Theoretical and experimental investigation of optical couplers in SOI

Integrated Optics in SOI need a concept for passive optical couplers, which is highly insensitive to fabrication tolerances. For this purpose directional couplers as well as couplers based on Multi-Mode-Interference (MMI) have been investigated, both theoretically and experimentally. BESOI material with a top layer thickness of 11 mu m is optimally adjusted to single-mode-fibers resulting in a field mismatch lass of 0.17 dB. For fabrication tolerances of 5 %, only MMI-couplers showed an insensitive behaviour with additional losses of 0.4 dB and a crosstalk (CRT) of - 30 dB at a wavelength of 1.3 mu m. A particular interest was paid to the additional coupling in the access region of the coupler, since there is no possibility to its compensation in this concept. The potential application of couplers in combination with phase shifters was studied theoretically as well. Realized MMI-based cross-couplers showed additional losses of 1 dB and a CRT of -17 dB, the best values for SOI-based couplers reported so far.