Phase-only computer-generated holograms generated by spatial light modulators (SLMs) can be used to diffract an incoming coherent beam into numerous focused spots distributed arbitrarily in three dimensions—which can be used, for example, in beam steering, optical interconnects, and optical tweezers. But coming up with the phase values for the SLM pixels is difficult, and is usually arrived at by a combination of calculation and iteration. Researchers at the University of Gothenburg and the Chalmers University of Technology (both in Göteborg, Sweden) have developed an approach that allows many more focused spots to be created with almost no extra computational cost compared to conventional approaches.
An extension of the commonly used Gerchberg-Saxton algorithm, the new technique is a single-plane fast-Fourier-transform-based design method, in which the calculations—which are done for an intermediate “iteration plane”—use more of the information from that plane than do previous methods, with the only restriction being that the converging or diverging beamlets at that plane can’t substantially overlap. The technique was tested experimentally with a liquid-crystal SLM, producing up to 25 grid-free spots in three dimensions, and not only Gaussian spots, but Laguerre-Gaussian spots as well (the figure shows simulated spots on the left and experimental spots on the right; the noncircular symmetry of the experimental Laguerre-Gaussian spots is caused by astigmatism from a curved SLM, not by the algorithm). Contact David Engström at [email protected].