We report on our progress developing long wavelength high power laser diodes based on the InGaAsP/InP alloy system emitting in the range from 1400 to 2010 nm. Output power levels exceeding 50 Watts CW and 40% conversion efficiency were obtained at 1470 nm wavelength from 20% fill factor (FF) bars with 2 mm cavity length mounted on water cooled plates. Using these stackable plates we built a water cooled stack with 8 bars, successfully demonstrating 400 W at 1470 nm with good reliability. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. For lasers emitting in the range from 1930 to 2010 nm we achieved output power levels over 15 W and 20 % conversion efficiency from 20% FF bars with 2 mm cavity length on a conductively cooled platform. Life testing of the 1470 nm lasers bars over 14,000 hours under constant current mode has shown no significant degradation.
We report on the performance of our high power and high efficiency 14xx nm lasers in different formats as packaged on conduction cooled packages using soft solder. Single emitters exhibited output powers as high as 6 watts, while six emitter minibars output 20 W, and 20% fill factor (ff) bars provided over 40 W of output power. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. These same 20% ff bars output close to 90 W when operated quasi CW (QCW). Preliminary life testing of these bars for over 5000 hours under constant current mode has shown no significant degradation.
The slow axis (SA) divergence of 20% fill-factor, 980nm, laser diodes (LDs) have been investigated under short pulsed
(SP) and continuous (CW) operation. By analyzing the data collected under these two modes of operation, one finds that the SA divergence can be separated into two components: an intrinsic divergence and a thermally induced divergence. At low injected current and power, the intrinsic SA divergence is dominant while at high power their magnitudes are approximately equal. The thermal gradient across the broad stripe is negligible under SP operation and, the SA divergence increased at a much slower rate as a function of injected current, thereby increasing the brightness of the LD by 2X. SRL has redesigned microchannel coolers that remove the thermal gradient under CW operation thereby eliminating the thermally induced SA divergence resulting in LDs that are 2X brighter at 300W/bar.
We present a novel, high-power stack of 20% fill-factor, 976nm, laser-diode bars, each directly attached to an enhanced lateral-flow (ELF), copper-based, water-cooled heat-sink. The heat-sinks contain mounting screws that form a kinematic mount to minimize detrimental mechanical-stress on the diode bars while also providing beneficial, double-side cooling of the bars. A stack of 18-bars, emitting 2.54kW, was constructed to validate the technology. Using standard optics and a polarization multiplexer, a 320μm diameter, 0.3NA focus is achieved with a 6-bar stack that robustly couples 450W, with a ~67% coupling efficiency, from a passive, 400μm, 046NA doubleclad fiber.
We report the development of a fiber-coupled diode laser module with high spatial and spectral brightness. Four
arrays of diode laser bars are multiplexed using polarization and narrow-band wavelength combination. The module
achieves 500 W of output power from a 200 μm, 0.2 NA fiber. The output spectrum, composed of contributions
from more than 150 emitters, is narrowed using VBGs and has nearly 100% content within +/- 1.5 nm of 975 nm at
High brightness, laser-diode bars are required for efficient coupling into small-core optical-fibers. Record power and
brightness results were achieved using 20% fill-factor, 980nm, 1cm-wide, 4mm cavity-length bars. Lifetimes of single
bars, operated CW at 200W and 20°C, exceed 1000hr. Due to superb thermal management, the power conversion
efficiency (PCE) exceeds 60% at 200W output power. Similar lifetime and PCE were obtained for a 3-bar stack
emitting 600W output power.
We report the development of a fiber-coupled diode laser module with high spatial and spectral brightness. Four arrays
of diode laser bars are multiplexed using polarization and narrow-band wavelength combination. The module achieves
500 W of output power from a 200 μm, 0.2 NA fiber. The output spectrum, composed of contributions from more than
150 emitters, is narrowed using VBGs and has nearly 100% content within +/- 1.5 nm of 975 nm at full power.
The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has been significantly improved in the last few years, with commercially available modules now rivaling the brightness of lamp-pumped Nd:YAG lasers. We report progress in the development of these systems for a variety of applications, such as material processing and pumping of solid state and fiber lasers. Experimental data and simulation results for wavelength stabilized outputs from 200 µm diameter fibers at 975 nm for power levels greater than 200 W will be presented. The enabling technology in these products is supported by key developments in tailored diode laser bars with low slow axis divergence, micro-optics, diode laser packaging, and modular architecture.
Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the
past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser
pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm
fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved
for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence
while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.
State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source-conduction cooled
diode laser stacks. The intensity of a QCW vertical stack is limited by the peak power of each diode bar and the bar
pitch. The minimum bar pitch of the existing laser diode stacks on the market is about 400um. In this paper, we present a
unique vertical diode laser stack package design to achieve a bar pitch as low as 150um, which improves the intensity of
the stack by nearly 3 times. Together with the state-of-art diode laser bar from Coherent, greater than 30kW/cm<sup>2</sup> peak
power density is achieved from the emitting area of the vertical stack. The p-n junction temperature of the diode bars in
the device under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability
results for these diode laser stacks are also reported.
Developments in Nd-based lasers pumped on the <sup>4</sup>I<sub>9/2</sub>→<sup>4</sup>F<sub>3/2</sub> transition have led to
demands for increased power, brightness, and spectral stability from diode pump sources.
We describe the development of fiber coupled diode pump sources that generate >120W
of power from a 400μm, 0.22NA fiber at 88Xnm wavelengths. In order to maintain
spectral purity at these high powers, we investigated the use of Volume Bragg Gratings to
stabilize the wavelength of these multi-bar systems. A detailed study of the trade-offs
between facet reflectivity and VBG reflectivity was conducted in order to determine an
optimal combination that balances output power and locking range.
In complement to the developments in 88Xnm pumping, recent interest in eye-safe fiber
lasers have resulted in the development of Tm-doped fiber lasers pumped at 79X
wavelengths. We describe the development of fiber coupled products with >80W from a
200μm, 0.22NA fiber, including the use of optimized bar geometries to improve fiber
We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO,
diamond composite and other advanced materials. We present the results of residual bonding stress from various solders
and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of
threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for
a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability
under on-off life-testing and thermal cycling tests.
The packaging of high power diode laser bars requires a high cooling efficiency and long-term stability. Due to the
increasing output power of the diode laser bars the cooling performance of the packaging becomes more important.
Nowadays micro channel heat sinks seem to be the most efficient cooling concept in regard to high power applications.
The active area of the p-side down mounted laser bar is located directly above the micro channels. In other applications
where conductive cooled heat sinks are used the bars are mounted on copper CS mount, CuW submount or high
All these packaging ideas use wire bonds or thin copper sheets as a n-contacts. The thermal advantage of these contacts
can be neglected.
N-contact cooling is typically used to achieve new records of optical output power in the labs.
These studies analyze the properties of an additional n-contact cooling. The cooling performance of a package cooled on
both sides can be improved by more than 20% when compared with typical wire bonds or metal sheets.
Different packaging styles with metal sheets, heat spreaders (expansion matched) and active n-side cooling are
investigated. The effect of n-side cooling with regards to the fill-factor and cavity length is analyzed also.
The first part of this paper approaches the topic theoretically. Simulations are carried out and show the advantages and
differences of different package styles in comparison to bar geometries variations. The second part of the studies
characterizes and analyses fabricated samples made out of copper in view of cooling performance, handling, and induced
stress. The results of different bar geometries and packaging styles are compared and guidelines for n-side cooling are