Point diffraction interferometer

This design makes the PDI extremely useful when environmental isolation is not possible or a reduction in the number of precision optics is required.

The zeroth order (the low frequencies in Fourier space) then passes through the hole and interferes with the rest of beam.

PDI systems are valuable tool to measure absolute surface characteristics of an optical or reflective instruments non destructively.

These problems are largely overcome in the phase-shifting point diffraction interferometer designs, in which a grating or beamsplitter creates multiple, identical copies of the beam that is incident on an opaque mask.

In the grating-based instance, phase-shifting is accomplished by translating the grating perpendicular to the rulings, while multiple images are recorded.

The continued developments in phase shifting PDI have achieved accuracy orders of magnitude greater than standard Fizeau based systems.

The end face is narrowed down to resemble a cone and is covered with metallic film to reduce the light spill.

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: 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.

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.

Figure 1: Basic layout of a PDI system, where the reference beam is generated by a pinhole etched onto a semitransparent film
Figure 2: Fizeau interferometer requires a reference optics. It is very important that the reference optics(flat) be near perfect because it heavily influence the measured surface form of a test object.
Figure 3: Phase-shifting point diffraction interferometer design proposed by Gary Sommargren [ 10 ]
Phase Shifting Point Diffraction Interferometer
Figure 4: Two-beam phase-shifting point diffraction interferometer, where the reference beam can be independently regulated for phase shifting and contrast regulation
Absolute surface form obtained by phase-shifting interferometry using an industrial point diffraction interferometer manufactured by Difrotec.
Absolute surface form obtained by phase-shifting interferometry using an industrial point diffraction interferometer manufactured by Difrotec [ 16 ]