672
23 Electricity-to-Light Conversion
Fig. 23.46 Historic
development of threshold
current density (at room
temperature, extrapolated
for infinite cavity length
and injection stripe width)
for various laser designs,
‘DH’: double
heterostructure,
‘SCH–QW’: separate
confinement
heterostructure with
quantum wells. ‘QD’:
quantum dots. Dashed
lines are guides to the eye
1960
1970
1980
1990
2000
2010
10
1
10
2
10
3
10
4
SCH-QW
Year
strained
QW
2
DH
QD
Fig. 23.47 Photon number
versus pump current for a
model laser. Adapted
from [1991]
10
6
10
10
10
10
10
10
10
10
10
5
4
3
2
1
0
-1
-2
-3
10
-10
10
-8
10
-9
10
-7 10
-6
10
-4
10
-5
10
-3 10
-2 10
-1
10
-4
10
-3
10
-2
10
-1
0.5
10
-5
=1
be calculated from the laser rate equations and is depicted in Fig. 23.47. For β = 1, all emitted power
goes into the laser mode regardless of whether emission is spontaneous or stimulated. The definition
of threshold in such ‘nonclassical’ lasers with large β is discussed in detail in [1990].
23.4.8 Output Power
The output power is given by the product of photon energy, the photon density in the cavity, the effective
mode volume and the escape rate:
P out = N ph
L w d
v g α m .
(23.30)
Thus, it is given by
P out = v g α m
τ ph
e
L w ( j − j thr ) =
e
α m
α m + α i
(I − I thr ) .
(23.31)