NOVELTY - Optical device (D1) comprises an active layer (130) which comprises at least two outer barriers and at least one coupled quantum well, where each of the coupled quantum well is sandwiched between the outer barriers and comprises at least three quantum well layers and at least two coupling barriers interposed between the quantum well layers, and the coupling barriers have a potential energy which is higher than a ground level and is lower than energy levels of the outer barriers.    USE - The optical device is useful as a reflective optical modulator or a transmissive optical modulator (all claimed), and in semiconductor devices including optical filters, photodiodes, solar cells, light emitting devices, optical communication systems, optical interconnections or optical operators.    ADVANTAGE - The optical device exhibits both low driving voltage and high optical absorption strength.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) an optical device (D2) comprising a lower reflective layer (120), a first active layer on the lower reflective layer, a middle reflective layer on the first active layer, a second active layer on the middle reflective layer, and an upper reflective layer (140) on the second active layer, where at least one of the first active layer and the second active layer comprises the outer barriers and the coupled quantum well; and    (2) an optical device (D3) comprising a first active layer which comprises at least two outer barriers and at least one three quantum well structure, and a second active layer which comprises at least one single quantum well structure, where each of the three quantum well structure comprises a first quantum well layer, a first coupling barrier, a second quantum well layer, a second coupling barrier and a third quantum well layer, and each of the single quantum well structure comprises a fourth quantum well layer interposed between the outer barriers.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the reflective optical modulator having a three coupled quantum well structure.    Reflective optical modulator (111) First contact layer (100)    Lower reflective layer (120)    Active layer (130)    Upper reflective layer (140)