Method of changing the birefringence of an optical waveguide by laser modification of the cladding

An optical waveguide having stress induced regions comprising:

• a length of light transmissive material having a core and a cladding and a core-cladding interface, said optical waveguide having a stress induced region comprising a plurality of spaced stress pillars within the cladding, wherein the number of stress pillars is between 10 and 5,000 per millimeter;
• and wherein said regions are proximate the core such that an operating mode of the waveguide, that mode being made birefringent, does not significantly overlap with the stress induced region, such that any reflectivity of that mode which may occur due to the stress induced region is less than 1% when light guided in that mode is incident on the stress induced region.