A planar optical 3-dB coupler is proposed that uses only a single region of mode conversion followed by a region of adiabatic modal evolution. These two regions are provided by an overlay with fast and slow tapers, which makes the device synchronous at a single point. Device length is noncritical. The device was fabricated with sputtered barium silicate and fused silica films and tested experimentally. Maximum power transfer of 40% was limited by a fabrication problem with the fast taper, but agreement with theory was obtained. An approximate coupled mode representation of the coupling between local normal modes is used to estimate power transfer at an abrupt transition and to provide design requirements for the fast and slow tapers. The theoretical concepts developed are applied to describe the operation of conventional directional 3-dB couplers and channel modal evolution 3-dB couplers which use branching waveguides.

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