewed in a cross section perpendicular to said longitudinal direction of the photonic crystal fibre, termed a transversal cross section, and at least one of said micro-structural elements is located between said at least one stress element and said centre of said core region, and wherein said location of said at least one stress element relative to said core region and said micro-structural elements and said second and third coefficients of thermal expansion are adapted to provide a stress induced birefringence in said core region of said photonic crystal fibre.