Abstract
An auto-referenced interferometric method for calibrating phase modulation of parallel-aligned liquid crystal (PAL) spatial light modulators (SLM) is described. The method is experimentally straightforward, robust, and requires solely of a collimated beam, with no need of additional optics. This method uses the SLM itself to create a tilted plane wave and a reference wave which mutually interfere. These waves are codified by means of a binary diffraction grating and a uniformly distributed gray level area (piston) into the SLM surface. Phase shift for each gray level addressed to the piston section can then be evaluated. Phase modulation on the SLM can also be retrieved with the proposed method over spatially resolved portions of the surface. Phase information obtained with this novel method is compared to other well established calibration procedures, requiring extra elements and more elaborated optical set-ups. The results show a good agreement with previous methods. The advantages of the new method include high mechanical stability, faster performance, and a significantly easier practical implementation.
Highlights
Parallel-aligned liquid (PAL) crystal spatial light modulators (SLM) are devices for wavefront manipulation that can be considered as linear retarders with spatially-resolved programmable phase-shift
Gamma curves can be defined by the user and the average phase response within large areas of the SLM can be adjusted to a 2π range
Illuminating the previously described phase structures with a collimated beam produces an interferometric scheme: The binary grid acts as a diffraction grating and, ideally, splits 80% of the energy of the collimated beam into the diffraction orders ± 1 (40% each), with angles, dependent on the grid period, that bring one of them to overlap with the beam reflected on the uniform half of the SLM producing a fringe pattern
Summary
Parallel-aligned liquid (PAL) crystal spatial light modulators (SLM) are devices for wavefront manipulation that can be considered as linear retarders with spatially-resolved programmable phase-shift. The use of a lens for overlapping small spots, the need of a microscope objective for magnifying the fringes, and the fact that the interfering beams travel along paths relatively far away compared to the wavelength (typically millimeters compared to hundreds of nanometers) before overlapping implies that fringe patterns become very sensitive to any change on their path difference introduced by mechanical vibrations and even air turbulence Another major difficulty arises when aligning the system since errors when overlapping the focal spots produced by the two beams can reduce considerably the contrast of the fringes and can affect the measurements. This apparent simple procedure is time consuming, requires precise alignment involving several optical components for reasonable phase estimation.
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