A laser diode (1) for outputting light of a single mode comprises a substrate layer (5), a cladding layer (6) and a compound light propagating layer (7) comprising first, second, third and fourth layers (9 to 12). A wave guiding region (15) of refractive index lower than its adjacent regions is defined by the third layer (11) by two quantum wells (16) positioned at the anti-node of light of a single mode which is propagating in the third layer (11) for propagating and amplifying the single mode light. A central ridge (17) locates the wave guiding region transversely of the direction of light propagation. The wave guiding region (15) can also be defined by shaping the top cladding layer (6) by forming an elongated central channel through the central ridge (17).