Overcoming the penetration depth limit in optical microscopy:Adaptive optics and wavefront shaping

Despite the unique advantages of optical microscopy for molecular specific high resolution imaging of living structure in both space and time,curent applications are mostly limited to research settings.This is due to the aberrations and multiple scattering that is induced by the inhomogeneous refractive boundaries that are inherent to biological systems.However,recent developments in adaptive optics and wavefront shaping have shown that high resolution optical imaging is not fundamentally limited only to the observation of single cells,but can be significantly enhanced to realize deep tissue imaging.To provide insight into how these two closely related fields can expand the limits of bio imaging,we review the recent progresses in their performance and applicable range of studies as well as potential future research directions to push the limits of deep tissuse imnaging.

Wavefront reconstruction algorithm for wavefront sensing based on binary aberration modes

We propose a new algorithm for wavefront sensing based on binary intensity modulation. The algorithm is based on the fact that a wavefront can be expended with a series of orthogonal and binary functions, the Walsh series. We use a spatial light modulator（SLM） to produce different binary-intensity-modulation patterns which are the simple linear transformation of the Walsh series. The optical fields under different binary-intensity-modulation patterns are detected with a photodiode.The relationships between the incident wavefront modulated with the patterns and their optical fields are built to determinate the coefficients of the Walsh series. More detailed and strict relationship equations are established with the algorithm by adding new modulation patterns according to the properties of the Walsh functions. An exact value can be acquired by solving the equations. Finally, with the help of phase unwrapping and smoothing, the wavefront can be reconstructed. The advantage of the algorithm is providing an analytical solution for the coefficients of the Walsh series to reconstruct the wavefront. The simulation experiments are presented and the effectiveness of the algorithm is demonstrated.

Application of optical diffraction method in designing phase plates

Continuous phase plate（CPP）,which has a function of beam shaping in laser systems,is one kind of important diffractive optics.Based on the Fourier transform of the Gerchberg-Saxton（G-S） algorithm for designing CPP,we proposed an optical diffraction method according to the real system conditions.A thin lens can complete the Fourier transform of the input signal and the inverse propagation of light can be implemented in a program.Using both of the two functions can realize the iteration process to calculate the near-field distribution of light and the far-field repeatedly,which is similar to the G-S algorithm.The results show that using the optical diffraction method can design a CPP for a complicated laser system,and make the CPP have abilities of beam shaping and phase compensation for the phase aberration of the system.The method can improve the adaptation of the phase plate in systems with phase aberrations.

Complete Solution of Sun Tracking for Heliostat

A general solution of sun tracking for an arbitrarily oriented heliostat towards an arbitrarily located target on the earth is published. With the most general form of solar tracking formulae, it is seen that the used azimuthelevation, spinning-elevation tracking formulae etc. are the special cases of it. The possibilities of utilizing the general solution and its significance in solar energy engineering are discussed.

A qualitative approach to aberrations of optical elements

A qualitative method to analyze wavefront aberrations is presented.Aberrations of the primary type,expressed in their matrix forms,are used to write the generalized ray transfer matrix of an optical component.The aberrations were treated collectively by examining the pseudospectra of an augmented matrix constructed from the aberration matrices.Results show that aberrations can be distinguished and relative strengths pronounced using this qualitative method.

Magnetic fluid based deformable mirror for aberration correction of liquid telescope

A magnetic fluid based deformable mirror(MFDM) that could produce a large stroke more than 100 μm is designed and demonstrated experimentally with respect to the characteristics of the aberration of the liquid telescope. Its aberration correction performance is verified by the co-simulation using COMSOL and MATLAB. Furthermore, the stroke performance of the MFDM and the decentralized linear quadratic Gaussian(LQG) mirror surface control approach are experimentally evaluated with a prototype of MFDM in an adaptive optics system to show its potential application for the large aberration correction of liquid telescopes.

Point spread function estimation from projected speckle illumination

The resolution of an imaging apparatus is ideally limited by the diffraction properties of the light passing through the system aperture,but in many practical cases,inhomogeneities in the light propagating medium or imperfections in the optics degrade the image resolution.Here we introduce a powerful and practical new approach for estimating the point spread function(PSF)of an imaging system on the basis of PSF Estimation from Projected Speckle Illumination(PEPSI).PEPSI uses the fact that the speckles’phase randomness cancels the effects of the aberrations in the illumination path,thereby providing an objective pattern for measuring the deformation of the imaging path.Using this approach,both wide-field-of-view and local-PSF estimation can be obtained by calibration-free,single-speckle-pattern projection.Finally,we demonstrate the feasibility of using PEPSI estimates for resolution improvement in iterative maximum likelihood deconvolution.