Weiqian Zhao

Bipolar absolute differential confocal approach to higher spatial resolution

By use of a superresolution pupil filtering technique to achieve a lateral optical superresolution and a differential confocal microscopy technique to achieve an axial resolution at the nanometer level, we propose a high spatial resolution bipolar absolute differential confocal approach for the ultraprecision measurement of three-dimensional microstructures. The feasibility of the proposed approach has been proved by use of a shaped annular beam differential confocal microscopy system. The experimental results indicate that the lateral and axial resolutions of the shaped annular beam differential confocal system are better than 0.2 mum and 2 nm, respectively, when lambda=632.8 nm, epsilon=0.5, uM=6.95, and with a 0.85 numerical aperture.

Tri-heterodyne confocal microscope with axial superresolution and higher SNR

Based on the characteristic of a confocal microscope (CM) that the offset of a pinhole along an optical axis changes the axial intensity response phase, a novel tri-heterodyne confocal microscope is built up by dividing the CM measurement light path into three paths, and using three sets of focusing lenses, detectors and pinholes placed behind, on and before the focal plane to form three detection systems, thereby achieving the axial superresolution imaging and high Signal Noise Ratio (SNR) through pairwise heterodyne subtraction of three intensity signals with given phases received by the three detection systems and data processing. Simulation and experimental results indicate that the new tri-heterodyne confocal microscope reduces the full width at the half maximum of CM axial response curve by more than 50%, results in the significant improvement of CM anti-interference capability, and enables CM to be more suitable for high accuracy bipolar absolute measurement of 3D microstructures and surface contours.

A new laser heterodyne confocal probe for ultraprecision measurement of discontinuous contours

In order to further improve the lateral resolution required for ultraprecision measurement of discontinuous surface contours, a new laser heterodyne confocal probe (LHCP) has been proposed for use in making ultraprecision bipolar absolute measurements. The new probe follows the principle of reflection confocal microscopes (RCM), and uses the property of RCM light intensity curves being almost invariant with the offset of a pinhole. It also uses a heterodyne confocal light path arrangement and intensity normalization technique to improve the linearity and resolution of RCM and to suppress the common-mode noise caused by the disturbance in light source intensity, different environmental conditions and electric drifts of detectors. Analyses and experimental results indicate that, when a microscope objective of 40 ? 0.65 is used, the LHCP has a measurement range of 7 ?m and a resolution of better than 2 nm. After nonlinear compensation, its residual nonlinear error is less than 13 nm in the full range. The application of the LHCP to ultraprecision measurement of discontinuous contours made it possible to measure the inner and outer contours of a groove in a revolving body at higher precision.

SEST: A new error separation technique for ultra-high precision roundness measurement

In order to further improve the rotation accuracy of ultraprecision roundness measuring instruments, a new single-step rotation error separation technique (SEST) is proposed to accurately separate instrument spindle rotation error and workpiece roundness error. This is done by first selecting an appropriate rotation angle and rotating the workpiece through a small angle with respect to the instrument spindle, and then measuring mixed errors A(n) and B(n) including workpiece error g(θ) and spindle rotation error z(θ) before and after rotation, and finally achieving accurate separation of z(θ) and g(θ) through Fast Fourier Transformation and harmonic analysis. Theoretical analysis and simulation results indicate that the signals in the harmonic range 1–100 upr can be totally separated from the wide range of harmonics measured when rotation angle error Δα < 0.01° is achieved by optimizing the rotation angle α. In comparison with the multi-step separation technique commonly used in roundness measuring instruments, SEST can be used to totally eliminate any harmonics singularity in the range of 1–100 upr and to make the error separation system very simple, shorten the separation process and reduce the separation time.

Optical probe using differential confocal technique for surface profile

A non-contact optical probe for surface profiling with up to 2nm position resolution over 100micrometers measurement range has been developed, which includes a confocal light path for non-contact position and a capacitance sensor for Z axis displacement measurement. The principle of the optical probe is based on differential confocal technique. The differential light- intensity distribution depends on the confocal axial response (or depth discrimination) properties. Using the diffraction theory, the mathematical analysis of the method has been performed. Validity of the mathematical theory analysis of the differential confocal technique is experimentally verified.

