Page 172 - High Power Laser Handbook

140 Diode Lasers High-Power Diode Laser Arrays 141

range of 9 to 16 psi. With an increasing number of diodes in the stack,
the water supply must switch from a single-sided supply to a double-
sided supply, because the supply cross section for the water is limited
by the mini-channel heat sink to an inlet diameter of about 5 mm.
A pitch that is greater than or equal to 1.5 mm allows the attachment
of the fast-axis collimation lens directly to the mini-channel heat sink
via a glass submount. This method helps achieve the best beam point-
ing for the individual beam from the stack to less than 0.2 mrad.

6.5 Device Performance

6.5.1 Wavelength, Power, Efficiency, and Mode of Operation
Today, commercially available wavelength offerings range from 400 to
2200 nm. The highest power/bar is in the 880 to 980nm range, because
this is the peak electro-optical efficiency range of high-power diode
laser bars (as shown in Fig. 6.1). For example, in CW operating mode,
at 980 nm laser diodes mounted on mini-channel-cooled heat sinks
with AuSn bonding are now approaching 200 W/bar. However, in the
1800 to 2200 nm range, the maximum power of diode laser bars is usu-
ally less than 10 W. The practical limitations of waste heat removal
from the diode bar limit its maximum performance. In this mode of
operation, for maximum efficiency and lifetime, individual emitters
on the 10-mm-wide laser diode bar are spaced so that thermal cross-
talk and threshold current are minimized, while maximizing slope
efficiency. For example, the most commonly used configuration for a
60-W, 808-nm wavelength bar is a 30 percent fill factor (19 emitters in
which each emitter is 150 mm wide on a 500-mm pitch) and a 2-mm
cavity length. This configuration allows for collimation of both fast
and slow axes with commercially available microlenses.
However, in QCW mode, which is typically defined as duty cycles
of less than 3 percent and pulse widths of less than 500 ms, the peak
powers can reach in excess of 400 W/bar. This is because the average
power is very low, and the thermal load on the laser bar is a tiny frac-
tion of CW mode operation. Therefore, in QCW mode, the peak power
is only limited by the optical intensity limits at the laser diode bar
facet. Because facet optical intensity, and not thermal load, is the limit-
ing factor, the laser diode bars operating in QCW mode typically have
a much higher emitter count in a 10-mm bar (which is a much higher
fill factor); fill factors of up to 80 percent are not uncommon. The
higher emitter count (fill factor) spreads the peak power over more
emitters, thus reducing peak power intensity on each emitter facet.
6.5.2 Beam Quality and Brightness
Despite the many advantages of high-power diode lasers, such as
high electro-optical efficiency, compactness, and very high powers,