Phase-only SLM steers beam more accurately with global phase offsets

A digitally controlled 1-D phase-only spatial light modulator (SLM) is a very fast, nonmechanical way to steer a laser beam, but the digitized pixel-phase settings that are calculated for a chosen SLM design can result in significant steering inaccuracies in the vicinity of certain angles (for example, near half the maximum steering angle). A group of scientists at the University of Gothenburg and Chalmers University of Technology (both in Göteborg, Sweden) have discovered a way to boost the accuracy of the calculations: they introduce a global phase offset (in other words, one that is the same for all the SLM pixels) whose value is optimized for each steering angle.

The global phase offset is chosen so that the resulting mean slope of the digitized staircase-shaped phase function matches the aimed slope as closely as possible. The researchers found that the resulting global offset appears to vary almost randomly as a function of angle–except in the vicinity of the most-difficult angles (like half the maximum angle), where it varies more slowly and in a somewhat parabolic manner. In theory, the new method reduces the steering inaccuracy from 15% of the diffraction-limited spot size to 2.4% of the spot size; experiments with a 128-pixel SLM and a HeNe laser beam verified the theory. Contact David Engström at [email protected].