A mode converter, comprising: an input multimode waveguide, an output multimode waveguide, and a first conversion waveguide, wherein: the input multimode waveguide is configured to receive a first signal, and a mode of the first signal is a first mode; the first conversion waveguide has an input coupling waveguide with a first effective refractive index, and has an output coupling waveguide with a second effective refractive index; and the first effective refractive index is corresponding to an effective refractive index of the first signal that is in the first mode and that is in the input multimode waveguide, and the second effective refractive index is corresponding to an effective refractive index of the first signal that is in a second mode and that is in the output multimode waveguide; the first conversion waveguide is configured to: perform, by using the input coupling waveguide, evanescent wave coupling on the first signal that is in the first mode and that is transmitted in the input multimode waveguide, and couple the first signal to the second mode of the output multimode waveguide by using the output coupling waveguide, so as to obtain the first signal in the second mode; and the output multimode waveguide is configured to output the first signal in the second mode, characterized in that the first conversion waveguide further comprises an input bent waveguide, a connection waveguide, and an output bent waveguide; the input coupling waveguide is configured to perform the evanescent wave coupling with the first signal transmitted in the input multimode waveguide, so that the first signal that is in the first mode and that is in the input multimode waveguide is coupled to a third mode of the input coupling waveguide; the output coupling waveguide is configured to perform the evanescent wave coupling with the output multimode waveguide, so that the first signal that is in the third mode and that is output by the output coupling waveguide is coupled to the second mode of the output multimode waveguide; the input bent waveguide is configured to prevent the connection waveguide from approaching the input multimode waveguide; the output bent waveguide is configured to prevent the connection waveguide from approaching the output multimode waveguide; and the connection waveguide is configured to separately connect the input bent waveguide and the output bent waveguide. the input multimode waveguide is configured to receive a first signal, and a mode of the first signal is a first mode; the first conversion waveguide has an input coupling waveguide with a first effective refractive index, and has an output coupling waveguide with a second effective refractive index; and the first effective refractive index is corresponding to an effective refractive index of the first signal that is in the first mode and that is in the input multimode waveguide, and the second effective refractive index is corresponding to an effective refractive index of the first signal that is in a second mode and that is in the output multimode waveguide; the first conversion waveguide is configured to: perform, by using the input coupling waveguide, evanescent wave coupling on the first signal that is in the first mode and that is transmitted in the input multimode waveguide, and couple the first signal to the second mode of the output multimode waveguide by using the output coupling waveguide, so as to obtain the first signal in the second mode; and the output multimode waveguide is configured to output the first signal in the second mode, characterized in that the first conversion waveguide further comprises an input bent waveguide, a connection waveguide, and an output bent waveguide; the input coupling waveguide is configured to perform the evanescent wave coupling with the first signal transmitted in the input multimode waveguide, so that the first signal that is in the first mode and that is in the input multimode waveguide is coupled to a third mode of the input coupling waveguide; the output coupling waveguide is configured to perform the evanescent wave coupling with the output multimode waveguide, so that the first signal that is in the third mode and that is output by the output coupling waveguide is coupled to the second mode of the output multimode waveguide; the input bent waveguide is configured to prevent the connection waveguide from approaching the input multimode waveguide; the output bent waveguide is configured to prevent the connection waveguide from approaching the output multimode waveguide; and the connection waveguide is configured to separately connect the input bent waveguide and the output bent waveguide.