High-power broad-area diode lasers are the most efficient light sources, with 90-μm stripe GaAs-based 940-980 nm single emitters delivering > 10 W optical output at a power conversion efficiency ηE(10 W) > 65%. A review of efforts to increase ηE is presented here and we show that for well-optimized structures, the residual losses are dominated by the p -side waveguide and nonideal internal quantum efficiency ηi . The challenge in measuring efficiency to sufficient precision is also discussed. We show that ηE can most directly be improved using low heat sink temperature THS with ηE(10 W) reaching > 70% at THS = -50 °C. In contrast, increases in ηE at THS = 25 °C require improvements in both material quality and design, with growth studies targeting increased ηi and reduced threshold current and design studies seeking to mitigate the impact of the p-side waveguide. “Extreme, double asymmetric” (EDAS) designs are shown to substantially reduce p-side losses, at the penalty of increased threshold current. The benefit of EDAS designs is shown here using diode lasers with 30-μm stripes, (in development as high beam quality sources for material processing). Efficiency increases of ~ 10% relative to conventional designs are demonstrated at high powers.

Efficient High-Power Laser Diodes

We present the results of performance modeling of diode-pumped solid state laser beamlines designed for use in Laser Inertial Fusion Energy (LIFE) power plants. Our modeling quantifies the efficiency increases that can be obtained by increasing peak diode power and reducing pump-pulse duration, to reduce decay losses. At the same efficiency, beamlines that use laser slabs of Yb:YAG or Yb:S-FAP require lower diode power than beamlines that use laser slabs of Nd:phosphate glass, since Yb:YAG and Yb:S-FAP have longer storage lifetimes. Beamlines using Yb:YAG attain their highest efficiency at a temperature of about 200K. Beamlines using Nd:phosphate glass or Yb:S-FAP attain high efficiency at or near room temperature.

Comparison of Nd:phosphate glass, Yb:YAG and Yb:S-FAP laser beamlines for laser inertial fusion energy (LIFE) [Invited]

Fluid flow and heat transfer characteristics of liquid cooling microchannels in LTCC multilayered packaging substrate

A next-generation microchannel cooler has been developed for packaging laser diode arrays th at eliminates many of the problems associated with typical copper-based cooling designs.  The coolers are built on well-established Low-Temperature Cofired Ceramic technology and provide excellent thermal performance. They do not require the use of deionized water. This work highlights the strengths of the new cooler technology. The results of a long-term, high-flow-rate test which demonstrates the excellent erosion resistance of these coolers are presented. Three devices have been tested for 2500 hours at a flow rate of 0.25 GPM and show minimal signs of erosion. This data is compared to a similar test conducted with copper coolers. Several design parameters are also addressed for the ceramic coolers. The available form and fit characteristics are addressed, as is the custom-configurable nature of the devices. Keywords: laser diode, microchannel, cooling, diode array, LTCC, ceramic, deionized water, erosion

Next-generation microchannel coolers

Experimental study and numerical modeling of micro-channel cooler with micro-pipes for high-power diode laser arrays

One of the primary challenges of the Laser Inertial Fusion Engine (LIFE) project is the cost and availability of the laser
diode arrays needed to pump the solid-state laser gain media in the system. Current projections indicate that the arrays
need to be available for approximately one cent per Watt of output power, which is one to two orders of magnitude
cheaper than currently available.
This work focuses on potential manufacturing approaches to meet the projected specifications of the LIFE project.
Special attention will be paid to requirements related to power density (25 kW/cm 2 ), bar pitch (150 - 400 microns),
output wavelength (87x), and fast-axis divergence (+/- 4 degrees). A summary of the supply limitations and cost
ramifications of each requirement is presented. Also discussed are potential supply chain limitations that are anticipated
as a result of the immense size of the LIFE project.

Low-cost diode arrays for the LIFE project

Northrop Grumman/Cutting Edge Optronics has developed a line of microchannel-cooled laser diode arrays in which the coolant is electrically isolated from the current path. As a result, these arrays do not require the use of deionized water. The thermal performance of these arrays is presented and, in one case, shown to far exceed the performance of standard copper microchannel-cooled packages. This sets them apart from other non-DI microchannel coolers currently on the market which typically have worse thermal performance than copper MCCs.These newly-developed coolers are compatible with other technologies developed by NGCEO that enable long lifetimes under harsh operating conditions. In particular, these arrays can be assembled using hard-solder technology that enables power-cycled and high-current operation. The soldering technology also generates arrays that are easily lensed and satisfy a wide range of direct diode needs, including welding, marking, and complex line generation applications.These microchannel-cooled arrays represent an important next step in the solid state laser industry. They allow for much simpler (non-deionized) cooling systems while maintaining excellent thermal performance and lifetime.Northrop Grumman/Cutting Edge Optronics has developed a line of microchannel-cooled laser diode arrays in which the coolant is electrically isolated from the current path. As a result, these arrays do not require the use of deionized water. The thermal performance of these arrays is presented and, in one case, shown to far exceed the performance of standard copper microchannel-cooled packages. This sets them apart from other non-DI microchannel coolers currently on the market which typically have worse thermal performance than copper MCCs.These newly-developed coolers are compatible with other technologies developed by NGCEO that enable long lifetimes under harsh operating conditions. In particular, these arrays can be assembled using hard-solder technology that enables power-cycled and high-current operation. The soldering technology also generates arrays that are easily lensed and satisfy a wide range of direct diode needs, including welding, marking, and complex line generation applications.These micro...

Reliable cooling of high-power laser diode arrays

Northrop Grumman Cutting Edge Optronics has developed a family of arrays for high-power QCW operation. These
arrays are built using CTE-matched heat sinks and hard solder in order to maximize the reliability of the devices.
A summary of a recent life test is presented in order to quantify the reliability of QCW arrays and associated laser gain
modules. A statistical analysis of the raw lifetime data is presented in order to quantify the data in such a way that is
useful for laser system designers.
The life tests demonstrate the high level of reliability of these arrays in a number of operating regimes. For single-bar
arrays, a MTTF of 19.8 billion shots is predicted. For four-bar samples, a MTTF of 14.6 billion shots is predicted. In
addition, data representing a large pump source is analyzed and shown to have an expected lifetime of 13.5 billion shots.
This corresponds to an expected operational lifetime of greater than ten thousand hours at repetition rates less than 370
Hz.

Reliability of high-power QCW arrays

Northrop Grumman / Cutting Edge Optronics has developed three designs for microchannel-cooled laser diode arrays in
which the coolant is electrically isolated from the current path. As a result, these arrays do not require the use of
deionized water. The thermal performance of two of these designs is presented and, in one case, shown to far exceed
the performance of standard copper microchannel-cooled packages.
Also presented is a microchannel cooler made from ceramic material. This design leverages existing technology to
create a low-cost, high-performance alternative to copper-based microchannel coolers. This approach offers the greatest
promise for future development due to the vast assortment of existing capabilities that have already been developed for
similar ceramic structures used in the electronics industry.

Ed Stephens

Papers

Next-generation microchannel coolers

Low-cost diode arrays for the LIFE project

Reliable cooling of high-power laser diode arrays

Reliability of high-power QCW arrays

Elimination of Deionized Cooling Water Requirement for Microchannel-Cooled Laser Diode Arrays