Optoelectronic Semiconductor Devices Principals and Characteristics

3.4.3 
EFFICIENCY AND OUTPUT POWER
 
The behaviors of laser diodes are quite similar to the edge-emitting LEDs, and a part of the spontaneous 
emission power in the active layer is emitted from both facets. the efficiency is quite low, and the total 
output power from both facets is less than a few percent of the power emitted in the active layer. 
At the lasing threshold, stimulated emission exceeds spontaneous emission, and above the threshold, the 
efficiency increases. The output power corresponds to the mirror loss from the viewpoint of the gain-loss 
relation shown in Figure 
21.
 and the current injected through the pn-junction induces gain. 
Taking output power from both mirror facets into consideration, the external quantum efficiency, η
d
, %, is 
given by

---

1 2
1 2
1
1
2
ln
2
1
1
ln
2
out st
d
id
id
i
P
L
R R
h
l
q
L
R R
ν
η
η
η
α
−






∆



 

 




=
=

∆






+ 
 




 




(19) 
where η
id
is called the internal differential quantum efficiency, %, and is given by
/
/
act st
id
P
h
l q
ν
η
−
∆
=
∆
(20) 
where P
act-st
is the lasing power due to the stimulated emission within the active layer. By using the 
external differential quantum efficiency, the lasing output power per facet (in W or mW) can be given as
1
1
2
2
th
F
th
out st
d act
d
J
J
I
I
P
h
S
h
q
νη
νη
−
−
−
=
=
q
(21) 
where
S
act
- the area of the active layer, 
I
F
- the forward injected current, 
I
th
- the threshold current observed. 
In addition to the external differential quantum efficiency, the slope efficiency (or responsivity) of laser 
diodes, η
s-st
, is also used quite often and is given by the first derivative of the output power with respect to 
the current, I
F
: 
( )
1
/
0.62
, (
)
2
out st
d
s st
d
F
dP
h
q
W A or mW mA
dI
m
η
η
η ν
λ µ
−
−
=
=
=
(22) 
Now, if we use the slope efficiency, lasing output power from formula (21) is simplified: 
(
1
2
F
th
out st
d
s st
F
th
I
I
P
h
I
q
νη
η
−
−
−
=
=
)
I
−
(23) 
The formulas (21) and (23) are showing the ideal increase of the optical input power above the lasing 
threshold.