Copper Laser Mirror Thermal Distortion Compensation

Solution Overview

Problem

High power carbon dioxide slab lasers experience significant thermal distortion and beam pointing variations due to temperature changes, primarily caused by the bimetallic effect between copper mirrors and aluminum holders, leading to undesirable changes in the laser's output beam performance.

Innovation Solution

The implementation of stainless steel strips with a lower coefficient of thermal expansion than copper, mounted on the mirror elements, to counteract the bimetallic effect and minimize thermal distortion, by balancing the differential expansion rates and heating effects, thereby stabilizing the mirror curvature and beam pointing.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If copper mirrors are mounted on aluminum holders, then the mirror assembly is mechanically stable and easy to manufacture, but thermal distortion occurs due to the bimetallic effect causing beam pointing variations

Engineering Contradiction:

Improvemirror assembly manufacturingVSAvoidbeam pointing stability

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The patent changes the material parameter (coefficient of thermal expansion) by replacing aluminum holders with stainless steel holders that have a lower coefficient of thermal expansion, thereby reducing thermal distortion and improving beam pointing stability while maintaining manufacturing simplicity

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent uses a composite structure combining stainless steel holder with copper mirror element, where the stainless steel provides thermal stability and the copper provides optical reflectivity, creating a material system that balances thermal and optical requirements

Inventive Principle:

Principle #40Composite materials

2Reliability

If stainless steel strips with lower coefficient of thermal expansion are added to counteract bimetallic effect, then beam pointing stability improves, but device complexity increases

Engineering Contradiction:

Improvebeam pointing stabilityVSAvoidmirror assembly structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent extracts the problematic aluminum holder material and replaces it with stainless steel, removing the source of excessive thermal expansion rather than adding compensating elements, thereby improving stability without significantly increasing complexity

Inventive Principle:

Principle #2Taking out (Extraction)

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 results in a significant reduction of beam pointing variations, improving the stability of the laser output beam by over an order of magnitude, making it suitable for industrial applications without adding complexity or cost, as demonstrated by experimental data showing reduced deflection from 2200 to 100 microradians per degree Celsius.

Implementation Method 1

The implementation of stainless steel strips with a lower coefficient of thermal expansion than copper, mounted on the mirror elements, to counteract the bimetallic effect and minimize thermal distortion, by balancing the differential expansion rates and heating effects

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 2

High power carbon dioxide slab lasers experience significant thermal distortion and beam pointing variations due to temperature changes, primarily caused by the bimetallic effect between copper mirrors and aluminum holders

Methodology Applied

Scientific EffectBimetallic effect: Bi-Metallic Strip

Implementation Method 3

Direct thermal heating of the mirror's reflecting surface by the laser beam occurs because the high reflecting thin films deposited on the mirror's surface have a very small but finite absorption which heats the surface of the mirror

Methodology Applied

Scientific EffectAbsorption (EM radiation): Absorption (EM radiation)

Data Source

PatentUS7965757B2Thermal distortion compensation for laser mirrors

Publication Date: 2011.06.21 COHERENT INC

AI summary

A laser mirror assembly is disclosed with improved pointing stability. An elongated mirror includes a concave reflecting portion. A pair of elongated planar portions extend parallel to the concave reflecting portion on either side thereof. The planar portions are stepped down from the reflecting portion. The mirror is formed from copper. A pair of stainless steel strips are connected to planar portions. The bimetallic effect between the copper mirror and the stainless steel strips operates to counteract the warping of the mirror due to differential heating effects which arise during operation. In an alternate embodiment, a pair of aluminum strips are mounted on the rear surface of the mirror.