A quantum cascade laser has an active layer (24), a first and second cladding layer (23, 26), and a optical guide layer (25). The active layer (24) has a plurality of injection quantum well regions (90) and a plurality of light-emitting quantum well regions (80). The each of the injection quantum well regions (90) and the each of the light-emitting quantum well regions (80) are alternatively stacked. The first and second cladding layers (23, 26) are provided to interpose the active layer (24) from both sides, and have a refractive index lower than an effective refractive index of each of the light-emitting quantum well regions (90). The optical guide layer (25) is disposed to divide the active layer (24) into two parts. The optical guide layer (25) has a refractive index higher than the effective refractive index of the each of the light-emitting quantum well regions (80), and has a thickness greater than the thickness of all well layers of quantum well layers. The optical guide layer in the centre of the active layer increases optical confinement of the transverse optical mode into the active region and simultaneously the electrical threshold current of the quantum cascade laser is reduced.
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