Real-time PGC Demodulation Normalization in Sinusoidal Phase Modulation Interferometer
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Solution Overview
Problem
Existing sinusoidal phase modulation interferometry methods face challenges in achieving real-time normalization of phase generated carrier (PGC) demodulation, leading to nonlinear errors due to phase modulation depth changes, phase delays, and gain variations, which affect measurement accuracy, especially in static object measurements and dynamic environments.
Innovation Solution
A real-time normalization apparatus and method using a dual-modulation approach with high-frequency sinusoidal and low-frequency triangular modulations applied to an electro-optic phase modulator, allowing for periodic variation of quadrature signals and real-time normalization through ellipse fitting, enabling precise displacement measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If internal modulation is used to generate phase carrier, then the structure is compact and no extra modulator is needed, but amplitude modulation is introduced resulting in phase demodulation error
Solution Approach 1:
The patent separates the phase modulation function from the amplitude modulation function by using external modulation. The electro-optic phase modulator is specifically dedicated to phase modulation, while the amplitude information is extracted separately through envelope detection, avoiding the coupling of amplitude and phase modulation errors
Solution Approach 2:
The patent introduces an envelope detector as an intermediary component to extract amplitude information from the modulated signal. This intermediary allows separate processing of amplitude and phase information, enabling compensation for amplitude modulation effects without affecting phase demodulation accuracy
2Ease of operation
If off-line normalization is performed before experiment start, then the method is simple to implement, but it is not applicable to static objects and the normalization coefficients become inaccurate during measurement due to drifting
Solution Approach 1:
The patent implements continuous real-time normalization during the entire measurement process. The normalization coefficients are continuously updated based on the current signal characteristics, ensuring that the normalization remains accurate even when phase modulation depth drifts or measurement conditions change, making it applicable to both static and dynamic objects
Solution Approach 2:
The patent uses feedback mechanisms where the demodulated signals are continuously monitored and used to update the normalization coefficients in real-time. This feedback loop ensures that any drift in phase modulation depth or changes in measurement conditions are automatically compensated, maintaining measurement accuracy throughout the experiment
3Productivity
If fundamental and second-harmonic terms are selected for demodulation, then the measurement dynamic range is wide, but phase delay and gain variations cause nonlinear errors
Solution Approach 1:
The patent dynamically adjusts normalization parameters based on the actual signal characteristics during measurement. By continuously adapting the normalization coefficients to match the current phase modulation depth and signal amplitude, the system maintains linear demodulation accuracy across the entire measurement dynamic range, even when operating conditions vary
Solution Approach 2:
The patent transitions from static off-line normalization to dynamic real-time normalization. The normalization process continuously adapts to changing measurement conditions, ensuring that the demodulation remains linear and accurate across the full dynamic range of the measurement system
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 solution eliminates nonlinear demodulation errors and achieves nanometer displacement measurement accuracy, applicable to both static and dynamic objects, with simplified optical configuration and avoidance of polarization-related errors.
Implementation Method 1
external modulation directly modulates the optical path difference of the interferometer through an external modulator such as the piezoelectric tranducer or the electro-optic phase modulator
Implementation Method 2
The internal modulation modulates the output wavelength by changing the current of the laser diode, achieving the modulation of optical path difference indirectly
Implementation Method 3
sinusoidal phase modulation interferometry applies sinusoidal modulation on the optical path difference of an interferometer
Data Source
AI summary
The present invention discloses a real-time normalization apparatus and method of the PGC demodulation in a sinusoidal phase modulation interferometer. An optical setup containing a measuring interferometer and a monitoring interferometer is constructed. An electro-optic phase modulator is placed in the common reference arm of the two interferometers. High-frequency sinusoidal wave modulation and low-frequency triangular wave modulation are applied to the electro-optic phase modulator at the same time. Sinusoidal modulation is used for generating phase carrier, and PGC demodulation is performed to obtain quadrature signals containing the phase information to be measured. Triangular wave modulation makes the quadrature signals change periodically. Ellipse fitting is performed on the Lissajous figure corresponding to the quadrature signals, and real-time normalization of the PGC demodulated quadrature signals is achieved. By calculating the variation of the phase difference between the two interference signals, the measured displacement is obtained, and nanometer scale displacement measurement is achieved.