Improving Shack-Hartmann wavefront sensor by using sub-wavelength annular apertures
2014
Out of the many wavefront sensing techniques, Shack Hartmann wavefront sensor remains the most
popular and the most versatile. Its optical configuration utilized a micro-lens array to measure the
directivity of the light beam associated with each micro-lens. In this design, smaller size of micro-lens
leads to angular resolution improvement. However, smaller size micro-lens typically is associated to
shorter depth of focus, which makes it difficult to focus on sensor array properly. In addition, the size
of micro-lens array is limited by the diffraction limit. In today’s technology, micro-lens with
dimensions in size of a few hundred of microns is possible. This dimension posts the limitation of the
angular resolution possible for Shack Hartmann wavefront sensor. To alleviate the compromise
between the angular resolution and the depth of focus, a sub-wavelength annular aperture (SAA)
structure was developed to generate Bessel light beams. That is, the SAA performs similar functions as
that of the micro lens array in traditional wave front sensors. It is shown that this design maintains a
sub-wavelength focusing capability while achieves tens of micron depth of focus in the far-field region,
which leads to an improved wavefront sensor. Both simulation and experimental results are detailed.
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