3.4 BASIC CHARACTERISTICS OF LASER DIODES
The three essential components in a laser system are an amplifying medium,
an optical feedback and a
pumping source.
The operation of an ILD can be described as follows:
•
[1]Electrons are injected from the
n-side of a junction to the
p-side and holes - from the opposite
direction.
•
[2]In
the depletion region, electrons are near the bottom of the conduction band, while holes are
near the top of the valence band.
•
[3]Photons are emitted when electrons and holes recombine.The injected current above the
threshold
induces stimulated emission, and coherent light is emitted from the laser diode.
•
[4]Electric charges return through the external circuit.
Lasers are classified as three-level or four-level systems. To determine how ILDs should be classified,
consider a DH injection laser with an energy diagram as shown in Figure
18.
Figure 18.: Energy diagram of heterostructure ILD.
[1]
As we can see from the Figure
18.
, lasing occurs in the narrow band-gap
semiconducting layer
sandwiched between two wide band-gap semiconductors. The difference between three- and four-level
systems involves whether level
E
a
is normally occupied. Since in an ILD, the
top of the valence band in
the active region is normally vacant, ILDs are considered four-level systems.
