Coupling losses in multimode waveguides due to misalignment

Suspended waveguides with discontinuities can be misaligned both longitudinally and laterally, thus enabling controllable light attenuation and switching. The effect of such misalignment on the transmission losses is investigated. Data collected in experiments with multimode waveguides show a monotonous decrease of the transmission with lateral displacement. In an optical train comprising a singlemode fibre, input waveguide, and output waveguide coupled to another singlemode fibre, a loss of 10 dB is experienced for a lateral misalignment equalling half the waveguide width, and 40 dB for a lateral misalignment equalling its full width. The data agree with a theoretical model to within 10%, where mode coupling, diffraction, smoothness, insertion loss and beam walk-off are represented. An oscillating modulation that was observed during axial misalignment has been found consistent with the formation of a Fabry-Perot cavity between the walls of the discontinued waveguide. With Si₃N₄ AR coating on the silicon waveguide facets a modulation depth of 1.22 dB was measured, indicating a reflectivity of 2%. It is possible to contain the losses by controlling the gap length and by maintaining a minimum lateral misalignment. Likewise, such a control may be conducive to signal attenuation.