Lateral shearing interferometry

Phasics^TM SID4 product line uses a technology based on a modified Hartmann test to measure wavefront distortions. Using the multi-wave lateral shearing interferometry formalism to analyse the recorded Hartmanngrams leads to increased resolution (at least by a factor 4) compared to all other gradient recovery based wavefront sensors (Hartmann test, Shack-Hartmann).
A 2D diffraction grating replicates the incident beam into 4 identical waves which are propagated along slightly different directions. The direction differences create interference patterns. In our case, this is made of sinusoidal fringes on a square grid.

After a few millimeter propagation, the 4 beams are slightly separated. When aberrations are present on the beam, the interference grid is distorted. The grid deformations are directly connected to the phase gradients. A spectral analysis using Fourier transforms allows the phase gradient extraction in 2 orthogonal directions. The phase map is finally obtained by integration of these gradients. Finally, you get one measurement point per interferogram fringe.
Since our 4-wave lateral shearing interferometry technology is based on a modified Hartmann test, each fringe is connected to one Hartmann aperture. Multi-wave lateral shearing interferometry allows us to move apertures as close as possible to each other and take into account the interferences between the waves generated from each aperture.

Unlike classical interferometers where a reference arm is mandatory, lateral shearing interferometry is self-referenced. Hence, measurement is particularly insensitive to environment vibrations.