Dual Laser Interferometer with Quadrature Error Compensation

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Solution Overview

Problem

Prior art interferometric measurement systems face errors in coarse and intermediate resolution measurements, limiting the accuracy of absolute distance measurements and the ability to determine the number of whole wavelengths included in the distance to a target, particularly due to common-mode and phase shift errors in quadrature detectors.

Innovation Solution

The system employs a dual laser interferometric setup with tunable lasers providing measurements at multiple wavelengths, combined with a quadrature detector configuration that compensates for phase shift errors and uses self-correction techniques for offset, amplitude, and orthogonality errors, along with simultaneous acquisition of signals to reduce common-mode errors, and wavelength calibration through a rubidium absorption cell.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a single wavelength is used in the interferometer, then the measurement resolution is improved, but the absolute measurement range is limited

Engineering Contradiction:

Improvemeasurement resolutionVSAvoidabsolute measurement range

Core Design Contradiction:

Measurement precisionVSLength of stationary object

Solution Approach 1:

The patent uses multiple wavelengths (e.g., 633nm and 532nm) to perform interferometric measurements. By changing the wavelength parameter and analyzing the phase differences at different wavelengths, the system determines absolute distance measurements over extended ranges while maintaining high resolution through synthetic wavelength techniques.

Inventive Principle:

Principle #35Parameter changes

2Productivity

If quadrature detectors are used to detect interference signals, then the measurement speed is improved, but phase shift errors and common-mode errors increase

Engineering Contradiction:

Improvemeasurement speedVSAvoidmeasurement accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The patent implements error compensation techniques where phase shift errors and common-mode errors detected by the quadrature detectors are measured and corrected through feedback processing. The system calculates and compensates for these errors to maintain high measurement accuracy while utilizing the fast detection capability of quadrature detectors.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent extracts and separates the error components (phase shift errors and common-mode errors) from the measurement signals. By identifying and removing these specific error components through signal processing, the system maintains measurement speed while improving accuracy.

Inventive Principle:

Principle #2Taking out (Extraction)

3Length of stationary object

If multiple wavelengths are scanned continuously to increase measurement range, then the absolute measurement range is improved, but the measurement time increases

Engineering Contradiction:

Improveabsolute measurement rangeVSAvoidmeasurement time

Core Design Contradiction:

Length of stationary objectVSLoss of time

Solution Approach 1:

The patent employs periodic wavelength scanning where the laser wavelength is tuned periodically across multiple wavelengths. By using the periodic nature of interference patterns at different wavelengths and applying phase unwrapping algorithms, the system achieves extended measurement ranges without requiring continuously slow scanning, thus reducing overall measurement time.

Inventive Principle:

Principle #19Periodic 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 approach enhances measurement accuracy and robustness, allowing for precise absolute distance measurements over large ranges with sub-nanometer resolution and improved reliability, even with weak signal reflections, by reducing residual errors and providing dynamic calibration.

Implementation Method 1

The object beam and reference beam constructively or destructively interfere, depending on the relative phase of the object beam compared to the reference beam

Methodology Applied

Scientific EffectInterference: Interference

Implementation Method 2

wavelength calibration through a rubidium absorption cell

Methodology Applied

Scientific EffectAbsorption Spectroscopy: Absorption Spectroscopy

Data Source

PatentUS7292347B2Dual laser high precision interferometer

Publication Date: 2007.11.06 MITUTOYO CORP

AI summary

An absolute distance measuring device based on laser interferometry may combine coarse, intermediate, and highest resolution measurement techniques to find the absolute distance to a sample surface with high resolution. The device may provide at least two laser wavelengths simultaneously, to allow reduction or elimination of certain common-mode error components, including dynamic error components. The device may scan at least one of the laser wavelengths over a relatively narrow range and may use quadrature detectors to provide enough signal data to allow certain self-corrections to be performed on the resulting scanned signals and measurements. A novel tunable laser and/or quadrature detector may provide advantages in combination with the device.