In a waveguide, the optical modes tend to be stronger where the refractive index is higher, so the vertical profile of the refractive index confines the optical mode in the middle layer, which is where light amplification takes place. In the case of horizontal cavity, edge-emitting laser diodes, the typical epitaxial structure includes layers of varying refractive indexes: a middle active layer with a given index of refraction, which is placed between two cladding layers with a lower index. To ensure low-threshold currents and high efficiency of the laser, it is important to optimize the transversal confinement of the optical modes. Semiconductor laser diodes find countless applications in telecommunication, medical/surgical systems, and other fields as compact high-power light sources that can be modulated quickly and are easily coupled to optical fibers.
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