Abstract
Speed and enhancement are the two most important metrics for anti-scattering light focusing by wavefront shaping (WS), which requires a spatial light modulator with a large number of modulation modes and a fast speed of response. Among the commercial modulators, the digital-micromirror device (DMD) is the sole solution providing millions of modulation modes and a pattern rate higher than 20 kHz. Thus, it has the potential to accelerate the process of anti-scattering light focusing with a high enhancement. Nevertheless, modulating light in a binary mode by the DMD restricts both the speed and enhancement seriously. Here, we propose a multi-pixel encoded DMD-based WS method by combining multiple micromirrors into a single modulation unit to overcome the drawbacks of binary modulation. In addition, to efficiently optimize the wavefront, we adopted separable natural evolution strategies (SNES), which could carry out a global search against a noisy environment. Compared with the state-of-the-art DMD-based WS method, the proposed method increased the speed of optimization and enhancement of focus by a factor of 179 and 16, respectively. In our demonstration, we achieved 10 foci with homogeneous brightness at a high speed and formed W- and S-shape patterns against the scattering medium. The experimental results suggest that the proposed method will pave a new avenue for WS in the applications of biomedical imaging, photon therapy, optogenetics, dynamic holographic display, etc.
Highlights
As photons are propagating through a scattering medium, their trajectories are randomly changed, so they cannot be focused to a micrometer-scale spot, which fundamentally limits the application of optical imaging, photon therapy, and optogenetics[1,2]
Since each micromirror in the digital-micromirror device (DMD) modulates light independently, we set the Hamming distance between the neighboring code values to 1 in the multi-pixel encoding process to mimic a continuous modulation on the light amplitude
The natural gradient provides a direction along which the feedback amplitude increases as well, and it can prevent the a Target plane b
Summary
As photons are propagating through a scattering medium, their trajectories are randomly changed, so they cannot be focused to a micrometer-scale spot, which fundamentally limits the application of optical imaging, photon therapy, and optogenetics[1,2]. If the optical property of the scattering medium is changed during the measurement, this method will fail and is not useful on highly dynamic scattering media[10,12]. By measuring a feedback signal, the iterative WS method optimizes the incident wavefront step by step and gradually increases the light intensity at the desired focus. The feedback signals can be light intensity[13,14], fluorescence[5,15,16], photoacoustic signal[17,18], etc This method can adjust the incident wavefront with respect to the current status of the dynamic scattering medium and has the potential to be applied in highly dynamic environments[12,19]
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