Point diffraction interferometer

Figure 3: Phase-shifting point diffraction interferometer design proposed by Gary Sommargren[10]

Gary Sommargren[11] proposed a point diffraction interferometer design which directly followed from the basic design where parts of the diffracted wavefront was used for testing and the remaining part for detection as shown in Figure 3. This design was a major upgrade to existing systems. The scheme could accurately measure the optical surface with variations of 1 nm. The phase shifting was obtained by moving the test part with a piezo electric translation stage.[12][13] An unwanted side effect of moving the test part is that the defocus also moves distorting the fringes. Another downsides of Sommargren's approach is that it produces low contrast fringes [14] and an attempt to regulate the contrast also modifies the measured wavefront.

In this type of point diffraction interferometer the point source is a single mode fiber. The end face is narrowed down to resemble a cone and is covered with metallic film to reduce the light spill. Fibre is arranged so that they generate spherical waves for both testing and referencing. End of an optical fibre is known to generate spherical waves with an accuracy greater than ${\displaystyle \lambda \diagup 2000}$.[15] Although optical fibre based PDIs provide some advancement over the single pinhole based system, they are difficult to manufacture and align.

Figure 4: Two-beam phase-shifting point diffraction interferometer, where the reference beam can be independently regulated for phase shifting and contrast regulation

Two-beam PDI provides a major advantage over other schemes by availing two independently steerable beams. Here, the test beam and reference beam are perpendicular to each other, where the intensity of reference can be regulated. Similarly, an arbitrary and stable phase shifts can be obtained relative to the test beam keeping the test part static. The scheme as shown in Figure 4 is easy to manufacture and provides user-friendly measuring conditions similar to Fizeau type interferometers. At the same time renders following additional benefits:

1. Absolute surface form of the test part.
2. High numerical aperture (NA = 0.55).
3. Clear fringe patterns of high contrast.
4. High accuracy of surface form testing (wavefront RMS error 0.125 nm).
5. Wave-front RMS repeatability 0.05 nm.
6. Can measure depolarising test parts.

The device is self-referencing, therefore it can be used in environments with a lot of vibrations or when no reference beam is available, such as in many adaptive optics and short-wavelength scenarios.

Absolute surface form obtained by phase-shifting interferometry using an industrial point diffraction interferometer manufactured by Difrotec[16]

Interferometry has been used for various quantitative characterisation of optical systems indicating their overall performance. Traditionally, Fizeau interferometers have been used to detect optical or polished surface forms but new advances in precision manufacturing has allowed industrial point diffraction interferometry possible. PDI is especially suited for high resolution, high accuracy measurements in laboratory conditions to noisy factory floors. Lack of reference optics makes the method suitable to visualise absolute surface form of optical systems. Therefore, a PDI is uniquely suitable to verify the reference optics of other interferometers. It is also immensely useful in analysing optical assemblies used in Laser based systems. Characterising optics for UV lithography. Quality control of precision optics. Verifying the actual resolution of an optical assembly. Measuring the wavefront map produced by X-ray optics. PS-PDI can also be used to verify rated resolution of space optics before deployment.