A method of writing a waveguide using an ultrashort laser beam is
disclosed. The laser beam is directed to a substrate in transverse
relation to a waveguide propagation axis to generate an ultrashort laser
pulse focus in the substrate. A refractive index is modified in an
affected region in the substrate along the waveguide propagation axis via
the ultrashort laser pulse focus, and the ultrashort laser pulse focus is
moved in a direction other than the waveguide propagation axis to
generate a widened affected region along the waveguide propagation axis.
The widened affected region has a cross-sectional profile capable of
supporting a fundamental mode of a signal having a telecommunications
infrared (TIR) wavelength, while the affected region has a
cross-sectional profile incapable of supporting the fundamental mode of
the signal having the TIR wavelength.