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This peculiar characteristic is due to the fact that in QC devices the
emission wavelength is determined by the geometry of the semiconductor
layers that compose the laser crystal.5.1 More precisely,
the laser transition is the transition of an electron inside sub-bands
from one upper quantum well level to a lower quantum well level. For
more details we would encourage the reader to consult semiconductor
physics text book.
Using two different semiconductor materials (InGaAs and AlInAs), a series
of potential wells and barriers for the electrons can be built. These
wells and barriers are so thin that the electrons are allowed only a
discrete set of energy levels. This situation is very similar to the
orbitals of an electron around a nucleus in the case of an atom. In the
case of the QC structure, the positions of the permitted energy levels
are determined by the thicknesses of the wells and barriers.
It is thus possible, using only one material system (InGaAs/AlInAs grown
on InP), to define laser transitions with energies ranging over a wide
span. The limits are set, on the short wavelength range, by the potential
difference between the wells and barriers, and on the other side by
intrinsic absorptions of the material.
In conclusion, a QC laser is a laser made with some sort of a specific
composit designed specifically for each wavelength but always composed
of the very same materials.