Enhancing laser beam directional stability by single-mode optical fiber and feedback control of drifts

A system, which uses a single-mode optical fiber to suppress the laser beam drifts, and uses feedback control to further reduce the drifts of a primary collimated beam, is proposed to enhance the directional stability of laser beams. The linear and angular drifts of laser beams are separated through light path arrangement and detected using quadrant position detectors, and are controlled using two-dimensional linear and angular drift feedback control systems according to their magnitudes detected. Theoretical analyses and experimental results indicate that the collimation accuracy of 0.7×10−7rad can be achieved at a collimation distance of 500 mm using single-mode optical fiber and separate control of linear and angular drifts.

Effect of fabrication errors on superresolution property of a pupil filter

Three analytical models have been established for superresolution parameters G(Ae), G(Te), and S(e) related to transmission function A(rho), phase function of phi(rho), and the structural parameters with fabrication errors of an N-zone circular-symmetrical superresolution pupil filter. These new models established, directly relate the superresolution parameters of an N-zone super-resolution pupil filter to its fabrication errors to make the quantitative analyses of the effect of fabrication errors easier, thereby providing a theoretical basis for the analysis, design, and fabrication of an N-zone super-resolution pupil filter. The models established for G(Ae), G(Te), and S(e) have been used to analyze the effect of the fabrication errors of a three-zone phase-only pupil filter on its superresolution property, to verify their validities.

Approach to higher spatial resolutions in a laser probe measurement system using a phase-only pupil filter

Phase-only pupil-filtering differential confocal measurement, a new approach, is proposed to improve the spatial resolution of a laser probe measurement system (LPMS) for ultraprecise measurement of microstructural workpieces. The proposed approach uses a lateral superresolution pupil filter for sharpening the main lobe of the Airy spot to improve the LPMS lateral resolution and uses the differential confocal measurement method to improve the LPMS axial resolution, thereby improving the LPMS spatial resolution. In addition to improving the spatial resolution, linearity, and antiinterference capability of a confocal measurement system, the proposed approach can be used for bipolar absolute measurement and improvement of the measurement range. Experimental comparison and analyses indicate that the lateral resolution of a phase-only pupil-filtering differential confocal system can be improved by 50% over that of an LPMS with the same parameters, and a lateral resolution better than 0.27 μm and an axial resolution better than 3 nm can be achieved when the wavelength of the incident beam is λ=632.8 nm, the numerical aperture of the measuring lens is NA=0.65, and μ M =4. It is therefore concluded that the phase-only pupil filtering differential confocal measurement method is a new approach to a higher spatial resolution of LPMSs and can be used for ultraprecise measurement of surface microcontours and microdimensions.

A High Precision Circuit Based on Digital Techniques for Roundness Measurement

In ultra high precision roundness measurement, a high accuracy and high integrated measuring circuit system equipped with inductive sensor are a developing trend. In order to meet the demand, a novel method to improve the accuracy of measuring circuit of amplitude modulation sensor using digital synthesizing, digital phase sensitivity detection (PSD), and digital set average techniques, is presented. The measuring circuit system based on the method is easy to realize and to operate. It can be plugged directly into an ISA Bus slot for use. Experimental results show that the stability of the sensor measuring system that consists of the measuring circuit system and a Taylor inductive sensor reaches nanometer accuracy. The digital measuring circuit of amplitude modulation can not only be applied widely in amplitude modulation measuring circuit for use of inductive sensors to improve the accuracy of the sensor measuring circuit system but also used in the one for use of capacitance sensors and magnetic grating sensors.

Development of research on super-precision measurement techniques for circle and cylindrical contour

At the beginning, it is presented that several theoretical and key technique problems among super-precision and nanometer measurement for circle and cylindrical contour, including: periodic reappearance of straight line datum movement; harmonic suppression in the measurement for circle contour; datum errors recognition and separation in the measurement for cylindrical contour; the problem of information integrality in the measurement for cylindrical contour. Then it is mainly introduced that the development research of super-precision measurement for circle and cylindrical contour, including: error separation technology with non-harmonic suppression; laser monitoring technology of straight line datum movement; error separation technology among datums, etc. Finally, give a vista of new promising measurement technique, such as the contour measurement based on confocal microscopy, optical measuring techniques with non-datums and so on.