Chenguang Liu

Super-aperture metrology: overcoming a fundamental limit in imaging smooth highly curved surfaces

The imaging of smooth, highly curved or tilted surfaces is widely recognized as one of the most challenging and unsolved problems in optical imaging and metrology today. The reason is that even when such surfaces are imaged using high aperture microscope objectives the steepness of the features causes the light to be reflected in such a way that it is not captured by the lens. This is true even in the limiting case of unity numerical aperture since the illuminating light may also be reflected in the forward direction. In order to overcome this fundamental problem we have developed a method whereby such specimens are covered with a readily removable organic fluorescent film thereby creating an isotropic scattering surface. We show that we are readily able to detect slopes with angles close 90° using a 0.75 NA objective – an 82% improvement over the theoretical aperture limit. Issues of variation in film thickness deposition are shown to be readily accommodated. This approach may be used with other fluorophore materials, organic or inorganic, since there is no need for biocompatibility in this application.

Sinc2 fitting for height extraction in confocal scanning

The essential step in determination of height in confocal microscopy is localization of the axial peak positions. A sinc2-fitting algorithm was developed to achieve a reliable and theoretically accurate method for height extraction in surface topography measurements. We demonstrate that the sinc2 model closely matches the rigorously calculated axial response for some typical cases, such as low- or high-aperture focusing when scanning a planar object. Compared with the existing methods, such as polynomial fitting and Gaussian fitting, sinc2 fitting can be used to easily determine the initial values of the fitted parameters. In addition, the method is computationally efficient and theoretically rigorous.

Digital differential confocal microscopy based on spatial shift transformation

Differential confocal microscopy is a particularly powerful surface profilometry technique in industrial metrology due to its high axial sensitivity and insensitivity to noise. However, the practical implementation of the technique requires the accurate positioning of point detectors in three‐dimensions. We describe a simple alternative based on spatial transformation of a through‐focus series of images obtained from a homemade beam scanning confocal microscope. This digital differential confocal microscopy approach is described and compared with the traditional Differential confocal microscopy approach. The ease of use of the digital differential confocal microscopy system is illustrated by performing measurements on a 3D standard specimen.

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.

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.

The α-β circular scanning with large range and low noise

A circular‐route scanning method called α‐β circular scanning is proposed and realized using sinusoidal signals with a constant phase difference of π/2. Experiments show that the circular scanning range of α‐β circular scanning is 57% greater than the rectangular scanning range of raster scanning within an effective optical field of view. Moreover, the scanning speed is improved by 7.8% over raster scanning because the whole sine signal is utilized in α‐β circular scanning whereas the flyback area of the saw‐tooth signal needs to be discarded in raster scanning. The maximum scanning acceleration decreases by a factor of 44, drastically decreasing the high noise, which should considerably elongate the lifetime of the galvanometers while inhibiting internal vibration. The proposed α‐β circular scanning technique could be used in scanning imaging, optical tweezers and laser‐beam fabrication.