An optical device includes an asymmetric dual quantum well (ADQW) structure which is comprised of a plurality (at least two) of different semiconductor quantum well layers coupled to each other. In the ADQW structure, the width of a deeper quantum well layer having a narrower band gap is made narrower than that of a less deeper quantum well layer having a wider band gap such that the shift of an exciton wavelength is scarcely caused due to the quantum confined Stark effect by the application of an electric field in a predetermined range. As a result, only a refractive index of the ADQW structure is changed, but an absorption factor is scarcely changed for a given range of wavelength by the application of the electric field. Further, there is provided a member for applying an electric field to the asymmetric dual quantum well structure.