Optical imaging can reveal the important biological information in the tissue and help human being better understand the biological structures, biochemical interactions, health condition in our body, but the effect of optical scattering in the biological tissue distorts the propagating trajectory of light.
As a result, all optical imaging techniques with high spatial resolution can be applied only in the superficial tissue in the body, unless finding a way to make the tissue "transparent" in vivo. For this reason, WS as one of the optimal solutions has been extensively studied. In this method, an optoelectronic device like DMD modulates the wavefront of incident light with high flexibility to control the trajectory of light in the scattering medium so that the scattering medium is like a transparent medium for optical imaging.
In a new paper published in Light Science & Application, Professor Jiamiao Yang at Shanghai Jiao Tong University, Professor Linxian Liu at Shanxi University, and Professor Yanyu Zhao at Beihang University proposed a new wavefront shaping (WS) method in which a digital micromirror device (DMD) with millions of optoelectronic units modulated light at a frame rate of ~ 20 kHz.
First, to solve the issue originated from the binary modulation of DMD, they proposed a multi-pixel encoding strategy in which they grouped multiple neighboring pixels into a single modulation unit to increase the accuracy of modulation. Second, they selected SNES as the optimizer, to circumvent the numerous inferior local minima in the WS problem. By calculating the natural policy gradient to maximize the optical feedback and then updating the parameters of sampling distribution, separable nature evolution strategy (SNES) could not only significantly shorten the period of optimization, but also prevent early convergence to an inferior local minimum, in comparison with the genetic algorithm.
"Comparing with the conventional GA method encoding each DMD pixel individually, the contrast with respect to a single focal spot and the speed of optimization could be improved by a factor of 16 and 179, respectively."
"Aided by the multi-pixel encoding SNES, the authors successfully generated 10 focal spots simultaneously and precisely controlled the energy distribution among these spots." they added.
"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." the scientists forecast.