Excitation of fundamental and low-order modes of optical fiber waveguides by gaussian beams. 1: tilted beams.

The optimum condition for coupling energy into an optical fiber waveguide is theoretically computed for an incident beam with a Gaussian intensity distribution and a curved wavefront. The dependence of the modal excitation efficiencies on the tilt angle of the incident beam and on the refractive index difference of core and cladding is discussed. Numerical results show that maximum efficiency for the HE(11) mode can be achieved when the beam radius is chosen to be approximately 0.65 times the core radius. Excitation efficiencies of low-order modes, HE(1m) (m >/= 2) approach 80% at their cutoff when the beam size is suitably larger than the core diameter.