Provided is an optical device including an active layer, which includes two outer barriers and a coupled quantum well between the two outer barriers. The coupled quantum well includes a first quantum well layer, a second quantum well layer, a third quantum well layer, a first coupling barrier between the first quantum well layer and the second quantum well layer, and a second coupling barrier between the second quantum well layer and the third quantum well layer. The second quantum well layer is between the first quantum well layer and the third quantum well layer. An energy band gap of the second quantum well layer is less than an energy band gap of the first quantum well layer, and an energy band gap of the third quantum well layer is equal to or less than the energy band gap of the second quantum well layer.