We demonstrate highly efficient lithium niobate thin film Michelson interferometer modulators with half-wave voltage length product of 1.4 V∙cm. Amorphous silicon grating couplers have been incorporated to achieve a 3.8-dB/port waveguide-fiber coupling loss. Devices with 1-mm phase shifter arms have a footprint of 2.5 mm × 1.7 mm. The demonstrated modulation data rates is up to 35 Gb/s.