Polarization-Separated Interferometer for High-Speed Phase Shifting
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Solution Overview
Problem
Conventional laser unequal path interferometers (LUPIs) face challenges due to the sensitivity of piezoelectric actuators to vibrations, temperature shifts, and low actuation rates, which lead to phase errors and reduced lateral resolution, and electro-optic phase modulators are bulky and prone to spatial inhomogeneities.
Innovation Solution
A polarization-separated, phase-shifted interferometer that eliminates moving parts by using orthogonally polarized, dynamically phase-shifted beams, allowing for high-speed phase modulation without mechanical components, utilizing fiber-coupled electro-optic phase modulators that operate at GHz rates with low voltages and are spatially homogeneous.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If piezoelectric actuators are used to move the reference mirror, then phase shifting can be achieved, but the system becomes sensitive to vibrations and temperature shifts, reducing reliability
Solution Approach 1:
The patent replaces the mechanical piezoelectric actuator system with an electro-optic phase modulator that uses the Pockels effect to achieve phase shifting electronically. This substitution eliminates mechanical moving parts and their associated sensitivities to vibration and temperature, while maintaining precise phase control capability through voltage-driven optical path length modulation.
Solution Approach 2:
The patent introduces an electro-optic phase modulator as an intermediary component between the light source and the reference arm. This mediator enables phase shifting through electro-optic effects rather than mechanical displacement, isolating the measurement system from environmental disturbances that affect mechanical actuators.
2Measurement precision
If piezoelectric actuators are used for phase modulation, then phase shifting is achieved, but the actuation rate is limited, reducing productivity
Solution Approach 1:
The patent replaces the slow mechanical response of piezoelectric actuators with the fast electronic response of electro-optic phase modulators. The electro-optic effect occurs virtually instantaneously when voltage is applied, enabling phase modulation at rates limited only by the electrical bandwidth rather than mechanical inertia, thus dramatically increasing actuation speed and measurement productivity.
3Productivity
If conventional electro-optic phase modulators are used, then high-speed phase modulation is achieved, but the devices are bulky and prone to spatial inhomogeneities, increasing device complexity
Solution Approach 1:
The patent modifies the operating parameters of the electro-optic phase modulator by using fiber-coupled configurations and optimizing the crystal orientation and dimensions. These parameter changes enable compact sizing while maintaining high-speed modulation capability and reducing spatial inhomogeneities through improved beam confinement and mode matching in the fiber-optic 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 provides robust, high-speed interferometry with reduced sensitivity to environmental disturbances, enabling precise surface topography measurement without phase wrapping ambiguities and improving lateral resolution and alignment accuracy.
Implementation Method 1
A polarization-separated, phase-shifted interferometer... uses orthogonally polarized, dynamically phase-shifted beams
Implementation Method 2
utilizing fiber-coupled electro-optic phase modulators that operate at GHz rates with low voltages
Implementation Method 3
A polarization-separated, phase-shifted interferometer... includes a polarizing beam splitter (PBS), first and second quarter-wave plates (QWPs)
Implementation Method 4
The reflected light interferes to produce an intensity fringe pattern sensed by the detector
Data Source
AI summary
A polarization-separated, phase-shifted interferometer can generate interferograms without moving parts. It uses a phase shifter, such as an electro-optic phase modulator, to modulate the relative phase between sample and reference beams. These beams are transformed into orthogonal polarization states (e.g., horizontally and vertically polarized states) and coupled via a common path (e.g., polarization-maintaining fiber) to a polarizing beam splitter (PBS), which sends them into separate sample and reference arms. Quarter-wave plates in the sample and reference arms rotate the polarization states of the sample and reference beams so they are coupled out of the PBS to a detector via a 45° linear polarizer. The polarizer projects the aligned polarization components of the sample and reference beams onto the detector, where they interfere with known relative phase to produce an output that can be used to map surface topography of the test object.