Optoelectronic Semiconductor Devices Principals and Characteristics

 
Figure 19.: Typical device characteristics for a 1300 nm-band InGaAsP/InP laser diode with cleaved 
facets. 
[4]
The operating characteristics of a typical 1300 nm-band InGaAsP/InP FP laser diode are shown in Fig 
19.
In the low bias range - that is, the low excitation range - spontaneous emission is dominant because the 
carrier density in the active layer is not high enough for forming a population (When there are external 
means available whereby the upper state may be populated preferentially, it is sometimes possible to 
create situations in which n
1
can be made greater then n
2
(where n
1
and n
2
are the number of atoms per 
unit volume), then a population inversion is said to exist.). As the bias is increases, the population 
inversion occurs in the active layer. 
Stimulated emission thus becomes dominant at a certain bias. This bias point is called the lasing 
threshold, and the corresponding current is called the threshold current. 
3.4.1 
THRESHOLD GAIN AND CURRENT
 
Threshold current is the most important and basic parameter of the laser diodes. Below the threshold, the 
emitted light in the active layer propagates along the layer because the refractive index of the active layer 
is higher then that of the cladding layers. Partially the propagating light is reflected at the mirror facet ant 
the rest of it is emitted outside from the facet. Propagation and reflection of the light is repeated within 
the optical cavity formed by the active layer and the two mirror facets. As a result, the light is lost and 
gained. 
The loss of the light is combined of the cavity loss (the absorption losses in the active layer and the 
cladding layers), the mirror loss caused by the mirror facet reflectivity being less than 100% (about 30% 
just after cleavage) and scattering loss caused by structural inhomogeneities (roughness of the 
heterointerface). 
The gain originates from the injection excitation. 
The gain coefficient at the threshold (threshold gain) can be given by Formula (4) 

---

1 2
1
1
ln
2
th
i
g
L
R R
α




=
+




 


(4) 
where
α
i
- the internal cavity loss and is dominated by free carrier absorption in the active layer, 
L - an active layer length, 
R
1
and R
2
- the reflectivities of two mirror facets. (Figure 
20.
).