Laser differential astigmatic probe for discontinuous profile measurement

In order to achieve the precise measurement of three-dimensional surface contours, pupil filtering laser differential astigmatic probe (PLDAP) is developed with axial resolution at nm level. PLDAP based on the principle of astigmatic measurement uses defocused signal obtained through the differential subtraction of two focal error signals and normalization of the detected intensity received from two detectors, which are placed at the same distance before and behind the focal plane of two cylindrical lenses, to achieve the high accuracy and bipolar absolute measurement of a large obliquity surface. The differential astigmatic polarized light path arrangement and intensity normalized technique used in PLDAP effectively suppress the bad effect on measurements caused by the disturbance of light source and ambient light in intensity, multi-reflection among optical elements and variation of optical property for the measurement surface, and obviously improve the anti-interference capability. Experimental results indicate that PLDAP developed has a resolution of better than 5 nm and a measurement range of ±10 μm when the parameter of measuring lens used is N_A = 0.65, and its stability is better than 2% and used for the surface with tilt angle of 20°. It can therefore be concluded that the technique provides a new technique approach for the absolute measurement and aiming trigger of discontinuous surface profiles.