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Calibration of spatial light modulators suffering from spatially varying phase response

Journal article
Authors David Engström
Martin Persson
Jörgen Bengtsson
Mattias Goksör
Published in Optics Express
Volume 21
Issue 13
Pages 16086-16103
ISSN 1094-4087
Publication year 2013
Published at Department of Physics (GU)
Pages 16086-16103
Language en
Links dx.doi.org/10.1364/OE.21.016086
https://gup.ub.gu.se/file/142567
Keywords Spatial light modulators, Phase modulation, Diffractive optics, Digital holography, Computer holography, Holographic optical element, Laser trapping, Optical tweezers, Optical manipulation
Subject categories Optics, Electrical Engineering, Electronic Engineering, Information Engineering, Medical Equipment Engineering

Abstract

We present a method for converting the desired phase values of a hologram to the correct pixel addressing values of a spatial light modulator (SLM), taking into account detailed spatial variations in the phase response of the SLM. In addition to thickness variations in the liquid crystal layer of the SLM, we also show that these variations in phase response can be caused by a non-uniform electric drive scheme in the SLM or by local heating caused by the incident laser beam. We demonstrate that the use of a global look-up table (LUT), even in combination with a spatially varying scale factor, generally does not yield sufficiently accurate conversion for applications requiring highly controllable output fields, such as holographic optical trapping (HOT). We therefore propose a method where the pixel addressing values are given by a three-dimensional polynomial, with two of the variables being the (x;y)-positions of the pixels, and the third their desired phase values. The coefficients of the polynomial are determined by measuring the phase response in 8×8 sub-sections of the SLM surface; the degree of the polynomial is optimized so that the polynomial expression nearly replicates the measurement in the measurement points, while still showing a good interpolation behavior in between. The polynomial evaluation increases the total computation time for hologram generation by only a few percent. Compared to conventional phase conversion methods, for an SLM with varying phase response, we found that the proposed method increases the control of the trap intensities in HOT, and efficiently prevents the appearance of strong unwanted 0th order diffraction that commonly occurs in SLM systems.

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