Feedback-based wavefront shaping for weak light with lock-in beat frequency detection

Abstract

Feedback-based wavefront shaping is a promising and versatile technique for enhancing the contrast of a target signal through highly scattering media. However, this technique can fail for low optical signals such as fluorescence and Raman signals or in a reflection setup because the trend in weak feedback signals can be easily overwhelmed by noise. To address this challenge, we develop a technique based on a single acousto-optic deflector (AOD) to create a signal with a selected beat frequency from optical signals that can serve as feedback, in which the phase distribution of various radio frequency components of the driving signal for the AOD is optimized for wavefront shaping. By shifting incident light frequency with the AOD, the feedback signal at a selected beat frequency can be measured with a high signal-to-noise ratio (SNR) by a lock-in amplifier, thus enabling the enhancement of weak target signals through highly scattering media. It is found that the method of lock-in beat frequency detection can significantly improve fluorescence imaging and Raman spectral measurements in a reflection setup, and thus could be potentially used for in vivo measurements.