Fig. 1.

Cross-sections of different kinds of TOPS. a TOPS based on strip waveguide with a metal heater on the top. b TOPS based on strip waveguide with doped-silicon heaters at both sides. c TOPS based on rib waveguide with doped-silicon heaters at both sides. d TOPS based on rib waveguide with a directly integrated doped silicon heater. e TOPS based on strip waveguide surrounded by a hybrid heater. f TOPS with air-gap trench and silicon substrate undercut. g TOPS based on the folded waveguide with a metal heater on the top. h TOPS based on multi-pass waveguides with a metal heater on the top