Fabrication of an Optical Waveguide-Mode-Field Compressor in Glass Using a Femtosecond Laser.

We report on fabrication of an optical waveguide-mode-field compressor in glass using a femtosecond laser. Our approach is based on building up a stress field within the waveguiding area which is realized by sandwiching the waveguide between a pair of laser-induced-modification-tracks. To induce an adiabatic conversion of the optical mode in the waveguide, the tracks are intentionally designed to be tapered along the waveguide. We show that our technique can allow for reducing the mode field size in a single mode waveguide from more than 10 μm to around 7 μm.