Phase microscopy for high-dynamic-range quantitative phase-contrast imaging of a transparent phase object was demonstrated. Using a common path Fourier domain optical coherence tomography system, this technique is capable of displacement measurement with a sensitivity of 34 pm. The limitation of 2pi ambiguity restriction was overcome by the use of a phase retrieval approach performed in spectral domain. Two-dimensional quantitative phase imaging of human neonatal dermal keratinocyte cells was demonstrated to evaluate the performance of the system for cell imaging.
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"Furthermore, by extracting the phase term from the interference signal to measure nanostructures, the phase of the OCT signal oscillates 2π rad at every shift of half a wavelength of optical path difference. This renders measurements of the displacement of sample to be limited to less than half a wavelength . "
[Show abstract][Hide abstract] ABSTRACT: efficiency and quality of solar cell devices. Currently, surface defect detection of solar cells can be achieved by several approaches such as machine vision, photoluminescence, and electroluminescence imaging techniques. Nevertheless, it is still difficult to inspect the inner structures of solar cells. In response to this need, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have become common techniques for investigating the nanostructures of solar cells. However, both carry the disadvantages of destructive imaging, high cost, and a small inspection area. Moreover, it is difficult to simultaneously estimate multiple optical properties with the aforementioned techniques.
In this study, we propose the use of phase-sensitive optical coherence tomography (PS-OCT) for the inspection of solar cells. We develop a two-reference-arm configuration to reduce the phase noise that intrinsically accompanies the OCT system. Based on the proposed approach to extract the amplitude and phase terms from OCT interference signals, the 3D microstructure of solar cells can be obtained while simultaneously probing the nanostructures on arbitrary planes of the solar cell. The OCT microstructural results show that the structures of different layers can be nondestructively visualized and quantitatively evaluated. From the phase signal, the inverted pyramid structure, which is commonly used for the reduction of interface reflection, can be visualized. Moreover, based on the two-reference-arm configuration, the optical reflection coefficient (ORC) can be estimated in order to evaluate the interface reflection from the surface of solar cell. Results show that PS-OCT can be a valuable tool for providing nondestructive inspection of the micro/nanostructures of solar cells.
Solar Energy Materials and Solar Cells 05/2015; 136:193-199. DOI:10.1016/j.solmat.2015.01.016 · 5.34 Impact Factor
"When the variation of FP cavity length is large—such that ϕi jumps from -π/2 directly to π/2 or from π/2 directly to -π/2—a phase unwrapping technique can be used to obtain continuous phase or displacement . "
[Show abstract][Hide abstract] ABSTRACT: During vitreoretinal surgery, the surgeon manipulates retinal tissue with tool-to-tissue interaction forces below the human sensory threshold. A force sensor (FS) integrated with conventional surgical tools may significantly improve the surgery outcome by providing tactile feedback to the surgeon. We designed and built a surgical tool integrated with a miniature FS with an outer diameter smaller than 1 mm for vitreoretinal surgery based on low-coherence Fabry-Pérot (FP) interferometry. The force sensing elements are located at the tool tip which is in direct contact with tissue during surgery and the FP cavity length is interrogated by a fiber-optic common-path phase-sensitive optical coherence tomography (OCT) system. We have calibrated the FS's response to axial and lateral forces and conducted experiments to verify that our FS can simultaneously measure both axial and lateral force components.
"Previous studies have also demonstrated that SD-OCT can achieve higher displacement sensitivity (DS) compared to the conventional SS-OCT approach. In addition, several methods have been proposed to improve the DS and increase the measurable optical path difference beyond half a wavelength for SD-OCT systems  . One such method involves the employment of a thin glass slide on top of the sample to provide a reference plane and produce the interference with the sample, while another includes the implementation of a phase unwrapping algorithm to overcome the 2π ambiguity restriction. "
[Show abstract][Hide abstract] ABSTRACT: In this letter, a phase-sensitive, swept-source optical coherence tomography (SS-OCT) system is implemented for the optical measurement of nanostructures. A new approach is proposed to reduce the phase errors, resulting from trigger jitter of the swept source and the asynchronization between the A-scan trigger and OCT signal at the data acquisition end, with a narrowband fiber Bragg grating to generate the accurate A-scan trigger. Furthermore, combining the common-path configuration with the proposed approach, the displacement sensitivity can be calculated to be 80 pm when the swept source is operated at 30 kHz. Finally, the conducting glass was scanned with the proposed approach to quantitatively measure the thickness of conducting layer. The results show that the proposed SS-OCT approach can make be a potentially useful tool for noninvasive, real-time inspection of nanostructures.