Dual-Homodyne Laser Interferometer Phase Modulation for Nanometer Displacement
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Solution Overview
Problem
Nanometer displacement measurement techniques face limitations in accuracy due to laser power drift and nonlinear errors in homodyne and heterodyne interferometry, respectively, which hinder precise measurements in ultra-precision applications.
Innovation Solution
A dual-homodyne laser interferometric nanometer displacement measuring apparatus and method utilizing phase modulation, where an electro-optic phase modulator converts DC interference signals into AC signals, allowing for sub-nanometer displacement measurement by detecting phase differences between interference signals without requiring polarizing optics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If homodyne interferometer is used for nanometer displacement measurement, then large measuring range and high resolution are achieved, but measurement accuracy deteriorates due to laser power drift and DC interference signals
Solution Approach 1:
The patent applies periodic action by using an electro-optic phase modulator to modulate the reference beam with a sinusoidal signal at frequency ω. This converts the DC interference signals into AC signals at frequency ω, enabling periodic sampling and processing of the interference patterns. The periodic modulation allows for accurate extraction of phase information through demodulation techniques, thereby improving measurement accuracy while maintaining the advantages of homodyne interferometry.
2Reliability
If heterodyne interferometer is used to overcome laser power drift, then measurement stability is improved, but measurement accuracy deteriorates due to first order nonlinear error from frequency mixing and polarization mixing
Solution Approach 1:
The patent changes the modulation parameter from frequency modulation (heterodyne) to phase modulation at a fixed frequency. By modulating the phase of the reference beam using an electro-optic phase modulator and detecting the phase difference between reference and measurement beams, the system achieves high measurement accuracy without the first order nonlinear errors associated with frequency mixing in heterodyne interferometers.
3Device complexity
If DC interference signals are directly subdivided for displacement measurement, then measurement process is simplified, but measurement accuracy deteriorates due to direct subdivision errors and non-quadrature interference signals
Solution Approach 1:
The patent applies preliminary action by modulating the reference beam phase before the interference signals are generated and detected. The electro-optic phase modulator pre-modulates the reference beam with a sinusoidal signal, ensuring that the interference signals contain the necessary frequency and phase information for accurate demodulation. This preliminary modulation simplifies the subsequent signal processing while achieving high measurement accuracy.
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 achieves sub-nanometer displacement measurement accuracy up to 8.8 pm, overcoming errors from DC drift and polarization nonlinearities, and simplifies the optical structure for applications in ultra-precision machining and micro-electronics manufacturing.
Implementation Method 1
an electro-optic phase modulator, a high voltage amplifier, a signal generator... The electro-optic phase modulator is connected with the signal generator through the high voltage amplifier. The sawtooth-wave voltage output by the signal generator is amplified by the high voltage amplifier and then applied to the electro-optic phase modulator.
Implementation Method 2
Laser interferometry is widely used in nanometer displacement measurement... The transmitted beam from the first beam splitter is incident on the second beam splitter and divided into transmitted and reflected beams... to form the reference and the measurement interference signals
Data Source
AI summary
The present invention discloses a dual-homodyne laser interferometric nanometer displacement measuring apparatus and method based on phase modulation. The linearly polarized beam with single wavelength emitted from a single frequency laser is projected onto a dual-homodyne laser interferometer consisting of four beam splitters and two retroreflectors to respectively form a measurement interference signal and a reference interference signal received by two photodetectors, respectively; an electro-optic phase modulator is placed in the optical path and a periodic sawtooth-wave voltage signal is applied to modulate the measurement and reference DC interference signals into AC interference signals; the measured displacement is obtained by detecting the variation of the phase difference between the two interference signals caused by the movement of the measured object. The present invention overcomes the error arising from DC drift in the homodyne laser interferometer and avoids the sinusoidal error caused by the direct subdivision of the interference signal or non-quadrature error of measurement interference signal. The present invention is applicable for the precision displacement measurement with sub-nanometer level accuracy in the fields of high-end equipment manufacturing and processing.