Tracking Laser Interferometer for Rotational Objects
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Solution Overview
Problem
Previous tracking type laser interferometers fail to effectively include rotational degrees of freedom due to the requirement that the spherical surface must be centered on the rotation axis, leading to loss of interference signal at all rotation angles if not centered.
Innovation Solution
A tracking type laser interferometer design utilizing coherent light, interferometer optics, quarter-wave retarding waveplates, a non-polarizing beam splitter, and polarizers to distinguish and measure the polarization of beams reflected from a flat reference surface and a spherical or cylindrical reference surface, enabling sub-nm interferometric resolution and sub-micron position measurement in all dimensions using two detectors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the spherical surface is not centered on the rotation axis, then the interferometer can measure objects with rotational degrees of freedom more flexibly, but the reflected beam does not propagate to the interference detector at all rotation angles causing loss of interference signal
Solution Approach 1:
A beam steering mirror is introduced as an intermediary component between the spherical reference surface and the interference detector. This mirror actively steers the reflected beam to ensure it always reaches the detector regardless of the sphere's rotational position, thereby maintaining signal continuity while allowing the sphere to rotate freely about an axis that does not require perfect centering
Solution Approach 2:
The system employs feedback tracking using position-sensitive detectors to monitor the reflected beam position and dynamically adjust the beam steering mirror to keep the beam centered on the detector. This active feedback mechanism ensures continuous signal acquisition during rotation without requiring precise mechanical centering
2Reliability
If the spherical surface is centered on the rotation axis, then the reflected beam propagates to the interference detector at all rotation angles maintaining signal continuity, but the interferometer cannot measure objects with rotational degrees of freedom effectively
Solution Approach 1:
The beam steering mirror serves as a mediator that decouples the mechanical centering requirement from the optical signal path. It allows the spherical surface to be positioned flexibly while maintaining reliable beam delivery to the detector through active steering
Solution Approach 2:
The system transitions from a static geometric centering requirement to a dynamic beam steering approach. The beam steering mirror actively adapts its angle in real-time to compensate for rotational movements, maintaining signal continuity without mechanical constraints
3Measurement precision
If quarter-wave retarding waveplates are used to rotate beam polarization by 90 degrees, then beams reflected from different surfaces have distinguishable polarization making them separable, but the device complexity increases
Solution Approach 1:
The system changes the polarization parameter of the light beams using quarter-wave retarding waveplates. By rotating the polarization by 90 degrees for beams reflected from different surfaces, the system creates distinguishable beam paths that can be separated and measured independently, enabling precise differential measurements
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
Enables accurate position measurement of the spherical surface in all dimensions, allowing for feedback tracking and automatic adjustment of the interferometer setup, achieving sub-nm interferometric resolution and sub-micron range position sensitivity.
Implementation Method 1
passing the first quarter-wave retarding wave-plate twice on the way back to the interferometer module, thereby rotating the polarization of the beam by 90 degrees
Implementation Method 2
uses the phenomenon of the interference which can be only observed when using coherent light
Implementation Method 3
a source emitting a coherent light beam, e.g. a laser source (homodyne or heterodyne)
Implementation Method 4
a spherically mounted retro-reflector as target
Data Source
Figure 1~2
AI summary
The method relates to the differential position measurement by laser interferometry of two elements while offering a rotational degree of freedom to one of the elements using a reflecting sphere as mirror for the laser beam. The method does not require this object to be aligned with the rotation axis, but instead can track the object in off-centered geometries. This is achieved by employing the pointing of the reflected beam from this sphere as feedback signal to realign the interferometer which then has a constant beam pointing to the center of the sphere in all cases. The method keeps the direction of the measurement constant. The method is suitable to homodyne and heterodyne types of laser interferometers technology.