Page 144 - High Power Laser Handbook

114 Diode Lasers Semiconductor Laser Diodes 115

80%
75%

70%
PCE 65%

60%
55% 35% fill factor
41% fill factor
50%
0 20 40 60 80 100 120
Optical output power (W)
Figure 5.13 PCE characteristics of a 100-W, 940-nm bar with room-
temperature cooling water.

demonstration of greater than 1000 W of power from a 940-nm bar
with 83 percent fill factor and double-sided cooling. 24
Significant advancements have been made on improving bar
power conversion efficiencies. Values of PCE greater than 70 per-
25
cent for 940-nm, 80 to 120-W bars were reported. 26–28 Figure 5.13
shows PCE as high as 76 percent for a 100-W, 940-nm bar.

5.9 High-Power, Single-Mode Laser Diodes
For many applications, a narrow-stripe diode laser operating in a
single lateral mode is necessary. The most widely used application is
a 980-nm pump source in erbium (Er)-doped fiber amplifiers (EDFAs).
Although a single-mode laser operates at lower absolute power than
multimode broad-area lasers with wider emitting apertures, it can
actually have higher brightness. The other important attribute of the
single-mode laser is its stable diffraction-limited far field, which is
essential for effective coupling into single-mode fiber. At high enough
powers, diffraction-limited operation of a single-mode laser can be
disrupted, which is usually observed as a nonlinearity, or “kink,” in
the light-versus-current characteristic. The kink power is an impor-
tant parameter of single-mode lasers that limits usable power from
the devices. A kink is usually accompanied by beam steering of sev-
eral degrees in the lateral far field, as shown in Fig. 5.14. 29
The beam steering is caused by coupling of fundamental and
higher-order lateral modes. 29,30 A model for coherent coupling of
31
fundamental and first-order mode has been proposed. This model
explains multiple kinks with increasing current by bringing the below-
threshold first-order mode in and out of resonance, thus drawing
power from the fundamental mode at each coherent kink. Increasing