Tilt Actuator Oscillation for Interferometer Retrace Error Reduction

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Solution Overview

Problem

Existing optical surface measurement techniques, such as interferometry, face challenges with short range retrace errors due to optical surface aberrations and misalignment issues, leading to measurement inaccuracies and the need for lengthy data collection times.

Innovation Solution

The proposed solution involves an interferometer system with a tilt actuator that oscillates the reference surface to capture multiple wavefront measurements at different angles, allowing for background subtraction and the elimination of short range retrace errors in real-time.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If phase shifting by mechanical longitudinal translation of the reference surface is used, then optical surface measurement can be performed, but measurement accuracy deteriorates due to retrace errors from vibration and air turbulence

Engineering Contradiction:

Improveoptical surface measurement accuracyVSAvoidmeasurement consistency

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The reference surface is oscillated at a known frequency and amplitude during measurement. This intentional vibration creates a time-varying optical path difference that can be mathematically separated from the static surface profile being measured, allowing retrace errors to be identified and eliminated through frequency domain analysis.

Inventive Principle:

Principle #18Mechanical vibration

Solution Approach 2:

The system uses feedback from the known oscillation parameters to process the interferometric data. By knowing the exact frequency and amplitude of the reference surface oscillation, the system can apply mathematical transformations to extract the true surface profile while rejecting dynamic errors introduced during measurement.

Inventive Principle:

Principle #23Feedback

2Measurement precision

If the number of acquired images is increased to reduce retrace error, then measurement accuracy improves, but data collection time increases

Engineering Contradiction:

Improveretrace error reductionVSAvoiddata collection time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The reference surface oscillates periodically during a single measurement cycle, allowing multiple phase samples to be acquired continuously over one or more oscillation periods. This periodic motion enables the system to gather sufficient data for accurate surface reconstruction without requiring multiple separate measurements, thereby reducing total data collection time while maintaining precision.

Inventive Principle:

Principle #19Periodic action

3Measurement precision

If a high precision displacement stage is used for phase shifting, then measurement accuracy improves, but device complexity and cost increase

Engineering Contradiction:

Improvephase shifting accuracyVSAvoiddisplacement stage complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The system replaces the need for a high-precision mechanical displacement stage with an oscillating reference surface mechanism. Instead of mechanically translating the reference surface with sub-micron precision, the invention uses a simpler oscillation mechanism that creates controlled dynamic errors, which are then eliminated through signal processing based on the known oscillation parameters.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

4Measurement precision

If cavity alignment is adjusted to correct retrace errors, then measurement accuracy improves, but system setup complexity increases

Engineering Contradiction:

Improveretrace error correctionVSAvoidsystem alignment ease

Core Design Contradiction:

Measurement precisionVSEase of operation

Solution Approach 1:

The system changes the operational parameters of the interferometer by introducing a controlled oscillation of the reference surface. This parameter change transforms the measurement process from a static alignment-critical operation to a dynamic measurement where misalignment errors can be identified and removed through frequency domain analysis of the oscillating signal.

Inventive Principle:

Principle #35Parameter changes

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 method provides a robust and accurate optical surface analysis by reducing short range retrace errors, improving measurement precision, and allowing for real-time background subtraction, thus enhancing the efficiency and reliability of optical surface characterization.

Implementation Method 1

combining a test wavefront reflected from the test surface with a reference wavefront reflected from a reference surface to form an optical interference pattern

Methodology Applied

Scientific EffectInterference: Interference

Implementation Method 2

a tilt actuator coupled to the reference surface configured to provide an angular displacement of the reference surface

Methodology Applied

Scientific EffectMechanical displacement: Displacement

Data Source

PatentUS12305976B2Optical system using enhanced static fringe capture

Publication Date: 2025.05.20 BMV OPTICAL TECH INC

AI summary

A background subtraction method and tilt stage device for eliminating contaminated or spurious interference patterns by reducing retrace errors. An optical reference surface secured in a pivoting mount coupled to a tilt actuator is configured to angularly displace the pivoting mount and optical reference surface. A microcontroller coupled to the tilt actuator controls the tilt displacement of the tilt actuator providing a plurality of wavefront measurements of the reference surface at a plurality of angles to provide a system and method for background measurement.