Page 142 - High Power Laser Handbook

112 Diode Lasers Semiconductor Laser Diodes 113

35
Th = 15C
30
25
Power (W) 20 CW

Pulsed
15
10
5

0
0 5 10 15 20 25 30 35 40
Current (A)
Figure 5.10 Light current characteristics of a 940-nm broad-area laser. 21

The highest powers and efficiencies have been reported for diodes
lasing in the 900 to 1000 nm wavelength range. These lasers have a
strained InGaAs quantum well active region. For diodes lasing in the
800 to 870 nm wavelength range, the quantum well material is typically
GaAs or AlGaAs, and the quantum well is lattice matched to the GaAs
substrate. These short-wavelength lasers have higher threshold current
density due to lower gain. They are more sensitive to temperature and
less efficient than 9XX-nm lasers. Higher photon energy also causes
lower COMD limits. Figure 5.11 shows power as high as 12.2 W from a
4.5-mm-long, 808-nm laser with 90-mm emitting aperture. Reliable
20
operating powers for 808-nm, 100-mm-wide broad-area laser diodes can
reach the 5 to 6 W range, but more typically operate at 2 to 3 W.

5.8 High-Power Bars
Laser diodes can be arranged as an array on a single chip called a
“bar.” Important bar parameters are the fill factor, which is the ratio
of the sum of emitter widths to the total bar width, and the pitch or
spacing of the emitters. The standard bar width is 10 mm, although
minibars with smaller widths are used for some applications. Low fill-
factor bars allow high power per emitter, because there is less thermal
cross talk between emitters. Linear power density as high as 85 mW/mm
was reported for 9XX-nm bars with low fill factors in the 9 to 15 percent
22
range. To increase total output power, higher fill factors can be
employed with a corresponding decrease in the linear power density
22
per emitter, dropping to 45 mW/mm for a 50 percent fill-factor bar.
Power as high as 325 W was reported for a 1-cm-wide, 920-nm bar
23
with 50 percent fill factor and proper cooling. Figure 5.12 shows