Developing novel wavefront shaping techniques through pupil conjugate mask engineering

Abstract

Understanding of optical scattering has primarily been led by physicists and has proven to be beneficial in the field of astronomical observation. Then it is now coming to fruition in the bioimaging field to conquer scattering caused by the inhomogeneous refractive index of biological tissue. This technique is named adaptive optics (AO) or wavefront shaping (WFS). Controlling optical aberrations for laser scanning fluorescence microscopy (LSFM) such as confocal microscopy or multiphoton microscopy (MPM) restores point-spread-function (PSF) to see deeper and offer sharper resolution by increasing energy transfer efficiency. Beyond mere resolution recovery, it has been discovered that by utilizing severely scattering media to reproduce the PSF of the system, it is possible to employ the scattering media as a new lens, so- called scattering lens. LSFM for deep tissue imaging is one of the best applications for AO or WFS. In the schematic of LSFM, a wavefront shaper is conjugated to pupil plane of the objective lens or sample plane. In this thesis, aiming to achieve efficient wavefront shaping through multiple scattering media in the LSFM regime, three novel approaches are introduced with a brief introduction of pupil conjugate wavefront shaping and scattering lens in chapter 1. In chapter 2, a method to increase enhancement factor for binary amplitude modulator, digital micromirror device (DMD) has introduced by exploiting multiple scattering and using symmetric amplitude mask (SM). As a result, the binary amplitude modulator, DMD, has been exploited as a binary phase modulator while maintaining its fast refresh rate. In this work, to the best of our knowledge, SM is newly adopted in wavefront shaping. Thus, we delved into the differences between a conventional mask and SM in wavefront shaping through scattering media in chapter 3. We first devised a generalized SM, specifically a periodically repeated mask (PRM). We then examined the corresponding speckle fields to find a relationship between the design of the wavefront mask and the characteristics of the focus produced by the wavefront mask. By adjusting the parameters of PRM, we obtained different shapes of foci within the same measurement time for optimization, controlling numerical aperture (NA) of scattering lens have been achieved. In chapter 4, an advanced WFS technique for MPM for deep tissue imaging has been proposed. Our method is based on a recently published modal-based method, dynamic adaptive scattering compensation holography (DASH) using phase stepping holography. In the paper, the fixed value of modulation power, f, hinders the optimal phase mask from converging to the optimum values. To overcome this drawback, we introduced an advanced version of DASH, FDASH, in which the f linearly decreases during the optimization process, resulting in a more accurate wavefront compared to the original method. This approach aligns with the principle that reducing perturbation or scanning power can aid in finding accurate solution, especially when the algorithm is scanning near global optimal value.