Aluminum-free diode lasers
produce higher power over longer
periods than conventional devices.
Merrill M. Apter and Arto
Salokatve / Laser Focus World
High-power diode laser bars and
stack arrays emitting in the 770- to
840-nm range currently are finding
mainstream applications. The devices
are commonly used, for example, as
pump sources for holmium (Ho),
holmium/thulium (Ho/Tm), and
neodymium (Nd) based solid-state
lasers. Within industrial materials
processing, avionics, medicine, and
reprographics, diode lasers
increasingly are being used. And
devices emitting specifically at 770
and 795 nm play key roles in a
promising new method for generating
polarized noble gases. These lasers,
respectively, optically pump
potassium and rubidium vapor and
produce highly polarized 3He, 21Ne,
or 129Xe gases via spin exchange.
Researchers currently are studying
how these gases might be used in
applications ranging from
high-resolution medical imaging and
physics experiments to mineral
exploration.
As demand for high-power laser
diode bars and stack arrays has
grown, so has demand for higher
output powers, brighter sources, and
longer lifetimes. But using
conventional aluminum gallium
arsenide (AlGaAs) laser diodes to
achieve these goals has proven to be
problematic because of the aluminum
in the active junctions of such
devices. While aluminum is a key
component in these laser
diodes—varying the amount of
aluminum in the active junction
during wafer fabrication changes the
peak emission wavelength—it also is
highly reactive. Compared to arsenic
and gallium, aluminum has a great
affinity for oxygen. The resulting
oxidation can quickly lead to
degraded laser diode performance and
eventual device failure.
Two widely documented problems
have been specifically tied to the
presence of aluminum in a laser
diode:
- Catastrophic optical
damage—This is the primary
early failure mode in
high-power, unpassivated AlGaAs
devices. Light emission spurs
oxidation, leading to increased
absorption at the facet. In
turn, local temperature builds,
cutting the effective bandgap
energy, and absorption at the
facet increases further. This
chain of events eventually
results in catastrophic optical
damage. Increasing the operating
current accelerates the damage
process, so an AlGaAs laser
diode is, by necessity, limited
in its output power.
- Dark-line and dark-spot
defects—AlGaAs laser diodes
can fall victim to these
defects, which consist of
nonradiative recombination
centers resulting from crystal
lattice flaws. Device
performance degrades rapidly if
these defects form near the
active region of a laser,
because they extend through the
material when the laser is
activated. When these defects
propagate through an AlGaAs
laser diode active junction,
they produce localized dark
lines or spots that prevent
light emission.
Seeking a way of overcoming these
limitations, in the late 1980s
researchers developed aluminum-free
laser diode material—indium gallium
arsenide phosphide (InGaAsP)—and
found it ideal for fabricating laser
diodes with higher output powers,
brighter sources, longer lifetimes,
and greater reliability than AlGaAs
laser diodes. Researchers discovered
that, like AlGaAs, the InGaAsP
composition could be altered to
force changes in lasing wavelength,
making InGaAsP devices capable of
handling most high-power laser diode
applications. In addition, due to
the elimination of antioxidation
measures in the InGaAsP wafers,
overall manufacturing costs for the
laser diodes were less than those
for AlGaAs devices.
We have been manufacturing
aluminum-free active area (AAA)
laser diodes in high volume for
nearly two years, with a large,
field-proven installation base now
in place. These devices are used in
numerous industrial materials
processing, scientific, medical, and
graphic arts applications. Creo
Products (Burnaby, British
Columbia), for example is using 20-
and 40-W 830-nm aluminum-free laser
diode bars in its thermal prepress
imaging systems—more than 1000 of
these systems are installed
worldwide (see Fig. 1). In addition,
in its installed base of 600-plus
solid-state, frequency-doubled
Nd:YVO4 lasers, Coherent Laser Group
uses our aluminum-free 808-nm laser
diode arrays as pump sources.
Performance advantages
The InGaAsP laser diodes
demonstrate impressive performance.
Experiments at room temperature show
that at the wavelength needed for
diode pumping (between 780 and 810
nm), aluminum-free devices function
reliably at output powers and
brightness levels at least twice as
high as those of AlGaAs lasers.
Also, at elevated ambient
temperatures (typically 35°C to
60°C), InGaAsP devices offer power
conversion efficiencies on the order
of 42%. In our ongoing
room-temperature lifetests of 13
1-cm aluminum-free laser bars
emitting 40 W (30% fill factor) at
810 nm continuous wave (CW) the
devices have already lasted 6000
hours. Though some commercial diode
laser manufacturers have
demonstrated output powers of around
40 W from AlGaAs laser diodes, the
published lifetime data for these
devices are not as impressive as
those for aluminum-free laser
diodes. We also recently introduced
the first commercial 1-cm wide 60-W
CW InGaAsP laser bars operating
between 790 and 830 nm. Further, our
research shows that in the
not-too-distant future it should be
possible to commercially produce
aluminum-free laser diodes that
reliably provide 80 to 100 W CW.
Regarding dark-line/dark-spot
migration and catastrophic optical
damage, InGaAsP laser diode material
is clearly superior to AlGaAs.
