Abstract
Low cost fabrication of photonic integrated circuits for future subscriber systems require efficient coupling of the single mode fibre to the chip as well as large alignment tolerances. This can be achieved by increasing the spot size of the waveguiude to the spot size of the fiber using an integrated spot size transformer. For buried waveguide structures such as lasers or amplifiers this can be done by gradually decreasing the waveguide thickness which leads to an adiabatic mode expansion /1-3/. More complex integrated photonic circuits, however, mostly have uncladded rib waveguides. If this type of waveguide is etched down in order to achieve spot size matching, the cutoff properties of the asymmetric layer structure make it impossible to achieve a large spot size and to make the waveguide-fibre coupling polarization independent. A concept for lateral tapering of uncladded waveguides has been proposed by Thurston and coworkers /4/. However, it is restricted to channel waveguides only. In addition, the design outlined in /4/ requires layers with refractive index differences as small as 0.01 which is not feasable for standard MOVPE growth techniques of InGaAsP/InP. In this work we theoretically investigate taper waveguides suitable for vertical tapering and propose a new waveguide design which can be used to fabricate tapers with polarization independent coupling to the fibre for arbitrary uncladded rib or channel waveguides.
© 1996 Optical Society of America
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