Point diffraction interferometer
A point diffraction interferometer is a type of common path interferometer. Unlike an amplitude splitting interferometer, such as a Michelson, which separates out an unaberrated beam and interferes this with the test beam, a common path interferometer generates its own reference beam.
The device is similar to a spatial filter. Incident light is focused onto a semi-transparent mask (about 0.1% transmission). In the centre of the mask there is a hole about the size of the Airy disc and the beam is focused onto this hole with a Fourier transforming lens. The zeroth order (the low frequencies in Fourier space) then passes through the hole and interferes with the rest of beam. The device is similar in operation to Zernike phase contrast microscopy.
Since the device is self-referencing it can be used in environments with a lot of vibrations or when no reference beam is available such as in many adaptive optics scenarios.
Phase-shifting [see Interferometry] versions have been created to increase measurement resolution. These include a diffraction grating interferometer by Kwon and the EUV Phase-Shifting Point Diffraction Interferometer.
The main criticism of the device is that when the beam becomes too aberrated the low frequency terms no longer pass through the hole. Thus the reference beam changes and it becomes difficult to unwrap the phase. Also light efficiency of the device is quite low because of the mask.
- Smartt, R. N.; W. H. Steel, (1975). "Theory and application of Point-Diffraction interferometers". Japanese Journal of Applied Physics 14: 351–356. doi:10.7567/jjaps.14s1.351. Retrieved 29 February 2012.
- Smartt, R. N.; Strong, J. (1972). "Point-Diffraction Interferometer". Journal of the Optical Society of America 62: 737. Retrieved 29 February 2012.
- Kwon, Osuk (February 1984). "Multichannel phase-shifted interferometer". Optics Letters 9 (2). doi:10.1364/ol.9.000059.
- Naulleau, Patrick (1999). "Extreme-ultraviolet phase-shifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy". Applied Optics 38 (35): 7252–7263. doi:10.1364/ao.38.007252.