Abstract
This paper presents an off-axis differential method for the improvement of a femtosecond laser differential chromatic confocal probe having a dual-detector configuration. In the proposed off-axis differential method employing a pair of single-mode fiber detectors, a major modification is made to the conventional differential setup in such a way that the fiber detector in the reference detector is located at the focal plane of a collecting lens but with a certain amount of off-axis detector shift, while the fiber detector in the measurement detector is located on the rear focal plane without the off-axis detector shift; this setup is different from the conventional one where the difference between the two confocal detectors is provided by giving a defocus to one of the fiber detectors. The newly proposed off-axis differential method enables the differential chromatic confocal setup to obtain the normalized chromatic confocal output with a better signal-to-noise ratio and approaches a Z-directional measurement range of approximately 46 μm, as well as a measurement resolution of 20 nm, while simplifying the optical alignments in the differential chromatic confocal setup, as well as the signal processing through eliminating the complicated arithmetic operations in the determination of the peak wavelength. Numerical calculations based on a theoretical equation and experiments are carried out to verify the feasibility of the proposed off-axis differential method for the differential chromatic confocal probe with a mode-locked femtosecond laser source.
Highlights
A confocal probe is an optical displacement sensor often employed in microscopy for form measurement of three-dimensional microstructures for the quality control of ultra-precision machined surfaces [1,2,3,4,5,6]
It should be noted that the work described in this paper method for the improvement of the signal-to-noise ratio (SNR), the measurement range and the is mainly intended to focus on the proposal of the preliminary idea of an off-axis differential method measurement resolutionof of femtosecond laserthechromatic confocal
A major modification has been made to the conventional differential chromatic confocal setup in such a way that the fiber detector in the reference detector unit has been placed to have an offset with respect to the optical axis of the collecting lens in the detector unit, rather than giving a defocus to the fiber detector
Summary
A confocal probe is an optical displacement sensor often employed in microscopy for form measurement of three-dimensional microstructures for the quality control of ultra-precision machined surfaces [1,2,3,4,5,6]. Sci. 2020, 10, 7235 defocus d; this modification improves the signal-to-noise ratio of the normalized chromatic confocal output to be obtained in the differential confocal setup. Theoretical and experimental examination are implemented to verify the to be1.obtained in theanalysis differential confocal setup This optical configuration is expected feasibility of the off-axis method theeasier differential chromatic confocal probe to make theproposed optical alignment of differential the differential confocalfor setup than the conventional setup shown in Theoretical analysis and experimental examination are implemented to verify with a mode-locked femtosecond laser source. It should be noted that the work described in this paper method for the improvement of the signal-to-noise ratio (SNR), the measurement range and the is mainly intended to focus on the proposal of the preliminary idea of an off-axis differential method measurement resolutionof of femtosecond laserthechromatic confocal. Reference detector unit off-axisdetector detector m in the newly proposed method
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