Page 141 - High Power Laser Handbook

110 Diode Lasers Semiconductor Laser Diodes 111

where W is the stripe width, R is the front facet reflectivity, d is the
quantum well thickness, and G is the transverse optical confinement
factor such that d/G is the equivalent spot size. Broad-area lasers are
able to achieve high output powers due to a wide stripe width W.
P COMD is a function of the active region material and facet passivation
2
techniques. 11–13 A P COMD value as high as 24 MW/cm was reported
for CW operation of broad-area InGaAs/AlGaAs laser diodes lasing
14
at 940 nm. The transverse size of the optical mode can be increased
15
using large optical cavity designs, which provide a low optical con-
finement factor G. The optical confinement factor can be further
reduced by designing an asymmetric waveguide structure and
16
using an optical trap layer. Another design tool for achieving high
17
optical power is to increase the laser cavity length. Larger cavity
length leads to lower electrical and thermal resistances, which in turn
result in reduced heating, higher efficiency, and higher thermal roll-
over powers. Efficient operation of long cavity length lasers requires
low internal loss of less than 1 cm . State-of-the-art broad-area lasers
–1
with stripe widths of approximately 100 mm have 4 to 5 mm cavity
length and reach more than 20 W optical power in CW operation. 18–20
Reliable operating power for these lasers is rated in the 10 to 12 W
range. Figure 5.9 shows 26.1 W of CW power achieved from a 5-mm-
long, 90-mm-wide, 940-nm InGaAs/AlGaAs laser. 14
Continuous wave power is limited not only by COMD but also
by thermal rollover, as self heating results in decreasing efficiency.
Pulse mode operation eliminates self heating, and much higher pow-
ers can be achieved. Figure 5.10 shows power as high as 32 W for
20-ms pulse duration and 1-kHz repetition rate for a 940-nm laser
diode with a 100-mm emitting aperture. 21

30

25

Power (W) 20 2204

15
3022
10

5

0
0 10 20 30 40
Current (A)

Figure 5.9 Experimental rollover light current characteristics for lasers with
AlGaAs barrier series. 14