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 Shearing Interferometry
 PHASICS 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 wave front 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 technology is based on a modified Hartmann test, each fringe is connected to one Hartmann aperture. Multi-wave 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, shearing interferometry is self-referenced. Hence, measurement is particularly insensitive to environment vibrations.
SID4 key features
High transverse resolution
Distortions of a grid of points can be measured in several ways. The simplest method consists in calculating the centroid of each point in given cells to determine its position. However, this technique does not take into account crosstalk between points, and cell size must be large enough to get an accurate statistical sampling.
To increase transverse resolution, an advanced analysis method is required. It is described above. The Fourier analysis is definitely well adapted for periodical signal processing. Moreover, the sinus shape of interference fringes allows to be very close to the Nyquist frequency, 2x2 pixel² per fringe. In our case, for dynamic purposes, we chose to use 4x4 pixel² per fringe. Thus with a standard 512x512 CCD camera, we can obtain 128x128 pixels² phase (and intensity) maps.
Achromaticity
Classical interferometer pitch is strongly dependent on wavelength. In our case, thanks to the use of a diffraction grating, multi-wave interferometers are achromatic : the chromaticity of the grating is exactly compensated by the interference chromaticity. The interferogram pitch is exactly equal to the grating pitch.
Therefore a SID4 is the perfect device for short pulse laser characterization
Tunable dynamic range
One key feature of the SID4 technology is its tunable dynamic range. This features has been inherited from the Hartmann test where you get higher sensitivity by moving the observation plane away from the aperture plate. Note that this property was lost when Shack used a micro-lens array and observed the Hartmanngram in the focal plane, ie at an infinite optical distance !
Therefore our SID4 products include a translation stage that let you set your sensitivity according to your needs.
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