Dark-line and dark-spot migration is
not a major problem for InGaAsP
devices because the hardening effect
of indium stabilizes the crystal
matrix. Catastrophic optical mirror
damage, as traditionally defined,
normally does not occur with InGaAsP
laser diodes. Instead, increases in
laser current lead to increases in
junction temperature, causing
carrier leakage from the junction
layer. Thermal rollover then
occurs—defined as output power
peaking at a particular point—with
additional increases in current
actually reducing output power.
Studies show that InGaAsP laser
diodes surpass catastrophic optical
mirror damage levels of 15 MW/cm2,
twice the power-density level of 800
nm-band AlGaAs devices. Working with
Jenoptik (Jena, Germany), our
engineers have achieved an output of
100 W before thermal rollover
occurrence from an 808-nm 1-cm-wide
InGaAsP bar (with 19 emitting
facets, each 1 mm long and 150 µm
wide).1
Other advantages of InGaAsP laser
diodes include significantly
increased device lifetimes, superior
power-conversion efficiency, and
lower beam divergence without
sacrificing performance. Parameters
used to define device lifetime are
rate of performance degradation and
time needed to lose a certain amount
of efficiency. For the current
commercial InGaAsP devices,
efficiency (at constant output
power) drops by less than 1%/1000
hours of operation at ambient
temperatures above 35°C. In the area
of beam divergence, current InGaAsP
devices are characterized by
fast-axis divergence of less than
30° (FWHM). For efficient AlGaAs
devices, typical fast-axis beam
divergence is about 40° (FWHM).
Commercial manufacturing
Given the advantages of InGaAsP
laser diodes, then, can these
devices be manufactured in the
volumes that commercial applications
demand? During the past few years we
have taken several steps to answer
this question. These include
ensuring reliable solid-source
molecular beam epitaxy (MBE)
production of InGaAsP wafers,
establishing a world-class, volume
production facility, and addressing
the carrier confinement issue.
Hence, we now offer aluminum-free
laser diodes as a mainstream
alternative to AlGaAs devices.
Our production facility is fully
vertically integrated for the
manufacture of InGaAsP laser diodes
in volume, with production
capabilities in Santa Clara and at
Coherent Tutcore Ltd. in Tampere,
Finland. Tutcore was established in
1991 as a spin-off from the
semiconductor group at Tampere
University of Technology and
continued the development of
Al-free-molecule beam-epitaxy growth
technology for optoelectronic
devices begun at the University. In
particular, Tutcore advanced the
state of the art in the fabrication
of single-mode Al-free diode lasers.
Coherent bought the the majority of
Tutcore in 1996 to bring the growth
technology and diode laser expertise
in-house.
Solid-source MBE growth reactors
can grow, at one time, 12 InGaAsP
wafers (2-in. diameter). These
wafers are of high quality,
exhibiting high wafer-to-wafer and
batch-to-batch process consistency,
and demonstrate high performance at
elevated ambient temperatures. And
because solid-source MBE is a highly
controlled, precise process, capable
of producing single-atomic-layer
resolution, it allows us to
manufacture laser diodes at optimal
near-infrared pump wavelengths for
the most demanding applications (Ho/Tm:YAG,
Nd:YLF, Nd:YAG, Nd:YVO4, rubidium,
and potassium vapor pumping).
We noted earlier that poor
carrier confinement is a rare
downside of InGaAsP laser diodes.
When an InGaAsP device incorporates
conventional cladding layers, the
active junction does not produce a
deep quantum well, meaning charge
carriers are not held as securely as
in AlGaAs devices. As the InGaAsP
device's current or operating
temperature increases, more and more
of the carriers escape. The result
is reduced slope efficiency (output
power/input current).
We addressed this problem by
engineering a proprietary device
architecture that significantly
increases carrier confinement in
InGaAsP laser diodes. Aluminum-free
devices (at 808 nm) based on this
architecture have been operated at
50 W quasi CW at temperatures as
high as 75°C and have maintained
high (>42%) electrical-to-optical
efficiency. This high-temperature
capability makes InGaAsP devices
suitable for a host of new
applications (such as military,
industrial, and medical). At the
same time, when these devices are
built into a system, their
high-temperature performance
translates into lower demands for
thermal management, which reduces
overall system costs.
These devices are setting new
standards for laser diode
performance. Looking ahead, many new
applications will be found for
high-power laser diodes, and InGaAsP
devices will undoubtedly spur a
major part of that growth.
REFERENCE
- F. Daiminger et al.,
"100 W CW Al-free 808 nm
Linear Bar Arrays," Proc.
CLEO 1997.
MERRILL M. APTER is
vice president, sales and
marketing, at the Semiconductor
Group, Coherent Inc., Santa
Clara, CA. ARTO SALOKATVE is
manager of growth and epitaxial
structures at Coherent Tutcore
Ltd., Tampere, Finland; e-mail:
arto.salokatve@tutcore.fi.
http://www.laserfocusworld.com /display_article/72704/12/none/none/Feat/DIODE-LASERS:-New-materials-promise-extended-life
