DEPS Abstract

Guest >> Sign In

DIRECTED ENERGY PROFESSIONAL SOCIETY

Abstract: 25-Symp-101

UNCLASSIFIED, PUBLIC RELEASE

Immersion-cooled, High- brightness Laser Diode Stacks with Lensing for Next Generation Laser Systems

High-brightness laser diodes are vital for next-generation laser systems in defense, inertial fusion, medical devices, and material processing. Increasing demands for higher power, brightness, and compact designs pose significant thermal management challenges, as higher power densities generate excessive heat that can degrade performance and reduce device lifetime. Previously, a >3x brightness increase was demonstrated in continuous-wave, laser diodes at a bar-to bar pitch of 0.390 mm using a novel interleaved immersion-cooling architecture developed at Lawrence Livermore National Laboratory, where microchannel-submounts replace conventional solid submounts. In conventional designs, the submounts in backplane-cooled stacks require a high bar-to-bar pitch due to the direct correlation between heat transfer surface area and pitch. However, in the novel immersion cooling design, fluid is pumped directly perpendicular to the front facet plane into the microchannels, allowing the bar-to-bar pitch to be reduced while simultaneously increasing heat transfer surface area. Due to low thermal resistance and optimized package design, interleaved immersion cooling demonstrates enhanced performance with longer cavity lengths, enabling a more compact device and offering significant SWaP advantages. A critical requirement for laser diode stacks is the integration of fast-axis collimation (FAC) lenses, as the light emitted from the laser bars is highly divergent. Lenses are typically positioned within a few hundred microns of the laser bar and are necessary to collimate the fast-axis beam. However, their close proximity to the laser bar in interleaved immersion-cooled laser diode stacks introduces new challenges, including restricted fluid flow into the microchannels, flow maldistribution, increased pressure drop, and reduced cooling efficiency. These potential flow maldistribution issues result in temperature gradients across the laser bar, leading to spectral broadening, wavelength shifts, and overall performance degradation.
To address these challenges, we propose a novel microchannel architecture that removes a section of the fins beneath the front facet to improve fluid distribution while accommodating FAC lenses. A custom-designed FAC array tailored to the laser bar geometry simplifies alignment and integration into the test manifold. An experimental prototype has been developed to evaluate this design in 1-bar, 3-bar, and 5-bar stacks of 940 nm continuous wave diodes bonded to microchannel heat sinks under fluid inlet restrictions. To assess the impact of flow distribution on wavelength stability, we will compare two microchannel designs: one without cutbacks (resulting in highly restricted flow) and one with the proposed cutback design. These results will also be compared to a control case with no lenses and uniform flow. Computational Fluid Dynamics will be used to guide microchannel design. Data on flow uniformity, temperature gradients, and spectral stability, will be collected to evaluate the impact of FAC integration on immersion-cooled laser diode systems. This study specifically investigates the relationship between flow distribution and wavelength shift, as well as spectral broadening caused by temperature variations across the laser bar due to flow maldistribution. By comparing the spectral performance of lensed and non-lensed immersion-cooled systems, we aim to demonstrate that the proposed design mitigates temperature gradients, enabling stable operation and >3x brightness. These findings will inform the development of scalable, compact, and high-performance laser diode systems for a wide range of applications.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 within the LDRD program (24-ERD-059). LLNL-ABS-872858

UNCLASSIFIED, PUBLIC RELEASE