Yuhang Wang

Improved differential confocal microscopy with ultrahigh signal-to-noise ratio and reflectance disturbance resistibility

Improved differential confocal microscopy is proposed to improve axial resolution and to enhance disturbance resistibility of confocal microscopy. The subtraction and sum values of the two defocusing detected signals are divided as the response function. Both ultrahigh signal-to-noise ratio (SNR) and wide range can be selectively obtained by controlling the defocusing amount of the two differential detectors more tightly with the reflectance disturbance resistibility. Since the detecting sensitivity of the proposed confocal microscopy is unrelated to the energy loss of the reflected beam, the multiplicative mode disturbance can be used to measure microstructures made of hybrid materials and overcome the power drift of a laser source during long scanning. In the case of ultrahigh SNR, the axial resolution reaches 1 nm when NA=0.75 and lambda=632.8 nm.

Stable and robust frequency domain position compensation strategy for Fourier ptychographic microscopy

Fourier ptychographic microscopy is a computational microscopy technique to achieve wide-field and super-resolution complex imaging which has been developed in recent years. The method is based on illuminating the sample by a light source array, and then computationally integrating different images correspondent to each of the sources, in the Fourier domain. Knowledge of the exact relative position of the light sources and the sample is critical for the quality of the final recovered image. In this paper, we present an iterative approach towards correcting the position in the Fourier domain based on Newtons method. Also, an analysis is presented which shows the relation between the position error and the deterioration of the final recovery quality. The effectiveness of the presented method in improving the quality of the final recovered image is demonstrated using simulation and experimental results. Moreover, the method is shown to be more stable and robust to noises in comparison with the state-of-the-art algorithm.

Measuring profile of large hybrid aspherical diffractive infrared elements using confocal profilometer

The discrete profiles of large hybrid aspherical diffractive infrared elements (HADIEs) are measured using a confocal profilometer, and are extracted using a polynomial regression method. In this study, a HADIE with a diameter of 80 mm is measured using a confocal profilometer with maximum horizontal and vertical measurement ranges of 120 mm and 12 mm, respectively. The experimental results indicate that the confocal profilometer can compensate for the fabrication error of the HADIE, which can be reduced from 7.20 µm to 1.05 µm. Furthermore, a profile error of 0.64 µm (measured peak-to-valley (P-V)) is obtained after correcting for the curvature error. The confocal profilometer is a nondestructive tool that can be used to measure the profile of HADIEs, and is particularly suitable in compensating for the fabrication error of fine optical elements.

Correction of phase-delay distortion for α–β circular scanning

α-β circular scanning with a large scanning field of view and high reliability can be widely applied in laser scanning imaging systems and laser processing. However, mechanical inertia of the galvanometers introduces phase delay and ultimately leads to scanning distortions in α-β circular scanning in both constant angular velocity scanning (CAVS) and constant line velocity scanning (CLVS). To compensate for the phase-delay distortions, two correction models are respectively proposed for CAVS and CLVS, which utilize phase-frequency relationships based on the galvanometers transfer function. Experimental results show that the presented models can effectively correct rotation distortion in CAVS and tortuosity distortion in CLVS. The correction of phase-delay distortion can improve the image quality and refine positioning accuracy in laser scanning systems.

Automatic, high-accuracy image registration in confocal microscopy

We proposed a high-accuracy image registration method of confocal microscopy for a large field of view and high resolution. The spatial information (edge information) and the entropy correlation coefficient have been both taken into account for higher accuracy of registration. The edge information is introduced to calculate the normalization correlation coefficient of the image. Then the normalization correlation coefficient and the entropy correlation coefficient of the original image have been used to improve the proposed similarity measures, the normalized mutual information with edge information (called EMI). Meanwhile, a parallel particle swarm optimization (pa-PSO) with the idea of conditional initialization and parallel cooperation is developed to speed up the convergence rate and further reduce the mismatch. Experiments verified that the registration accuracy can be up to 0.2 pixel and has better robustness to the noise.

focusing by an arbitrary opening paraboloid mirror and its application in confocal scanning microscopy

Focusing of the plane wave with radially polarized electric field by an arbitrary opening paraboloid mirror is analyzed using a rigorous vectorial diffraction theory, i.e., Stratton-Chu integral. In the vicinity of the focus, far-field approximation conditions are used to simplify the derived integrals with sufficiently high accuracy. It is found that a noticeable deviation of the approximate integral, as characterized by a phenomenon of focal shift, from the exact integral can be observed when the maximum focusing semi-angle α below π/9. For α=π/2, the radial spot size reduces to below 0.40λ if cutting off the central segment, larger than π/4, of the paraboloid mirror. The sharp focusing property of the paraboloid mirror has the potential application in super-resolution confocal scanning microscopy. Specific confocal scanning arrangements are provided and remarked.