Optoelectronic Semiconductor Devices Principals and Characteristics

 
Figure 16.: Basic configuration of double heterostructure laser diodes. 
[4]

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The basic configuration of double heterostructure lasers is shown in Figure 
16.
, where AlGaAs/GaAs and 
InGaAs/InP lasers are used as typical examples. Under the forward bias condition the minority carriers 
are injected into the active layer through the pn-junction. Because of the requirement for electrical 
neutrality, what actually occurs is double injection (majority carriers also diffuse into the active layer). 
For stimulated emission, a very high density of injected carrier (over 10
18
cm
-3
) is required to form a 
population inversion. Such a high density of the injected carriers is attained by making the active layer 
less than 150 nm thick. To confine the carriers and photons within the active layer, we can set the 
refractive index the way as in shown on the Figure 
16.
 This confinement will result in the occurrence on 
the stimulated emission at a high rate leading to lasing at a low input power. 
We can show that the separation of longitudinal modes of a Fabry-Perot cavity of length d is
2
2
dn
d n
d
λ
λ
λ
λ
∆ ≈
 −





(3) 
Where
n - refractive index of the material 
dn
d
λ
- is the material dispersion (Source of time dispersion arising from the fact that the refractive index is 
indeed a function of wavelength might well be called chromatic dispersion but it is most commonly 
refered to as material dispersion). 
For GaAs, material dispersion is between -1.5 and -2. Thus for a diode with length d of 500 µm, the 
longitudinal mode separation is in the range of 0.1 to 0.3 nm. The spectral separation of transverse modes 
is even narrower. However, higher-order transverse modes can be suppressed by introducing excessive 
losses to the higher-order modes, which is done by the index variation or gain stripes.