Yunxin Wang

Hybrid phase retrieval approach for reconstruction of in-line digital holograms without twin image

A hybrid phase retrieval approach is proposed to address the twin image problem in the reconstruction of in-line digital holograms. The approach is a variant iterative transform algorithm and exploits two mostly natural constraints of a sample, namely, the finite transmission and the finite support. Here, the initial sample support estimate is first refined by applying the finite transmission constraint with phase flipping. The approach provides better reconstruction than if only the finite transmission constraint is used and improve the convergence rate of Fienups algorithm owing to a better estimate of support especially for strong samples with complex structures. Both simulation and experimental results are presented.

Three-dimensional imaging of microstructure by dual-wavelength digital holography

In this paper, dual-wavelength digital holography is demonstrated by both numerical simulation and experiment. Simulations were done for two wavelengths of 500 nm and 550 nm. The objects are a wedge prism with a height of 2.5 μm and four abrupt steps. Three-dimensional (3D) reconstruction profiles of the original objects are obtained in the simulations. We find that dual-wavelength phase unwrapping is a fast and robust method for removing 2π discontinuities compared to software algorithm-based methods. In the experiment, two lasers of the different wavelengths 632.8 nm and 671 nm are used to obtain a larger beat wavelength. The object is a phase grating. In dual-wavelength phase unwrapping, two individual phase images are obtained by using each wavelength, respectively, and the phase image for beat wavelength is obtained by subtracting one single wavelength phase image from the other and then adding 2π whenever the resultant value is less than zero. In the final synthetic image, 3D profile of the phase grating is obtained after removing, the discontinuities by reducing the noise of the beat wavelength phase image. The experiment results demonstrate the effectiveness of the dual-wavelength phase unwrapping in digital holography.

Fingerprint acquisition for the criminal investigation by digital holography

Digital holography is proposed as a non-destructive fingerprint acquisition method in the criminal investigation field. Digital holography is a new imaging technology which combines the advantages of both the optical holography and digital processing. As an advanced optical diagnostic tool, digital holography allows to provide the quantitative phase information and the intensity information simultaneously. Based on the off-axis digital Fresnel holography, the principle of recording and reconstruction are analyzed, the reconstruction methods of the fingerprint amplitude and phase image are investigated, and the experimental results are presented. Both the theoretical analysis and the experimental results showed that digital holography does not destroy the on-site fingerprints, overcomes the shortcomings of traditional fingerprint acquisition methods, and as a simple and effective acquisition method can be applied to the criminal investigation field.

Effect of noise on the performance of image restoration in an optical sparse aperture system

The optical sparse aperture system achieves almost the same resolution as a conventional full filled aperture telescope but with a lower weight and cost. However, the greatly reduced modulation transfer function (MTF) causes significant blurring and loss of contrast in the observed image. The image restoration algorithms can correct the blurring if the actual optical transfer function (OTF) or point spread function (PSF) is given. Usually, the OTF and PSF captured by experiment is assumed to be the actual OTF and PSF, which are considered as the best match for image restoration. However, the noise contained within the captured PSF or OTF may prevent successful restoration. Here, we investigate the restored image by the captured PSF and the calculated PSF based on array configuration. The calculated PSF produced a better result than the captured PSF.

Non-invasive monitoring of living cell culture by lensless digital holography imaging

A non-invasive detection method for the status analysis of cell culture is presented based on digital holography technology. Lensless Fourier transform digital holography (LFTDH) configuration is developed for living cell imaging without prestaining. Complex amplitude information is reconstructed by a single inverse fast Fourier transform, and the phase aberration is corrected through the two-step phase subtraction method. The image segmentation is then applied to the automatic evaluation of confluency. Finally, the cervical cancer cell TZMbl is employed for experimental validation, and the results demonstrate that LFTDH imaging with the corresponding image post-processing can provide an automatic and non-invasive approach for monitoring living cell culture.

Autofocusing on pure phase object for living cell imaging in lensless Fourier transform digital holography

The lensless Fourier transform digital holography has been widely employed in microscopic imaging. It enables quantitative phase analysis for both reflection and transmission objects. The phase image is obtained in the numerical reconstruction procedure. The in-focus reconstruction distance could be determined according to the extremum of the autofocusing criterion function, which is commonly applied in finding the in-focus amplitude image of the object. Then the reconstruction distance for the phase image is considered to be equal to the one for the amplitude image. When the object is a pure phase sample, such as the living cell, the minimum value of the autofocusing criterion function should be found to determine the in-focus reconstruction distance. However, in the experiment, the in-focus amplitude image is often not an ideal uniform bright field, so this method will result in some deviation. In this contribution, two derivatives-based criterion functions are applied to the phase image directly to accomplish the in-focus phase contrast imaging, which is more intuitive and precise. In our experiments, the set-up of the lensless Fourier transform digital holography is established firstly. Then the living cervical carcinoma cells are detected. The phase aberration is corrected by two-step algorithm. The final autofocusing results verify the algorithm proposed in this paper.

