VTT-NTM has elaborated a laser diffraction interferometer for testing highly precise optical parts - new variant of the Linnik’s PDI interferometer ('Smart' interferometer) based on diffraction-made reference wavefront.

The new schematic made it possible to manufacture a sample of PDI for optical industry which can be safely used in optical plants in close vicinity to the polishing equipment without damage to measurements carried out on the interferometer.

Here is a view of the PDI interferometer industrial sample VTT-055 together with computer station. On the monitor there can be seen a fringe processing program window with the initial (acquired) interferogram.

The interferometer VTT-055 which is intended to measure errors of the surfaces and wave fronts under industrial conditions with the following accuracy:
P-V: λ/250
RMS: λ/200
Testing wavelength: 632.8 nm

The basic advantages of an offered laser interferometer with diffraction reference wave front are:

1) Entire absence of deformations of the reference wave front with keeping operational opportunities, such as classical interferometers obtain.

2) The principle of action considered in diffraction interferometer is based on use of light diffraction by a pinhole aperture, which can be rather simple and certainly is reliably reproduced in any conditions and any amount of compact devices. Thus we have an interferometer which does not require reference optical elements and at the same time may serve for testing optical surfaces and systems of the best accuracy, including testing reference elements of traditional interferometers.

3) The given interferometer also produces various means of increasing the accuracy of measuring data readout. Thus the increase in accuracy of readout = the increase in accuracy of measurement (while in traditional interferometers it is only the accuracy of unknown errors of the reference wave front).

Application:

• Testing DUV and EUV lithography optics,
• Certification of traditional interferometer e.g. Fizeau types
• Testing quality of optical materials and crystals, testing thin films (antireflection coating (ARC), barrier antireflection coating (BARC)) etc.