A method and a corresponding apparatus for side-coupling light into an optical waveguide, such as a double-cladding fiber, are described. The method includes emitting light from a light source and coupling the emitted light into a side of a waveguide that defines a slit disposed on the side of the waveguide at an angle relative to an axis of the waveguide. The emitted light is incident on a surface of the slit and the slit surface reflects the incident light into the waveguide.