Fabry-Perot Interferometer Mirror Bonding for Thermal Stability

Solution Overview

Problem

Fabry-Perot interferometers face significant geometric deformation due to thermal expansion and humidity-induced changes in the adhesive joints, which cannot be effectively corrected by the actuators, disrupting the interferometer's operation.

Innovation Solution

The interferometer design incorporates supporting elements with joints positioned at the middle plane of the mirror plates to minimize geometric deformation. This configuration ensures that deformations of the upper and lower surfaces of the mirror plates compensate each other, maintaining the mirror's planarity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If the mirror plate is bonded to base plates and actuators by adhesive joints, then the interferometer components are connected and supported, but thermal expansion and humidity-induced changes of the joints cause significant geometric deformation of the mirrors

Engineering Contradiction:

Improvebonding strengthVSAvoidmirror geometric deformation

Core Design Contradiction:

StrengthVSShape

Solution Approach 1:

The mirror plate is divided into multiple bonding regions (first bonding region and second bonding region) positioned at specific locations on the middle plane. This segmentation allows selective bonding at optimized positions that minimize the impact of joint deformation on the mirror's overall shape, while still providing sufficient bonding strength.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The bonding regions are positioned specifically at the middle plane of the mirror plate, where the local structural properties minimize stress-induced deformation. This local optimization ensures that the adhesive joints are located where they have the least impact on mirror geometry, while still providing adequate bonding strength at those specific locations.

Inventive Principle:

Principle #3Local quality

2Strength

If the joints are positioned to maximize bonding strength, then the mirror plate is securely attached, but the deforming forces from thermal expansion and shrinking cause significant geometric deformation

Engineering Contradiction:

Improvejoint bonding strengthVSAvoidmirror shape accuracy

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

The bonding regions are pre-positioned at the middle plane of the mirror plate before bonding occurs. This preliminary positioning ensures that when the adhesive joints are formed, the deforming forces from thermal expansion and shrinking act at locations that minimize impact on mirror shape accuracy, while still providing secure attachment.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The bonding regions are positioned specifically at the middle plane of the mirror plate, where the local structural properties minimize stress-induced deformation. This local optimization ensures that the adhesive joints are located where they have the least impact on mirror geometry, while still providing adequate bonding strength at those specific locations.

Inventive Principle:

Principle #3Local quality

3Strength

If the bonding regions are located away from the middle plane, then the joints provide adequate support, but the deforming forces cause bending of the mirror

Engineering Contradiction:

Improvesupport strengthVSAvoidmirror planarity

Core Design Contradiction:

StrengthVSShape

Solution Approach 1:

The bonding regions are positioned specifically at the middle plane of the mirror plate, where the local structural properties minimize stress-induced deformation. This local optimization ensures that the adhesive joints are located where they have the least impact on mirror geometry, while still providing adequate bonding strength at those specific locations.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The bonding regions are positioned in the middle plane dimension of the mirror plate, utilizing the thickness dimension to optimize joint location. By placing bonds at the middle plane rather than at the surfaces, the design exploits the intermediate dimension to minimize bending moments while maintaining support strength.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

4Shape

If the bonding regions overlap with the middle plane of the mirror plate, then geometric deformation is minimized, but the bonding area is reduced

Engineering Contradiction:

Improvemirror shape stabilityVSAvoidbonding strength

Core Design Contradiction:

ShapeVSStrength

Solution Approach 1:

The bonding regions are positioned specifically at the middle plane of the mirror plate, where the local structural properties minimize stress-induced deformation. This local optimization ensures that the adhesive joints are located where they have the least impact on mirror geometry, while still providing adequate bonding strength at those specific locations.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

Instead of bonding across the entire mirror plate surface, the invention uses selective bonding at specific regions located at the middle plane. This partial bonding approach is sufficient to minimize geometric deformation while maintaining adequate bonding strength, avoiding the need for excessive bonding area.

Inventive Principle:

Principle #16Partial or excessive action

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 design effectively minimizes stress-induced deviations in the mirror's shape, maintaining the interferometer's accuracy and operational stability by ensuring that local deviations in the mirror's shape remain within a predetermined limit, such as smaller than 10 nm.

Implementation Method 1

Thermal expansion and/or shrinking of the joints J0, J3 may cause deforming forces FX1, FY1, FY2

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 2

Adhesive joint may absorb moisture e.g. from ambient air, depending on the relative humidity of the ambient air

Methodology Applied

Scientific EffectHumidity-induced expansion: Absorption (physical)

Implementation Method 3

coupling the supporting and deforming forces to the mirror plate via bonding regions located at the middle plane may ensure that deformations of the upper surface of the mirror plate may substantially compensate deformations of the lower surface of said mirror plate

Methodology Applied

Scientific EffectStress compensation:

Data Source

PatentEP4127623B1Fabry-perot interferometer having supporting elements

Publication Date: 2025.05.28 TEKNOLOGIAN TUTKIMUSKESKUS VTT OY

Figure 1a~1b
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Figure 3

AI summary

A Fabry-Perot interferometer (300) comprises: - a first mirror plate (100) comprising a first semi-transparent mirror (M1), - a second semi-transparent mirror (M2) to define an optical cavity together with the first mirror (M1), and - one or more first supporting elements (S1) to support the first mirror plate (100), wherein the first mirror plate has a first substantially planar surface (SRF11) and a second substantially planar surface (SRF12) defining the maximum thickness (h100) of the first mirror plate (100), wherein the first mirror plate (100) is bonded to the one or more first supporting elements (S1) by three or more joints (J1), wherein each joint (J1) is bonded to the first mirror plate (100) at a bonding region (REG1), wherein the distance (d1) between each bonding region (REG1) and the first substantially planar surface (SRF11) is greater than 30% of the thickness (h100) of the mirror plate (100), and the distance (d2) between each bonding region (REG1) and the second substantially planar surface (SRF12) is greater than 30% of the thickness (h100) of the mirror plate (100).