Active and Interactive Teaching Based on Exploring Forefront Topics in Information Optics

Information Optics (i.e. Fourier Optics) is a compulsory professional course in the teaching program for juniors in the field of applied physics at Beijing University of Technology. Various methods are applied to information optics teaching in order to obtain satisfying teaching effect. Active and interactive teaching method based on exploring forefront topics was proposed and put into practice, especially for teaching “holography and holographic technology application” section of the courseU00100000in which the teaching activity was not restricted to classroom any more. A visiting to the exhibit of forefront production of holographic display was introduced as an episode in the teaching. The process of teaching was designed elaborately to an interactive activity between the teacher and students, and to stimulate students to cooperate. The teaching practice proves that the active and interactive teaching method is much favorable by students and successful in information optics teaching.

The measurement of average refractive index with substrate-calibrating by using complex frequency-domain optical coherence tomography method

Optical Coherence Tomography (OCT) was successfully applied in the microstructure imaging of biological tissuenafter being proposed firstly in 1991 by the researchers of MIT. As a novel optical imaging technology, it mainly usesninterference principles to achieve noninvasive and high resolution visualization of samples. OCT works analogously tonan ultrasound scanner, the major difference is that ultrasound pulses are replaced by broadband light. According tonwhether need for mechanical axial scan in the depth direction, it can be classified into the time-domain OCT (TD-OCT)nand frequency-domain OCT (FD-OCT). The FD-OCT system overmatches the TD-OCT in imaging speed because of itsndepth collection advantage. But in the reconstructive image of FD-OCT detection, the complex-conjugate ambiguity willnseriously deteriorate the imaging effect of tomogram. So the technique of removing the complex-conjugate image isnemployed that is called complex FD-OCT. The complex FD-OCT has widely application in many fields, especially innthe refractive index measurement. The refractive index is an important parameter characterizing light propagation in thenmedium. In the paper, we present a method to measure the average refractive index of the sample with substratencalibration by using complex FD-OCT method, in which we can calculate it without depending on the parameters ofnsystem such as spectral width of light source. Due to the measurement of average refractive index relative to the actualnthickness and optical length, it is necessary to obtain them of the sample experimentally. The complex FD-OCT methodncan easily achieved the optical length via measuring the positions of the sample’s front and rear surfaces. In thenexperiment, the coverslip (the borosilicate glass) is chosen as the sample and the calibration substrate. We make use ofnthe substrate to load the sample on it, and then the tomogram of the sample can be achieved by means of OCT’s lateralnscan in the edge of the sample and complex FD-OCT method. According to the experimental results, we can acquire thensample’s tomographic information and position of the substrate. The ratio of actual thickness and optical length can benindirectly calculated out with the pixel number obtained by analyzing the image data. So with only one time scan, we canncomplete the measurement of average refractive index of the sample without aid of other instruments.

Phase imaging for photorefractive holographic gratings with dual-wavelength digital holography

In this paper，the phase-type grating recorded in a Fe:Cu:LiNbO3 crystal is measured by dual-wavelength digital holography. In the experiment, a volume hologram, which is recorded in a 3-mm-thick Fe:Cu:LiNbO3 crystal by interference of two recording beams at the wavelength of 532 nm, is reconstructed to be imaging by dual-wavelength digital holography. Two lasers of the different wavelengths 660 nm and 671 nm are used to obtain a larger beat wavelength. Each laser output, which is spatially-filtered and collimated, is split into a reference and object beams in an interferometer setup based on the Mach-Zehnder configuration. In dual-wavelength phase unwrapping, two individual phase images are obtained by using each wavelength, respectively, and the phase image of beat wavelength is obtained by subtracting one single wavelength phase image from the other and then adding 2π whenever the resultant value is less than zero. In the final synthetic image, the discontinuities are removed after reduce the noise of the beat wavelength phase image. Thus, a 3D surface profile of the phase grating is obtained.

Simulations and experiments on optical focusing characteristics using an imaging fiber bundle

Imaging fiber bundle is a necessary element in a conventional endomicroscopy imaging system. The combination of a proximal spatial light modulator as a means of achieving beam scanning and an imaging fiber bundle for light delivery and collection enables the wavefront at the distal end of the fiber bundle to be synthesized, controlled and scanned. In this way the endomicroscope is very different from conventional systems which use proximal scanning mirrors or distal scanning heads. Thus, it is necessary to investigate the effect of primary parameters, such as diameter of each core, core-core separation and phase mask applied to the face of the imaging fiber bundle on the characteristics of focusing spot. These effects were simulated by numerically generating distal wavefronts and propagating them using the method of angular spectrum of plane-wave. The axial and lateral resolution and SNR were introduced to evaluate the characteristics of the focus. The imaging system could be optimized and reduced constraints on the imaging fiber bundle used based on these results.