Why is light confined to the depletion region in a simple homojunction Fabry-Perot laser diode, like the one schematically illustrated below? Does this region for some reason have a higher refractive index even though, it too, is GaAs? If so, Why? I cannot find anything on this. Thank you

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1 Answer 1


In laser diodes there are basically two ways of optical confinement

  • index guiding

    There the laser has a refractive index profile and the optical wave is confined because of this. This may be because the refractive index of the active region is different and/or because there is a ridge for the other lateral dimension.

  • gain guiding

    Here it is used that the gain is inhomgeneous and the light wave is only amplified there. This does not remove scattering/diffraction, but the gain might be high enough to achieve lasing. Due to Kramers Kronig this may go along with some index guiding as well (because if there is gain, there is also a refractive index change even if it is the same material).

Modern laser diodes are mostly index guided.

  • $\begingroup$ So, if the active region of the device pictured was just the depletion region of a GaAs p-n junction, the pictured device would not actually work? Even in the ideal case? $\endgroup$ Commented Apr 28, 2020 at 8:22
  • $\begingroup$ I am sure it can work and think the first semiconductor laser was of this kind. Quantum well lasers were invented later. $\endgroup$
    – Andreas H.
    Commented Apr 28, 2020 at 10:32
  • $\begingroup$ Why is light confined in the depletion region of such a device? $\endgroup$ Commented Apr 28, 2020 at 18:45

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