Fiber combining multiple pump sources for fiber lasers has enabled the thermal and
reliability advantages of distributed architectures. Recently, mini-bar based modules have been
demonstrated which combine the advantages of independent emitter failures previously shown in
single-stripe pumps with improved brightness retention yielding over 2 MW/cm2Sr in compact
economic modules. In this work multiple fiber-coupled mini-bars are fiber combined to yield an
output of over 400 W with a brightness exceeding 1 MW/cm2Sr in an economic, low loss
New-generation multi-mode 9xx mini-bars used in fiber pump modules have been developed. The epitaxial designs have
been improved for lower fast-axis and slow-axis divergence, higher slope efficiency and PCE by optimizing layer
structures as well as minimizing internal loss. For 915nm mini-bars with 5-mm cavity length, maximum PCE is as high
as ~61% for 35W operation and remains above 59% at 45W.
For 808nm, a PCE of 56% at 135W CW operation has been demonstrated with 36%-fill-factor, 3-mm-cavity-length,
water-cooled bars at 50°C coolant temperature. On passive-cooled standard CS heatsinks, PCE of >51% is measured for
100W operation at 50°C heatsink temperature. Leveraging these improvements has enabled low-cost bars for high-power,
Continued advances in high power diode laser technology enable new applications and
enhance existing ones. Recently, mini-bar based modules have been demonstrated which combine
the advantages of independent emitter failures previously shown in single-stripe architectures with
the improved brightness retention enabled by multi-stripe architectures.
In this work we highlight advances in a family of compact, environmentally rugged mini-bar
based fiber coupled Orion modules. Advances in PCE (power conversion efficiency) and reliable
operating power from a 9xx nm wavelength unit are shown from such modules. Additionally,
highly reliable fiber coupled operation and performance data is demonstrated in other wavelengths
in the 780 - 980 nm range. Data demonstrating the scaling this technology to 25W and higher
power levels will be given.
High-power, packaged diode-laser sources continue to evolve through co-engineering of epitaxial design, beam conditioning and thermal management. Here we review examples of improvements made to key attributes including reliable power, brightness, power per unit volume and value.
As GaAs based laser diode reliability improves, the optimum architecture for diode pumped
configurations is continually re-examined. For such assessments, e.g. bars vs. single emitters, it is
important to have a metric for module reliability which enables comparisons that are the most
relevant to the ultimate system reliability. We introduce the concept of mean time between emitter
failures (MTBEF) as a method for characterizing and specifying the reliability of multi-emitter
pumps for ensemble applications. Appropriate conditions for an MTBEF model, and the impact of
incremental changes of certain conditions on the robustness of the model are described.
In the limit of independent random failures of individual emitters as the dominant failure mechanism
it is shown that an ensemble of multi-emitter modules can be modeled to behave like an ensemble of
single emitter modules. The impact of thermal acceleration due to failed emitters warming other
emitters on a shared heat-sink is considered. Data taken from SP built multi-emitter devices bonded
with AuSn on CTE matched heat-sinks is compared with the MTBEF model with and without
correction for the thermal acceleration effect.
Here we present characteristic performance of laser-diode devices employing a novel CTE-matched heatsink technology
(where CTE is Coefficient of Thermal Expansion). Design variants of the composite-copper platforms include form-fit-compatible
versions of production CS (for standard 1-cm-wide bars) and CT (for single-emitter devices and mini-bars)
assemblies. Both employ single-step AuSn bonding and offer superior thermal performance to that of current production
standards. These attributes are critical to reliability at high powers in both CW and hard-pulse (e.g., 1sec on/1sec off)
The superior thermal performance of the composite-copper CS device has been verified in CW testing of bars where
85W is typically obtained at 95A (compared to 76W from production-standard, indium-bonded, solid-copper CS
devices). This result is especially significant as alternative CTE-matched bar platforms (e.g., those employing a sub-mount
bonded to a solid copper heatsink) typically compromise the effective thermal resistance in order to achieve the
CTE match (and often require two-step bonding). The close CTE match of the composite-copper CS results in relatively
narrow, single-peaked spectra. Initial step stress tests of eight devices in hard-pulse operation up to 80A has been
completed with no observed failures. Six of these devices have subsequently been operated in hard-pulse mode at 55A
for >4000 with no failures.
The CT variant of the composite-copper heatsink is predicted to offer a reduction in thermal resistance of nearly 30% for
a 5-emitter mini-bar (500-μm pitch). In first-article testing, the maximum achievable CW power increased from 20W
(standard CuW CT) to 24W (composite-copper CT). As with the CS devices, the composite-copper CT assemblies
exhibited characteristically narrower spectral profiles.
Leveraging improvements to device structures and cooling technologies, ultra-high-power bars have been integrated into
multi-bar stacks to obtain CW power densities in excess of 2.8 kW/cm2 near 960 nm with spectral widths of <4nm FWHM. These characteristics promise to enable cost-effective solutions for a variety of applications that demand very high spatial and/or spectral brightness. Using updated device designs, mini-bar variants have been employed to derive CW powers of several tens of Watts near 940 nm on traditional single-emitter platforms. For example, >37 W CW have been obtained from 5-emitter devices on standard CuW CT heatsinks with AuSn solder. Near 808 nm, a PCE of 65% with a slope efficiency of 1.29 W/A has been demonstrated with a 20%-fill-factor, 2-mm-cavity-length bar.
