To couple optical waveguides with differents cross-sections, nowadays adiabatic tapers are widely used. However, due to the on-going integration of photonic circuits, component size is becoming an important factor and adiabatic tapers will become too space-consuming. Instead of slowly (adiabatically) adapting one mode profile into another one, a more drastic scheme is studied.
Between the two different waveguides, a number of waveguide sections with random length and random width are placed. By changing these lengths and widths the mode-to-mode transmission is altered. Different optimisation algorithms stearing these changes result in good coupling efficiencies. A first is an evolutionary or genetic algorithm requiring no a priori knowledge of the system and a second a local, more deterministic, starting from well-known structures. Both algorithms lead to spot-size converters with theoretical transmissions that are much higher than conventional tapers of equal length.
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