Increasing enhancement factor in wavefront shaping by means of spatial filtering

Enhancement factor in wavefront shaping depends on the spatial resolution of a spatial light modulator (SLM) and on the modulation type. For a given resolution, phase modulation provides better enhancement than amplitude modulation, and thus is more preferable. In order to make wavefront shaping practical, it is important to provide the highest possible refresh rate of SLM. Liquid crystal SLMs are known for their outstanding phase modulation capabilities but their speed usually does not exceed a few hundred hertz. Meanwhile, digital micromirror devices (DMDs) can reach the refresh rates of 20 kHz that meets demands of practical applications. Although the refresh rate is high, this kind of the SLM can operate only in binary amplitude mode. In this paper we consider conversion of binary amplitude modulation into phase modulation by removal of zero spatial frequency from the modulated beam with respect to the application in wavefront shaping. We demonstrate experimentally increasing enhancement factor in 1.5 times (theoretically up to 2-fold). The advantage of considered method is its simplicity as it requires only a telescopic system and a simple spatial filter. In our case, it did not require any modification of the existing setup except the addition of the removable filter. Also, the considered method is suitable for any kind of spatial light modulator and is not limited to DMD. The obvious benefit of the modulation conversion is that we can use fewer modes to achieve the same enhancement factor and thus increase the focusing speed.