Page 140 - High Power Laser Handbook

110 Diode Lasers Semiconductor Laser Diodes 111

describe the increase of threshold current and the decrease of internal
efficiency with temperature T:

   T 
T
I ~exp  , η ~ exp −   (5.4)
th
 T 0  i  T 1
A significant effort has been made to increase the wall-plug efficiency
of high-power lasers. Design optimization is focused on all contribu-
tors to the power waste: lowering of internal loss, increase of internal
efficiency, and decrease of series resistance. A typical design tradeoff is
in the clad-layer doping. Higher doping leads to lower series resistance
and self heating, while also giving rise to higher internal losses due to
free carrier absorption. Figure 5.8 shows the results of a design optimi-
zation for a 20-mm-wide, 915-nm AlGa(In)As/GaAs laser diode that
peaks at a wall-plug efficiency of 73 percent at 25ºC operating tempera-
–1
9
ture. This was achieved by decreasing internal loss to less than 1 cm
and achieving high values of T = 198 K and T = 962 K.
0 1
5.7 High-Power Broad-Area Laser Diodes

The maximum power from a diode laser P max is proportional to the
internal optical power density at catastrophic optical mirror damage
P , according to the expression: 10
COMD
d    1 − R 
P =   W   P (5.5)
max G + COMD
   1 R 
75

60
Heat sink T = 5°C
Efficiency (%) 45 Heat sink T = 25°C

30

15

0
0 1 2 3 4 5 6 7 8 9 10 11 12
Current CW (A)
Figure 5.8 Dependence of CW power efficiency versus drive current
recorded for an L = 2 mm, W = 20 mm laser diode at 5°C and 25°C. 9