1. An optical device comprising: an active layer comprising two outer barriers and a coupled quantum well disposed between the two outer barriers, wherein the coupled quantum well comprises a first quantum well layer, a second quantum well layer, and a third quantum well layer, a first coupling barrier disposed between the first quantum well layer and the second quantum well layer, and a second coupling barrier disposed between the second quantum well layer and the third quantum well layer, wherein a thickness of the first quantum well layer and a thickness of the third quantum well layer, disposed at opposite ends of the coupled quantum well, are different from a thickness of the second quantum well layer disposed between the first quantum well layer and the third quantum well layer, and wherein an energy level of the first quantum well layer and an energy level of the third quantum well layer are different from an energy level of the second quantum well layer.