State-of-the-Art and New Developments

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Diode Lasers for Pumping Solid State Lasers – State-of-the-Art and
New Developments
Stefan Heinemann
Fraunhofer USA, Center for Laser Technology
ph: 734-738-0500, fax: 734-354-3335,
email: [email protected]
Abstract: The requirements for diode pumping of disk, slab and fiber lasers is discussed. The state-of-the art and means for
scaling of power and brightness to fit the specific laser designs will be reviewed.
©2008 Optical Society of America
OCIS codes: 140.2020 Diode lasers, 140.5560 Pumping
Introduction
Development of new solid-state lasers is guided mainly by optimizing the beam quality and the optical efficiency for
increased average output powers and/or shorter pulses. The volume and propagation of the required laser mode and
the pump mode must therefore be designed to closely match each other in the laser active medium. Each of today’s
major resonator designs - thin disk, fiber, slab and rod - therefore require a design of the pump diodes, which is
tailored for maximum mode overlap. Diode lasers are characterized by output power, brightness, cooling and
package design and their impact on optimized mode overlap and integration into the laser system is discussed. Slab
lasers require low brightness and high power pump diodes, whereas fiber lasers require high brightness and lower
power diode lasers. Disk and end-pumped rods require medium brightness and high power diodes.
Overview
Broad area emitters are the major diode design for efficient high power pumping of solid-state lasers. A diffraction
limited beam is emitted parallel to the pn junction (fast axis) from an aperture of about 1.2 m with a divergence
angle of less than 600 (1/e2). Larger apertures and smaller divergences were realized over the last several years
making lower NA fast axis collimation more efficient and improving the reliability due to decreased power densities
at the facet. Perpendicular to the pn junction (slow axis) multiple transverse modes are emitted with a typical
divergence of 100 (1/e2). The emitter width is defined by the photolithographic processing and is typically in the
range of 90 m to 200 m. The maximum output power per emitter is determined by the damage threshold of the
facet coating, catastrophic optical damage (COD) occurs for excessive power densities (>100mW/ m), and the
temperature of the pn-junction, typically not to exceed 1130F (450C) for long reliability. Sophisticated coatings, i.e.
NAM (non absorbing mirrors) [1] or E2 [2] significantly increase the COD level and >220mW/ m are reported for
commercial devices [3]. Elevated junction temperatures reduce the electro-optical efficiency of the diode thus
resulting in an even higher temperature and enhance the growth of lattice dislocations, likely to cross the laser active
area. Electro-optical efficiencies used to be around 50% but recently 71% are reported [4] and around 60% are
commercially available for 9xxnm devices. Diodes emitting at 808nm are less efficient (around 55%) than diodes
emitting at 9xxnm due to the materials used in the epitaxy, GaAlAs/GaAs for 808 versus InGaAlAs/GaAs for
9xxnm, and thus emit about 1/3 less power than 9xxnm diodes. Diodes emitting in the range from 1,500nm to
1,900nm (InGaAs/InP) show about 1/3 of the output power of 9xxnm devices. A high electro-optical efficiency, an
excellent mirror coating and a diode package with low thermal impedance are the main drivers for the improved
output power and reliability of today’s commercial diode lasers.
Packaging
Conduction cooled packages with a single bar reach about 80W cw, while actively cooled heatsinks, typically
micro-channel cooling plates made from copper, are commercially rated for about 120W (24mW/ m) cw per bar at
9xxnm, but >500W are achieved. SE with a 100 m emitter are rated for about 12W (240mW/ m), but >20W are
reported [5]. The advanced coatings of SE, developed for the telecom, no thermal crosstalk of individual emitters
and a package with low thermal impedance enable the superior output power per unit length of emitting area of the
SE compared to the bar. Mini-bars are again in between bars and SEs.
Thermo-mechanical design of the diode packaged on the heatsink is essential for high output power and high
reliability. Based on 120W from a single bar with a contact area of 20mm2 mounted p-side down, a heat density of
400W/cm2 must be handled. The coefficient of thermal expansion of the diode (about 6.8*10-6K-1) must be closely
matched to the one of the heatsink to minimize thermally induced stress. Standard diode packages deploy copper as
a heatsink for high heat conduction and soft solders, preferably Indium, to mitigate the stress resulting in a typical
thermal impedance of 0.6K/W. Copper microchannel coolers show a thermal impedance of about 0.4K/W leading
to a temperature gradient between the pn-junction and the coolant of about 32K for a 120W device. Cavitation,
relatively high pressure drops and electro-corrosion need to be addressed for microchannel coolers through proper
water management, typically Di water with a controlled ph value is used. The soft Indium solder allows a reliability
of 10,000+ hours under continuous operation, but shows electromigration under high stress, induced either by high
current densities or by hard pulsing of the diode. Hard solders, such as eutectic AuSn, in combination with
expansion matched heatsinks are becoming increasingly popular achieving 50,000+ hours lifetime at slightly higher
thermal impedance. The diode is soldered onto a submount, typically ALN or alike, which is than bonded with a
soft solder on the main heatsink.
