Point diffraction interferometry based on the use of two pinholes

Abstract

Point diffraction interferometer (PDI) has become the high degree of accuracy device. In the optical wavefront testing the measurement accuracy is much higher than 1.0 nm RMS. In the paper there is presented a new version of PDI with two independently controlled beams using a pinhole plate with two pinholes as a beam coupler instead of a single-mode fiber or single-pinhole plate. Theoretical analysis of the pinhole diffraction wavefront and double pinholes diffraction interference is given. The PDI is used to investigate an interferometer reference lens and compare measurement results. The device can test high NA, the interference is obtained in circularly polarized light, and fringe contrast is adjustable to measure surfaces with different reflectance. The measurement repeatability now has been sub-nm RMS (measured NA = 0.33). The experiment result provides guarantee for the measurement in the high degree of accuracy. In the double pinholes PDI, generating two ideal spherical waves through two pinholes, one wave is as the reference wavefront for interference test, another ideal wavefront is reflected to the pinhole plate by the test mirror, and the tested wavefront and reference wavefront bring interference. Advantages of such arrangement of the PDI are: high maximum numerical aperture (NA = 0.55), distinct fringe patterns of high contrast, high accuracy of surface figure testing and wave-front repeatability RMS error 0.3 nm.