An optical modulator device made on large core silicon fin waveguide platform and its fabrication methods. The optical device includes two silicon optical coupling waveguides each having a lower ridge and an upper ridge, two mode transformers respectively connecting the coupling waveguides with an optical modulator waveguide. The optical modulator waveguide has a silicon fin waveguide structure with a narrower fin structure on top of a wider lower ridge structure. Each coupling waveguide and the corresponding mode transformer form a two-stage horizontal taper structure, namely a taper in the lower ridge of the coupling waveguide and a taper of the mode transformer. The light travelling in the coupling waveguide with majority of light in the upper ridge can gradually shift to the lower ridge of the optical modulator where an electro-optic region is positioned. The electro-optic region changes its optical property in response to an applied electric field.