Beam Quality
Broad area diodes are divided in 3 groups – single emitters (SE), mini-bars, and bars. Bars are typically 10mm wide
and 1.5 to 2 mm long with a varying number and size of emitters determining its fill factor, output power and beam
quality. The beam quality is highly asymmetric with about 440mm*mrad (M2=1,457) in slow axis, and M2=1 in fast
axis. Micro-optical lens arrays are applied for bars with a fill factor of less than 50% improving the beam quality to
175mm*mrad (M2=583) but reducing the output power and increasing complexity and costs respectively. For a
symmetric beam quality, micro-optical components are used to cut the beam several times in slow axis and to
rearrange the sections on top of each other in fast axis direction. SEs are typically 0.4mm wide and 4-6mm long
with a 100 m broad emitter yielding a beam quality of 4.4mm*mrad (M2=15) in slow axis and thus represent the
diode with the best beam quality. Mini-bars comprise short bars or bars with low fill factor and are typically
configured with slow axis collimation lenses for a medium to high beam quality. Stacking in fast axis is done to
first scale the output power and secondly, symmetrizing the beam quality.
Brightness is defined as power density at the facet divided by the square of the numerical aperture. It is highest for
SE (200 kW/(mm*NA)2), lowest for a bar (0.2 kW/(mm*NA)2) and in between for a mini-bar. However, the
absolute power is much higher for bars than it is for SE. Bars are commercial available as stacks with typically up to
12 bars stacked on top of each other resulting in 1,400W from an emitting area of 10mm by 20mm. The dark zones,
caused by the heatsinks, are optically filled by symmertrization of beam quality or by optical interleaving of
multiple stacks. High output powers with SEs require handling of many diodes and lenses.
Many laser designs deploy fiber coupled pump diodes for homogenous pumping with a rotational symmetry.
Optical symmetrization and a high optical fill factor in near and far field are required. Bars and stacks typically
deploy slow axis microlens arrays and symmetrization techniques but the brightness is limited as described above.
SEs offer the highest brightness and eliminate the need for any optical symmetrization, which is replaced by a
tailored mechanical arrangement of the SEs, thus maximizing optical efficiency and minimizing complexity. More
than 100 SEs can be arranged for coupling into a 200 m fiber, NA=0.2 and >350W are demonstrated from the fiber
[6]. Further refinement of components and assembly techniques as well as wavelength multiplexing are under
development and will soon enable kilowatt systems making the direct diode a viable alternative to many lamppumped solid-state lasers.
Classification of Pump Diodes
Low brightness and high power are based on stacks delivering up to 1kW from a 400 m fiber, high brightness
diodes are based on mini-bars or SEs delivering typically 30W (200W) from a 100 m (200 m) fiber, both at a
single wavelength and fiber NA=0.22. Dense wavelength multiplexing based on Volume Bragg Grating is deployed
for scaling the output power in the pump band at identical beam quality. 600W are reported from a 200 m fiber,
NA=0.2 [7]. Laser designs requiring a high brightness pump source, such as fiber lasers, are pumped with fiber
coupled single emitters, whereas slab lasers are pumped with stacks. Disk lasers require a medium brightness and
are pumped with (beam shaped) stacks. End-pumped rods require a higher brightness and are pumped with beam
shaped bars, stacks or often fiber coupled devices. Side pumped rods require only a low brightness and are pumped
with bars.
Summary
Output power, beam quality and reliability of diode lasers were greatly improved over the last several years. Driven
by efficiency improvements and hard solder, output powers now range from 10W from a single emitter to >1kW
from stacks. Package designs are becoming more standardized with the bar and the single emitter being the
dominant configuration. Reliabilities from 20,000 to 200,000 hours are commercially available. Fiber coupled
devices now reach the beam quality and output power of lamp pumped solid-state lasers. Power scaling through
(dense) wavelength multiplexing and advanced diode designs will push the performance and brightness of diode
lasers even further in the near future approaching those of even some lamp pumped systems. The variety and
performance of todays’ diodes offer new possibilities for optimizing solid-state laser designs.
References
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