INDEX-AND-DIMENSIONAL TAPER AND ITS APPLICATION TO PHOTONIC DEVICES

A mode-field-converting optical channel waveguide with tapered refractive index and cross-sectional dimensions is proposed. In this waveguide, both the refractive index and cross-sectional dimensions of a core are adiabatically varied along a propagation direction in such a way that the normalized frequency V of the waveguide may be kept constant. Since radiation loss caused by waveguide imperfections is strongly dependent on the V value, the waveguide has good mode-field-converting capability without causing high radiation loss. Layer thickness and index of a core of a silica waveguide was tapered by modulating a raw material gas flow in a flame hydrolysis deposition method, and a channel width taper was formed by a photolithographical technique. In order to confirm the usefulness, taper waveguides have been used, 1) to achieve good field matching between a fiber and a guided-wave optical switch, and 2) to fabricate a fiber-compatible monolithic 32 x 32 star coupler in a small chip.