TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: In the optical device (D1), each coupled quantum well is a three coupled quantum well which 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, where the first coupling barrier and the second coupling barrier have the potential energy which is higher than the ground level and is lower than the energy levels of the outer barriers. The second quantum well layer has a higher material thickness than the first quantum well layer, and the first quantum well layer has a higher material thickness than the third quantum well layer. The first coupling barrier and the second coupling barrier have a material thickness such that tunneling of electrons and holes occurs. The optical device (D1) further comprises: n-contact layers and p-contact layers which are disposed on a lower surface and an upper surface of the active layer, respectively; and a lower reflective layer and an upper reflective layer which are disposed on a lower surface and an upper surface of the active layer, respectively. The lower reflective layer has a reflectivity which is higher than the upper reflective layer, or equal to the upper reflective layer. In the optical device (D1), at least one of the lower reflective layer and the upper reflective layer further comprises at least one microcavity layer, where when lambda is a resonant wavelength of the optical device (D1) or (D2), each of the active layer and the microcavity layer in the device (D1) or each of the first active layer, the second active layer, and the microcavity layer in the device (D2) has an optical thickness which is an integer multiple of lambda /2, and each of the upper reflective layer and the lower reflective layer is a distributed Bragg reflector layer which comprises a repeated stack of alternating first and second refractive index layers having an optical thickness of lambda /4 and different refractive indices. The microcavity layer is made of a same material as one of the alternating first and second refractive index layers. The active layer comprises a first active layer on the lower reflective layer and a second active layer on the first active layer, where the first active layer 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, the second active layer comprises a fourth quantum well layer, a third coupling barrier, a fifth quantum well layer, a fourth coupling barrier, and a sixth quantum well layer, and the second quantum well layer in the first active layer has a different material thickness than the fifth quantum well layer in the second active layer. The first quantum well layer and the third quantum well layer in the first active layer have material thicknesses equal to the fourth quantum well layer and the sixth quantum well layer in the second active layer, respectively. The active layer comprises a first active layer having at least one three coupled quantum well structure, where each of at least one three coupled 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, where the second active layer has at least one single quantum well structure, and each of at least one single quantum well structure comprises a single fourth quantum well layer interposed between the outer barriers. In the optical device (D2), each coupled quantum well in the first active layer 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 coupled quantum well in the second active layer comprises a fourth quantum well layer, a third coupling barrier, a fifth quantum well layer, a fourth coupling barrier, and a sixth quantum well layer, where the second quantum well layer in the first active layer has a different material thickness than the fifth quantum well layer in the second active layer, or a material thickness equal to the fifth quantum well layer in the second active layer. The second quantum well layer has a higher material thickness than the first quantum well layer, the first quantum well layer has a higher material thickness than the third quantum well layer, the fifth quantum well layer has a higher material thickness than the fourth quantum well layer, and the fourth quantum well layer has a higher material thickness than the sixth quantum well layer. The first coupling barrier through the fourth coupling barrier has a potential energy which is higher than the ground level and is lower than the energy levels of the outer barriers. The first active layer further comprises a single quantum well structure having at least two outer barriers and at least one single quantum well layer, where each of the single quantum layers is sandwiched between the outer barriers. The second active layer further comprises a single quantum well structure having at least two outer barriers and at least one single quantum well layer, where each of the single quantum layers is sandwiched between the outer barriers. The lower reflective layer and the upper reflective layer in the device (D2) are doped to a first conductive type, and the middle reflective layer is doped to a second conductive type which is electrically opposite to the first conductive type. The first coupling barrier and the second coupling barrier in the device (D3) have a material thickness of 1-1.5 nm, and the material thickness of the first coupling barrier and the second coupling barrier in the device (D3) is 1.5 nm. In a wavelength band of 850 nm, the quantum well layers comprise gallium arsenide, the coupling barriers comprise aluminum gallium arsenide of formula (AlyGa1-yAs), and the outer barriers comprise aluminum gallium arsenide of formula (AlxGa1-xAs). In a band of a wavelength of 1550 nm, the quantum well layers comprise at least one of indium gallium arsenide (InrGa1-rAs) (a1), indium gallium aluminum arsenide of formula (In1-r-yGarAlyAs) (a2), and indium gallium arsenic phosphate of formula (In1-rGarAszP1-z) (a3), and the coupling barriers and the outer barriers comprise at least one of indium gallium aluminum arsenide (a2) and indium gallium arsenic phosphate (a3).x = greater than 0 to less than or equal to 1; andr, z and y = greater than 0 to less than 1.