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

Abstract

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 µm 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.