In the paper methods and results of testing optical surface form with the use of the point diffraction interferometer (PDI) D7 are presented. This interferometer can produce much higher accuracy of testing optics of various kinds than traditional interferometers, and validation of its industrial applicability is the scope of the paper. An overview and analysis of techniques making using D7 in manufacturing process easier and faster are given, the accuracy which can be achieved with common requirements of optical industry is evaluated at level of λ/150 – λ/250 PV and λ/600 – λ/1000 RMS. Measurements by subapertures for consequent stitching of subaperture wavefronts with desired overlay are considered. Conclusions followed by further perspectives of the described instrument are given.
Testing of an X-ray mirror by a point diffraction interferometer (PDI) D7 with two beams is described. Thanks to the two independent test and reference beams, mirrors metrology using the D7 coupled with accessory optics becomes straightforward and reliable. Therefore procedure of systematic error removal and sub-aperture measurements with stitching are simplified. In this paper, we describe the main technique to achieve high accuracy of stitching sub-aperture wavefronts, followed by further perspectives of the described instrument.
Advanced figuring technology has enabled manufacturing of high accuracy optics for precision applications. The measurement technologies to verify them are largely based on Fizeau interferometry, which is limited in terms of accuracy because of external accessories such as reference flat. Lack of appropriate verification method is adversely affecting the manufacturing and optimization of precision optics. In this paper, we explore a fundamentally different interferometry arrangement, D7 produced by Difrotec. A phase shifting point diffraction interferometer (PSPDI) and present measurement results for concave spheres with an accuracy of λ/1000 PV, and compared this full-shot result with wavefront maps obtained by subaperture stitching (SAS) to verify stitching accuracy. We also describe measurement of asphere cavity using SAS, with higher accuracy, λ/500 RMS, discuss strategies to measure concave/convex spheres and aspheres with R-number ≥ 0.5 with nanometer accuracy, and conclude with perspectives on the future applications of PSPDI D7.
Almost hidden residual defects of a test surface can be revealed using high precision instrument such as a point
diffraction interferometer (PDI). In general, PDI is engaged to display the figure of a surface or wavefront with subnanometer
accuracy paying attention to low-frequency configurations. Such technique is suited to test EUV or X-ray
optics. The tool described in the paper is able to map absolute profile deviations of several angstroms and therefore it
provides a new vision of a surface under test of various quality, e.g. detects specific characteristics which immediately
disclose either lapping or diamond turning has been used to form the substrate. Such inspection may help optimize the
processes in early stage of shape forming before final configuring.
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.
Interferometric inspection of optical surfaces and wavefronts requires permanently increasing accuracy. Therefore
interferometric equipment is being improved and improved continuously. Point-diffraction interferometers (PDI) with an
“inbuilt” reference wavefront originating from light diffraction by a pinhole aperture are potentially capable to produce
the highest possible accuracy of a surface figure or wavefront characterization. The most mentioned configurations and
their versions like Linnik-Smart and Sommargren schemes produce low-contrast spare-striped fringe patterns instead of
full-contrast distinct interferograms, like e.g. produced by Fizeau interferometers, with clear phase shifting (PS) data
flow.
The concept of a PDI presented in this paper is to provide two perpendicularly outgoing wavefronts – test and reference
ones – with mutual intensity regulation and arbitrary and stable phase shifts of one wavefront relative to the other. Such
concept is also targeted to provide user-friendly measuring conditions similarly to interferometers which are in common
use.
Advantages of such arrangement of the PDI are: high numerical aperture (NA = 0.55), clear fringe patterns of high
contrast, high accuracy of surface figure testing with wave-front RMS error 0.125 nm and wave-front RMS repeatability
0.05 nm. Performance of the PDI is illustrated by tables of repeatability and test surface profile plots for different
azimuthal angles.
For interferometric testing of polished surfaces and wavefronts with the best physically accessible accuracy it is good to
use a perfect wavefront reference originating from light diffraction by a pinhole aperture in a point-diffraction
interferometer (PDI). It is evident that phase shifting (PS) interferometric measurements with the use of the PDI should
be fulfilled adequately to perfectness of its wavefront reference unless high accuracy expectations will not be met.
High accuracy of the reference beam phase shifting of the two-beam PDI is produced by a two staged alignment
procedure being performed on-line when PS fringe patterns (frames) are being saved. This procedure is a time frequency
filtering of intensity function of each pixel performed in order to extract regular sinusoids from a set of erroneous and
noisy signals. The results of wavefront retrieval from the saved set of PS frames using any N bucket algorithm by the
Durango software in both cases – not aligned and aligned – are compared.
This research helps keep the PDI accuracy corresponding to perfectness of the wavefront diffraction reference. Also this
research may help in PS measurements performed by other types of interferometers where phase shifts are realized by
test part movement.
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