Complex beam sculpting with tunable acoustic gradient index lenses

Abstract

Spatial Light Modulators (SLMs) have been successfully used for beam sculpting in the area of optical manipulation, however in some applications their associated pixelation, slow switching speeds, and incident power limitations can be undesirable. An alternative device that overcomes these problems is the Tunable Acoustic Gradient index (TAG) lens. This device uses acoustically induced density and refractive index variations within a fluid to spatially phase modulate a transmitted laser beam. The acoustic waves within the fluid are generated via a piezoelectric transducer. When driven with a frequency-modulated signal, arbitrary optical phase modulation patterns can be generated at regular time intervals. The resulting sculpted beam is best observed using a pulsed laser synchronized to the frequency-modulated signal of the TAG lens. As this device is purely analog, there is no pixelation in the phase modulation pattern. Also, because the only major requirement on the fluid is that it be transparent, it is possible to select fluids with high damage thresholds and high viscosities. High damage thresholds allow the TAG lens to be used in high power applications that would be unsuitable for an SLM. High viscosities provide fast damping of transient density variations and increase switching speeds between patterns. Discussion here will be limited to axially symmetric beam sculpting, however the results can be generalized to asymmetric cases.