Biomedical Engineering Reference
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device to device, and provide a structure in which emission is immune to
external reflections. The rare-earth-doped laser device may meet all of these
requirements.
This type of current injection laser was reported [50] to provide a stable sin-
gle longitudinal mode of operation in the near-infrared 1.55 μm wavelength.
The device uses a rare-earth erbium dopant in the active layer, chosen so
that the emission wavelength of 1.5322 μm from the trivalent rare-earth ion
is shorter than the band edge emission of the host semiconductor at 1.55 μm.
This results in a narrow spectral emission superimposed on the broad gain
peak of the host semiconductor (see Figure 4.23).
ζ c
Rare-earth
doped active
layer
n
CB
E
FC
E
FC
Excited
state
Excited
state of
rare-earth
ions
Ground
state
Ground
state
E
FV
E
FV
VB
C
V
p
N-cladding
layer
P-cladding
layer
(a)
(b)
Density of states ζ (
E
)
Lasing
mode
Cavity
losses
(c)
Wavelength
FIGURE 4.23
Rare-earth-doped semiconductor injection laser structure. (a) bandgap diagram (b) energy
densities (c) gain curve.
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