Page 133 - High Power Laser Handbook

102 Diode Lasers Semiconductor Laser Diodes 103

100

Multiple-transverse mode
(~100 µm wide aperture)
10 power growth
15%/year
CW power (W)

1
Heavy telcom
investments
Single-transverse mode
(~3 µm wide aperture)
0.1
1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
Year of introduction
Figure 5.1 Growth in multimode and single-mode reliable continuous wave (CW)
power for 9XX-nm.

5.3 High-Power Laser Diode Attributes
Several attributes distinguish the high-power laser class from other
semiconductor lasers and are important to their utility. First is the
rated optical power level at a specified reliability point. This is a key
tradeoff in almost all high-power laser designs, because almost all
high-power lasers can operate above the rated power, though at the
expense of lower reliability. Power levels in the 10-W to 20-W continu-
ous wave (CW) range are commercially available from a single-aperture
source. The maximum power is typically limited by the linear power
density (i.e., the power divided by aperture width) at the laser facet,
where the light exits the confinement of the semiconductor waveguide
and diffracts into air. More optical power may be obtained from a single
chip either by increasing the width of the emission aperture or by
forming monolithic arrays of these emitters on a laser “bar.” Although
these techniques increase the total power, they do so at the expense of
brightness. Optical power is balanced by the etendue, or the two-
dimensional spot size of the light in physical and numerical aperture
space. The brightness of the laser, defined as the optical power divided
by the etendue, is the physical parameter that dictates the extent to
which various beam-combining methods may be used to form a single,
higher-power beam from multiple single emitters. Recent advances in
higher-brightness performance have come from both increases in reliable
optical power from the chip and reduction of the etendue, especially
in the far-field divergence of light emission.