Page 176 - High Power Laser Handbook

144 Diode Lasers High-Power Diode Laser Arrays 145

bar metallization, bonding, and heat sink design have cumulatively
contributed to long-term reliability.
Diode laser bars operating in QCW mode routinely have lifetimes
of greater than 1 Gigapulses at peak powers of 200 W or higher. Long-
term reliability of a laser diode bar is a function of three primary fac-
tors: (1) operating temperature, (2) operating power, and (3) operating
current density. For example, a 50-W, 808-nm CW laser bar mounted
on a passively cooled heat sink operating at 25°C will last twice as
long as the same bar operating at 35°C. If the same bar is operated at
60 W instead of 50 W (i.e., the same heat sink operating temperature),
then the junction temperature at the laser bar solder interface will rise
by approximately 5 to 7°C above the 50 W operation junction tem-
perature, which will reduce its lifetime. Furthermore, at 60-W opera-
tion, the current density is also higher, which accelerates aging of the
bulk semiconductor material.
However, advances in the use of aluminum-free active regions
and the increase of characteristic temperatures T and T have allowed
0
1
5
the diode laser bar to operate at higher junction temperatures with-
out compromising efficiency. Advancements in antireflection (AR)
coatings and facet passivation have increased the catastrophic optical
mirror damage (COMD) threshold of emitters, which has allowed
higher power per emitter in both CW and QCW modes of operation.
The use of hard solder, such as AuSn, and of coefficient of thermal
expansion (CTE)–matched heat sinks with lower thermal impedance
has allowed the diode bar to operate reliably at higher powers.

6.6 Product Performance
Without first collimating the beam with a cylindrical lens, the large
beam divergence (> 40°) perpendicular to the p-n junction (i.e., the
fast-axis direction) allows only a limited number of applications.
Side pumping of solid-state laser crystals, in which the diodes can
be placed in very close proximity to the laser crystal, is one of those
rare cases where the divergence is of benefit for uniform illumination
of the crystal. The divergence in the lateral direction of a diode laser
bar typically depends on the drive current or the current density, as
the beam is first gain guided and to some extent index guided by the
established temperature profile at higher output powers. The lateral
divergence takes on values of between 4 and 10 degrees. These values
for the divergence in both directions, as well as the dimensions of the
emitting area, result in an astigmatic beam. The beam parameter
2
product (full angle × diameter) is about 2 mm-mrad (M about 1.3) in
2
the fast-axis direction and up to 1700 mm-mrad (M about 1000) in
the slow-axis direction, which is too large for most applications. The
beam quality in the slow-axis direction can be further improved by
using an array of cylindrical lenses to collimate the individual emitters