Broad-Area Laser Bars for 1 kW-Emission

Demonstrating increased efficiency and narrowed far field

Karow, Matthias M.

• Publisher: Cuvillier Verlag
• Year: 2022
• eBook ISBN:
  9783736966260
• Print ISBN:
  9783736976269
• Edition: 1

Industrial laser systems for material processing applications rely on the availability of highly efficient, high-brightness diode lasers. GaAs-based broad-area laser bars play a vital role in such applications as pump sources for high-beam-quality solid-state lasers and, increasingly, as direct processing tools.This work studies 940 nm-laser bars emitting 1 kW optical power at room temperature, identifying those physical mechanisms that are currently limiting electrical-to-optical conversion efficiency as well as lateral beam quality. In the process, several diagnostic studies on bars with varied lateral-longitudinal design were carried out. The effects of technological measures for performance optimization were analyzed, yielding a new benchmark in efficiency and lateral divergence.The studies into altered resonator lengths of 4 and 6 mm as well as fill factors between 69 and 87 % successfully reduce both the voltage dropping across the device and power saturation at high currents, enabling 66 % efficiency at the operation point. Concrete measures how to reach efficiencies ?70 % are presented thereafter, showing that doubling the efficiency value of the first 1 kW-demonstration in 2007 – amounting to 35 % – is in near reach.Investigation of the beam quality bases on a herein proposed and realized concept, in which the far field is resolved for each individual bar emitter. In this way, it is possible to determine how far-field profiles vary along the bar width and how much these variations affect the overall bar far-field. Further, such effects specific to bar structures can be separated into non-thermal and thermal influences. The effect of mechanical chip deformation (bar smile) as well as neighboring-emitter interaction has been investigated for the first time in active kW-class devices, yielding a lateral divergence as low as 8.8° at the operation point.

Table of contents

• Abstract
• 1. Introduction
• 1.1. The global laser market
• 1.2. State of the art
• 1.3. Structure of this work
• 2. Theoretical Background
• 2.1. Concept and physics of broad-area laser bars
• 2.2. Definition and governing effects of beam quality in broad-area lasers
• 2.3. Strained diode-laser chips
• 2.4. Thermal aspects of diode-laser operation
• 3. Experimental Methods
• 3.1. Fabrication of broad-area laser bars
• 3.2. Characterization techniques
• 4. Diode-Laser Bars for Highly Efficient 1 kW-Emission
• 4.1. Vertical structure for high-efficiency operation
• 4.2. Thermal transient
• 4.3. Varied resonator length
• 4.4. Impact of the lateral design
• 4.5. Conclusions
• 5. Efficient 1 kW-Emitting Bars Exhibiting Narrow Far Field
• 5.1. Mechanical chip deformation (bar smile)
• 5.2. Emitter cross heating
• 5.3. Bar edge emitters
• 5.4. Emitter sub-structure
• 5.5. Conclusions
• 6. Conclusions and Outlook
• Appendices
• A. Beam-Path Calculation Using the Matrix Formalism
• B. Thermal Wavelength Dependence of Longitudinal Modes
• Bibliography
• Public ContributionsPublic Contributions
• Acknowledgements
• Biographical Sketch