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Point diffraction interferometer

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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.

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.

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