Laser Diode Submount CTE Mismatch Stress Control
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Solution Overview
Problem
Existing high power semiconductor laser diode devices face challenges with heat dissipation and mechanical stress due to mismatched thermal expansion coefficients between laser diodes and coolers, leading to reliability issues and suboptimal optical performance.
Innovation Solution
A submount/cooler assembly with a controlled coefficient of thermal expansion (CTEeff) that intentionally applies stress to the laser bar, differing from the CTEbar by a predetermined amount, allowing for optimized mechanical properties and improved reliability and spectral purity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If a CTE-matched submount is used to reduce thermal expansion mismatch, then mechanical stress is reduced, but solder interface stability deteriorates under thermal cycling
Solution Approach 1:
The patent introduces a stress relief layer as an intermediary between the laser bar and submount. This layer acts as a mediator that decouples the thermal expansion mismatch from the solder interface, allowing the submount to be CTE-matched for stress reduction while the stress relief layer protects the solder joint from thermal cycling damage.
Solution Approach 2:
The patent segments the mounting structure into distinct functional layers: laser bar, stress relief layer, submount, and cooler. This segmentation allows each layer to be optimized independently - the submount for CTE matching and stress reduction, the stress relief layer for protecting the solder interface, and the cooler for heat dissipation.
2Ease of manufacture
If soft solder is used for mounting, then ease of assembly is improved, but operational reliability deteriorates at high power levels
Solution Approach 1:
The stress relief layer is positioned beforehand between the laser bar and submount to cushion and absorb thermal expansion stresses before they reach the solder interface. This pre-positioned protective layer enables the use of soft solder for easy assembly while preventing its known weakness under thermal cycling.
3Reliability
If hard solder is used for mounting, then operational reliability is improved, but mechanical stress increases
Solution Approach 1:
The stress relief layer serves as a compliant intermediary that allows hard solder to be used for reliable high-power operation while preventing excessive mechanical stress from being transmitted to the laser bar. The stress relief layer absorbs the stress that would otherwise be imposed by the rigid hard solder joint.
4Temperature
If direct mounting to copper cooler is used, then heat dissipation is improved, but thermal expansion mismatch increases
Solution Approach 1:
The patent segments the thermal path into multiple layers with different CTEs: laser bar, stress relief layer, CTE-matched submount, and copper cooler. This segmentation allows the copper cooler to provide excellent heat dissipation while the intermediate layers progressively accommodate the thermal expansion mismatch.
Solution Approach 2:
The patent changes the CTE parameter of the submount to match the laser bar, creating a gradual transition in thermal expansion properties from the laser bar through the submount to the copper cooler. This parameter matching reduces thermal stress while maintaining effective heat dissipation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the reliability and optical performance of high power laser diodes by controlling stress, achieving higher rated output power with reduced smile values and improved beam shaping, while maintaining stability and polarization purity.
Implementation Method 1
the large mismatch in thermal expansion coefficient (CTE) between the commonly used laser diodes and the cooler
Implementation Method 2
The laser bar is directly attached to the copper cooler using a 'soft solder', e.g. In, InAg, or InSn
Data Source
AI summary
A high power laser source comprises a bar of laser diodes having a first coefficient of thermal expansion CTEbar on a submount having a second coefficient CTEsub and a cooler having a third coefficient CTEcool. The submount/cooler assembly shows an effective fourth coefficient CTEeff differing from CTEbar. This difference leads to a deformation of the crystal lattice of the lasers' active regions by mechanical stress. CTEeff is selected to be either lower than both CTEbar and CTEcool or is selected to be between CTEbar and CTEcool. The submount may either comprise layers of materials having different CTEs, e.g., a Cu layer of 10-40 μm thickness and a Mo layer of 100-400 μm thickness, or a single material with a varying CTEsub. Both result in a CTEsub varying across the submount's thickness.