This paper gives an overview of recent product development and advanced engineering of diode laser technology at
Spectra-Physics. Focused development of device design, heat-sinking and beam-conditioning has yielded significant
improvement in both power conversion efficiency (PCE) and reliable power, leading to a family of new products. CW
PCEs of 60% to 70% have been delivered for the 880 to 980 nm wavelength range. For 780 to 810 nm, PCE are typically
between 50% and 56%. Comprehensive life-testing indicates that the reliable powers of devices based on the new
developments exceed those of established, highly reliable, production designs.
For the progress of ultra-high power bars, CW output power in excess of 1000 W and 640 W have been demonstrated
from single laser bars with doubled-side and single-side cooling, respectively. Spatial power density of greater than 2.8
kW/cm2 and FWHM spectral widths of 3.5 nm have been obtained from laser stacks.
Successful thermal and stress management of edge-emitting GaAs-based diode lasers is key to their performance and
reliability in high-power operation. Complementary to advanced epitaxial structures and die-fabrication processes, next-generation
heatsink designs are required to meet the requirements of emerging applications. In this paper, we detail the
development of both active and passive heatsinks designed to match the coefficient of thermal expansion (CTE) of the
laser die. These CTE-matched heatsinks also offer low thermal resistance, compatibility with AuSn bonding and
improved manufacturability. Early data representing the performance of high-power devices on the new heatsinks are
included in the presentation.
Among the designs are a water-cooled, mini-channel heatsink with a CTE of 6.8 ppm/°C (near to the nominal 6.5
ppm/°C CTE of GaAs) and a thermal resistance of 0.43 °C/W (assuming a 27%-fill-factor diode-laser bar with a cavity
length of 2 mm). The water flow in the heatsink is isolated from the electrical potential, eliminating the possibility of
electrolytic corrosion. An additional feature of the integrated design is the reduction in required assembly steps.
Our next-generation, passive, CTE-matched heatsink employs a novel design to achieve a reduction of 16% in thermal
resistance (compared to the predecessor commercial product). CTE's can be engineered to fall in the range of
6.2-7.2 ppm/°C on the bar mounting surface. Comparisons between simulated performance and experimental data (both
in CW and long-pulse operation) will be presented for several new heat-sink designs.
Ongoing optimization of epitaxial designs, MOCVD growth processes, and device engineering at Spectra-Physics has
yielded significant improvement in both power conversion efficiency (PCE) and reliable power, without compromising
manufacturability in a high-volume production environment. Maximum PCE of 72.2% was measured at 25 °C for 976-
nm single-emitter devices with 3-mm cavity length. 928 W continuous-wave (CW) output power has been demonstrated
from a high-efficiency (65% maximum PCE) single laser bar with 5-mm cavity length and 77% fill factor. Eight-element
laser bars (976 nm) with 100&mgr;m-wide emitters have been operated at >148 W CW, corresponding to linear power
densities at the facet >185 mW/&mgr;m. Ongoing life-testing, in combination with stepped stress tests, indicate rates of
random failure and wear-out are well below those of earlier device designs.
For operation near 800 nm, the design has been optimized for high-power, high-temperature applications. The highest
PCE for water-cooled stacks was 54.7% at 35°C coolant temperature.
Micro-channel heatsink assemblies made from bonding multi-layered etched metal sheets are commercially available
and are often used for removing the high waste heat loads generated by the operation of diode-laser bars. Typically, a
diode-laser bar is bonded onto a micro-channel (also known as mini-channel) heatsink then stacked in an array to create
compact high power diode-laser sources for a multitude of applications. Under normal operation, the diode-laser waste
heat is removed by passing coolant (typically de-ionized water) through the channels of the heatsink. Because of this,
the heatsink internal structure, including path length and overall channel size, is dictated by the liquid coolant properties.
Due to the material characteristics of these conductive heatsinks, and the necessary electrically serial stacking geometry,
there are several restrictions imparted on the coolant liquid to maintain performance and lifetime. Such systems require
carefully monitored and conductive limited de-ionized water, as well as require stable pH levels, and suitable particle
filtration. These required coolant systems are either stand alone, or heat exchangers are typically costly and heavy
restricting certain applications where minimal weight to power ratios are desired.
In this paper, we will baseline the existing water cooled Spectra-Physics MonsoonTM heatsink technology utilizing
compressed air, and demonstrate a novel modular stackable heatsink concept for use with gaseous fluids that, in some
applications may replace the existing commercially available water-cooled heatsink technology. We will explain the
various benefits of utilizing air while maintaining mechanical form factors and packing densities. We will also show
thermal-fluid modeling results and predictions as well as operational performance curves for efficiency and power and
compare these data to the existing commercially available technology.
High power diode lasers have demonstrated reliable output power of more than 50W per diode far beyond 10,000 hours. Record output power of more than 300W per diode laser bar has been reported. The improved reliability of the semiconductor material demands a review of the performance of the actively water cooled heatsink with regards to the expected lifetime. Results from corrosion tests at various water conditions for durations of more than 13,000 hours predict an extended usage of water-cooled heatsink beyond 20,000 hours without significant performance change.